FABRICATION OF CERAMIC COMPONENTS USING MOLD SHAPE DEPOSITION MANUFACTURING

FABRICATION OF CERAMIC COMPONENTS USING MOLD SHAPE DEPOSITION MANUFACTURING A DISSERTATION SUBMITTED TO THE DEPARTMENT OF MECHANICAL ENGINEERING AND THE COMMITTEE ON GRADUATE STUDIES OF STANFORD UNIVERSITY IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY Alexander G. Cooper August 1999

Copyright by Alexander Cooper 1999 All Rights Reserved ii

I certify that I have read this dissertation and that in my opinion it is fully adequate, in scope and quality, as a dissertation for the Doctor of Philosophy. Fritz Prinz (Principal Advisor) I certify that I have read this dissertation and that in my opinion it is fully adequate, in scope and quality, as a dissertation for the Doctor of Philosophy. Mark Cutkosky I certify that I have read this dissertation and that in my opinion it is fully adequate, in scope and quality, as a dissertation for the Doctor of Philosophy. Kosuke Ishii Approved for the University Committee on Graduate Studies: iii

Abstract Mold Shape Deposition Manufacturing (Mold SDM) is a new process for the fabrication of geometrically complex structural ceramic components. Mold SDM builds fugitive molds which are then used to produce ceramic parts via gelcasting. This thesis describes the development of the Mold SDM process, including process steps, materials selection and planning strategies. A prototype Mold SDM machine, which was built to automate the mold building phase of the process, is shown. Preliminary process characterization results are presented, as well as a range of current and potential applications for ceramic, metal and polymer parts made using Mold SDM. The high strength, stiffness and low density of ceramic materials make them attractive for structural applications, and their excellent high temperature properties make them particularly attractive in high temperature applications. The two primary obstacles to the wider use of ceramics are their low toughness and the limitations of current manufacturing processes. As designers gain more experience with ceramics and the materials themselves become more reliable, the low toughness issue will become less of a limitation. Current production processes are limited in terms of achievable shape complexity and also suffer from long lead times. Rapid prototyping processes have been adapted to the fabrication of ceramic parts but they generally produce parts with poor surface quality. There is therefore a need for methods that can quickly produce high quality, geometrically complex ceramic parts. Mold SDM was developed to address this need. Mold SDM is based on Shape Deposition Manufacturing (SDM) and uses SDM techniques to build fugitive wax molds. SDM is an additive-subtractive layered manufacturing process. The subtraction step, accomplished by milling, improves surface finish and increases material flexibility relative to rapid prototyping processes. In order to increase the capabilities of the Mold SDM process and to improve the quality of parts produced, a number of new build techniques were developed. These new techniques can also be used with the basic SDM process. iv

The materials selection work involved identification and testing of a variety of materials in order to optimize the performance of the Mold SDM process. Key materials properties were identified and used to guide the selection process. Candidate materials were evaluated, primarily in terms of machinability, shrinkage, heat resistance and chemical compatibility. A number of preferred materials combinations were developed based on these results and these were used to produce a range of ceramic, metal and polymer parts. Based on experience accumulated during process development and part building, a number of new process planning strategies were developed. The manufacturability analysis technique presented can be used to determine if a given part is manufacturable using Mold SDM. Guidelines are given for optimum build orientation selection based on an analysis of part features. New decomposition techniques are proposed to take advantage of process capabilities such as multiple build directions. The construction and operation of the experimental Mold SDM machine is described. The machine was built by adding material deposition equipment to a milling machine to produce a machine capable of automating the mold building phase of the Mold SDM process. Process characterization in terms of feature size capabilities, achievable accuracy and surface roughness, build rates and final part properties is presented. Common part defects and their causes are explained and methods of avoiding them are described. The process characteristics are compared to those of competing ceramic manufacturing processes and rapid prototyping techniques. The primary application for Mold SDM has been the fabrication of complex structural ceramic components for use in gas turbine engines. During the course of this research a number of other applications and potential applications have been identified for ceramic, metal and polymer parts made via Mold SDM. Characteristics of ideal applications are identified and used to evaluate these applications. Some proof of concept parts are shown. v

Acknowledgments I would like to thank my advisor, Professor Fritz Prinz for providing the environment which made all of this work possible. I would also like to thank him for his energetic support and for always encouraging me to aim higher. I would like to thank my thesis reading committee members, Professors Kosuke Ishii and Mark Cutkosky for their time and effort in providing insightful feedback and advice. I would like to thank Professor Tom Kenny for serving on my defense committee and Professor James Jucker for Chairing it. I would like to thank my colleagues in the Rapid Prototyping Laboratory and also the Center for Design Research for their contributions to this research and their friendship. I would particularly like to thank Dr. John Kietzman, Alex Nickel, Rudi Leitgeb, Dr. John Fessler, Tassos Golnas, Dr. Ju-Hsien Kao, Sangkyun Kang, Dr. Jim Stampfl, Byong-Ho Park, Miguel Pinilla, Dr. Mike Binnard, Eric Chamness, Dr. Robert Merz, Dr. Krishnan Ramaswami and Tom Hasler. Sylvia Walters and Lynn Hoschek provided essential administrative support and encouragement throughout this time. Thanks also go to my colleagues at Carnegie Mellon University, Dr. Lee Weiss, Larry Schultz and Dr. Gennady Neplotnik, for their advice and assistance. Dr. John Lombardi and David Dent, from Advanced Ceramics Research Inc., and Professor Paul Calvert, from the University of Arizona, provided much assistance, particularly on materials related issues. I would like to thank the Defense Advanced Research Projects Agency, in particular Dr. Bill Coblenz, for supporting this research under contracts N00014-96-I-0625 and N00014-98-I-0734. Part of my studies was also funded by a Department of Energy Integrated Manufacturing Pre-Doctoral Fellowship. Finally, I would like to thank all of my friends and family for their support and encouragement over the years. vi

Table of Contents Table of Contents...vii List of Tables...xii List of Figures...xiv 1 Introduction...1 1.1 Overview.........................................................1 1.1.1 Process Development..........................................1 1.1.2 Materials Selection and Optimization...............................2 1.1.3 Process Planning..............................................2 1.2 Thesis Outline.....................................................3 1.3 Individual / Group Research Statement..................................4 2 Background...5 2.1 Ceramics.........................................................5 2.1.1 Structural Ceramics............................................6 2.1.2 Issues.......................................................10 2.2 Ceramic Manufacturing Processes.....................................13 2.3 Ceramic Forming Processes..........................................15 2.3.1 Powder Pressing..............................................15 2.3.2 Slip Casting..................................................16 2.3.3 Tape Casting.................................................16 2.3.4 Extrusion.....................................................17 2.3.5 Injection Molding...............................................17 2.3.6 Green Machining..............................................17 2.3.7 Gelcasting....................................................18 2.4 Rapid Prototyping Of Ceramic Parts....................................20 2.4.1 Overview.....................................................20 2.4.2 Stereolithography (SLA).........................................22 2.4.3 Fused Deposition Modeling (FDM).................................23 2.4.4 Selective Laser Sintering (SLS)...................................24 2.4.5 Three Dimensional Printing (3DP).................................25 vii

2.4.6 Laminated Object Manufacturing (LOM)............................26 2.4.7 Direct Photo Shaping (DPS)......................................27 2.4.8 Computer Aided Manufacturing of Laminated Engineering Materials......27 2.4.9 Sanders Prototype.............................................28 2.5 Evaluation of Existing Processes.......................................28 2.6 Objectives for SDM and Mold SDM of Ceramics...........................29 3 Process Development...30 3.1 Shape Deposition Manufacturing (SDM).................................30 3.1.1 Process Description............................................30 3.1.2 Comparison to Other Rapid Prototyping Processes....................32 3.1.3 SDM Application Examples......................................34 3.1.4 Fabrication of Ceramic Green Parts Using SDM......................35 3.2 Mold Shape Deposition Manufacturing (Mold SDM)........................35 3.2.1 Process Description............................................35 3.2.2 Mold SDM Compared to SDM and Other RP Processes................37 3.3 New Build Techniques...............................................37 3.3.1 Freeform Layer Geometries......................................38 3.3.2 Overcut-Fill-Trim-Backfill........................................40 3.3.3 One Time Surface Machining.....................................42 3.3.4 Cut Through Machining.........................................44 3.3.5 Incremental Casting............................................44 3.3.6 Multiple Build Directions.........................................46 3.4 Fabrication Of Pre-Assembled Mechanisms..............................47 3.5 Process Example...................................................51 4 Materials Selection...54 4.1 Materials Requirements..............................................54 4.1.1 Mold Building Materials.........................................55 4.1.2 Part Materials.................................................59 4.2 Materials For SDM Of Ceramic Green Parts..............................61 4.3 Materials For Mold SDM.............................................63 4.3.1 Initial Materials Combination.....................................64 4.3.2 Reducing Cracking And Delamination..............................65 4.3.3 Lower Temperature Wax Deposition...............................66 4.3.4 Improving Gelcasting Slurry Cure Conditions.........................67 viii

4.3.5 Switch To A Lower Temperature Wax..............................67 4.3.6 Solvent Removal Of Wax........................................68 4.3.7 Investigation of Alternative Waxes.................................69 4.3.8 Wax Optimization..............................................72 4.3.9 Other Materials Considered......................................73 4.4 Materials Testing...................................................73 4.4.1 Machinability Testing...........................................73 4.4.2 Shrinkage Testing..............................................79 4.4.3 Heat Resistance...............................................85 4.5 Current Mold SDM Materials..........................................87 4.5.1 Mold Materials................................................87 4.5.2 Support Materials..............................................87 4.5.3 Part Materials.................................................88 4.6 Future Mold SDM Materials...........................................100 4.6.1 Mold Materials................................................100 4.6.2 Support Materials..............................................101 4.6.3 Part Materials.................................................101 5 Process Planning...103 5.1 Background.......................................................103 5.1.1 Planning for Rapid Prototyping Processes...........................104 5.1.2 Planning for the SDM and Mold SDM Processes......................105 5.1.3 Planner Development History.....................................107 5.1.4 The Current Planner............................................108 5.2 Process Planning Enhancements......................................111 5.3 Manufacturability Analysis............................................112 5.3.1 Manufacturability Rules for SDM and Mold SDM......................113 5.3.2 Manufacturability Analysis Example................................118 5.4 Orientation Selection................................................120 5.4.1 Orientation Selection For Rapid Prototyping Processes................121 5.4.2 Orientation Selection For Mold SDM...............................122 5.5 Decomposition Strategies............................................124 5.5.1 Improved General Decomposition.................................125 5.5.2 Decomposition For The New Build Techniques.......................128 5.5.3 Decomposition Strategies For Multiple Build Directions.................129 5.6 Pitch Shaft Planning Example.........................................130 ix

5.6.1 Horizontal Build Orientation......................................131 5.6.2 Vertical Build Orientation........................................134 5.6.3 Multiple Build Orientations.......................................136 6 Process Automation...140 6.1 Reasons for Automation..............................................140 6.2 Automation of the Mold SDM Process...................................141 6.3 The Experimental Mold SDM Machine...................................142 6.3.1 Hardware Description...........................................142 6.3.2 Issues.......................................................147 7 Process Characterization...150 7.1 Objectives........................................................150 7.2 Characterization....................................................151 7.2.1 Tolerances and Surface Quality...................................151 7.2.2 Feature Sizes.................................................157 7.2.3 Build Rate....................................................159 7.2.4 Part Properties................................................165 7.3 Defects and Methods for Eliminating Them...............................167 7.3.1 Defects Resulting From Mold Fabrication...........................167 7.3.2 Defects Resulting From Casting...................................173 7.3.3 Defects Resulting From Curing...................................175 7.3.4 Defects Resulting From Demolding................................176 7.3.5 Defects Resulting From Drying, Burnout and Sintering.................178 7.4 Comparison With Other Processes.....................................179 7.4.1 Materials.....................................................179 7.4.2 Speed.......................................................180 7.4.3 Accuracy.....................................................181 7.4.4 Sintering.....................................................183 8 Applications...184 8.1 Application Objectives...............................................184 8.2 Mold SDM Process Characteristics.....................................185 8.3 General Application Characteristics.....................................186 8.4 Applications For Ceramics............................................187 8.4.1 Heat Engines.................................................187 x

8.4.2 Structural Applications..........................................189 8.4.3 Ceramic Mechanisms...........................................190 8.5 Applications For Metals..............................................191 8.6 Applications For Polymers............................................191 8.7 Prototyping and Short Run Production Applications........................192 9 Conclusions...194 9.1 Summary.........................................................194 9.2 Contributions......................................................195 9.3 Future Work.......................................................198 References...200 Glossary...210 xi

List of Tables Background Table 2.1: Typical properties of structural ceramics...............................9 Table 2.2: Typical properties of structural alloys.................................9 Table 2.3: Material properties of silicon nitride and 4340 steel......................11 Table 2.4: Calculated failure loads............................................13 Table 2.5: Properties reported for ceramics made by SLA..........................23 Table 2.6: Properties reported for ceramics made by FDC.........................24 Table 2.7: Properties reported for ceramics made by 3DP.........................26 Table 2.8: Commercial licensees of 3DP.......................................26 Table 2.9: Properties reported for ceramics made by LOM.........................27 Materials Selection Table 4.1: Waxes tested for use in Mold SDM...................................69 Table 4.2: Measured shrinkage values for various waxes..........................84 Table 4.3: ACR gelcasting slurry properties.....................................89 Table 4.4: Processing conditions for the silicon nitride.............................90 Table 4.5: Polymer part materials.............................................95 Process Planning Table 5.1: First orientation edge list...........................................119 Table 5.2: Second orientation edge list........................................120 Table 5.3: Third orientation edge list..........................................120 Process Automation Table 6.1: Milling machine specifications.......................................143 Table 6.2: Material deposition and curing equipment specifications..................144 Process Characterization Table 7.1: Polymer part material shrinkage values...............................152 Table 7.2: Part size measurements...........................................153 Table 7.3: Inlet nozzle size measurements.....................................154 Table 7.4: Summary of surface roughness measurements.........................155 Table 7.5: Factors driving process accuracy....................................156 xii

Table 7.6: Estimated part and feature size capabilities............................160 Table 7.7: Summary of main build time drivers..................................163 Table 7.8: Properties of polymeric part materials used in Mold SDM..................166 Table 7.9: Surface quality achieved using RP processes..........................182 xiii

List of Figures Background Figure 2.1: Ashby charts illustrating material property ranges.......................7 Figure 2.2: Beam with a surface flaw loaded in 3-point bending.....................11 Figure 2.3: Ceramic fabrication process sequence...............................13 Process Development Figure 3.1: Example SDM build sequence......................................32 Figure 3.2: Example Mold SDM build sequence.................................36 Figure 3.3: Inclined sharp edge..............................................39 Figure 3.4: Decomposition using a horizontal splitting plane........................39 Figure 3.5: Decomposition using non-horizontal splitting plane......................40 Figure 3.6: Thin walled cylinder build sequences.................................40 Figure 3.7: Conventional milling sequence......................................41 Figure 3.8: Overcut-fill-trim-backfill sequence...................................42 Figure 3.9: One time surface machining........................................43 Figure 3.10: One time surface machining example...............................43 Figure 3.11: Cut through machining...........................................45 Figure 3.12: Incremental casting.............................................45 Figure 3.13: Multiple build directions example part...............................46 Figure 3.14: Multiple build direction build sequence example.......................47 Figure 3.15: Pre-assembled mechanisms made using Mold SDM....................48 Figure 3.16: Multiple cavity mold for a turbine mechanism part......................48 Figure 3.17: Turbine mechanism fabrication sequence............................49 Figure 3.18: Multimaterial turbine.............................................49 Figure 3.19: Sintered alumina turbine mechanisms...............................50 Figure 3.20: Sintered alumina turbine before fixture removal........................50 Figure 3.21: CAD model of the shrouded fan....................................51 Figure 3.22: Shrouded fan build steps.........................................52 Figure 3.23: Finished polyurethane shrouded fan................................53 Materials Selection Figure 4.1: Green silicon nitride slidey made by SDM.............................63 Figure 4.2: Blistered alumina part.............................................65 xiv

Figure 4.3: Machinability spiral test pattern.....................................75 Figure 4.4: Machinability failure modes........................................76 Figure 4.5: Machinability without air...........................................77 Figure 4.6: Machinability using room temperature air.............................78 Figure 4.7: Machinability using cold air........................................79 Figure 4.8: Wax shrinkage modes............................................80 Figure 4.9: Shrinkage as a function of the fraction of machinable wax................84 Figure 4.10: Heat resistance test geometry.....................................86 Figure 4.11: Alumina impellers and turbines....................................90 Figure 4.12: Miniature alumina turbines........................................91 Figure 4.13: Alumina pitch shaft..............................................91 Figure 4.14: Alumina vane doublet............................................92 Figure 4.15: Silicon nitride impellers and pitch shaft..............................92 Figure 4.16: Silicon nitride inlet nozzle.........................................93 Figure 4.17: Silicon nitride impellers...........................................94 Figure 4.18: Polymer impellers...............................................96 Figure 4.19: Polyurethane turbines and sectioned mold...........................96 Figure 4.20: Polyurethane shrouded fan.......................................97 Figure 4.21: Multicolor polyurethane Inchworm..................................98 Figure 4.22: Silicone artery model............................................98 Figure 4.23: Multimaterial polymer mechanisms.................................99 Figure 4.24: Gelcast steel parts..............................................99 Process Planning Figure 5.1: RP process planning.............................................104 Figure 5.2: Mold SDM planning..............................................106 Figure 5.3: Process planning flow chart........................................110 Figure 5.4: Edge concavity classification.......................................114 Figure 5.5: General V-shaped edges..........................................115 Figure 5.6: Edges with a vertical surface.......................................116 Figure 5.7: Edges with both surfaces leaning to the same side......................116 Figure 5.8: Edges with horizontal or below horizontal surfaces......................116 Figure 5.9: Manufacturability of sharply curved surfaces...........................117 Figure 5.10: Example part geometry..........................................118 Figure 5.11: First build orientation............................................119 Figure 5.12: Second build orientation..........................................119 xv

Figure 5.13: Third build orientation............................................120 Figure 5.14: Decomposition using planar layers.................................125 Figure 5.15: Decomposition using non-planar layers..............................126 Figure 5.16: Compact consolidation...........................................126 Figure 5.17: Compact consolidation using the CAG...............................127 Figure 5.18: Pitch shaft geometry.............................................131 Figure 5.19: Horizontal build orientation........................................131 Figure 5.20: Horizontal orientation decomposition................................132 Figure 5.21: Cylindrical section build sequence..................................133 Figure 5.22: Pitch shaft built in a horizontal orientation............................133 Figure 5.23: Vertical build orientation..........................................134 Figure 5.24: Vertical orientation decomposition..................................135 Figure 5.25: Pitch shaft built in a vertical orientation..............................135 Figure 5.26: Pitch shaft divided into 2.5D sections................................136 Figure 5.27: Interaction between sections 1 and 3................................137 Figure 5.28: Multiple build orientation build sequence.............................138 Figure 5.29: Pitch shaft built using multiple build directions.........................139 Process Automation Figure 6.1: Mold SDM machine..............................................143 Figure 6.2: Mold SDM machine schematic......................................144 Figure 6.3: Machine control schematic.........................................146 Figure 6.4: Teflon casting frame..............................................147 Figure 6.5: Casting using containment walls....................................148 Process Characterization Figure 7.1: Engine vanes...................................................159 Figure 7.2: Silicon artery models.............................................159 Figure 7.3: Miniature alumina turbines.........................................160 Figure 7.4: Pitch shaft build time breakdown....................................163 Figure 7.5: Surface ripples..................................................168 Figure 7.6: Slumped tops of uprights..........................................168 Figure 7.7: Impeller mold sections showing thermal distortion.......................169 Figure 7.8: Mold with cracks due to residual stress buildup.........................170 Figure 7.9: Christmas Tree surface profile......................................171 Figure 7.10: Christmas Tree effect experiment..................................171 xvi

Figure 7.11: Voids resulting from incomplete mold filling...........................174 Figure 7.12: Mold features that are difficult to fill.................................174 Figure 7.13: Flaking and cracking of green part during curing.......................177 Applications Figure 8.1: Vane doublet from Rolls-Royce Pegasus engine........................187 Figure 8.2: M-DOT miniature gas turbine engine.................................188 Figure 8.3: Inlet nozzle from the M-DOT engine.................................189 Figure 8.4: Pitch shaft from a missile guidance system............................189 xvii

1 Introduction 1.1 Overview The goal of this research was to develop a new manufacturing process for the fabrication of geometrically complex, high quality structural ceramic components. The primary application is for components for use in the hot sections of gas turbine engines. In order to accomplish this a number of areas had to be addressed including process development, materials selection and process planning strategies. 1.1.1 Process Development Existing ceramic manufacturing processes have a number of limitations. They are not well suited to the fabrication of complex parts, for example engine vanes with internal cooling channels. Most processes are based on some type of tooling, which is expensive to fabricate and usually has a long lead time. This makes design and testing iteration for ceramic parts difficult and expensive. Research into the use of rapid prototyping processes to produce structural ceramic components has not been particularly successful because of the limitations in surface quality. Parts made from defect sensitive materials, such as ceramics, need to have smooth surfaces in order to achieve the best mechanical properties. 1

One of the primary goals of this research was therefore to develop a new manufacturing method that would combine the shape complexity advantages of the rapid prototyping methods together with the quality of the traditional manufacturing processes, without suffering from any of the disadvantages of either. The result was Mold Shape Deposition Manufacturing (Mold SDM). 1.1.2 Materials Selection and Optimization Having identified a promising process, it was necessary to identify a set of materials that could be used to implement the process. Because the Mold SDM process is based on Shape Deposition Manufacturing (SDM), the starting materials were chosen from the range of materials already used to build polymer parts using SDM. As the process developed, experience indicated that these materials could be further optimized, or replaced by new materials, to improve the quality of parts produced. As a result of this work, essential material characteristics were identified and used to identify and evaluate a range of new materials. These were then tested and several best combinations were selected for use with Mold SDM. 1.1.3 Process Planning Initially, Mold SDM planning was essentially identical to SDM planning. However as parts were built it became apparent that there were a number of limitations inherent in the planning process. Parts were not built exactly as specified, but were instead built as close approximations. Sharp corners, for example, were often built with small fillets. The context of features was also not considered in the planning process and this made it difficult to optimize the build strategy to take advantage of part features to build better quality parts. Experience gained from building parts was used to formulate a number of new planning and build techniques to improve part quality. A number of new part decomposition schemes were also developed to take advantage of process capabilities. 2

1.2 Thesis Outline Chapter 2 presents background information on ceramic materials and applications and then explains the range of current manufacturing processes and what their limitations are. The chapter concludes with an outline of the goals for this research, based on the limitations of current manufacturing technologies. Chapter 3 explains the Mold SDM process and how it was developed as an extension of the original SDM process. The chapter also presents a number of new build techniques which were developed to improve part quality, build rate and the range of manufacturable features. Chapter 4 describes the materials selection process. The materials requirements and issues for the Mold SDM process are presented in terms of the process steps. The history of the material selection work is described together with a number of material testing procedures which were developed to evaluate the suitability of the materials. The current best material selection is presented and a number of example parts are shown. Chapter 5 introduces a manufacturability analysis as well as a number of new process planning strategies. These are all based on the concept of exact manufacturability, where a part is built exactly as specified, with no geometrical approximations due to systemic process limitations. The new planning strategies are designed to improve part quality as well as to reduce the build time and simplify fabrication. Process automation issues are addressed in Chapter 6. These typically involve new process strategies that make the process easier to automate. This chapter also describes the Mold SDM machine which has been built to automate the mold building phase of the process. Chapter 7 describes initial characterization of the Mold SDM process. Estimates of process capabilities are given based on measurements made on parts built using Mold SDM. A number of common part defects are illustrated and methods for avoid them are presented. The Mold SDM process capabilities are also briefly compared to those of other processes. Chapter 8 examines applications for Mold SDM. The process is evaluated from an appli- 3

cations point of view to identify the general characteristics of applications that are ideal for Mold SDM. The chapter then presents specific current applications and explores some of the potential future applications. Chapter 9 contains brief conclusions and pointers to possible future research directions. 1.3 Individual / Group Research Statement The Stanford University Rapid Prototyping Laboratory is a group environment where collaboration and interaction are strongly encouraged. Consequently much of the research is the result of work performed by multiple individuals. All of the work presented in this thesis was performed in this environment. The basic SDM process was developed at Carnegie Mellon University and further refined at Stanford University. The Mold SDM process was my idea and I carried out all of the early development work. The new build techniques are entirely my own work and were developed based on experience gained during part building. Initial Mold SDM process development used materials that had for the most part been previously identified by researchers at Carnegie Mellon University and John Kietzman. As the process developed further, I investigated a variety of new materials which had not been used previously. All of the materials testing procedures presented are my own work. The manufacturability analysis and new planning strategies are almost entirely my own work. Discussions with Dr. Ju-Hsien Kao and Miguel Pinilla, as well as their development of the Compact Adjacency Graph, did influence this work. The construction of the Mold SDM machine was a group effort. I was principally responsible for the development of the initial concept and for equipment selection. I also participated in various phases of the machine construction. 4

2 Background This chapter presents background information on ceramic materials and the issues associated with the fabrication of ceramic components and their use. The chapter begins with a brief overview of ceramic materials in general, and then discusses ceramic materials used in structural applications. Traditional ceramic part fabrication processes are explained and compared with some of the newer Rapid Prototyping processes which have been developed recently for the fabrication of ceramic parts. The chapter ends with a definition of the goals for this research based on the limitations and issues of the existing ceramic fabrication processes. 2.1 Ceramics In the most general terms ceramics are defined as inorganic, non-metallic materials. Naturally occurring materials, such as rocks and minerals, are usually excluded. Inorganic glasses can be considered to be a subset of ceramics, but they differ in that they do not display long range order in their structure. Ceramics are typically oxides, carbides, nitrides or borides. In most applications they are used in polycrystalline form rather than as single 5

crystals. Ceramic materials have a tremendous range of properties which are useful for mechanical, electrical, optical and thermal applications. Currently the most widespread uses for ceramic materials are in electrical applications where ceramics are used because of their dielectric, piezoelectric, magnetic and superconductive properties. 2.1.1 Structural Ceramics Structural ceramics are ceramics that are used in applications that involve mechanical loading. Compared to structural metals, ceramics are attractive because of their low densities and high strengths, high stiffnesses and excellent creep resistances. As the use temperature increases, ceramics become more attractive because they maintain their mechanical properties to higher temperatures than metals do. For high temperature applications ceramic materials also have the advantage that they do not contain strategic materials such as refractory metals. Ceramics usually have excellent chemical and oxidation resistance, as well as good abrasion resistance because of their hardness. The major limitations of ceramics for structural applications are their low toughnesses and high costs. The two Ashby charts shown in Figure 2.1 show where ceramics lie with respect to other engineering materials in terms of their properites [Ashby 92]. The first chart, Figure 2.1a, shows fracture toughness versus strength. Ceramcs are the strongest class of materials. This is even more evident if specific strength is considered, because of the relatively low densities of ceramics compared to metallic alloys. However the fracture toughnesses of ceramics are inferior to all but the most brittle alloys. The second chart, Figure 2.1b, shows strength versus operating temperature. This figure clearly illustrates the benefits of ceramics as they are not only the strongest materials, but they also have the highest operating temperatures. Silicon Nitride Silicon nitride is one of the primary structural ceramics because of its high strength, particularly at high temperatures, high thermal shock resistance and good oxidation resistance at high temperatures. Silicon nitride is the ceramic material of choice for use in high tem- 6

Fracture Toughness (MPa m 1/2 ) 1000 100 10 1 0.1 0.01 0.1 Polymer Foams Polymers 1 10 100 1,000 Strength (MPa) a Alloys Composites 10,000 10,000 Strength (MPa) 1,000 100 Ceramics 10 1 0.1 Composites Polymers Polymer Foams 0 200 500 1000 Temperature ( C) b Alloys Elastomers Ceramics Figure 2.1: Ashby charts illustrating material property ranges perature structural applications such as heat engine components. Application examples include gas turbine engine vanes and blades, turbocharger rotors, internal combustion engine valves and high temperature seals. Silicon nitride components can be used at temperatures up to about 1350 C. Its high strength, hardness and thermal shock resistance also make silicon nitride attractive as a cutting tool material. Silicon nitride is also used in general structural applications which do not experience high temperatures. Examples include ball bearings, thread guides and parts for paper making machines. Alumina Alumina is most often used in electrical and electronic applications because it is an excellent insulator and is relatively inexpensive. However it is also used in structural applications because of its good mechanical properties and relatively low cost. It is not as strong as silicon nitride and has much less thermal shock resistance. Alumina is very chemically resistant, and since it is an oxide ceramic it is oxidation resistant, which makes it ideal for use in harsh chemical environments. Alumina is also used in biomedical applications, such as hip prostheses, because of its 7

excellent biocompatibility, corrosion resistance, wear resistance and strength. Alumina has been used to make cutting tools since the 1920s but because of its low thermal shock resistance it is usually toughened using silicon carbide whiskers or zirconia. Silicon Carbide Silicon carbide is used in many of the same structural applications as silicon nitride. It has good mechanical properties, maintaining its strength to temperatures up to 1400 C, high thermal shock resistance and good oxidation resistance at high temperatures. Because of its excellent wear resistance silicon carbide is used in applications such as sand blasting nozzles, seal rings and bearings. Its high hardness makes it attractive for use in armor applications, and together with its excellent thermal shock resistance, as a cutting tool material. Silicon carbide is also frequently used as an abrasive in the form of grinding wheels and powders. Zirconia Partially stabilized zirconia, which is toughened by a phase transformation mechanism [Evans 90], is the toughest of the structural ceramics with a fracture toughness approximately twice that of silicon nitride. Above about 800 C, a phase transformation negates the toughening effect, so zirconia is usually used in lower temperature applications. Zirconia is used in the same biomedical applications as alumina. Advantages of zirconia over alumina are its higher toughness and its strength which can be approximately double that of alumina. Zirconia is also used in refractory applications, as grinding media, extrusion dies and as wear resistant components. Cubic zirconia is an excellent ionic conductor and is used for oxygen sensor applications. Property Comparisons Table 2.1 lists some of the properties of these ceramic materials and Table 2.2 lists the same properties for a selection of typical structural metals [ASM 97, ASM 91, ASM 90a, 8

ASM 90b, ASM 82]. Properties shown are measured at room temperature unless shown otherwise. The maximum operating temperatures are approximate values since these depend greatly on the loading and service life required. The maximum use temperature for the 7075-T6 aluminum is shown as 100 C because the strength drops sharply at temperatures above 100 C. Property Density (g/cm 3 ) Strength (MPa) Elastic Modulus (GPa) Fracture Toughness (MPa /m) Thermal Expansion (10-6/K) Thermal Conductivity (W/mK) Maximum Use Temperature ( C) Alumina Silicon Nitride Silicon Carbide Zirconia (PSZ) 3.97 3.19 3.21 5.56-6.1 276-1034 414-1000 96-825 600-700 380 304 207-483 205 2.7-4.2 4.1-6.0 4.8-6.1 8-9 7.2-8.6 3.0 4.3-5.6 8.9-10.6 27.2 9-30 63-155 1.8-2.2 1300 1350 1400 800 Table 2.1: Typical properties of structural ceramics Property Density (g/cm 3 ) Yield Strength (MPa) Elastic Modulus (GPa) Fracture Toughness (MPa /m) Thermal Expansion (10-6 /K) Thermal Conductivity (W/mK) Maximum Use Temperature ( C) Aluminum (7075-T6) Titanium (Ti6 Al4 V) Steel (AISI 4340) Inconel (IN-738) 2.80 4.43 7.65 8.11 503 830-1103 740-1860 950 71 113.8 210 201 16.5-27.5 44-110 53-110 - 23.4 8.6 11.5 11.6 @ 93 C 130 6.6-6.8-17.7 @ 538 C 100 315 300 800 Table 2.2: Typical properties of structural alloys 9

2.1.2 Issues Despite their attractive properties ceramics are currently limited in their applications by a number of factors: Manufacturability: because of the limitations of current manufacturing processes it is difficult to make complex shapes in ceramic materials [Panzarino 96]. In most cases, due to the variability in sintering shrinkage (typically ± 2%) and the need for defect free surfaces, ceramic parts need to be machined and/or ground after sintering. Because of the hardness and defect sensitivity of ceramics this is a difficult and expensive process. Low Toughness: because of their low toughness ceramics are very defect sensitive. This is explained in more detail below. Design Rules: in a ceramic part failure occurs when the critical stress of one of the defects in the part is exceeded. All ceramic parts contain defects and since there is no practical way to determine the flaw distribution in a particular part, failure is probabilistic. Design practice must therefore be based on the probability of the existence of a given flaw distribution in the parts. This is a very different design approach to the traditional approach used for metal parts and because of this designers need to adapt their design strategies to ceramics. Cost: ceramic parts are expensive because of the high purity starting materials and the carefully controlled processing steps required. Surface Quality Requirements Under mechanical loading ceramic materials usually fail by fracture, although at high temperatures effects such as creep may also contribute to failure. Brittle failure results from the lack of ductility in ceramic materials at low temperatures. Brittle failure is governed by the principles of fracture mechanics. In any component there will be a population of defects spread throughout the component. These defects will be of various configurations, such as cracks and voids, and will be present in a range of sizes. For any given loading condition, each defect will have a critical stress above which it will grow rapidly and cause failure of the part. Part failure therefore occurs when the critical 10

stress of one of the defects is exceeded. There are two general types of defect: surface and volume defects. Surface defects are located on the surfaces of the part and are typically small cracks, scratches and surface roughness. Volume defects occur inside the part and are typically cracks, voids and foreign particle inclusions. The critical stress for a defect depends on its size and configuration. Larger and sharper defects generally have lower critical stresses. Sharp cracks, for example, are worse than spherical voids. The following simple calculation illustrates the sensitivity of ceramic materials to defects. The failure loads for a beam with a 100 µm crack on the underside will be calculated for a 3-point bending case as shown in Figure 2.2. For comparison purposes the failure loads F a = 100 µm h,b = 10 mm L = 20 mm L = 20 mm Figure 2.2: Beam with a surface flaw loaded in 3-point bending will be calculated for a beam made of silicon nitride and for one made of 4340 high strength steel. The properties of these materials are shown in Table 2.3. Material Failure Strength (MPa) Fracture Toughness (MPa m 1/2 ) 4340 steel 860 (yield) 100 Silicon nitride 860 5 Table 2.3: Material properties of silicon nitride and 4340 steel 11

The 4340 steel was selected because it is a structural steel with a strength in the range of typical strengths for silicon nitride. The strength of the silicon nitride was set equal to that of the 4340 steel for the purposes of this comparison. Note that the failure strength used for the 4340 steel is its yield strength. The failure loads can be calculated as follows. Ignoring the defect, the beam failure load can be calculated from simple beam theory: The maximum stress found on the top and bottom surfaces of the beam is: σ = 3FL --------- bh 2 The failure load is therefore given by: F = bh 2 -------- σ 3L For failure by fracture the following equation relates the critical stress to the crack size: K IC = σk πa Here K is the stress intensity factor, which for this configuration is very nearly 1. K IC is the critical stress intensity required for failure. This equation can be rearranged to give the failure load due to fracture by using the maximum stress equation from above: F = K IC bh 2 ----------------- 3L πa Table 2.4 lists the calculated failure loads. The 4340 steel beam will fail by yielding at a load of 14.3 kn. Failure by fracture occurs at a much higher load and will therefore not be seen in practice. The silicon nitride beam does not yield but fails at 4.7 kn due to fracture. 12

Compared to the steel beam, which has the same strength, this is only about a third the failure load and it results from a defect only 100 µm in size. The steel beam will also fail gracefully by yielding rather than by suddenly fracturing. Material Yield (kn) Fracture (kn) 4340 steel 14.3 94 Silicon nitride - 4.7 Table 2.4: Calculated failure loads This simple calculation illustrates why it is so important to produce defect free ceramic parts. Because of the lack of ductility, ceramic parts can t relieve the high stresses around their defects and thus the defect population controls the mechanical strength of ceramic parts. This is the reason why surface quality is so critical for structural ceramic parts. 2.2 Ceramic Manufacturing Processes Most ceramic parts are currently made in a two step process as shown in Figure 2.3. First, ceramic powders are formed into green parts. These are porous bodies consisting of tightly packed ceramic powder particles. Then to achieve full density the green parts are sintered at high temperature. During the sintering process the powder particles fuse together and produce a strong, dense ceramic part. Ceramic Powder Form Green Part Sinter Finished Part Figure 2.3: Ceramic fabrication process sequence For advanced ceramics applications the starting ceramic powders are usually high purity powders with particle sizes of 1 micron or less. The small particle size is needed because the mechanical properties of ceramics increase with decreasing grain size. Finer powders are also easier to sinter because they sinter faster and at lower temperatures. The forming step compacts the powder into a porous shape, or green part. A binder, typi- 13

cally a polymer, must normally be added to the powder to give the green part some mechanical strength so that it can withstand handling. A wide range of forming processes are in use, some of the more common are described briefly in the following sections. The goal of all these processes is to achieve maximum packing density and uniformity because these factors directly impact the sintering process. Higher green density makes sintering easier and reduces the sintering shrinkage. Non-uniform green density results in non-uniform sintering rates and shrinkage which can lead to warping and cracking during sintering. Green parts generally contain fluids and organic components in addition to the ceramic powder. These are leftover from the forming process and must be removed before the ceramic powder can be densified by sintering. The organic components and ceramic powder typically form a porous network structure with the fluid filling the pores. The fluid is removed first by drying. This must be performed in a carefully controlled fashion to avoid damage to the part caused by capillary forces. Once the fluid has been removed the organic components are burnt out to leave a porous ceramic structure. At this point the ceramic particles have begun to fuse but the links are very weak. Because the drying and burnout steps tend to be time consuming it is desirable to minimize the amount of liquid and organic components in the green parts. Sintering is a high temperature process where the individual particles fuse together to form a dense part. In order to achieve high sintered densities, green parts typically need to have densities of 50% or greater. Higher density parts usually sinter more easily and shrink less during sintering. Packing uniformity is important because uneven shrinkage during sintering usually causes warping. Areas of low packing density may fail to sinter to full density leaving voids in the sintered part. Because the green parts are usually only 50-60% dense there is significant shrinkage during sintering. To compensate for this the green parts are made correspondingly larger. In most cases ceramic parts are machined or ground after sintering to achieve required geometrical tolerances. The amount of shrinkage and process variability makes it very difficult to achieve tight tolerances without this. 14

The following sections briefly describe the main forming processes currently being used in the manufacture of ceramic components and also describe recent work on using Rapid Prototyping processes to fabricate ceramic parts. 2.3 Ceramic Forming Processes The following sections are largely based on the ASM Engineered Materials Handbook, Volume 4 [ASM 91], which contains overview articles and selected references for all of these processes and is an excellent source of general information. 2.3.1 Powder Pressing Low Temperature Powder Pressing Processes These processes all form green bodies by compacting ceramic powders under pressure. A small amount of binder added to the powder holds the particles together so that the green body maintains its shape after removal of the pressure. In dry pressing, ceramic powders are compacted inside dies by applying pressure in a uniaxial direction using punches. Geometries are usually limited to prismatic shapes although parts with multiple holes, levels and diameters can be produced using more elaborate punches. Die wall friction and geometric effects tend to result in different compaction stresses being exerted on different areas of the powder, resulting in non-uniform pressed densities. These density variations can cause problems during sintering. More uniform compaction can be achieved using cold isostatic pressing. In this process the powder is placed inside a flexible mold which is sealed and placed into a pressure chamber. Hydrostatic pressure is applied to the mold using a fluid. Advantages of this process are very even pressed densities and few size limitations. The main disadvantage is poor shape and dimensional control because it is difficult to control exactly how the flexible mold will deform during pressing. Powder pressing is an economical process for producing relatively simple shapes in quantity. Examples of parts made by pressing include spark plug insulators and grinding 15

wheels. Hot Isostatic Pressing (HIP) Hot isostatic pressing is mechanically similar to cold isostatic pressing except that the process is performed at high temperature such that the powder sinters to form a dense ceramic part. The pressure increases the sintering rate allowing the material to be sintered at a lower temperature than in pressureless sintering. Lower temperature sintering can reduce grain growth and this tends to produce stronger parts. The pressure also has the effect of shrinking any voids, again resulting in stronger parts. Due to the equipment required, hot isostatic pressing is a very expensive process. However since it produces the best mechanical properties it is used for high performance applications, such as engine components, which must withstand high stresses. 2.3.2 Slip Casting A slip is a suspension of colloidal powders in an immiscible liquid, usually water. In slip casting a slip is cast into a porous mold, usually made of plaster. Capillary suction draws the liquid into the mold and leaves a layer of particles deposited against the walls of the mold. Pressure can be applied to the mold to increase the rate. Hollow objects can be made by allowing a certain layer thickness to form before draining the rest of the slip from the mold. Solid objects can be made by allowing the whole volume to solidify. Slip casting is an economical process for the fabrication of objects such as crucibles and chinaware. A disadvantage of this method is that it is relatively slow, taking tens of minutes to hours to cast a part. 2.3.3 Tape Casting In tape casting a fluid suspension of ceramic particles is cast to form a thin film on either a fixed or moving substrate. Film thickness is controlled by a doctor blade. The fluid is removed by evaporation to leave a film of green material. Tape casting is ideally suited to making thin ceramic sheets which are used in many electronics applications. Holes and other features can be punched or cut in the tape prior to 16

sintering. Multiple sheets can be stacked to produce more complex objects, however the process produces essentially two dimensional parts. 2.3.4 Extrusion Ceramic extrusion is very similar to plastic and metal extrusion processes. A powder and binder formulation with sufficient plasticity is forced through a die which gives it shape. Extrusion is a very effective method for fabricating objects of constant cross section such as tubes, pipes, plates and honeycombs. In order to achieve sufficient plasticity in the material a large amount of organics must be added and this makes the drying and burnout steps long and difficult. The high abrasivity of the material also causes rapid die and machine wear. The cost and lead time associated with the fabrication of dies is also a limitation. 2.3.5 Injection Molding Ceramic injection molding [Mutsuddy 95] is similar to plastic injection molding. A heated powder and binder formulation is forced into a mold under pressure and allowed to cool before being removed from the mold. A polymer or wax binder is usually used for injection molding and this produces high strength green parts, but as with extrusion, the drying and burnout steps are long and difficult. The tooling can also be expensive and usually has long lead times. Tool life will be shortened by the abrasivity of the materials used. 2.3.6 Green Machining The starting point for green machining is a green ceramic blank made by another forming process. The blank is machined to shape using milling, turning, drilling or other machining processes. The blank might be simply a block from which the part will be cut, or it might be a shape produced by another forming process. For example green machining may be used to drill holes in an extruded part. Because CNC machining is well developed green machining can be used to make complex shapes fairly quickly and easily. However since ceramic materials are very hard and abra- 17

sive tool life will be very short, even when using diamond cutting tools. 2.3.7 Gelcasting Gelcasting is a relatively new process for ceramic part fabrication originally developed at Oak Ridge National Laboratory [Janney 90, Omatete 91]. A slurry is made consisting primarily of ceramic powder, a monomer and a solvent. The slurry is cast into a mold and the monomer is polymerized to form a green part consisting of ceramic particles embedded in a polymer network. The solvent occupies the pore spaces in the green part. Advantages of gelcasting include the relatively low fraction of binder which makes burnout and sintering faster and easier. The good mechanical strength of the green parts allows them to be handled without too much risk of damage and makes them well suited to green machining [Nunn 96]. Gelcast parts also exhibit very isotropic sintering shrinkage which makes it easier to achieve the desired sintered shape. Gelcasting has been used to produce alumina, silicon nitride, SiAlON, silicon carbide, zirconia and fused silica ceramic parts [Young 91], as well as metallic parts using materials such as superalloys and tool steels [Janney 95, Janney 98]. There are four potential means for producing molds for use in gelcasting: Non-fugitive molds Cast fugitive molds Machined fugitive molds Rapid Prototyped fugitive molds Non-fugitive molds are molds that not destroyed in the process of making parts and can therefore be used multiple times. Examples of non-fugitive molds are the metal molds used in injection molding or die casting. Fugitive molds are molds that can only be used once because they are destroyed in the course of making a part. Fugitive molds are typically melted or dissolved after use and this gives the advantage of being able to produce intricate shapes that could not be removed from non-fugitive molds. Metal tooling for gelcasting will last longer than it would for injection molding because the material is not being forced into the mold under high pressures. As a result aluminum 18

tooling is often used for gelcasting since it is less expensive than tool steel. However, as with injection molding the tooling will be expensive and will have significant leadtimes. It is also difficult to produce very complex shapes because the part must be removable from the mold. Slides can be used to produce more complex features but these increase the cost of the mold. Cast fugitive molds are molds made by casting into non-fugitive molds. Typically wax molds are made by injection molding using metal tools. In this case the metal tools would have very long lifetimes since waxes are typically not abrasive. As with non-fugitive molds there is significant cost and leadtime involved in making the tools for producing the wax molds. The second replication step also reduces the accuracy of the final part. Instead of injection molding, the fugitive molds could also be directly machined. This eliminates the need for metal tooling and the associated cost and lead time. Also since it is relatively easy to reprogram the milling machine to produce a new mold geometry this process is much more flexible. It also eliminates the second replication step introduced by casting the fugitive molds. The disadvantage is that this process will be slower because machining is slower than injection molding. Using a rapid prototyping process to fabricate fugitive molds is another alternative [Jamalabad 96]. This also eliminates the need for expensive, long leadtime tooling and provides the flexibility to quickly change to new mold configurations. There are two principal limitations to this approach. The poor surface finish of commercial RP systems is an issue because with a mold the important internal surfaces are not usually accessible for manual smoothing. For the molds to be fugitive they must be made from an easily removable material, such as a wax, and not all RP processes can make parts from such materials. Fused Deposition Modeling and the Sanders process are the most likely candidate processes. These are described below. 19

2.4 Rapid Prototyping Of Ceramic Parts 2.4.1 Overview Rapid Prototyping (RP) processes, also known as Solid Freeform Fabrication (SFF) processes, build parts in a layerwise fashion. There are a number of reasons for adopting this approach: Shape complexity: Because parts are built layer by layer, every point in or on the part will be easily accessible at some point during the build process. There will be no tool access limitations as is typical in machining, or shape restrictions due to the necessity of being able to remove the part from the mold as occurs in molding processes. This greater accessibility means that complex geometries can be fabricated because each section of the part can be reached and shaped as necessary. In theory arbitrarily complex shapes can be built although in practice there are limitations. Elimination of tooling: RP processes do not use custom tooling because they build each layer using a general fabrication technique which does not depend on the particular geometry of the layer. This is similar to the difference between molding and machining. In molding a different mold is required for each geometry whereas with machining the same cutting tool can be used to produce a wide range of geometries by simply changing the cutting path. Simplified process planning: By decomposing complex parts into simpler sections the planning process is greatly simplified. In most cases 3-dimensional parts are decomposed into 2-dimensional layers which can be manipulated far more easily. Automated fabrication: The preceding features of RP processes tend to make them relatively easy to automate. Because of the shape complexity capability, RP processes are able to accept and build almost any part as is. Most processes have limitations on the types of geometry that can be built and might therefore require design modifications to make parts manufacturable. Tooling is typically fabricated using different processes than those used to fabricate parts, so eliminating the tooling eliminates additional equipment and process requirements. Most RP machines can be implemented as machines roughly the size of an office photocopier. The simpler process 20

planning means that most of the planning can be done automatically without the need for human intervention or detailed process knowledge. Programming for CNC machining operations requires a skilled operator familiar with the intricacies of the machining process. RP processes typically suffer from three drawbacks, which may or may not be important in any particular application: Material limitations: Most RP processes depend on particular material properties or behaviors for their function. For this reason the range of materials that can be used in each process is usually fairly limited. In many cases materials with good engineering properties are not compatible with the process requirements. Surface quality: In most cases parts are fabricated as a series of 2.5-dimensional layers and this results in a stairstep effect on all inclined part surfaces. Reducing layer thickness reduces the size of the steps but increases build time. When smooth surfaces are required manual finishing is necessary and this usually results in loss of accuracy. Internal surfaces are usually not accessible and so cannot be smoothed. Material quality: In addition to the limitations in which types of materials can be used there are material quality issues associated with building parts in layers. If there are any bonding defects between layers the properties of the part will be degraded. There may also be anisotropy in properties due to the lack of symmetry between the build direction and the orthogonal directions in the build plane. RP processes are typically able to build parts from polymeric materials in build envelopes up to roughly 400 mm cubes in a matter of hours or days. Geometric complexity does not significantly affect build rate which is usually determined principally by part volume. Some processes are able to build parts from materials other than polymers but polymers are still the most common RP part materials. The following sections briefly describe the RP processes which have been used to build ceramic parts and highlight some of their strengths and weaknesses. 21

2.4.2 Stereolithography (SLA) Stereolithography, from 3D Systems, uses an ultraviolet laser to selectively cure liquid polymerizable resins [Jacobs 92]. A platform is located in a vat of resin such that a thin layer of resin covers it. The laser scans across the surface and selectively cures areas of the resin to form one layer of the part being built. The platform is then lowered by one layer thickness, recoated with resin and the process repeats. Although the maximum layer thickness achievable is limited by the cure depth of the resin this is not usually an issue since much thinner layers must be used for accuracy reasons. Typical layer thicknesses are between 25 and 200 µm. Once all the layers have been built the part is removed from the vat, cleaned and postcured. Support for overhanging features is provided by an open framework structure built using part material. Once the part is finished the support structures must be manually cut away from the part. The supported surfaces must usually be manually finished to remove the remains of the support structures. SLA parts exhibit a stairstep finish on inclined surfaces as a result of being fabricated from 2.5 dimensional layers. This surface roughness is typically smoothed by manual sanding which is labor intensive, time consuming and reduces part accuracy. A smoothing approach based on coating parts by dipping in resin has shown potential in improving surface smoothness, however there are issues with reduction in accuracy [Reeves 95]. SLA has been adapted for the fabrication of ceramic green parts by using resins filled with ceramic particles [Griffith 96]. An issue with ceramic filled resins is the reduction in cure depth due to scattering.with silica and alumina cure depths of 200 µm were achieved with solids loadings between 40 and 55%. Silicon nitride does not work as well and even at 20% solids loading the cure depth was only 10 µm. Scattering also results in horizontal spreading of the beam below the surface of the resin which widens the cured area. This has been shown to help improve surface finish on inclined surfaces [Himmer 97]. An alternative approach is based on lamination rather than recoating [Himmer 97]. Instead of using the standard recoater which spreads a layer of liquid resin over the previous layer, this approach makes a tape from a high viscosity resin and then laminates each layer on 22

top of the previous layers. After lamination, the tape is selectively cured using the laser. Once the build cycle is complete the uncured resin is washed away. This approach is still in the experimental stages. Properties of ceramics made using SLA are shown in Table 2.5 [Zimbeck 96]. The sintered silicon nitride was only 90% dense which accounts for the low strength reported. Notice also that the maximum cure depths are much higher than the previously reported values although the target of 200 µm was not reached for silicon nitride or zirconia. Material Maximum Cure Depth (µm) Flexure Strength (MPa) Alumina >838 360 Silicon Nitride 102 412 Zirconia 152 530 Table 2.5: Properties reported for ceramics made by SLA 2.4.3 Fused Deposition Modeling (FDM) Fused Deposition Modeling, from Stratasys, is based on extrusion of thermoplastic polymers [Stratasys 91]. FDM machines contain two deposition nozzles, one for part material and one for support material. Nozzles are typically 250-500 µm in diameter. Each layer is built up by sequentially depositing part material and then support material areas. After each layer is completed the part is lowered by one layer thickness and the process repeats. Once the part is complete the mechanically weak support structures are manually broken away from the part. A recent development is a water soluble support material [Lombardi 98]. This eliminates the need for manual support removal and makes it possible to build parts with complex internal cavities and fine features that might be damaged by the forces involved in manual support removal. A variation on FDM is Fused Deposition of Ceramics (FDC), where a ceramic filled polymer is used as the part material to produce ceramic green parts. FDC has been used to make a variety of silicon nitride, alumina and silica parts [Agarwala 96a, Agarwala 96b], as well as lead zirconium titanate (PZT) piezoelectric components [Bandyopadhyay 97, 23

Lous 98]. Porous alumina bioceramic implants have also been made with a fugitive mold based approach using FDM [Bose 98]. An issue with FDC is the fabrication of void free parts [Agarwala 96a]. When making non-structural polymer parts internal voids are not critical since they will not be seen. In ceramic parts, voids have a detrimental impact on mechanical properties and must be eliminated. A number of techniques have been developed to eliminate voids but these also tend to reduce part accuracy because they are based on overfilling which can cause bulging of the perimeter [McIntosh 97]. Strengths reported for ceramics made by FDC are shown in Table 2.6 [Agarwala 96a, Agarwala 96b]. Material Strength (MPa) Silica 12.5 ± 1.7 Silicon Nitride 824 ± 110 Table 2.6: Properties reported for ceramics made by FDC 2.4.4 Selective Laser Sintering (SLS) Selective Laser Sintering, from DTM Corporation, is based on laser sintering of powders [Deckard 88, Nutt 91]. In this process a laser scans over a powder bed and selectively sinters the powder particles. After each layer the powder bed is lowered by one layer thickness, recoated with powder and the process repeats. The powder bed provides sufficient support such that a separate support material is not needed. Once the part is complete it is extracted from the loose powder surrounding it and any remaining powder is blown off or out of internal cavities. SLS has been used to produce ceramic parts by using either ceramic particles coated with binder or a mixture of ceramic and binder particles. Alumina parts were made using the former approach followed by an infiltration step using an alumina colloid. After sintering maximum strengths obtained were around 14 MPa due to the low sintered densities of about 55% [Subramanian 95]. A silicon / silicon carbide composite was produced by SLS of binder coated SiC particles. 24

The binder was pyrolized to convert it to carbon and then molten silicon was infiltrated into the porous structure [Stierlen 98]. Strengths of up to 70 MPa were reported. SLS has been used to produce ceramic investment casting molds. Partially stabilized zirconia molds for titanium casting were made by SLS of stabilized zirconia which was then infiltrated with unstabilized zirconia before being sintered [Harlan 98]. Zirconium silicate molds were made for casting of an aluminum silicon alloy [Klocke 98]. SLS has also been used to produce porous bioceramic materials with properties similar to natural bone [Vail 98]. 2.4.5 Three Dimensional Printing (3DP) Three Dimensional Printing, developed at MIT, has several variants. The two main approaches used for fabricating ceramic parts are the dry powder bed 3DP and slurry based 3DP processes. The dry powder bed approach, which was developed first, builds parts by selectively printing binder into a powder bed [Sachs 92]. To build a layer a new layer of powder is spread over the surface of the build area. A binder is then printed into the powder bed using a process similar to inkjet printing. The printed areas become the part material, the unprinted areas function as support during the build process. After all the layers have been built the part is removed from the loose powder surrounding it. The dry powder bed approach was initially targeted at the fabrication of ceramic molds for investment casting. For the fabrication of ceramic parts, however, this approach is not ideal. To make the powder flow sufficiently for recoating the micron sized particles must be agglomerated into 50 µm agglomerates. These do not pack well and limit green densities to about 35%. Green parts with such low densities do not usually sinter fully and therefore a pressing stage is required to increase green density before sintering. Slurry based 3DP was developed to overcome the problem with low green densities [Grau 97]. In this approach, layers are deposited by spraying a layer of slurry over the build area. The slurry is dried and then binder is selectively printed to define part areas. Once the build is complete the unprinted powder is redispersed to free the finished green part. This 25

approach can produce green parts with densities up to 67%. Properties of ceramics made using 3DP are shown in Table 2.7 [Cima 95]. Material Strength (MPa) Alumina 400 Silicon nitride 570 Zirconia toughened alumina 475 Table 2.7: Properties reported for ceramics made by 3DP The 3DP process has been licensed to a number of companies for a variety of uses as shown in Table 2.8. Company ExtrudeHone Soligen Specific Surface TDK Therics Z Corporation Application(s) The ProMetal division builds metal tooling using 3DP Used as part of their Direct Shell Production Casting (DSPC) technique to produce ceramic molds for metal casting Ceramic filters for industrial applications unknown Biopharmaceutical company developing drug delivery and medical products Build 3DP machines that produce parts using a starch based material Table 2.8: Commercial licensees of 3DP 2.4.6 Laminated Object Manufacturing (LOM) In Laminated Object Manufacturing, from Helisys Corporation, parts are built by stacking and bonding together layers made from a paper-like construction material [Feygin 91]. Each new layer is formed by feeding a new section of construction material over the build area. This material is bonded to the previous layers using a laminating roller. A laser then cuts the outline of the layer into the construction material and crosshatches the remaining area. Once all layers have been built the crosshatched areas are broken away from the part manually. This decubing process is difficult around delicate parts and in internal cavities. A variation is Curved Layer LOM where the part is built on top of a curved mandrel which controls the curvature of the layers [Kalmanovich 96]. A flexible membrane is used to apply pressure to the curved layers during lamination. The mandrel is fabricated using 26

conventional LOM processing techniques. LOM has been used to fabricate ceramic parts using ceramic green tapes instead of paper. Monolithic alumina, silicon nitride and silicon carbide have been made [Griffin 94, Pope 97, Klosterman 97], as well as silicon carbide/silicon carbide ceramic matrix composites [Klosterman 97, Klosterman 98]. Because LOM builds parts from sheets of material it is well suited to the fabrication of composites containing fibers. By orienting the sheets one can control the fiber orientation and thus the properties of the composite. Reported strengths of ceramic parts made using LOM are shown in Table 2.9 [Griffin 94, Klosterman 97, Pope 97]. Material Strength (MPa) Alumina 310 Silicon carbide 200-275 Silicon nitride 920 Table 2.9: Properties reported for ceramics made by LOM 2.4.7 Direct Photo Shaping (DPS) Direct Photo Shaping, developed at SRI International, is similar to SLA except that it cures an entire layer at a time and was developed specifically for the fabrication of ceramic parts [Ventura 97]. For each layer a coat of UV curing part material is spread over the build envelope. A projection system then selectively exposes and cures the entire layer through a mask. This has the advantage of being faster that SLA, however SLA can use different scan patterns to control shrinkage and warping if necessary [Jacobs 92, Ullett 94, Jayanthi 94]. Strength values reported for silicon nitride produced using DPS are slightly above 1 GPa [Ventura 97]. 2.4.8 Computer Aided Manufacturing of Laminated Engineering Materials (CAM-LEM) CAM-LEM, developed at Case Western Reserve University, is similar to LOM in that it builds parts by laminating sheets of material [Mathewson 95]. There are two principal dif- 27

ferences between CAM-LEM and LOM. CAM-LEM uses a pick and place approach where part sections are cut from sheets of material and then robotically placed on top of the previously built layers before being laminated. CAM-LEM also uses two different materials, a part and a support material. Two advantages of CAM-LEM over other RP processes are its ability to use different thicknesses of sheet material, and its ability to do tangent cutting, using 5-axis laser cutting, to produce non-vertical edges. This allows parts to be built more rapidly from thicker layers and with better surface finish than is possible with purely 2.5 dimensional layers [Zheng 96]. CAM-LEM has been used to make ceramic parts by using ceramic green tapes as the build material. Alumina and silicon nitride parts have been made but no materials property values have been reported [Liu 96, Liu 97]. 2.4.9 Sanders Prototype The Sanders process is an inkjet printer based process that builds parts from a proprietary thermoplastic part material and wax support material [Stoddart 97]. Both materials are deposited as a series of fine droplets in a printing type process. After each layer has been built it is planed to the final height. Layer thicknesses can be between 0.013 and 0.13 mm. Because of its high accuracy and relative slowness, the Sanders process is best suited to fabricating small, accurate parts such as investment casting patterns for jewelry. Although the Sanders process can t build ceramic green parts directly it has been used to produce fugitive molds which were then used to produce PZT piezoelectric ceramics and composites [Safari 97]. This approach could also be used to produce other ceramic parts although the speed of the Sanders process may be a limitation. 2.5 Evaluation of Existing Processes Traditional fabrication processes are most appropriate for high volume production of relatively simple parts. Processes such as powder pressing and tape casting can produce parts 28

such as spark plug insulators and electronic substrates at high rates. Extrusion and injection molding can produce more complex shapes but they rely on expensive tooling and also suffer from high binder content in the green parts which makes binder burnout slow and difficult. Binder burnout for an injection molded turbocharger rotor (about 170 mm in diameter and weighing about 700g) can take as long as 11 days [Takatori 93]. Gelcasting is an elegant method for producing ceramic parts. The green parts are strong, but because they contain relatively little binder, the burnout process is fairly quick and easy. Gelcast parts also exhibit very uniform sintering shrinkage making it easier to predict final part size. For complex parts, the challenge is fabricating the molds for gelcasting. RP techniques overcome the shape complexity limitations of the traditional fabrication processes but they tend to suffer from poor surface quality. For functional structural ceramic parts this is a very serious limitation since part strength is strongly dependent on surface roughness. An attractive feature of RP processes is the rate at which arbitrary geometries can be produced, typically in a matter of hours. However without the required surface quality the benefits of these processes can't be exploited. 2.6 Objectives for SDM and Mold SDM of Ceramics Based on the limitations of the existing processes, the goal for SDM and then Mold SDM of ceramics was to improve the green part fabrication process. In particular the goal was to develop a method to produce high quality green parts with high shape complexity. For this to be commercially viable it would also be very desirable to do this rapidly and at low cost. 29

3 Process Development This chapter introduces the Mold Shape Deposition Manufacturing (Mold SDM) process. It begins with a brief description of Shape Deposition Manufacturing (SDM), the process from which Mold SDM is derived, and explains the limitations which led to the development of Mold SDM. The description of Mold SDM covers the process steps and sequence but not the materials issues which are addressed in the next chapter. A variety of new build techniques are presented as improvements to the basic SDM and Mold SDM build strategies. These increase the range of buildable features and improve the quality of the final part. The chapter ends with a process example illustrating the build steps required to build a shrouded fan part using Mold SDM. 3.1 Shape Deposition Manufacturing (SDM) 3.1.1 Process Description Shape Deposition Manufacturing (SDM) is a patented layered manufacturing process that was originally developed at Carnegie Mellon University [Merz 94a, Merz 94b, Prinz 94]. 30

Whereas most Solid Freeform Fabrication (SFF) techniques decompose parts into relatively thin 2.5-dimensional planar layers of uniform thickness, SDM uses three-dimensional layers that may be of arbitrary thickness and need not be planar. This difference is significant for two reasons. First, part accuracy is independent of layer thickness and therefore the build rate can often be increased by using thicker layers. Second, since SDM layers are 3-dimensional, it is possible to produce smooth inclined surfaces with none of the stairstep effect common to other SFF processes. SDM is an additive-subtractive process. Material is added by a variety of material dependent deposition processes such as laser welding, extrusion or casting. Material removal is normally accomplished by either 3- or 5-axis computer numeric control (CNC) milling although electric discharge machining (EDM) can be used with electrically conductive materials. The material removal step makes it possible to produce smooth, accurate surfaces. Also, since the material does not need to be deposited to final shape directly it is possible to use more rapid near-net shape deposition processes. SDM uses a sacrificial support material to provide fixturing and support for overhanging features during the build process. Once all the layers have been built the support material is removed, typically by etching or melting, to leave the finished part. An example SDM build sequence for a simple three layer part is shown in Figure 3.1 (The process of decomposing the part to identify the layers is described in Chapter 5). Each step represents one cycle of material deposition and material removal operations. The first layer of the part has non-undercut surfaces on both sides and is built by first depositing and shaping the part material (1) and then depositing support material around it (2). The second layer has undercut surfaces on both sides so the support material must be deposited (3) before the part material (4). The last layer has an undercut on the left side only. In this case the left side support material must be deposited first (5), followed by the part material (6) and finally the right side support material (7). The build order for the various sections of a layer is determined by the vertical stacking order. Sections that are underneath others must be built first. At this point the part is complete and the final step is to remove the support material (8). 31

1 2 3 4 Support Material Part Material 5 6 7 8 Note that step 7 is not strictly necessary. Since the last support segment is never used to support anything it does not need to be built. Figure 3.1: Example SDM build sequence 3.1.2 Comparison to Other Rapid Prototyping Processes The key feature that differentiates SDM from other RP processes is the material removal, or machining, step. Some processes, for example Sanders, feature a planing step to control layer thickness and smooth the top surface prior to building the next layer. However only SDM uses machining to shape all surfaces, either by direct machining for non-undercut surfaces, or by replication from machined surfaces for undercut surfaces. The use of machining eliminates the dependence of accuracy on layer thickness. RP parts typically suffer from the stairstep effect which produces poor surface quality on inclined surfaces. The stairstep can be reduced by reducing the layer thickness but this results in a proportional increase in build time. Alternatively, manual smoothing can be used to remove the stairsteps but this will reduce part accuracy and is also a very labor intensive process. Machining produces smooth, accurate surfaces irrespective of the layer thickness. Removal of the accuracy constraint on layer thickness allows much thicker layers to be used in most cases to increase the build rate. Machining also removes the dependence of accuracy on the material deposition method. Purely additive processes must deposit material to net shape and this typically necessitates the use of deposition methods that accurately deposit small increments of material. When machining is used to produce the final shape more rapid near-net shape deposition pro- 32

cesses may be used. The greater process flexibility may allow lower cost deposition equipment to be used. It may also enable the use of materials which can not be deposited to net shape. This greater flexibility in processes and materials also allows SDM to build parts from a wide range of materials, some of which are not available to other RP processes. Metal parts for example can be built directly using SDM. Other RP processes such as SLS can also build metal parts but these must be made by first building a green body and then either sintering it or infiltrating it with another material. These extra process steps increase build time and tend to reduce overall accuracy and tolerances. For processes that use support material or support structures, the support removal method is a key issue. For RP processes such as SLA, FDM and LOM this involves manually cutting away support structures. This may be difficult for fragile parts, or on parts with hard to reach internal cavities. SLS uses unsintered powder as support and it can be removed using compressed air, although it may be difficult among fine features or in internal cavities. The cohesive powder bed used in the slurry-based 3DP process serves as the support material. Placing the finished part in an ultrasonic bath redisperses the powder bed and frees the part. Ideally the support material is removable without the application of force, reducing the risk of damage to delicate parts, and without the need for manual intervention. This is typically achieved by having support material that can be removed by melting, chemical etching or a combination of both. These support removal processes also have the advantage that they can remove support material from internal cavities which may not be accessible using tools. This is the approach used in the Sanders and SDM processes. In metal SDM the copper support material is removed by etching in nitric acid. In polymer SDM the wax support material is removed using a heated organic solvent, although melting can be used to removed most of the wax before using the solvent to minimize the amount of solvent required. Melting alone is inadequate because it does not remove wax well from small passages and corners, and tends to leave a waxy layer on the part. SDM can be used to built multi-material objects as long as compatible part materials can 33

be found. In metal SDM, multi-material parts have been built using Invar, stainless steel and copper [Fessler 97]. However the copper and Invar must be completely embedded in stainless steel to prevent them from being damaged during the support material removal step. In polymer SDM parts have been made using several different polymers, for example combinations of polyurethane and polyurethane foam in different sections of the same part [Kietzman 99]. Embedded components can also be incorporated into SDM parts. The build process can be interrupted while components, such as sensors or actuators, are placed in position. As the build process continues these components become embedded inside the part. A range of wearable computers and electro-mechanical mechanisms have been built using this approach with polymer SDM [Weiss 96, Binnard 99, Cham 99]. Pre-assembled mechanisms can be fabricated using SDM. Other RP processes such as FDM and SLS can also produce mechanisms. Processes that use solvent support removal are advantageous because it is easier to penetrate and remove support material from the fine clearances between moving parts. 3.1.3 SDM Application Examples Metals SDM has been used to build a variety of metal parts [Merz 94a, Merz 94b]. SDM has also been used to build a multimaterial injection molding tool with internal cooling channels and copper regions for improved heat distribution, as well as pre-assembled mechanisms [Fessler 96]. More recently functional gradient parts have been built which feature composition gradients between Invar and stainless steel [Fessler 97]. Invar was used to reduce the thermal distortion in the part and the stainless steel produced a durable outside surface. Polymers SDM has been used to build several wearable computers that consisted of polymer housings with embedded electronic components [Weiss 96]. A variety of polymer parts, including pre-assembled mechanisms and multimaterial mechanisms, have also been made 34

[Kietzman 97]. The capability to embed components during part fabrication has been used to fabricate electro-mechanical mechanisms with embedded actuators and sensors [Binnard 99, Cham 99]. This application area is being investigated at the Center for Design Research at Stanford University as a new approach to the fabrication of robotic devices. 3.1.4 Fabrication of Ceramic Green Parts Using SDM SDM was found to be unsuitable for building ceramic green parts, principally due to the limitations of the materials combinations used. These issues are discussed in the next chapter. Because of these problems with the materials, alternative approaches for fabricating ceramic green parts were being investigated and the result was the creation of the Mold SDM process. 3.2 Mold Shape Deposition Manufacturing (Mold SDM) Gelcasting is a versatile method for making quality ceramic parts, however due to the poor interlayer bonding it was not possible to build parts incrementally in layers. But by using the waxes and soldermasks, as part and support materials respectively, it would be possible to build complex fugitive wax molds using SDM. These molds could then be used for gelcasting. This combination of process and materials was the starting point for Mold SDM. The following sections describe the Mold SDM process steps. Materials issues are covered in the next chapter. 3.2.1 Process Description Mold SDM makes molded parts using fugitive wax molds built using SDM techniques. Figure 3.2 illustrates how the example part used in the SDM example above would be built using Mold SDM. Although the part is the same as above this build sequence uses a more efficient decompo- 35

sition which reduces the number of steps required to build the part. Steps 1 through 6 in 1 2 3 4 Mold Material Support Material Part Material 7a 8a 5 6 7b 8b Figure 3.2: Example Mold SDM build sequence the SDM example are accomplished in steps 1 through 3 in this example. Figure 3.2 illustrates the five phases of the Mold SDM process: 1. Mold building (1-4): the mold is built using SDM techniques. In the case of Mold SDM however, the support material takes the shape of the part. The mold material encases the support material and forms the mold itself. Note also step 4, where the last layer of mold material is deposited to form the top of the mold. 2. Support removal (5): once the mold is complete the support material is removed, typically by dissolution, to open up the mold cavity. 3. Casting (6): the part material is cast into the mold cavity and allowed to cure or set. Vacuum degassing is used to eliminate trapped air in the casting. 4. Mold removal and finishing (7-8): once the part material has fully cured the mold must be removed, typically by melting or etching, and any finishing operations may be performed. There are a number of options at this stage of the process and two possibilities are shown here. The first option is the remove the mold (7a) and then perform finishing operations (8a). However for fragile or delicate parts it may 36

be beneficial to perform the finishing operations while the part is still protected by the mold material (7b). The mold is removed once the finishing operations are complete (8b). 3.2.2 Mold SDM Compared to SDM and Other RP Processes Since Mold SDM is based on SDM it shares many of the same advantages and disadvantages over other RP processes. The principal advantage of Mold SDM over other layered manufacturing processes is that the final part is cast monolithically. This is beneficial for two reasons: the finished part will not contain any layer boundaries. Layer boundaries can be a source of weakness due to incomplete bonding or the presence of foreign particles. This is particularly useful for flaw sensitive materials such as ceramics. the finished part will not contain any of the residual stresses that typically result from layered manufacturing. The mold may contain residual stresses, but these will not be transferred to the cast part. The lack of residual stresses in the finished part will reduce the tendency for distortion. The elimination of the need for interlayer bonding in the part material allows Mold SDM to use materials which can t be used in SDM. The ACR gelcasting formulations are one example, as are some polymer materials which have poor self adhesion [Kietzman 98]. Mold SDM has three disadvantages over SDM. First, a third compatible material, the mold material, is required. The additional materials compatibility and processing requirements may restrict the range of materials that can be used. Second, there are extra casting and mold removal steps which increase process time. Finally, mold filling issues may limit part geometry, although in many cases sprues and vents can be added to ensure complete mold filling. 3.3 New Build Techniques This section describes a variety of new build techniques which were developed to increase 37

the capabilities of the Mold SDM process. Since these are in fact variations on the basic SDM process they are applicable to SDM also. While in general 5-axis machining can be used, the Mold SDM machine was built around a 3-axis CNC milling machine due to the lower cost of the machine. For this reason most of the build techniques presented here assume that only 3-axis milling is used. The goals of the new build techniques are to increase: accuracy build rate the range of buildable features 3.3.1 Freeform Layer Geometries Part decomposition for RP processes has traditionally consisted of dividing the part into a number of equal thickness planar slices. Because SDM uses shaping methods capable of producing complex geometries this type of decomposition will typically result in parts being decomposed into more elementary units than is necessary. A natural extension of the process is therefore to remove the restriction that layer boundaries must be horizontal and planar. Instead layer boundaries should be shaped to conform with the part geometry: Using freeform layer geometries will generally reduce the number of layers required to build a part. This is beneficial for two reasons. First, the part build time is reduced because fewer deposit and machine cycles are required. Second, part quality is improved because there are fewer layer boundaries in the parts produced. Layer boundaries are often sources of defects due to incomplete bonding or the presence of foreign particles. Note that in Mold SDM this is only an issue for the mold since the final part is cast monolithically. Using freeform layer geometries also increases the range of manufacturable features. Certain features are impossible to build exactly using horizontal planar layer boundaries. The inclined sharp edge shown in Figure 3.3 is an example of such a feature [Merz 94a]. This feature is similar to a section of a screw thread. Manufacturable segments can only contain either undercut or non-undercut surfaces, not a mixture of both (vertical surfaces can be treated as either as necessary). The edge is bordered by an undercut face above and a non- 38

undercut face below. As shown in Figure 3.4, it is not possible to decompose the solid using horizontal planes such that these faces are separated. The two pieces still contain both undercut and non-undercut faces. This feature is therefore not manufacturable. Sharp Edge Top Side Non-undercut Surface Front Undercut Surface Figure 3.3: Inclined sharp edge Undercut Splitting Plane a b Non-undercut Figure 3.4: Decomposition using a horizontal splitting plane Figure 3.5 shows how this feature can be decomposed into buildable segments using a non-horizontal splitting plane aligned along the edge. The lower section now contains non-undercut faces and the top section undercut faces. A support segment would be built after the lower section to support the undercut surface above the edge. While some features can only be manufactured using these more general layer boundary geometries, others features that were manufacturable may become easier to build. One example, a tall thin cylinder, is shown in side view in Figure 3.6. Ideally the cylinder would be built in one layer to maximize build rate and eliminate layer boundaries. However to do this using traditional SDM methods would require a very high aspect ratio endmill, either to cut the inner diameter of the cylinder, if depositing part material first (Figure 39

Splitting Plane Undercut Non-undercut a b Figure 3.5: Decomposition using non-horizontal splitting plane 3.6a), or to cut the cylinder itself, if depositing the support material first (Figure 3.6b). High aspect ratio endmills may not be available and are in any case less desirable because their lower stiffness makes it more difficult to achieve good surface finishes. a b c Figure 3.6: Thin walled cylinder build sequences An alternative is to build the cylinder using a horizontal radial build sequence as shown in Figure 3.6c. In this case the support material inside the cylinder is built first. The part material is then built up around the support material and machined. Finally the outer support material is deposited. Large diameter, low aspect ratio tools can be used in all shaping operations because there is always plenty of tool access from outside. 3.3.2 Overcut-Fill-Trim-Backfill This technique makes it possible to build sharp internal corners in situations where con- 40

ventional milling will leave corner radii. Corner radii are produced because the cutting tools have finite diameters and can't reach right into sharp corners. Using smaller diameter endmills will reduce, but not eliminate, the corner radius. Figure 3.7 illustrates in top view how a sharp corner would be approximated by conventional milling. The starting condition is the block of material shown on the left of the figure with the dashed line showing the desired corner geometry. In Steps 1 and 2 the corner is machined with successively smaller diameter endmills. Each time the corner becomes sharper, but a radius remains. In Step 3 the finished geometry, after deposition of the part material, contains the corner radius. 1 2 3 Figure 3.7: Conventional milling sequence This corner radius can be avoided by reversing the build order shown in Figure 3.7. If the part material is deposited first then the corner can be machined from the outside and a sharp corner can easily be produced. The support material can then be deposited around it, producing the desired sharp corner geometry. Because it is simpler, reversing the build order, when possible, is preferable to the overcut-fill-trim-backfill method. In cases where there are both convex and concave corners the overcut-fill-trim-backfill method must be used. The overcut-fill-trim-backfill method is illustrated in Figure 3.8. The desired corner geometry is the same as above. Step 1 is to machine the pocket and overcut one side of the corner, in this case the top side. The entire right side of the pocket has now been machined to its final geometry. After filling the cavity with part material in Step 2, the top side is machined to its final shape as shown in Step 3. Backfilling the area left after trimming produces the finished sharp corner geometry as shown in Step 4. This technique increases the range of features which can be built exactly. Although a right 41

1 2 3 4 Figure 3.8: Overcut-fill-trim-backfill sequence angle edge is used in this example, the technique applies to sharp edges in general. It is also not limited to vertical sharp edges. For metals, and electrically conductive materials in general, this technique may not be necessary since EDM machining can be used to produce sharp concave edges. The disadvantages of this technique are that process planning is made more difficult and build rate is reduced due to an increase in the number of process steps required. 3.3.3 One Time Surface Machining Even when using non-planar layer boundaries there will frequently be cases where part surfaces must be split between two or more compacts. If each section of the surface is built separately then there will almost certainly be slight discontinuities between the sections. The most common reason for such discontinuities is distortion of the part caused by material deposition processes. To avoid these discontinuities all sections of a surface should be finish machined together in one step if possible. Figure 3.9 illustrates this technique. During the decomposition the part shown on the left will be split to separate undercut and non-undercut surfaces. If this is done using planar layers then the non-undercut surface on the right side of the part will be split at the point indicated by the arrow. If the part is built in the standard fashion, as shown in the upper sequence, then the two sections of that surface will be machined at different times and this might result in a slight discontinuity at the location indicated by the arrow. If instead the lower section of the surface is rough machined, leaving some extra material, as shown in the lower sequence, then it can later be finish machined together with the upper section. This will eliminate the discontinuity. 42

Figure 3.9: One time surface machining Since this technique involves machining into previously built layers, issues such as tool access and feature interference must be considered. An example of the use of this technique is the fabrication of a hollow cylinder with its axis oriented horizontally, as shown in Figure 3.10. The cylinder is split into a lower and an upper half by the decomposition and must therefore be built as two halves rather than as a single piece. In the sequence shown in Figure 3.10 the ends of the lower half of the cylinder would normally be finish machined in Step 2 and the ends of the top half would be finish machined in Step 4. Because the upper and lower halves of the ends are machined at different times there will usually be a slight discontinuity in the surface between the upper and lower halves. To avoid this the ends of the lower half can be rough machined in Step 2, leaving some extra material. Then in Step 4 the ends of both the upper and lower halves can be finish machined together. This will require cutting down into the support material for the lower half during Step 4. 1 2 3 4 5 Figure 3.10: One time surface machining example 43

3.3.4 Cut Through Machining In traditional SDM the material shaping operations are performed without consideration for the layers below the current layer. Cutting paths are generated based on the current layer geometry only. Each compact is machined to final shape immediately after being deposited and is not machined subsequently. Part quality can be improved by considering the current layer's geometry together with that of the underlying layers when generating cutting paths. In some cases, it may be easier to generate efficient cutting paths if the cutter is allowed to penetrate below the base plane of the current layer. An example of this is the case shown in Figure 3.11. Figure 3.11a shows the starting condition, where a layer of the part material has just been deposited. The desired final geometry is shown in Figure 3.11b. This curved surface is most efficiently machined using a ball endmill (outline shown in Figure 3.11b), however, because of the tip radius it is not possible to reach the very bottom of the curved surface. One can either leave small fillets on either side, as shown in Figure 3.11c, or gouge into the previous layer, as shown in Figure 3.11d. Leaving fillets will result in significant part inaccuracy. Gouging into the previous layer will do no damage unless a previous part material compact below the surface is gouged. An alternative would be to use a flat endmill to cut the entire surface. A flat endmill can reach the very bottom of the curved surface without gouging. However flat endmills do not cut curved surfaces efficiently. To get a smooth surface requires using a very small stepover which will take a long time. Another option would be to start with a ball endmill and then finish the fillets that are left using a flat endmill. If there are no part features in the gouge area then the simplest method is to use a ball endmill to cut the entire surface by allowing it to gouge into the previous layer (as in Figure 3.11d). 3.3.5 Incremental Casting When building parts, especially those involving thick layers, it is often necessary to deposit large amounts of material over previously machined geometry. If the deposition process occurs at high temperature, as is the case with wax casting, then it is possible that 44

c a b d Figure 3.11: Cut through machining features being deposited over will soften and distort, resulting in reduced part accuracy and surface quality. Incremental casting minimizes this distortion by first casting one or more thin protective layers before casting the bulk of the material. The thin layers have sufficiently low heat content that they cool rapidly without causing significant distortion. Once a sufficient thickness has been built up this will protect the underlying geometry when the bulk of the material is cast. Figure 3.12: Incremental casting Figure 3.12 illustrates the difference between incremental casting and the traditional build process. For the partially built part shown on the left the next step is to deposit support material. The upper sequence shows the traditional approach where a sufficient height of support material is deposited in one step and then machined to shape. The lower sequence 45

illustrates the incremental casting approach where a thin protective layer of support material is deposited first. Once this layer has cooled sufficiently the remainder of the support material is deposited. 3.3.6 Multiple Build Directions RP processes normally use only one build direction during the fabrication of a part. This is typically the vertical direction, working upwards from the bottom of the part. Based on specific features parts can be oriented in any direction relative to the build direction to make them easier to build. In many cases there are good build directions for certain sections of the part but no good build direction for the entire part. In these cases it would be useful if different sections of the part could be built using different build directions. Note that this is the only build technique presented in this section that requires 4- or 5-axis milling in order to be able to orient the part along its different build directions. Figure 3.13 shows a simple part that could benefit from the use of multiple build directions. In SDM and Mold SDM circular sections are most easily built when their axis is aligned with the build direction. This allows the entire circular section to be made in one step rather than as a lower half and an upper half. In the part shown in Figure 3.13 there is no orientation where the axes of both circular sections can be aligned with one single build direction. Using a single build direction this part must be built using at least two layers. Figure 3.13: Multiple build directions example part If multiple build directions are possible then the part can be built in one layer. The hori- 46

zontal section is machined from above (1), the vertical section is machined from the right (2), then material is deposited from each direction and again machined from each direction (3 and 4). If this were an SDM part then it would be finished at this point. In the case of Mold SDM, mold material would then be deposited from each direction to close the mold and casting features would be machined as appropriate. 1 2 3 4 Figure 3.14: Multiple build direction build sequence example Using multiple build directions reduces build time because the number of layers can be reduced, thus reducing the number of deposit and machine cycles required. The use of 5- axis machining also results in better surface quality and reduced cutting time. The disadvantages are that planning becomes more complex due to the greater range of build options and that material deposition from multiple directions is more complex. 3.4 Fabrication Of Pre-Assembled Mechanisms Mold SDM can be used to fabricate pre-assembled mechanisms. Figure 3.15a shows a set of three pre-assembled turbine mechanisms. This part consists of a rotor with a captive shaft passing through its center. Figure 3.15b shows a sectioned part. The shaft was built together with the rotor and no assembly operations were required. The turbines shown in Figure 3.15 are made of polyurethane but in general Mold SDM can make mechanisms from any part materials, including ceramics and metals. To build a pre-assembled mechanism one must build a part with two or more interlocking sections that are not joined. In Mold SDM this is accomplished by building multiple cavity 47

Figure 3.17 illustrates the sequence of steps involved in the fabrication of the turbine mechanism. A multi-cavity mold is fabricated (1) and part material is cast into the mold cavities (2). If desired, different materials can be cast into the different mold cavities. After mold removal (3) the casting features are removed to leave the finished part (4). Casting features include the casting reservoir and the various runners used to supply the mold cava b Figure 3.15: Pre-assembled mechanisms made using Mold SDM molds. Each part section requires a separate mold cavity. Figure 3.16 illustrates the mold configuration for the turbine part shown above. The mold cavities for the rotor and shaft are separate, however in this case they are both supplied from the same casting reservoir at the top of the mold. Reservoir Runners Rotor Shaft Figure 3.16: Multiple cavity mold for a turbine mechanism part 48

ities. 1 2 3 4 Figure 3.17: Turbine mechanism fabrication sequence Figure 3.18a illustrates a set of three multimaterial turbines that were made by casting different materials into the shaft and rotor cavities. The shaft is made of polyurethane and the rotor is made of epoxy. Figure 3.18b shows a turbine mold that was sectioned after casting both part materials. a b Figure 3.18: Multimaterial turbine Figure 3.19 illustrates three sintered alumina turbines. In the case of ceramic mechanisms there is an extra consideration which is that the sections of the mechanism must be kept separate during sintering so that they do not fuse together. This is achieved by leaving the casting reservoir and runners attached to the part until after sintering. The part material in the casting reservoir acts as a base and the runners fixture the sections of the part to the base and hold them in alignment during sintering. After sintering the runners and base are cut away. Figure 3.20 shows a sintered alumina turbine before removal of the casting features used as fixtures. It was sintered in the orientation shown. 49

Figure 3.19: Sintered alumina turbine mechanisms Figure 3.20: Sintered alumina turbine before fixture removal One of the key issues with the fabrication of mechanisms using this approach is the minimum achievable clearance between sections of the mechanism. For joints and bearings in particular clearances are critical. Since the mold cavities must be separate, the minimum achievable clearance is dictated by the minimum thickness of mold material that can be built. This depends on the strength of the mold material, the processing conditions and also the configuration, in particular the span, of the thin section. Several sets of polymer turbine parts were made to test the minimum achievable clearances. Using a 25% Kindt-Collins Master File-a-wax and 75% Kindt-Collins Master Protowax mix, turbines with radial clearances as low as 200 µm were built. The success rate at this clearance level was small and most parts ended up fused. A 250 µm clearance had better than a 50% success rate. In these parts the span of the thin mold section was 2 mm, equal to the thickness of the top and bottom discs of the rotors. 50

3.5 Process Example To illustrate the use of the Mold SDM process and the new build techniques, this section describes the process steps required to build the shrouded fan shown in Figure 3.21. Process planning is covered in Chapter 5 and will not be addressed here. Since the part is symmetric, for clarity only the section highlighted in the figure will be used in the figures accompanying the description. Figure 3.21: CAD model of the shrouded fan The design calls for sharp corners where the ends of the blades meet the hoops. To build these features requires use of the overcut-fill-trim-backfill method. The blades are built first, overcutting beyond their ends such that they extend into the space where the hoops will later be built. Once the blades are finished the hoops are built. Machining the hoops performs the trim operation on the ends of the blades. The first step is to machine the geometry of the undersides of the blades into the substrate. Because this is an inclined surface it is machined using a ball endmill. The cutting path extends all the way across to the sides of the blade so that the fillets left by the endmill are beyond the edge of the final blade surface. The geometry resulting from this step is shown in Step 1 in Figure 3.22. 51

Figure 3.22: Shrouded fan build steps Note that as shown in the figure the geometry has been machined down into the substrate, rather than in material deposited on top of the substrate. Either alternative may be used, but by machining down into the substrate one avoids one material deposition operation. In general in SDM and Mold SDM the first operation will be to machine geometry in the substrate rather than to deposit a first layer of material. After machining the underside geometry, the blade is built up using support material and 52

its top surfaces are machined, again using a ball endmill (2). Once the blades have been covered with mold material the hoops can be machined into the substrate (3). Machining the hoops trims the ends of the blades and leaves a sharp edge all around the ends of the blades. The hoops are then be filled with support material (4) and a final layer of mold material is deposited to seal the top of the mold. Machining the casting features into this last layer of mold material finishes the mold construction and exposes the support material ready for etching (5). After etching (6) the mold is ready for casting (7). Postprocessing after casting can be done in a number of different ways. In this example the mold material is removed (8) and then finishing operations, such as casting feature removal, are performed, leaving the finished part (9). Another option would be to perform the finishing operations while the part is still embedded in mold material. This could be particularly useful for fragile parts which may not be able to stand cutting forces or might be difficult to fixture without damaging the part. Figure 3.23 shows a finished shrouded fan. Figure 3.23: Finished polyurethane shrouded fan 53

4 Materials Selection This chapter describes the materials issues and materials selection process for Mold SDM. It begins with a general discussion of the materials issues that are important for a successful implementation of the process. The materials selection work for SDM of ceramic parts is described briefly because problems encountered there resulted in the development of the Mold SDM process. The Mold SDM materials selection history is described and a number of materials testing procedures that were used to evaluate candidate materials are presented. The chapter concludes with the current preferred materials combinations and brief descriptions of some of the potential next generation materials. 4.1 Materials Requirements There are two categories of materials used in Mold SDM: those that are used to build molds and those that are cast into the finished molds to produce parts. The two categories have different property requirements because of their different uses. Property requirements can also be roughly divided into two groups: those that are neces- 54

sary to make the Mold SDM process work, and those that are desirable because they improve the process by making it better, faster or cheaper. Examples of desirable properties are low material cost and low toxicity. 4.1.1 Mold Building Materials The materials used to build molds must work for the basic mold building process steps. These will always include material deposition, material shaping and material removal. Because of this the mold materials and the support materials will share many of the same material property requirements, but each will have its own function specific requirements also. The following sections describe some of the material requirements in general terms. More specific information and examples will be presented in later sections describing the current material combinations as well as materials tried previously. Properties Related To Deposition Low shrinkage: one of the key requirements for successful material deposition is low shrinkage. Deposition processes almost always involve some material shrinkage. When a material is deposited in the molten state it will shrink on solidification and there will also be an effect due to the coefficient of thermal expansion. Materials that cure chemically usually shrink and may also cause stresses from thermal expansion due to exothermic curing reactions. The net result of this shrinkage is the accumulation of residual stresses which tend to cause warping and delamination as parts are built up. Low viscosity: low viscosity is important for feature replication. High viscosity materials will not be able to flow into and replicate small details and therefore part accuracy will suffer. For materials deposited in the molten state the viscosity of the melt is a function of temperature and this in part dictates the deposition temperature required. Good wetting: good wetting is also necessary for replication of fine details. Materials that do not wet each other will not be able to replicate each other closely and part accuracy will suffer. 55

Strong interlayer bonding: strong interlayer bonding is necessary for two reasons. First, it reduces the tendency for layers to delaminate due to the buildup of residual stress. Second, it is necessary to prevent machining forces from causing delamination. For materials deposited from the melt interlayer bonding depends on the amount of remelting. For materials that cure chemically the interlayer bonding depends on chemical bonding between layers. Despite this it may be desirable to have some delamination to help release built up stresses, in much the same way that controlled microcracking can be used to increase material toughness. However this would be problematic for machining operations. Thermoset versus thermoplastic: since many of the material deposition and curing processes involve high temperatures, thermoset materials are attractive due to their superior temperature resistance. They will not melt and deform when other materials are deposited onto them. However thermoset materials tend to be chemically more stable which makes them difficult to remove using solvents (which is the only option since they can't be removed by melting). In most cases solvent removal was found to be impossible or impractical due to slow removal rates or the hazardous solvents required. Deposition and curing temperatures: temperatures required for successful deposition must be compatible with other materials. Excessive temperatures can cause remelting and softening which may lead to feature damage and distortion. Large temperature changes and gradients will also cause residual stresses due to thermal expansion. Faster curing materials usually exhibit more exothermic cures. Cure depth (if applicable): for materials that are cured by exposure to light, for example ultraviolet light, the cure depth is an important parameter. In general larger cure depths are better because thicker layers can be built more rapidly. However many materials cure exothermically and greater material thicknesses result in higher temperatures. Filled versus unfilled: in general unfilled materials are preferred because there is no possibility of the filler settling out while the material is being used. Fillers are often added to materials to reduce shrinkage. A worst case scenario is where the filler settles to the bottom of the cast area before the material solidifies. Then the lower sections 56

have an excessive filler concentration relative to the upper sections which results in low shrinkage at the bottom and high shrinkage at the top which will tend to make the cast material curl upwards at the edges. Many fillers are also abrasive and will shorten cutting tool life. Working time (if applicable): for materials that cure chemically after the addition of a hardener the working time is the amount of time that the material can be manipulated before it begins to set. Longer working times are generally desirable because it is easier to cast materials and deair them fully. However working times usually scale with cure times and shorter cure times are desirable for build rate. In practice a 10 minute working time is almost always sufficient. Melt characteristics: different waxes behave differently on melting. Some melt and quickly form a low viscosity liquid phase, others soften gradually as temperature increases and require significantly higher temperatures to achieve low viscosity. Both types are useful in Mold SDM. Those that melt to form a low viscosity liquid are best suited for casting because the deposition temperature can be adjusted as necessary without changing the viscosity significantly. However these waxes are not as good when cast onto because they will flow and deform more easily. Waxes that soften gradually can be used to create containment walls for casting because their viscosity can be adjusted by changing the deposition temperature such that they do not flow much before solidifying. This enables the fabrication of high aspect ratio walls. Properties Related to Machining Toughness: low toughness materials are difficult to machine effectively because they tend to chip easily, particularly along edges and corners, making it hard to achieve good finishes. Machining more slowly and carefully avoids most of the problems but greatly increases machining time. High aspect ratio features will always be difficult to produce because they will tend to break off due to the cutting forces. Softness: soft sticky materials are difficult to machine because the chips produced by the cutting tool stick to the tool and workpiece and clog it up. Once the cutting tool clogs up the chips rub against the surface being cut and ruin the surface finish. Using forced air to remove chips helps, and in some cases cold air can be used to increase the 57

material's hardness. In the case of waxes, higher melting point waxes are usually harder and tougher and so machine better. Abrasivity: this is particularly applicable to filled materials since ceramics and glasses are often used as filler materials. Due to their hardness and abrasivity these types of fillers will reduce cutting tool life. Abrasive fillers will also reduce the life of the machine tool by wearing out bearings and other mechanical components. Properties Related to Material Removal Melting point: if material is removed by melting then the melting point is an issue because that dictates the temperature that the material which is not being removed must withstand. In general lower temperatures are more desirable because they impose less constraints on the other materials being used. Melt characteristics: materials that melt and immediately form a very low viscosity liquid phase are best because they will run off most easily. Materials that soften gradually will require higher temperatures for removal which may be problematic for the other materials being used. Solvent resistance: ideally there are mutually exclusive solvent systems for each material that must be removed. That allows each material to be removed individually without affecting the remaining materials. No residue: materials should not leave any kind of residue after removal. Residue on the inside of the mold may get transferred into the part during casting. Residue left on the part after mold removal may affect the part and subsequent processing operations, such as sintering. Mold and Support Material Specific Requirements Requirements specific to the mold and support materials are that the mold material be unaffected by the support material removal process. This requirement is best satisfied by identifying a solvent for the support material that does not affect the mold material. The support material can also be removed by melting, as in the Sanders process, but this is not as desirable because it is very difficult to achieve complete removal of the support material from small features and internal cavities. 58

4.1.2 Part Materials Mold SDM part materials are the materials that are cast into the finished molds to produce the final parts. Whereas the mold and support material requirements are primarily process based, the property requirements for the part materials depend on both the process and the application requirements. The process requires that the part material be castable and that it not be damaged during the mold removal step. Application requirements typically include mechanical, thermal or electrical properties. Part materials are usually first chosen based on their ability to meet the application requirements, then they are tested to determine if they are compatible with the Mold SDM process requirements. In some cases when the part material is not compatible with process requirements it is possible to alter the process slightly to make it work, in other cases a different part material must be found. The following sections describe the main material requirements for Mold SDM part materials. Properties Related to Casting and Curing Viscosity: lower viscosity materials are easier to cast because they flow through the small passages in the molds more easily. If pressure is required to force the part material into the mold then mold strength may become an issue, particularly for small fine features. Lower viscosity materials are also easier to deair because bubbles can move through the liquid and up to the surface more easily. Wetting: mold filling is also dependent on how well the part material wets the mold material. If wetting is poor the part material will have difficulty flowing into fine features in the mold. Applying pressure during casting will help but this complicates the process. Working time: as explained above, longer working times make casting easier because there is more time to fill the mold and deair it. Longer working times usually correlate with longer cure times, but this is not as much of an issue with part materials because there is only one casting operation per part, instead of one per mold layer as there is with mold and support materials. For casting purposes the combination of working time and viscosity is important. Materials with higher viscosities can be used 59

successfully as part materials if they have a sufficiently long working time that the mold can be fully filled before the material begins to set. Cure conditions: whatever conditions are required to cure the part material must be compatible with the mold material. Materials are often cured by heating them, in which case the cure temperature must be sufficiently low that it does not cause the mold to soften or melt. The cure exotherm must also be taken into account. Cure exotherm: many materials cure exothermically and since the molds are made of wax this can be an issue if enough heat is generated that the mold softens or melts. Faster curing materials generally exhibit higher cure exotherms. Chemical compatibility: part materials must be chemically compatible with the mold material and the mold material must not chemically inhibit the curing of the part material or affect the part material's properties. Shrinkage on cure: part materials must shrink as little as possible on cure for two reasons. First, if there is significant shrinkage then the cured part may not accurately duplicate the geometry of the mold cavity. Sections of the part might shrink away from the mold surface and cause sink holes on the part surface. Second, if the part shrinks it may break itself or the mold due to the stress created. Properties Related to Mold Removal Temperature resistance: mold removal usually involves exposure to high temperature, either to melt off the mold directly or because a heated solvent bath must be used. It is therefore important for the part material to be temperature resistant. If the part softens during the demolding operation it is possible for it to warp or slump and thus reduce the accuracy of the final part. Solvent resistance: a solvent is often used either as the primary means of mold removal or in addition to melting to complete the removal process. Part materials must be resistant to the solvent so that they are not damaged during this step. Strength and toughness: mold removal is easier to accomplish when the part material is strong and tough because more mechanical force can be used if required. More vigorous agitation, for example, improves the mold material removal rate. Parts must usually be handled during this process and the stronger they are the less likely they are 60

to be damaged. When heat is used to remove the mold, the thermal expansion of the mold during heating, before the mold begins to soften, can cause damage to the part, so the stronger the part the better. 4.2 Materials For SDM Of Ceramic Green Parts Materials selection occurred in two distinct phases. The first phase, covered in this section, was during the initial work on using SDM to produce ceramic green parts directly. The second phase, covered in the next section, began when Mold SDM replaced SDM as the process for making ceramic green parts. The initial materials combination for SDM of ceramic green parts was a wax support material together with a ceramic gelcasting formulation developed by Advanced Ceramics Research (ACR) of Tucson, AZ. Early experiments indicated that the interlayer bonding between gelcast layers was insufficient to withstand machining forces. This frequently resulted in part delamination during machining. A number of surface treatment options, including the use of hydrogen peroxide to activate the surface before casting, failed to improve the bonding. Part of the reason for the poor bonding was that the green material contained relatively little polymer and the polymer was fairly inert after curing. By reformulating the gelcasting slurries it might have been possible to increase interlayer bonding but this was not pursued because the changes would greatly increase the toxicity of the slurries. This would make the process much less attractive. An alternative to the gelcasting slurries was required and this resulted in the development, by ACR, of a range of experimental ceramic filled waxes. These were similar to formulations used in ceramic injection molding. Testing indicated that the wax previously used as a support material would no longer be suitable. Due to the very high viscosities of the ceramic filled waxes these had to be deposited by high pressure extrusion at high temperatures. The high temperatures and forces involved caused significant deformation in the wax support material. The high viscosities also made it very difficult to fill fine features and often resulted in voids in deposited material. 61

Plasters were tested as a replacement for the wax support material because of their temperature resistance. Plasters were found to be unsuitable for a number of reasons. The main issue was removability. Pure plasters could be dissolved using a mix of sodium hydroxide and EDTA, however the rate of dissolution was far too slow to be practical. In an attempt to improve the removability, mixes were made using plaster and starch as well as plaster and sugar. The hope was that the water soluble starch or sugar would dissolve quickly, leaving a porous plaster network that could be easily dissolved or broken up using ultrasonic agitation. Unfortunately the mixes did not set fully and were therefore not machinable. Another potential mixture was plaster and calcium carbonate. In this case it was hoped that using acid to dissolve the calcium carbonate would produce bubbles that would break up the plaster. However the samples again did not set properly and remained soft and weak, making machining impossible. Pure plasters also tended to machine poorly. When the plaster was still wet the cutting tools tended to clog up. If the plaster was dried it became brittle and fine features tended to chip. Dried plaster was also virtually impossible to cast onto because it wicked the water from the newly cast layer. This made it very difficult for the new plaster to spread over the surface and also resulted in very poor bonding. Figure 4.1 shows a green silicon nitride slidey made using SDM. The part material (darker) is silicon nitride filled wax and the partially removed support material is plaster. Two of these parts were made. The first part was destroyed by the sodium hydroxide and EDTA solution used to etch the plaster, the second part is the one shown here. The detail view (b) illustrates a number of voids in the part caused by the difficulty of getting the highly viscous ceramic filled wax to adequately fill corners. Rayite 100, a machinable plaster made by US Gypsum, was also tested. It contained a polymer component to give it toughness and this produced excellent machinability results. Efforts to dissolve the Rayite 100 using sodium hydroxide and EDTA were unsuccessfully. This is believed to be partly due to the polymer component resisting dissolution and protecting the plaster. Another alternative for a support material was a water soluble thermoplastic polymer iden- 62

a b Figure 4.1: Green silicon nitride slidey made by SDM tified by ACR. This material had a higher melting point than the waxes and was also stronger. However since both it and the ceramic filled waxes were thermoplastic and very viscous, it was inevitable that there would be remelting and distortion during deposition. The high viscosities also meant that relatively expensive high pressure deposition equipment was required. A further limitation was that the ceramic filled waxes tended to swell and crack during dissolution of the support material. 4.3 Materials For Mold SDM The first part of this section is an overview of the Mold SDM materials selection process. The discussion covers the range of materials that were investigated and explains the associated issues. Later sections focus on the current Mold SDM materials and show a number of parts that have been made using the various part materials. The following sections outline the sequence of the materials selection process for Mold SDM. The initial phases of this process were the result of the materials selection for SDM described above. Materials compatibilities had already been determined and a change of perspective, from SDM to Mold SDM, made these materials the starting point for Mold SDM. 63

4.3.1 Initial Materials Combination The initial materials combination for Mold SDM was the set of materials tried during the work on SDM of ceramic green parts. The Kindt-Collins Master File-a-wax became the mold material, the soldermask mix became the support material and the gelcasting formulation was used as the part material. The wax had to be the mold material because the soldermask was not compatible with the gelcasting formulation. Attempts to gelcast into soldermask resulted in bubbles and cracking in the gelcast test parts. The soldermask would be removed by dissolution in water and the mold would be removed by melting. The overall process sequence would be as follows: 1. Build the mold using wax and soldermask. 2. Dissolve soldermask in water. 3. Gelcast alumina slurry into the mold. 4. Cure slurry at 100 C for 1 hour. 5. Remove mold by melting it off at up to 150 C in air. 6. Dry, burn out and sinter the green part There were four problems with this material combination: 1. The File-a-wax had high shrinkage which caused distortion and cracking. As the size and number of layers deposited increased this became more problematic. This would be a severe limitation for large or complex parts. 2. The File-a-wax has a high melting point and must be deposited at about 200 C for it to be sufficiently fluid to flow and fill fine features. At these temperatures features in the soldermask are severely distorted and this would make it very difficult to make accurate parts. 3. The curing conditions for the gelcasting slurry were not ideal. The slurry had to be heated to 100 C and cured at that temperature for 1 hour. This not only takes a long time, but the temperature is high enough that the integrity of the mold becomes an issue since the File-a-wax melts at 105-110 C. At the cure temperature the wax will have softened sufficiently that residual stresses built up during the build process might cause cracking of the mold. 64

4. Mold removal by melting did not work. Because of the high viscosity of the File-awax, mold removal was very difficult, even at temperatures as high as 200 C, because the wax did not melt and run off the parts easily. At these temperatures drying of the green parts was also a concern. If drying occurs too rapidly the green parts can be damaged by cracking. It was also found that melting left a thin film of wax over the part and this caused blistering and cracking during burnout and sintering. The sintered alumina part shown in Figure 4.2 suffers from this effect. Figure 4.2: Blistered alumina part 4.3.2 Reducing Cracking And Delamination Initial attempts to solve the first problem involved two approaches. The first was annealing of cast layers, the second was reduction of the temperature differential between ambient temperature and the deposition temperature. Annealing was performed by depositing a wax layer at 200 C and then, after cooling to room temperature, heating it back up to 100 C for 1 hour. It was hoped that the stresses caused by the deposition process would be relieved by plastic flow. Instead, the substrate cracked. This approach was not pursued further because it did not appear particularly promising and would be awkward to use in practice. For the second approach, reduction of the temperature differential, since reducing the deposition temperature resulted in incomplete bonding and delamination, the only option was to increase the ambient temperature. Experiments were performed where a substrate was preheated in an oven to 90 C, deposited onto using a casting temperature of 160 C, main- 65

tained at temperature for between 30 minutes to 1 hour, and then slow cooled to ambient temperature. Multiple layers could be deposited in this fashion but this was not a very practical approach since it would require a heated build chamber, which would be awkward to incorporate into the planned Mold SDM machine. The conclusion from these early experiments was that the File-a-wax was not suitable for use as a mold material because it had too high a deposition temperature and shrank too much on solidification. However, because of its excellent machinability attempts were made to lower the deposition temperature in the hopes that this would reduce the amount of heat distortion and reduce the residual stresses produced. 4.3.3 Lower Temperature Wax Deposition Reducing the deposition temperature of the wax would be advantageous for two reasons. At lower temperatures there would be less softening of the soldermask and thus reduced feature damage. Also, stresses caused by thermal expansion would be reduced since the temperature differential would be smaller. Deposition experiments were performed by depositing a number of wax layers and counting how many layers it took to produce a crack or delamination. Wax layers were cast as squares, with edge lengths of approximately 40 mm. It was found that the deposition temperature could be lowered from 200 C to 180 C. Further reduction in deposition temperature resulted in delamination due to insufficient bonding between layers. As described above, preheating of the substrate had enabled lower temperature deposition. A simpler alternative to using a heated build chamber was to use preheating of the surface only. This was accomplished using a hot air gun to heat the top surface of the substrate immediately before casting. Experiments indicated that the File-a-wax could be successfully deposited at 160 C using this method. While there was some success with reducing the deposition temperature, this approach was not successful. Feature damage was still a problem and larger parts would still warp and crack due to residual stress buildup. 66

4.3.4 Improving Gelcasting Slurry Cure Conditions Except for the gelcasting curing conditions issue, all the above issues could be resolved by switching to a wax with a lower deposition temperature and less shrinkage. Investment casting waxes typically fit these property requirements quite well because they are designed to have low shrinkage, for better pattern reproduction, and usually have melting points between 50 and 70 C. Unfortunately, since the gelcasting slurry must be cured at 100 C, these lower melting point waxes couldn't be used as the mold material. Therefore the first priority was to modify the gelcasting slurry such that it could be cured below about 60 C. The gelcasting slurry is made up of two parts: the slurry itself, which contains the ceramic particles and monomer, and an initiator, which is added to the slurry immediately prior to casting, to cause polymerization of the monomer. Advanced Ceramics Research suggested a number of alternative initiators for use with the existing slurry. These were tested and it was found that the slurry could now be cured in 20 minutes at temperatures as low as 50 C, thus enabling the use of a lower temperature wax as the mold material. 4.3.5 Switch To A Lower Temperature Wax The first lower temperature wax tested was Kindt-Collins Master Protowax. This wax was chosen because it had previously been used successfully at Carnegie Mellon University for the fabrication of wearable computers using SDM. A summary of this work is given in John Kietzman s thesis [Kietzman 99]. The Protowax is an investment casting wax with a melting point of about 60 C. It has much lower shrinkage and forms a low viscosity liquid phase almost immediately upon melting. Protowax can be cast at 90-100 C, reducing thermal damage to the soldermask, and cracking is almost completely eliminated due to its lower shrinkage. Although the wax film issue still remains for mold removal, the low viscosity of the Protowax allows it to run off parts much more easily and at much lower temperatures. Using Protowax as the mold material addressed the first two issues by having a lower shrinkage and a lower casting temperature, and improved the mold removability by imme- 67

diately becoming very fluid on melting. The Protowax also introduced some issues which had not existed before, so at this point the main process issues were: 1. Mold removal is still problematic because melting leaves a thin film of wax on the green parts and this causes blistering during drying, burnout and sintering. 2. The Protowax is not nearly as machinable as the File-a-wax. Machining fine features and high aspect ratio slots is very difficult and this will affect the kinds of features which can be built using Mold SDM. 3. Because the Protowax has a low melting point, curing of soldermask layers on top of wax can cause softening and melting of wax features due to the exothermic nature of the curing process. The heat generated by the UV light used for curing the soldermask makes this worse. 4.3.6 Solvent Removal Of Wax The first issue was addressed by using a solvent to finish the wax removal process. John Kietzman investigated a number of industrial cleaners and solvents for use in polymer SDM to remove the wax support material [Kietzman 99]. The best solvent was found to be BioAct 280 Precision Cleaner, produced by Petroferm Inc.(Fernandina Beach, FL). This is a solvent that operates at, or above, the melting point of the wax and can be used to completely remove wax from parts. Testing indicated that this solvent was not harmful to the green gelcast parts. There was concern initially that the solvent might dissolve or damage the polymer binder phase in the green parts, or that it might alter the drying and burnout properties. However this was found not to be the case and green parts could be fully cleaned using the solvent, and then sintered with no ill effects. A secondary advantage of using the solvent was that the wax removal process could be performed at lower temperatures, thus reducing the risk of cracking the green parts. Protowax removal in air in an oven must usually be performed at temperatures in the 90-120 C range. Using the solvent the wax can be removed below 80 C and the process is also much faster. 68

4.3.7 Investigation of Alternative Waxes The lower machinability and melting point issues seemed to indicate that a wax with properties intermediate between those of the File-a-wax and Protowax would be best. Ideally the wax would have the excellent machinability of the File-a-wax, the low shrinkage of the Protowax and an intermediate melting point. In an effort to identify this optimum wax a wide range of commercially available waxes were tested and evaluated for machinability, shrinkage and deposition temperature. Table 4.1 lists a selection of the waxes that were evaluated. Material Melting Point ( C) Comments The Kindt-Collins Company Master File-a-wax 110 machinable wax with excellent machinability, high shrinkage Master Protowax 60 pattern wax with low shrinkage, poor machinability Aluminum hard pattern wax 80 aluminum filled wax for pattern repair Calwax 252-B machinable wax 108 general purpose machinable wax, poor interlayer bonding 126 casting wax 85 investment casting pattern wax, very sticky, poor machinability Corning Rubber Inc. Blue Inlay wax 70 pattern building wax, poor machinability Freeman Mfg. & Supply Company Freeman machinable wax 108 machinable wax, excellent machinability, high shrinkage Kerr Perfect Purple wax 75 pattern building wax, slightly tacky Master pattern carving wax, soft 110 pattern making wax, excellent machinability, high shrinkage Master pattern carving wax, medium 110 pattern making wax, excellent machinability, high shrinkage Master pattern carving wax, hard 115 pattern making wax, excellent machinability, high shrinkage M. Argüeso & Co., Inc Rigidax, Type WI-GREEN 90 fixturing wax, filled with glass fibers, high viscosity melt, sticks to machine Rigidax, Type WI-NMF RED 90 uses a non-metallic filler Table 4.1: Waxes tested for use in Mold SDM 69

These waxes can be divided into two general categories: machinable waxes investment casting waxes Machinable waxes are designed to be used for CNC program verification. They are therefore highly machinable and do not contain any abrasive fillers that might wear out cutting tools. These waxes are strong and tough, with melting points in the range from 90 to 110 C. These waxes exhibit high casting shrinkage, but this is not an issue in normal use because the wax is used in blocks as supplied and is not recast. Investment casting waxes are used to make investment casting patterns. These are typically made by injection molding so low shrinkage on solidification is desirable for pattern accuracy reasons. These waxes have lower melting points, typically between 50 and 70 C, and on melting they rapidly become fluid to make pattern removal easier. Machinability is not critical and most of these waxes machine fairly poorly. The Kindt-Collins Master File-a-wax and Protowax were the first two waxes tried because they had been used previously at Carnegie Mellon University. Together they represent both ends of the spectrum of waxes investigated for use in Mold SDM and were used as baseline waxes for comparison purposes. The initial materials search focused on identifying a machinable wax that had lower shrinkage and a lower melting point than the File-a-wax. The Freeman and Calwax machinable waxes were the main candidates. The Freeman machinable wax was found to be virtually identical to the File-a-wax and offered no measurable benefits. The Calwax machinable wax was not quite as machinable as the File-a-wax, although it was still very good. The problem with the Calwax was that it had poor interlayer bonding and tended to delaminate. Since machinable waxes all appeared to have similarly high shrinkage, the next search was for a pattern wax with better machinability and a slightly higher melting point than the Protowax. There are several types of pattern waxes. The lowest melting point waxes are very soft and can be formed by hand, or using slight application of heat. These waxes are much too soft to machine well and melt at too low a temperature for use in Mold SDM. 70

The intermediate melting point waxes, such as the Protowax, can be machined, carved or injection molded to form patterns. The highest melting point waxes are designed for hand carving and these are in many ways very similar to the machinable waxes. The Kerr Master pattern carving waxes are an example of the high melting temperature pattern waxes. These machined very well but also exhibited high shrinkage on solidification. These waxes did not provide any benefits over the machinable waxes already tested. The Calwax casting wax, Corning Blue Inlay wax and the Kerr Perfect Purple wax are examples of the intermediate melting point pattern waxes. Although they all have higher melting points than the Protowax, none of them machined noticeably better. Another type of wax was the Kindt-Collins aluminum hard pattern wax. This is a wax used for repairing or filling in features in patterns. It contains about 30% by weight of aluminum powder and it was hoped that this would reduce the shrinkage of the wax. Unfortunately the wax had a high melt viscosity which made it very difficult to fill fine details during casting. The Rigidax waxes shown at the bottom of the table are used as fixturing waxes for machining. The Rigidax is cast around parts to hold them in place and provide support for delicate features. After machining the wax is removed from the parts. The Rigidax waxes, in particular the WI-GREEN, were used to build a number of parts and they worked quite well. In their pure form the Rigidax waxes have relatively low shrinkage, comparable to that of the Protowax, but their melt viscosity is much higher, since they contain fillers. The Rigidax machines reasonably well but without cold air it clogs fairly easily. These waxes are also quite brittle which results in machined surfaces being finely pitted. To improve the machinability, mixes of Rigidax and Protowax were tried at Carnegie Mellon University. It was found that using 10% Protowax increased the toughness of the Rigidax and resulted in smooth machined surfaces without significantly affecting the other properties. The fillers used in the Rigidax waxes caused a number of problems. The high melt viscosity made it difficult to deposit material and obtain good replication of fine features. Most of the varieties used abrasive fillers, such as glass fibers, which would shorten tool life and 71

cause wear to the milling machine. All other Mold SDM materials cause essentially no wear to the tools or the machine, which reduces the amount of periodic maintenance and tool changing required. The fillers are also significantly more dense than the wax and this results in settling when the material is molten. The automatic dispensing equipment used on the Mold SDM machine does not have build in stirrers so the wax was continually circulated through a loop and back into the tank to provide some mixing. A serious issue with the Rigidax was that the chips generated during machining stuck to and accumulated on all the internal surfaces of the Mold SDM machine. This would have long term maintenance and reliability implications for the machine. 4.3.8 Wax Optimization For Shrinkage, Machinability and Deposition Temperature Based on the survey of commercially available waxes it was determined that there were no waxes that met the Mold SDM process requirements. The only remaining option was to develop a custom wax which would provide a better trade-off in terms of properties. Since the wax formulation expertise was not available in house the next best alternative was to have an outside supplier formulate a custom wax. Wax manufacturers, including several of the companies listed in Table 4.1, were contacted about developing custom waxes. Development costs were prohibitive, often because minimum material quantities were very large, often over 70 kg. It would also be necessary to perform a number of iterations before the optimum formulation could be identified and this would be expensive and time consuming. The alternative was to develop a custom wax in house, by combining the commercially available waxes to try to improve the property trade-off. The File-a-wax and Protowax were chosen as the two component waxes and these were mixed in various proportions to arrive at a wax with properties intermediate between the two. The machinabilities and shrinkages of the various mixes were evaluated as described below. The net result of the testing was that it was found that the machinability increased gradually as the fraction of File-a-wax increased, but that the shrinkage remained low until about 35-45% File-a-wax at which point it increased rapidly to a higher level. Based on these results an optimum 72

combination consisting of 25% File-a-wax was selected as the best mold material. 4.3.9 Other Materials Considered A mold or support material option that was not investigated was low melting point metal alloys, such as Wood's metal. These alloys can be made with melting points as low as that of mercury. A benefit of a metal over a non-metal would be the higher thermal conductivity. This might allow sufficiently fast heat dissipation that distortion from casting of hot materials would be reduced. Metals also tend to machine well. However these alloys typically contain metals such as mercury, bismuth, antimony, lead and indium which are toxic and expensive. Furthermore, any metallic residue left on green parts would cause problems during sintering. For these reasons low melting point alloys were considered but were never tested. 4.4 Materials Testing 4.4.1 Machinability Testing The goal of the machinability testing was to obtain a quantitative measure of how well the materials used in Mold SDM could be shaped by CNC milling. This information could then be used to select the best material from those tested as well as to evaluate new candidate materials in the future. Machinability Standards Because of the huge number of possible machining operations and configurations, and the large variety of different materials that can be machined there are no general machinability testing standards [Mills 83]. A variety of machinability testing standards exist but these are not applicable to the materials used in Mold SDM and they do not produce the desired data. ISO 3685-1977 is a general machinability test procedure for evaluating the machinability of metals using single point cutting tools in a lathe. ASTM E 618-81 [ASTM 81] addresses the machinability of 73

ferrous alloys using automatic screw machines. ASTM D 1666-87 [ASTM 87] describes a method of evaluating the performance of wood machining operations. All of these test methods quantify either how fast the operation can be run while achieving a given level of surface finish and tool wear rate, or they are used to determine relative tool performance for a given operation. Reports in the literature indicate that in many cases custom machinability tests are developed to suit the particular conditions of interest [Metcut 80]. The machining issues of interest in Mold SDM are different than those found in machining metals: Tool wear is minimal because the materials being machined are in general not abrasive. The notable exception to this are filled waxes which may contain metal, glass or even ceramic fillers. Cutting rate is usually limited by either chipping of the workpiece, or by the cutter becoming clogged up with chips. Neither of these factors is a significant limitation in the machining of metals. For these reasons a new machinability test was developed to evaluate machinability as it applies to Mold SDM. The Mold SDM Machinability Test The test consists of machining a constant depth slot in a single pass. Initially a 120 mm long linear slot was used. This test was found to be impractical for the higher shrinkage waxes because it was not possible to cast such a large area of wax without it cracking or delaminating. All machining tests were conducted on recently cast wax to duplicate the conditions seen in the actual process. The current test consists of machining a constant depth spiral slot inside a 50 mm diameter post of material. By using a spiral slot it is possible to get a relatively long slot in a small area of material and thus avoid the cracking and delamination problems. Figure 4.3 shows the geometry of the spiral in relation to the outline of the post of material. The spiral is made up of a set of connected circular arcs. The numbers on the right side of the figure are 74

the y-coordinates of the ends of the arcs (the x-coordinates are all 0). The test begins outside the post of material at point A and ends at point B. 20 14 8 B -2-8 A -14-20 Figure 4.3: Machinability spiral test pattern The posts are made by casting wax into 50 mm diameter pockets cut into a 1/2 thick Teflon plate which is placed on top of a wax substrate. After cooling the teflon frame is removed and the tops of the posts are planed flat. The standard cooling time is 4 hours in ambient air with no forced cooling. The slot is machined in a single pass using a 1/8 (3.175 mm diameter) two flute carbide endmill. The depth of cut is equal to the diameter of the endmill. The standard spindle speed and feed rate are 6000 rpm and 1000 mm/minute respectively. Most tests varied the feed rate while keeping the spindle speed constant. Three coolant options were used: no coolant, room temperature air and cooled air. The no coolant option involves machining the slot in ambient air with no forced airflow to help remove chips. Room temperature air and cooled air both use forced airflow, directed at the tip of the endmill by one nozzle, to help remove chips. The cooled air is produced using a vortex cooler. The machinability was evaluated by examining the quality of the slots produced. As 75

shown in Figure 4.4, two failure modes were identified: chipping and clogging. Chipping (a) is where the top edges of the slot are chipped or broken. Clogging (b) is where the endmill clogs up with chips and leaves debris stuck to the walls and floor of the slot. a b Figure 4.4: Machinability failure modes The machining test is based on machining a constant depth slot in one pass. This was chosen because it represents the worst case situation: the endmill is cutting with its full width and there is no open area to the side where chips can escape. The maximum amount of chips are being generated and there is minimum clearance for them to be removed. A 1/8 two flute carbide endmill was chosen for use in these tests. A 1/8" endmill was chosen for to reasons: it is a size commonly used in part building and its small size reduces sample dimensions. Two flute endmills are used in almost all Mold SDM machining operations because they provide more clearance for chip removal than three or four flute endmills. Experience had indicated that the machinability of the wax varied with time as the wax cooled. All samples were machined 4 hours after casting. This cooling time was chosen because it was sufficiently long for the waxes to cool fully and also because it was not so long as to be completely unreasonable from a process rate standpoint. In practice cooling might be accelerated using refrigeration. 76

Machinability Results Figure 4.5 shows the machinability results when no coolant is used to blow away the chips. As the fraction of the Kindt-Collins Master File-a-wax increases, the machinability increases as can be seen from the greater feed rates that can be used. This is as expected since the File-a-wax is designed to be a machinable wax. Mixtures containing less than about 35% File-a-wax cannot be machined because the cutter clogs up and produces very poor surface quality. 3000 UNMACHINABLE Feed Rate (mm/min) 2000 1000 Chip Clog MACHINABLE 0 10 20 30 40 50 60 70 80 90 100 Percent Kindt-Collins Master File-a-wax Figure 4.5: Machinability without air The curve for the chipping failure mode lies entirely above the clogging curve and therefore chipping is never a limiting factor. The higher feed rate achieved at 20% File-a-wax on the chipping curve is due to the coarseness of the measurements and also the difficulty of obtaining consistent results with low scatter. Feed rates were varied in increments of 250 mm/min and the final value for each composition level was the lowest value obtained. In some cases the results were fairly consistent, in others values varied by as much as 1000 mm/min. 77

To improve the machinability one can use compressed air to help clear away the chips and thus reduce the amount of clogging. Figure 4.6 shows the machinability results when using room temperature air to blow away chips. As can be seen from the plot the clogging has been greatly reduced and it is now possible to machine waxes containing as little as 5-10% File-a-wax without clogging the cutter. The dashed line shows the previous boundary between the machinable and unmachinable regions, and as can be seen from the figure machinability has been improved throughout the composition range. 3000 Feed Rate (mm/min) 2000 1000 UNMACHINABLE Clog Chip MACHINABLE 0 10 20 30 40 50 60 70 80 90 100 Percent Kindt-Collins Master File-a-wax Figure 4.6: Machinability using room temperature air At higher fractions of File-a-wax, above about 30%, the machinability is now limited by chipping. Despite the improvements due to the use of room temperature air the machinability of the low fraction of File-a-wax waxes is still not very good because of the low feed rates required. Since the limiting factor for those waxes is clogging, one option is to use cold air as coolant to increase the hardness of the wax. The results are shown in Figure 4.7. As can be seen from the figure, clogging has been greatly reduced and it is in fact no longer the limiting factor. At all compositions chipping is now the limiting factor. Use of cold air 78

reduces the toughness of the wax and thus increases the tendency to chip. 3000 Feed Rate (mm/min) 2000 1000 Clog UNMACHINABLE Chip MACHINABLE 0 10 20 30 40 50 60 70 80 90 100 Percent Kindt-Collins Master File-a-wax Figure 4.7: Machinability using cold air Note also that the use of cold air has only improved the machinability for compositions between 0 and 10% File-a-wax. At most other compositions the increased tendency to chip has actually reduced machinability. Based on these results one can see that room temperature air provides the most benefit in terms of machinability because it reduces the clogging and does not affect chipping. Cold air is only beneficial for waxes with very low fractions of File-a-wax, below about 10%, where clogging is the limiting factor. For all other compositions the cold air increases the tendency to chip and thus reduces the machinability. 4.4.2 Shrinkage Testing The goal of shrinkage testing was to obtain a quantitative measure of the shrinkage that occurs as wax is cast and allowed to solidify. Shrinkage causes residual stresses in parts and these result in the interlayer discontinuities, warping and in extreme cases, delamination or cracking. Reducing shrinkage, or identifying materials with low shrinkage, will 79

improve part quality and accuracy. Shrinkage measurements were only made on the wax materials because the soldermask materials exhibited negligible shrinkage. Background The volumetric shrinkage of waxes can be measured by casting a known volume of material and then measuring its volume after cooling. Since the wax will usually solidify in a non-uniform shape it is necessary to use a method such as water displacement to measure the volume. Volumetric shrinkage values for several of the waxes were measured by Professor Paul Calvert and Sajiv Boggavarapu of the University of Arizona in Tucson, AZ. The results indicated that all the unfilled waxes had volumetric shrinkages in the range of 8-12%. The newer filled waxes that have been developed at the University of Arizona and at Advanced Ceramics Research can have volumetric shrinkages of 4% or less. The best results so far have been for a wollastonite filled wax blend made at Advanced Ceramics Research that has a volumetric shrinkage of 2-3%. Volumetric shrinkage, however, is not representative of the situation in Mold SDM. On cooling, waxes were found to exhibit a range of shrinkage modes the two extremes of which are shown in Figure 4.8. The dashed outline shows the shape of the volume of material immediately after casting. The shaded area is the final geometry. The mode shown on the left is the more isotropic shrinkage mode where all sides shrink inwards. In the mode shown on the right, the sides shrink in very little and most of the volume change is accommodated by the top surface. Figure 4.8: Wax shrinkage modes These shrinkage modes are at least partly due to the cooling conditions, particularly the relative heat transfer rates for each surface. Surfaces that cool more rapidly will form solid 80

skins which will resist deformation. Slower cooling surfaces will be able to distort and accommodate shrinkage in other areas of the part. If the surface is cooled rapidly, then as the still molten core solidifies and shrinks it will tend to pull in the surfaces, resulting in concave surfaces. When building parts in layers the more isotropic shrinkage mode is undesirable. This is because the area of the base shrinks as the sides shrink inwards. Because the base of the newly cast material bonds to the previous layer, this shrinkage must be accommodated by a combination of distortion and residual stress buildup. When the shrinkage is accommodated mostly by the top surface there will be minimal stress and distortion at the interface between the newly cast material and the previous layer. It will however be necessary to cast a thicker layer such that the top surface can be machined away and still leave enough material to form the required layer thickness. The Mold SDM Shrinkage Test The shrinkage test consists of casting wax into a container and then measuring the final size of the cast block once it has cooled. Only the shrinkage in the XY plane is measured, because as described above, shrinkage in the Z direction does not contribute significantly to part distortion. Initial tests were performed by casting wax into a teflon frame. Teflon was chosen as the container material because it would be easy to remove the wax samples once they had cooled. There were two problems with this approach. First, the teflon frame has a significant coefficient of thermal expansion and will change size when the hot wax is cast into it. As the wax and frame cool the frame will change size again. With the container changing size during the experiment it is not possible to get reliable results. The second issue was that when hot wax is cast into a container at room temperature, the wax rapidly forms a skin from contact with the cold surface and then as the interior solidifies significant warping occurs and a wrinkled surface results. The irregular surfaces and warping make it difficult to measure repeatable shrinkage values. To address these two issues an Invar frame was used. The frame was machined from Invar 36 plate. It was 10 mm thick and 75 mm square. A 50 mm square window was machined 81

out of the center of the plate. A 1/4 endmill was used to finish the sides of the window, resulting in corner radii of 3.175 mm (1/8 ). The supplier s datasheet for the Invar 36 indicates that over the range from room temperature to 200 C, the Invar has a thermal expansion of less than 0.02%. About half of that occurs above about 130 C. In the worst case therefore, the inside dimension of the Invar frame will change by less than 10 µm over the temperature range used during shrinkage testing. The shrinkage test procedure was as follows: 1. Preheat the Invar frame, resting on a Teflon plate, to the casting temperature in an oven in air. 2. Maintain at the casting temperature for 1 hour. 3. Cast wax into the Invar frame and allow the oven to equilibrate at the casting temperature (takes a few minutes). 4. Turn off the oven and allow it to slow cool (takes several hours to reach room temperature). After cooling the wax sample was removed from the Invar frame and left to stabilize for one week. This was necessary because it was found that the wax samples shrank further over the course of a few days after casting. This is believed to be due to slow crystallization effects. Sample dimensions were then measured in three places for both the X and Y directions and the results were averaged. At least four samples were made for each material. If large variations were found in the results further samples were made to verify if the variation was due to experimental error or if it was simply due to variability in the material. The wax mixes consisting of File-a-wax and Protowax were all made using material from the same batches to avoid any variations due to batch to batch variations in properties. Some of the issues with this measurement technique include: Effects of long term shrinkage: even after the wax has fully cooled, it still continues to shrink slightly over a period of several days to a week. During part building these 82

long term effects should not be significant since parts should be built over shorter time spans. Effect of cooling rate: the cooling rate may affect the shrinkage. Waxes are frequently partially crystalline so slower cooling rates should increase the amount of crystallization and thus increase the shrinkage because crystalline areas are more densely packed. The shrinkage values measured by this test should therefore be higher than those experienced during the process where cooling is more rapid. Effect of the Invar frame and Teflon base plate: during Mold SDM processing the wax will not be cast onto a Teflon plate and will not be inside an Invar frame. The wax will be cast onto and surrounded by either wax or soldermask. Differences in heat transfer rates may affect the results, however since all container materials are preheated to the casting temperature and this is a slow cooling test, the effects of the thermal properties of the container materials should be minimized. Shrinkage Results Shrinkage values were measured for a range of mix ratios of the File-a-wax and Protowax as well as for some of the other waxes tested for use in Mold SDM. Figure 4.9 shows the variation in shrinkage as a function of the fraction of File-a-wax for the mixtures and Table 4.2 lists shrinkage values for the other waxes that were tested. The results shown in Figure 4.9 are qualitatively as expected, with the machinable wax showing the highest shrinkage. However the key result is that the shrinkage drops dramatically in the 30-40% File-a-wax range. Shrinkage measurements were initially made at 25% intervals and then further measurements were made near the step in order to define it better. Because of the sudden step in the shrinkage it is desirable to use wax mixes with File-a-wax fractions below about 30%. Further reduction in the File-a-wax content will further reduce the shrinkage, but then the machinability may become an issue as described above. Table 4.2 shows that the Rigidax waxes, and in particular the WI Green variety, have very low shrinkages. This is partly due to the fact that these are filled waxes and the fillers help reduce shrinkage. Unfortunately their other properties are not as good. The machinability 83

3.0 Shrinkage (%) 2.0 1.0 0 10 20 30 40 50 60 70 80 90 100 Percent Kindt-Collins Master File-a-wax Figure 4.9: Shrinkage as a function of the fraction of machinable wax Material Shrinkage (%) Kindt-Collins Master Protowax 0.47 ± 0.04 Kindt-Collins Master File-a-wax 2.60 ± 0.07 Freeman machinable wax 2.91 ± 0.09 Rigidax, WI Green 0.26 ± 0.02 Rigidax, NMF Red 0.68 ± 0.05 Kindt-Collins 3230A 0.83 ± 0.02 Table 4.2: Measured shrinkage values for various waxes of these materials is quite good but they are quite brittle and therefore the machined surfaces tend to be finely pitted. The chips generated during machining also stick to the insides of the milling machine and make it hard to clean. The fillers also increase the casting viscosity and make it difficult to accurately replicate fine features without using high casting temperatures. The Kindt-Collins 3230A wax is a blend of 25% Protowax and 75% File-a-wax that was made by Kindt-Collins. Its shrinkage of 0.83% is higher than the 0.68% value measured 84

for mixes made at Stanford University. This difference can be at least partially attributed to the variability between batches of material. 4.4.3 Heat Resistance One of the key issues in Mold SDM is thermal distortion caused by casting hot materials over existing features. Existing features soften due to the heat input and then distort due to gravity and other forces present during deposition. As the cast material cools, stresses are caused by thermal contraction as well as phase changes, and these also contribute to distortion. Heat resistance testing has focused solely on the heat resistance of the soldermask support material during wax deposition. The heat resistance of the wax during soldermask deposition is not as much of an issue. During soldermask deposition, curing of the soldermask can cause remelting of the wax mold material. Heat is generated by the exothermic cure reaction and the UV light source also emits some IR radiation which will cause heating. However if adequate cooling is used and the soldermask is cured in thin layers then remelting of the wax can be avoided. If in future a water soluble wax is used as a soldermask replacement then the heat resistance of both the mold and support material waxes will need to be investigated. The Mold SDM Heat Resistance Test The Mold SDM heat resistance test consists of depositing wax over features machined in soldermask and then measuring the resulting distortion. This test measures the combined effects of a variety of parameters that affect heat resistance and surface replication: Deposition temperature: higher temperatures generally cause more remelting, softening and distortion. Viscosity: higher viscosities will impair feature replication and may result in higher forces exerted on the substrate features as material flows during deposition. Shrinkage effects: shrinkage during solidification and cooling may cause distortion and these effects will be difficult to separate from thermal effects. 85

Figure 4.10 shows the geometry that is machined into a block of soldermask. It consists of four square posts. Wax is then deposited over the soldermask features at various casting temperatures. The soldermask is dissolved and the replicated wax pattern is examined. Epoxy is then cast over the wax patterns and the wax removed in order to create a more permanent replica of the deformed soldermask geometry. 8 5 3 3 8 5 3 mm deep Figure 4.10: Heat resistance test geometry Results Tests were performed at 10 C intervals from 80 C to 130 C. Two identical samples were made at each temperature. After removal of the soldermask it was immediately evident that the 80 C casting temperature was too low because the wax had solidified before it had had time to fully flow into the machined slots. The result was rounded edges with a corner radius of approximately 0.5 mm. There was negligible distortion of the soldermask posts although their top surfaces were very slightly domed due to heat distortion. The edges were approximately 30 µm lower than the centers. At 90 C and above there were no replication problems due to insufficient casting temperature. The distortion seen in the soldermask posts was limited to the top edges around the posts. Distortion was approximately constant over the temperature range and did not show a significant increase with increasing deposition temperature. Maximum height differences between the edges and the centers of the tops were about 60 µm. 86

The conclusion from this heat resistance testing is that the soldermask is quite heat resistant when exposed to wax deposition conditions. The distortion seen in parts is much greater than the results of this testing would predict. The primary cause of the greater distortion seen in practice is probably incompletely cured soldermask. The fact that the distortion often manifests itself as ripples on the surfaces, with spacing between the ripples on the same order as the layer thickness, and the fact that the lower sections of layers tend not to cure as fully suggests that this is probably the cause. 4.5 Current Mold SDM Materials 4.5.1 Mold Materials The current preferred mold material is a mixture of 75% Kindt-Collins Master Protowax and 25% Kindt-Collins Master File-a-wax. As shown above, this wax has superior machinability to the Protowax and shrinks significantly less than the File-a-wax. Its melting point is intermediate between the two and offers sufficient heat resistance. When building parts that do not require intricate machining the Protowax is sometimes used as the mold material to take advantage of its lower shrinkage. It is also used if possible when building larger parts, again because of its lower shrinkage. Another option is to use a combination of materials to build a mold. A wax with a higher File-a-wax content could be used where machinability is required, and then Protowax could be used in areas where machinability is not as critical. One implementation of this approach would be to build a mold consisting of a File-a-wax shell inside a bulk Protowax block. This approach has not yet been tested since simpler alternatives are more desirable because they minimize process and process planning complexity. 4.5.2 Support Materials The current preferred support material is a mixture of 80% Electrolite ELC 4497 and 20% Dymax 9-20113F. This mixture was developed during the polymer SDM work performed by John Kietzman [Kietzman 99]. The mixture was found to be necessary to achieve a 87

good combination of toughness and machinability. The Electrolite ELC 4497 by itself was very brittle and chipped easily when machined. The Dymax 9-20113F was fairly rubbery and tended to machine very poorly due to clogging of the cutting tools. By combining the two it was possible to increase the toughness of the Electrolite ELC 4497 sufficiently for it to machine acceptably. The major limitations of this material are: Low cure depth: the maximum cure depth is on the order of 1.5 mm which means that thick layers must be built up from a number of thin layers. This increases build time. Poor machinability: when machined gently the soldermask produces a very fine, high quality finish. However more aggressive machining often causes chipping. Poor machinability increases the build time and may limit the types and sizes of features that can be built. High viscosity: the mixes have a fairly high viscosity so it takes some time for the material to flow into fine features. Also any trapped air, or bubbles, must be removed by degassing which is awkward. It also appears that the two materials may react when mixed because the viscosity of the mix gradually increases over time. Dissolution rate: the soldermask dissolves slowly in water. A 1 cm cube would take several hours to dissolve in still or slightly agitated water. When access is restricted, as it is when dissolving soldermask from inside a mold cavity, the dissolution rate is slower. At this point there is also no way to determine if all of the soldermask has been dissolved. Development of a chemical test to test for the presence of soldermask would be useful. Presently the soldermask dissolution step is done very conservatively to ensure complete removal and this increases the time taken to build parts. 4.5.3 Part Materials Mold SDM has been used to make parts from a range of part materials including ceramics, polymers and most recently metals. The primary goal was to make ceramic parts and this has been the focus throughout the research. Polymer parts are easier and quicker to make, because they do not require the drying, burnout and sintering post processing steps, and so 88

they can be used for mold verification purposes, in addition to producing polymer parts. Metal parts can also be made in much the same way that ceramic parts are made by using metal gelcasting. Ceramic Part Materials All ceramic parts made using Mold SDM have been made by gelcasting (see Chapter 2, Section 2.3.7). The initial material was a proprietary non-aqueous alumina gelcasting slurry developed by ACR. A silicon nitride slurry has also been developed, based on the alumina formulation. Due to differences in the surface chemistries of alumina and silicon nitride the organic components are slightly different in each slurry. The silicon nitride slurries are more difficult to make and do not currently have as high solids loadings as the alumina slurries. Table 4.3 shows a comparison of the properties of these slurries. Material Solids Loading (%) Cure Conditions (time,temp) Alumina slurry 55 30 minutes @ 55 C Silicon nitride slurry 52 30 minutes @ 55 C Table 4.3: ACR gelcasting slurry properties Table 4.4 lists typical processing conditions for the silicon nitride gelcasting slurries used in Mold SDM. The sintered alumina impellers shown in Figure 4.11a are an example of the impeller design. This design was used for test part throughout the process development work because it was a relatively complex part that was also easy and fast to build. The impeller design consists of two planar disks between which are six curved blades. This design is intended to be similar to a pump impeller part. The sintered alumina turbines, shown in Figure 4.11b, are a variation on the impellers. These use a modified impeller with eight blades as the rotor and have a shaft passing through the center of the rotor. The shaft and rotor are separate pieces such that the rotor is free to spin about the shaft. These parts were built pre-assembled and were the first ceramic mechanisms made using Mold SDM. 89

Operation Curing Drying Burnout Sintering Conditions 50-60 C for 2 hours, parts are hermetically sealed and evacuated to exclude air drying is performed in air as follows: 4 hours @ 86 C 1.5 hours @ 96 C 2 hours @ 155 C 15 hours @ 165 C 2 hours @ 186 C burnout is performed in air using the following time-temperature profile: 80 C/hour to 160 C 10 C/hour to 300 C 2 hours @ 300 C 6 C/hour to 400 C 2 hours @ 400 C 12 C/hour to 500 C 18 C/hour to 600 C 1 hour @ 600 C sintering is performed in a nitrogen atmosphere at temperatures between 1700 and 1750 C. Control of the atmosphere is critical to obtain the best results. Table 4.4: Processing conditions for the silicon nitride a b Figure 4.11: Alumina impellers and turbines Figure 4.12 shows three miniature turbines that were built by Shelley Cheng. These are shown in front of a full size turbine part. The miniature turbines are about 7 mm in diameter and all sections are 0.5 mm thick. Figure 4.13a shows a sintered alumina pitch shaft. This was originally a Hughes part from a missile guidance system that was produced by CNC machining of titanium billets. 90

Figure 4.12: Miniature alumina turbines Changing to a ceramic material would reduce the weight and inertia of this part while increasing its temperature resistance. Initial attempts to sinter pitch shafts resulted in significant warping due to sagging of the thin sections. To minimize the distortion parts were built with an integrated frame that could be cut off after sintering. Figure 4.13b shows the configuration of the green part with the frame. The top section of the frame keeps the uprights aligned and the central pillar prevents the top section from sagging. a b Figure 4.13: Alumina pitch shaft The sintered alumina vane shown in Figure 4.14 is a simplified version of a vane doublet from a Rolls-Royce Pegasus gas turbine engine. This part was built to test the ability of Mold SDM to produce larger parts, such as the actual vane doublet, and also to test the use of 5-axis cutting for part building. The design was simplified to reduce the build time since this was only a test part. This part was built in three layers using 5-axis cutting to machine 91

the blade surfaces. Figure 4.14: Alumina vane doublet The impellers shown in Figure 4.15a are some of the first silicon nitride parts made using Mold SDM and were postprocessed and sintered at ACR. These parts were medium grey with white marks on the part surfaces due to the sintering procedure. Silicon nitride parts can have colors ranging from beige to almost black, depending on the sintering conditions, and if the atmosphere is not well controlled white stains can occur on the surfaces. a b Figure 4.15: Silicon nitride impellers and pitch shaft The pitch shaft, shown in Figure 4.15b, has a much more even surface color and is a pale beige rather than a grey like the impellers. This is a result of optimization of the sintering process. The part is not perfect however, as there is some flaking along some of the part edges. 92

Figure 4.16 shows a top and a bottom view of a sintered silicon nitride inlet nozzle part. This is a part for the M-DOT miniature gas turbine engine (see Chapter 8, Section 8.4 for further details). a b Figure 4.16: Silicon nitride inlet nozzle An aqueous silicon nitride gelcasting formulation developed at Oak Ridge National Laboratories (ORNL) was also tested for use in Mold SDM. The advantage of an aqueous formulation is reduced toxicity since there is no organic solvent in the slurry. However for silicon nitride in particular there may be issues with the water affecting the surface of the silicon nitride powders over time. In practice this does not appear to be a problem as the ORNL slurries have produced silicon nitride with strengths over 1GPa. An issue with these slurries for use in Mold SDM is the mold removal procedure. Boiling of the solvent, or any other component, of the slurry will cause damage to the green part. Because the ORNL slurries use water, the mold removal process must be performed at temperatures below about 85 C. The ACR formulations use a non-aqueous solvent which has a boiling point above 200 C. The restriction on the mold removal process limits which waxes can be used as mold materials, although at this point the best mold waxes have lower melting points and are compatible with the ORNL gelcasting formulations. An issue with the ORNL formulations is completeness of curing. The surface of the green part is often not fully cured where it is in contact with the mold surfaces. This is believed to be due to cure inhibition caused by a component of the mold wax and is most pronounced in molds made using the Protowax. Longer cure times at higher temperatures 93

seem to eliminate the problem, but this effect has not been investigated in detail because attention has focused on the ACR formulations. Figure 4.17 illustrates a pair of sintered silicon nitride impellers made using the ORNL gelcasting slurry. The impeller on the left shows the effect of incomplete curing of the slurry. The sections are thinner and the edges are rounded giving the part a slightly skeletal look. Figure 4.17: Silicon nitride impellers Polymer Part Materials Most polymer part materials used in SDM can also be used in Mold SDM. The main limitation is the combination of viscosity and cure time. Because SDM casts part materials into relatively open areas materials with higher viscosities and shorter cure times can be used. These materials might be difficult to use in Mold SDM because of the difficulty of achieving complete mold filling. Table 4.5 lists the main polymer part materials used in Mold SDM. All of these materials, except the silicone, were originally identified for use in polymer SDM either by researchers at Carnegie Mellon University or by John Kietzman, whose thesis contains a extensive description of the selection and evaluation of these materials [Kietzman 99]. The Adtech LUC 4180 was originally used at Carnegie Mellon University for the fabrication of wearable computers using SDM. Because its compatibility with the process was known, it was a natural starting polymeric part material. The main limitation of this material for use in SDM was the poor interlayer bonding which resulted in parts that tended to 94

Material Strength (MPa) Mixed Viscosity (cps) delaminate. Since Mold SDM parts are cast monolithically this issue is avoided. The main issue for use with Mold SDM was the viscosity and how well it would fill complex molds. In most cases this was not a problem because the material has a sufficiently long working time, on the order to 10 minutes, to allow effective mold filling. The Ciba TDT 205-3 polyurethane was selected for its more rapid cure. In SDM, where building each layer involves a part material casting step, the material s cure time has a significant effect on process rate. With slow curing materials, such as the LUC 4180 polyurethane, the cure time can account for most of the build time. Since Mold SDM part fabrication involves only one part material casting operation, short cure times are not as great a concern. This material has a very short working time of less than 5 minutes, however it also has a very low viscosity so mold filling is not usually a problem. The TDT 205-3 was found to be incompatible with the Rigidax waxes. Surfaces in contact with the Rigidax did not cure fully and had a wet look after demolding. Cure Conditions Adtech LUC 4180 polyurethane 55 800-900 12 hours at room temperature Ciba TDT 205-3 polyurethane 23 80 2 hours at room temperature Adtech 501/530 epoxy 42 3500 24 hours at room temperature Dow Corning Sylgard 184 silicone 6.2 3900 24 hours Table 4.5: Polymer part materials The Adtech 501/530 epoxy was selected as an alternative to the LUC 4180 because of its good adhesion. This is not an issue for Mold SDM since parts are cast monolithically. Because of the high viscosity and slow cure time, this material is less attractive than the LUC 4180 for Mold SDM applications. However because it has a long working time, of at least 15 minutes, it has been used successfully in many complex molds made using Mold SDM. This material has slightly better heat resistance than the other two materials and thus there is less tendency to soften and distort during the mold removal process. The Dow Corning Sylgard 184 silicone was used for making the artery model. This material was selected by Mary Draney of the Stanford University Mechanical Engineering 95

Department because its properties were suitable for the testing procedures that would be employed. The main issue was the elastic modulus of the material so that it would deform measurably under pulsed flow conditions. Transparency was necessary so that tracer particles in the flow could be monitored. Figure 4.18 shows a number of Ciba TDT 205-3 polyurethane (a) and Adtech 501/530 epoxy impellers. a b Figure 4.18: Polymer impellers Figure 4.19 shows a set of LUC 4180 polyurethane turbines (a) together with a sectioned mold made of 25/75 wax (b). These turbines have been made with radial clearances between the shaft and rotor as small as 200 µm. a b Figure 4.19: Polyurethane turbines and sectioned mold 96

Figure 4.20 shows two views of a Ciba TDT 205-3 polyurethane shrouded fan. This part was designed to be similar to a fan that might be found in a typical gas turbine engine. It demonstrates the ability of Mold SDM to produce fine features with smooth surfaces. Figure 4.20b shows a close-up of several of the blades that shows the smooth curved surfaces of the blades and the well defined sharp trailing edges. a b Figure 4.20: Polyurethane shrouded fan Figure 4.21 shows a multicolor polyurethane Inchworm that was built by David Miller and Brad Levin. The Inchworm consists of a flexible backbone and two ratcheting wheels. By pressing down on the backbone the front wheel moves forwards while the rear wheel is held fixed. Releasing the backbone allows the rear wheel to move forwards in turn. Unfortunately the ratchets on the wheels did not work because the clearances used in the design were too large. The Inchworm is made of LUC 4180 polyurethane. The backbone is white, which is the natural color of LUC 4180, and the wheels were made red by adding a red color to the polyurethane. Figure 4.22 illustrates a proof of concept silicone artery model. This part was built for Professor Charles Taylor and Mary Draney of the Mechanical Engineering Department at Stanford University. The objective was to demonstrate the ability of Mold SDM to rapidly produce complex artery models in silicone for use in flow measurement experiments. Because the parts would be used for flow experiments it was important that the surfaces be smooth so that surface roughness would not alter the flow characteristics. Manual finishing of models produced by RP processes would be very difficult because of the difficulty 97

a b of reaching the internal surfaces. Figure 4.21: Multicolor polyurethane Inchworm Figure 4.22: Silicone artery model Figure 4.23 shows two different multimaterial polymer mechanisms. The turbines (a) consist of a modified impeller with eight blades with a shaft passing through the center. The rotors are made of Adtech 501/530 epoxy, the shafts are made of Adtech LUC 4180 polyurethane. The slidey (b) is made of the same materials and consists of a rectangular frame with a circular disk inside. The disk is free to rotate and to move back and forth in the frame. Metal Part Materials Metal parts can be made by metal gelcasting [Janney 95, Janney 98]. Metal gelcasting is like ceramic gelcasting except that a powdered metal is used. Because most metals have 98

a b Figure 4.23: Multimaterial polymer mechanisms much higher densities than ceramics there are settling issues with metal gelcasting. The metal powders available also typically have larger particle sizes which again increases the tendency of the particles to settle. Powders with smaller particle sizes are becoming more widely available so this should improve the viability of metal gelcasting in the future. Metal gelcasting is currently not as developed as ceramic gelcasting. Figure 4.24 shows two green 17-4 PH steel parts made by Katsu Sakamoto using metal gelcasting. Figure 4.24a shows an impeller part similar to the ceramic and polymer impeller parts previously shown. Figure 4.24b is a pump impeller. In both cases the casting features are still attached to the parts. a b Figure 4.24: Gelcast steel parts 99

4.6 Future Mold SDM Materials 4.6.1 Mold Materials The current wax mold materials are quite good and can be used to make quality parts, particularly if the parts are planned in such a way as to minimize the effects of the materials limitations. However, there is always room for improvement. Reduced shrinkage and increased machinability would be very useful for increasing part quality and build rate. Based on the materials search to date, it is fairly certain that there are no commercially available waxes that meet the needs of the Mold SDM process. It is therefore necessary to use the knowledge gained to date to develop a custom mold material. ACR and researchers at the University of Arizona at Tucson are both investigating new wax based materials for use as mold materials. ACR is working to develop a wax with properties similar to the current Kindt-Collins mix. One of the issues with commercial waxes is that not all of the ingredients are necessary. For example dies, which may cause chemical incompatibilities or curing inhibition, are not needed because for Mold SDM the appearance of the wax is not important. By eliminating these unnecessary components and controlling the important ones more closely it should be possible to develop a superior wax. Initial waxes have shown improved machinability and greatly reduced cure inhibition of the gelcasting slurries. However high melting points and high shrinkage continue to be problematic. The University of Arizona researchers are investigating the possibility of using a variety of fillers to minimize the shrinkage of the wax. Many commercial waxes contain fillers to improve their properties but these often have associated issues. Ceramic and metal fillers, for example, are abrasive and reduce tool life. They are also much denser than the wax itself and so tend to settle if the molten wax is not constantly agitated. Organic fillers are more promising because their density more closely matches the wax and they are not abrasive. A key issue with fillers is the increase in melt viscosity. Initial waxes have shown encouraging reductions in volumetric shrinkage and good machinability. 100

4.6.2 Support Materials The soldermask support material is an effective support material despite several key limitations. Its high viscosity makes it difficult to deposit because it does not flow into fine features and tends to trap air bubbles. Because of its low cure depth it takes a long time to deposit thick layers and there is always the risk of incomplete curing in corners where there is poor light access. The cured soldermask is also quite brittle and must be machined carefully to avoid chipping the surface. Because the soldermask is moisture sensitive it must be protected from ambient moisture. Efforts to improve the machinability and cure depth were not very successful. The existing soldermask mixture was found to be the best combination. A new soldermask material recently developed by ACR has a very low viscosity and much better machinability. Initial results are promising but there has not yet been any extensive testing. Water soluble waxes have also been tested as possible soldermask replacements. Many of these contain fillers and tend to crack when cast, but a Kindt-Collins water soluble wax, KC 2283-A Premium grade non-filled, was recently found that works quite well. The disadvantage of using two thermoplastic mold building materials is that feature damage due to mutual remelting is difficult to avoid, although some of the new build styles, for example incremental casting, can help minimize this problem. 4.6.3 Part Materials The ceramic gelcasting slurries have been continuously refined by ACR during the course of this research. Work has focused on reducing viscosity to make casting easier, maximizing the solids loading to make sintering easier and increasing the batch to batch consistency and repeatability of the slurries. Batch to batch consistency is still an issue as viscosity can be quite different from batch to batch. Characterization work at ACR is making good progress in understanding the issues involved. The most important issue with the gelcasting slurries is the mechanical properties of parts made using them. Strengths are gradually increasing as the slurries and their processing conditions are optimized. Measured strengths are currently below the industry average but 101

recent results are very promising. Up until now alumina and silicon nitride slurries have been used. In future silicon carbide and zirconia formulations might be needed for other applications. Metal gelcasting is currently not as well developed as ceramic gelcasting, but initial experiments have shown promise. 102

5 Process Planning This chapter introduces three enhancements to the process planning techniques for the SDM and Mold SDM processes. The first is a manufacturability analysis which can identify whether a certain part orientation is buildable. The second is a qualitative method for identifying optimum build orientations from sets of potential build orientations. The third is a number of new decomposition approaches that are proposed to take advantage process flexibility based on part features. All of these techniques are intended to improve the quality of parts produced, and where possible to simplify the planning process. 5.1 Background Process planning consists of generating the information required to fully specify the manufacturing steps needed to build a particular part. In general process planning for layered manufacturing processes consists of three distinct sequential steps: 1. Geometric decomposition 2. Operation planning 103

3. Machine code generation The geometric decomposition step divides the part model into manufacturable sections. Operation planning generates the parameters, such as tool paths, required to build each of these sections of the part. In the last step these parameters are translated into machine commands which run the part building machine. Each of these steps is usually highly process dependent. 5.1.1 Planning for Rapid Prototyping Processes Most RP processes build parts by stacking planar 2.5-dimensional layers of uniform thickness. Figure 5.1 illustrates the sequence of planning and manufacturing steps for this general type of process. 1 2 3 4 5 6 Figure 5.1: RP process planning The first step, given a part model (1), is to determine where support structures are required. Support structures are usually needed to support undercut surfaces. After adding support structures (2) the model is decomposed into layers (3). Layers are usually planar and of a uniform thickness. Curved-Layer LOM is an exception where uniform thickness curved layers are used [Kalmanovich 96]. The decomposition produces a set of 2-dimen- 104

sional cross-sections (4). These represent the areas that must be filled with part and support material to form each layer. A path planning algorithm then generates a suitable scanning pattern that will fill these areas. Patterns typically involve tracing the perimeter and then filling the interior using a variety of raster patterns. In some cases the interior fill pattern is not 100% dense. This reduces build time and saves material, possibly at the expense of part strength. The last step is to build the part by executing the deposition patterns, layer by layer, until the whole part has been fabricated (5). In some processes, such as Sanders, each layer is planed after deposition to ensure correct thickness and a flat surface for the next layer to be deposited onto. Removal of the support material leaves the finished part (6). Notice that because the layers are created as 2.5 dimensional geometries the inclined surfaces of the part exhibit a stairstep effect. Reducing the layer thickness will improve the approximation but will not eliminate it. Reduced layer thickness increases the layer count and thus the build time. To increase surface quality without reducing the layer thickness CAM-LEM uses an approach called tangent cutting [Zheng 96]. Instead of cutting out 2.5 dimensional sections of material, 5-axis laser cutting is used to cut the edges such that they are inclined and tangent to the part surface. This produces much smoother surfaces and allows thicker layers to be used. This approach is only applicable to processes that use a cutting or machining process. This technique can t be used in SLA or FDM, for example, without major changes to the process and equipment. RP process planning usually starts with a geometric model in the STL file format. The STL format represents the part surface as a set of triangular facets, each of which also has a normal vector to indicate which side is outside. Because the surface is faceted some geometric information is lost. Curved surfaces in particular are no longer easily identified. Large numbers of facets are needed to represent curved surfaces accurately and this typically results in large file sizes. There can also be model quality issues if there are any gaps where facets don't join exactly. 5.1.2 Planning for the SDM and Mold SDM Processes Since Mold SDM uses SDM techniques both processes are essentially the same in terms 105

of planning. The main difference is that Mold SDM planning involves the creation of the mold geometry based on the given part geometry. This involves generating the negative of the part, with sufficient material around the part to form a structurally sound mold, and then adding gates and vents for casting. Figure 5.2 parallels Figure 5.1 above and illustrates the process planning and fabrication steps used when building the part with Mold SDM. 1 2 3 4 5 6 Figure 5.2: Mold SDM planning The first step is to take the given part model (1) and generate the mold geometry (2). The part geometry becomes support material. An oversize bounding box is created around the part with sufficient material around the part such that the mold is structurally sound. This material becomes the mold material. Note that support structures do not need to be created explicitly as in the RP planning case. This is because the mold has vertical sides, which do not need support, and all other features are embedded inside and are therefore automati- 106

cally supported. Casting features are created. In this simple case a gate is created with a reservoir above it. The model is then decomposed (3). Here the process differs significantly from the RP planning process. Instead of decomposing the part using evenly spaced horizontal planes, here the decomposition is much more flexible. Layer boundaries may be of arbitrary shape and need not be planar, horizontal or evenly spaced. The resulting decomposed model (4) preserves the geometry of the original part. Smooth inclined surfaces are represented as smooth inclined surfaces and are not approximated as a series of planar cross sections. This allows the operation planning to generate deposition and cutting paths that will build the specified geometry and not an approximation of it (5). This is the second major difference between SDM planning and RP planning. Instead of generating deposition paths to fill areas, SDM planning must generate both that type of path, to deposit materials, and fully 3-dimensional cutting paths to shape the material after deposition. The 3-dimensional cutting path generation is considerably more difficult to automate than the 2-dimensional raster patterns for deposition, however the geometry produced is of a much higher quality in terms of surface smoothness and accuracy. Once all the sections of the part have been built the support material is removed to leave the finished mold (6) which can be used for casting. Whereas RP processes are based on the faceted STL geometry, SDM uses the original CAD model which contains an exact representation of the desired geometry. Curved surfaces will be specified as curved surfaces rather than as sets of facets. As a result there is no loss of geometric information between the CAD system used to design the part and the SDM manufacturing system. 5.1.3 Planner Development History The first SDM planner [Merz 94a] was developed at Carnegie Mellon University and was based on the NOODLES [Gürsöz 88] geometric modeling engine, which was also developed there. NOODLES is a linear geometric modeling engine which means that it must approximate curved surfaces using a number of planar facets (as is the case in STL files), and this was a limitation of this original planner. The geometric decomposition algorithm began by classifying each surface of the model as 107

either undercut or non-undercut. Layer slice heights were then chosen where the surface changed from non-undercut to undercut. Layers were further decomposed into part and support material segments. All the resulting segments were then arranged according to build order. This implementation of the planner used horizontal planes to divide the model into layers and was therefore limited to planar layers of variable thickness. Features within each layer, however, did not need to be 2.5 dimensional so inclined surfaces could be build accurately. Due to the limitations of NOODLES this planner was also not able to represent curved surfaces exactly. A second generation planner [Ramaswami 97] was developed around the ACIS geometric modeling engine from Spatial Technologies Inc., Boulder, CO. ACIS is a non-linear geometric modeling engine which enables it to represent curved surfaces as curved surfaces and not sets of facets. Although this planner was used for many parts there were limitations, particularly in the sophistication and robustness of the cutting path generation algorithms. An alternative planning approach, called design by composition [Binnard 98], was developed at Stanford University. In this approach instead of decomposing a model to obtain manufacturable sections, a model is built up by combining pre-defined manufacturable primitives. These primitives, which are currently limited to prismatic shapes, are combined using a set of rules, based on manufacturing process constraints, to produce more complex shapes. Given a set of manufacturable primitives the system can generate a valid manufacturing sequence for the combination, but manufacturability of the combination is not guaranteed. Manufacturability is not guaranteed because it is possible to create a part with features which are too small to machine for example. 5.1.4 The Current Planner The current planner is built on the Unigraphics CAD system and its geometric modeling engine, called Parasolids. There are several reasons for the change from ACIS to Unigraphics: 108

Improved cutting path generation: this was one of the main weaknesses of the previous planners, particularly for freeform surfaces. Unigraphics has one of the most sophisticated CAM packages available for cutting path generation. Accessibility of CAD/CAM functionality: because the planner is a software add-on it must be able to access the core functionality of the system that it is built around. Not all CAD/CAM systems make this functionality accessible from outside. Unigraphics provides both a scripting language, called GRIP, and an API, called UG/OPEN, which external applications can use. Complete CAD/CAM functionality: geometric modeling capabilities will always be required for model creation and manipulation. Manual cutting path generation capabilities are needed while the automated planner is under development and may also be required subsequently to handle special cases that the planner can't. Unigraphics provides both of these functions whereas ACIS provides neither since it is a core package on which applications can be built. AutoCAD is the most popular ACIS based CAD package but it does not have any built in CAM capabilities, and the available CAM packages are not as sophisticated as Unigraphics. The planning process is illustrated in Figure 5.3. The input is a CAD model of the part to be fabricated. Given this model the first step is to determine the build orientation for the part. There will typically be many potential build orientations, and these must be analyzed for manufacturability so that the best orientation can be identified. This will typically be an iterative process where each orientation is evaluated in turn. If there are no manufacturable orientations then it may be necessary to modify the original CAD model to make it manufacturable. Alternatively the part can be built to approximate the model as closely as possible given the limitations identified in the manufacturability analysis. Once the build orientation has been selected the model must be decomposed into manufacturable sections, called compacts. A compact is a section of a part that can be built in one deposition and machining cycle. Once all the compacts have been identified a set of operations must be generated to manufacture each one. This involves deposition and cutting path generation as well as specification of curing, cooling and preheating operation parameters. 109

CAD Model Orientation Selection Process Planner Manufacturability Analysis Decomposition Operation Planning Machine Code Generation Fabrication Figure 5.3: Process planning flow chart The final output from the planner is an operation sequence and a set of operation parameter files. The operation sequence describes which operations must be performed and in what order they must be performed in order to build the part. The parameter files contain all the information required to fully specify each operation. Typical parameters include CNC programs, materials to be deposited, curing and cooling times and preheat power levels and times. The current planner, as well as the previous ones, focus primarily on the decomposition, operation planning and machine code generation steps. Machine code generation, which is very tedious, is relatively easily automated since it consists mostly of translating operation specifications, particularly deposition and cutting paths, into machine readable formats. Decomposition and operation planning are much more difficult tasks and they are targets for automation because they are time consuming and require detailed process knowledge and experience. Encoding this process knowledge in the planner relieves the operator from needing a deep understanding of the details of the process. 110

5.2 Process Planning Enhancements Of the planning steps shown in Figure 5.3, the planner currently focuses on the more tedious and computationally intensive decomposition, operation planning and machine code generation steps. The higher level strategy issues such as orientation selection and non-planar decomposition are not addressed. The new build techniques presented in Chapter 3 will also affect the planning strategy. Previously unbuildable features may now be buildable, at least in certain orientations, and decomposition strategies may also be affected, particularly by multi-step strategies such as overcut-fill-trim-backfill. Another limitation of the current planner is that it does not build parts exactly as specified in the CAD model. The principal manifestation of this is that sharp concave corners are often manufactured with a slight fillet because it is not always possible to machine square concave corners. In many cases the new build strategies now enable SDM and Mold SDM to build parts exactly, without any approximations due to machining limitations. The next generation planner will need to make use of this expanded process capability so that it can automatically identify the optimum build strategies and manufacture parts exactly as specified. The first step in implementing these more sophisticated planning approaches is to develop a scheme to identify which features are or are not manufacturable. A general scheme is presented in the manufacturability analysis section below. Because manufacturability depends primarily on the relationships between surfaces it is highly dependent on part orientation. The section on orientation selection discusses some of the issues that are important in orientation selection, particularly part features and how these affect the difficulty of manufacturing parts. A complete and general algorithm for orientation selection has not yet been developed because this is a highly complex problem that depends greatly on the specifics of part geometry and also the intended use of the parts. New decomposition strategies are presented that will address some of the planning needs for the new build techniques and also help improve overall part quality. Most layered manufacturing processes, including SDM and Mold SDM, have been developed around the 111

concept of a single build direction. If this constraint is eliminated and multiple build directions are allowed then the decomposition strategy changes also. 5.3 Manufacturability Analysis The goal of the manufacturability analysis is to determine if a given geometric model can be built exactly in a particular orientation. This means that the part produced must not contain any geometrical approximations or defects due to systemic limitations of the manufacturing process. For example all sharp edges and corners must be built sharp and without fillets. Because of the need to determine if a part can be built exactly, the manufacturability analysis for SDM and Mold SDM is different than that for typical RP processes. Except in special cases commercial RP processes can t build the exact part geometries and therefore there will always be a degree of geometric approximation.the primary example of this is in the fabrication of inclined surfaces where RP processes produce a stairstep effect rather than a smooth surface. Because of this approximation, the manufacturability analysis for RP processes typically concentrates on finding build orientations where the approximation error is minimized. A variety of methods can be used for this optimization. The volume error can be obtained by computing the geometry of the part that will be built and then comparing it to the starting model. Surface smoothness can be calculated based on the angle of a surface relative to the build direction. Surfaces that are parallel or perpendicular to the build direction are usually best. Because of the machining step in SDM and Mold SDM it is possible to produce smooth surfaces free of the stairstep effect. It is therefore theoretically possible to produce the exact geometry specified in the part model. However there are machining limitations, as well as other processing restrictions, which limit what features can be built. The manufacturability rules presented below can be used to identify these features and determine if a part can be built exactly with no approximation. 112

5.3.1 Manufacturability Rules for SDM and Mold SDM The manufacturability rules are based on the following four restrictions: 1. Only 3-axis milling is used for shaping 2. Only flat and ball endmills are used for shaping 3. Tool length is not taken into account 4. Interference between nearby features is not taken into account Only 3-axis milling is used because it is much simpler to generate cutting paths for 3-axis milling than it is for 4- or 5-axis milling. This simplifies process automation. The increased access required by 4- or 5-axis milling, for example access from the side of the part in addition to access from the top, will reduce the number of parts that can be built in parallel on the same substrate. In some cases one might only be able to build one part per substrate. This will reduce process throughput. Finally, almost any geometry can be made by 5-axis milling so this analysis would not be as necessary. Cutting tools are limited to flat and ball endmills because these are the most generally useful tool geometries. Conical endmills which taper to a point would be useful for cutting sharp corners but they are not a very general solution because they each have a specific taper angle. Additional restrictions are caused by limitations in tool length, or more specifically tool aspect ratio. Whereas deep, narrow grooves can theoretically be machined, in practice cutting tools with aspect ratios greater than 3 are difficult to find. In some cases when two features are very close together it is possible that in order to machine each feature the cutting tool would have to pass through the other nearby feature. In this case it may not be possible to build one or both of the features. If features are further apart than one tool diameter then this will not occur. Given the above restrictions, the two rules for determining manufacturability are as follows: 113

Rule 1: Find all sharp edges that are concave upwards and non-vertical. These can only be built exactly if one or more of the following is true: a. One of the two surfaces is vertical. b. Both surfaces lean to the same side of the edge. c. If the edge is horizontal and one of the surfaces is horizontal or below horizontal. Rule 2: Find all areas where the surface is curved, concave upwards and non-vertical. These areas can only be built exactly if one of the following is true: a. If the surface is inclined the radius of curvature must be greater than that of the smallest endmill available. b. If the axis of curvature is horizontal then there must be sufficient clearance for a flat endmill to be used to cut the sharply curved surface. Edges that are concave upwards are edges where the bisector of the included angle between the two surfaces points upwards. This is illustrated in Figure 5.4. The edges pierce the plane of the page but are not necessarily perpendicular to it (i.e. they are not necessarily horizontal edges). Edges 1 and 2 are concave upwards, edges 3 and 4 are concave downwards. Bisector Z 1 2 Surface 1 Surface 2 Figure 5.4: Edge concavity classification 3 4 For the purposes of Rule 1 it does not matter which is the material side of the edge. The material side only determines whether the feature will be machined in the part or the support material and does not alter the manufacturability of the edge. 114

Although Rule 1 is stated in terms of edges it should be applied at each point along each edge, not to entire edges. For example the circular edge at the end of a cylinder with its axis oriented horizontally has different manufacturability conditions for its upper and lower halves. Note that straight edges, such as the edges of a cube, will have the same manufacturability conditions along the whole edge. Figure 5.5 through Figure 5.9 show a range of typical edge configurations together with manufacturing sequences. Figure 5.5: General V-shaped edges The sharp edge shown in Figure 5.5 is a general sharp edge between two inclined surfaces. As above, the edge pierces the plane of the page but is not necessarily perpendicular to it. Two alternative build sequences are shown. In the upper sequence one attempts to machine the groove directly. However flat or ball endmills will leave a small chamfer or fillet at the bottom of the groove. In the lower sequence one machines one side of the groove first, then attempts to deposits the second material. Without support material the darker material cannot be deposited as shown, and even if it could be, the underside can then not be machined because it can t be reached from above. This is therefore an unbuildable feature. If one of the two surfaces is vertical then the edge becomes manufacturable. Two configurations are shown in Figure 5.6. In both cases one machines the non-vertical surface first. After depositing the second material one can then cut the vertical surface. This is Rule 1a. Alternatively if both surfaces lean to the same side of the edge then the edge is manufacturable as shown in Figure 5.7. This is because the two edges are effectively stacked vertically so that there are no accessibility problems. This is Rule 1b. 115

Figure 5.6: Edges with a vertical surface Figure 5.7: Edges with both surfaces leaning to the same side The edge configurations shown in Figure 5.8 are in general not manufacturable. These can only be built by machining both surfaces before deposition of the second material. Otherwise one will have to machine an unreachable undercut surfaces as was the case in the second build sequence in Figure 5.5. The two surfaces, together with the sharp corner, can be machined using flat endmills, but only if the edge is horizontal (i.e. perpendicular to the plane of the page). If the edge is inclined the endmill can't reach the edge without gouging one of the surfaces. This is Rule 1c. Figure 5.8: Edges with horizontal or below horizontal surfaces If the surface to be machined is concave and has a radius smaller than the smallest endmill it is in general not manufacturable exactly. This is the left case in Figure 5.9 where the 116

smallest ball endmill is too large to fit into the slot. This is Rule 2a, unless the axis of curvature is horizontal in which case Rule 2b applies. BALL FLAT FLAT Figure 5.9: Manufacturability of sharply curved surfaces In cases where the axis of curvature is horizontal it is possible to machine sharply curved surfaces using the corner of a flat endmill, as long as there is sufficient clearance for the endmill. Two example cases are shown on the right of the figure. This is Rule 2b. All of these manufacturability rules are based on tool accessibility for the cutting tools used to shape the surfaces. An alternative way to determine manufacturability is therefore to look at the kinds of features that can or can not be reached by the available cutting tool shapes and the available milling machine configuration (3-axis vs. 5-axis for example). For 3-axis machining, all machinable features must be non-undercut. Therefore in order for a feature to be manufacturable it must be possible to decompose it into a sequence of non-undercut features. However this condition is not sufficient because the V-shaped edge meets this requirement. In order for a feature to be machinable, there must be enough clearance around the feature for the cutting tool to reach the surface of the feature. This depends on the size, aspect ratio and shape of the cutting tool. This condition addresses the manufacturability of the V- shaped edge because only a conical cutting tool will be able to machine the edge. If the model can be built in the given orientation then one can either build the part in that orientation or look for other buildable orientations. If there are multiple buildable orientations, as will usually be the case, then one must determine which orientation is optimal. If the model can not be built in the given orientation then one could search for a different 117

orientation or modify some of the features that are not manufacturable. For example one could fillet geometries such as that shown in Figure 5.5. In some cases the approximate shape may be acceptable and the part could be built with reduced accuracy. 5.3.2 Manufacturability Analysis Example Two orientations of the part geometry for this example are shown in Figure 5.10. All faces are planar and all edges are sharp with no fillets. Since there are no curved surfaces only Rule 1 applies in this case. Figure 5.10: Example part geometry In the first orientation, shown in Figure 5.11, the base lies in the XY-plane. The numbered edges are all the edges that are concave upwards. Table 5.1 lists these edges and indicates which manufacturability rule applies to each. Figure 5.11: First build orientation This orientation contains one unbuildable V-shaped edge. To avoid this unbuildable edge one can rotate the model such that the edge has a vertical side. This orientation is shown in 118

Edge Rule 1 1.a 2 1.a 3 1.a 4 1.c 5 1.a 6 1.a 7 unbuildable 8 1.a Table 5.1: First orientation edge list Figure 5.12 and the edges are listed in Table 5.2. Z 4 7 Y X 6 5 7 1 2 3 2 1 Figure 5.12: Second build orientation Edge Rule 1 1.a 2 1.a 3 1.a 4 1.a 5 unbuildable 6 1.a 7 1.a Table 5.2: Second orientation edge list As with the first orientation, this one again contains a V-shaped edge which can't be built. Another alternative is to rotate the model such that the unbuildable edges are oriented ver- 119

tically. This orientation is shown in Figure 5.13 and the edge list is shown in Table 5.3. Z Y X 1 2 2 1 4 3 5 6 5 6 Figure 5.13: Third build orientation Edge Rule 1 1.a 2 1.a 3 1.a 4 1.a 5 1.a 6 1.a Table 5.3: Third orientation edge list In this case there are no unbuildable edges. Notice that in this orientation there are no inclined edges or surfaces. This is a purely 2.5D orientation. 5.4 Orientation Selection Given a set of build orientations, one must determine which is optimal. Due to the specific strengths and weaknesses of the process there may be certain orientations where the part is much easier to harder to build efficiently. The goal of this section is to outline a general method for identifying optimum build orientations based on part geometry and process capabilities. 5.4.1 Orientation Selection For Rapid Prototyping Processes Because RP processes use 2.5D layers and therefore suffer from the stairstep effect they 120

can't build parts exactly, except in special cases. Orientation selection for RP processes is therefore usually based on selecting the orientation which produces the closest approximation to the desired geometry or minimizes the build time. Given a geometric model of a part it is relatively easy to calculate the volumetric error that will result when the part is built using an RP process. The volumetric error can be computed for different orientations and the one with the lowest error can be selected as the optimum build orientation. Although this is computationally intensive it is straightforward. Simply calculating the volumetric error may not be sufficient. Because of the stairstep effect, the surface quality of parts produced by RP processes varies greatly with surface orientation. Horizontal and vertical surfaces are usually the smoothest and so the optimal orientation may be the one that minimizes the number of inclined surfaces. In some cases certain surfaces may be more important than others and these may dictate the selection of build orientation. The dependence of build time on part orientation varies by process. A general expression for calculating the build time is given by [Thomas 96]: T total = T pre + T layer i n i = 1 + () T post In this equation is the preprocessing time, is the time required to build the ith layer and T pre T post T layer() i is the postprocessing time. The layer build time can be broken up in a similar fashion with a preprocessing time, a build time and a postprocessing time. Processes such as DPS have a constant layer build time which does not depend on layer geometry. Therefore the part build time depends only on the number of layers which is dictated by the part height. Therefore optimum orientations for build time reduction are those that minimize the build height. In processes such as SLA, where each layer is created a point at a time, the laser scanning time to cure each layer depends on the part volume. The preprocessing time for each layer 121

is the recoating time, which is constant. Since the scanning time depends on the part volume, and is therefore fixed for any given part, increases in build rate must be obtained by reducing the number of layers to reduce the amount of time spent in recoating. So again, to reduce build time the build height must be minimized. Other issues that determine orientation may be process dependent. For example, when using LOM to produce composite parts with fiber reinforcement, the orientation of the fibers will dictate the build orientation because the process is limited to laying fibers in the horizontal plane, or along curved surfaces if Curved Layer LOM is used. 5.4.2 Orientation Selection For Mold SDM Orientation selection for Mold SDM is different from orientation selection for RP processes because in most cases a part will have orientations in which it can be built exactly. In general, however, there may several, one or no such orientations. If there are no exactly buildable orientations, then the part can either be redesigned to make it exactly buildable or an orientation selection approach similar to that used for RP processes may be used. The best orientation can be selected by computing the geometrical error or surface quality for various orientations. If there is only one exactly buildable orientation, then unless there are other issues this would be the obvious choice. If there are several exactly buildable orientations, then each must be evaluated based on the criteria presented below to determine which is the best orientation. In general the optimum orientation is highly part dependent, and may depend on the intended use for the part. For example there may be certain critical surfaces and a build orientation would be selected such that these surfaces could be built most accurately. For these reasons there is currently no general orientation selection algorithm for Mold SDM. Instead a number of criteria are presented which can be used to evaluate orientations. The weighting of these criteria will vary depending on the needs of the part and the application. There are three parameters that can be improved by appropriate orientation selection: 122

Speed: build times can usually be minimized by selecting the build orientation that results in the fewest layers. This is not always the case though. In some cases it may be faster to build a part using more simpler layers rather than fewer more complex layers. For example, because machining time is usually a significant fraction of the total build time it may be preferable to orient the part to make machining as easy and fast as possible. Machining several layers with purely 2.5D features will almost always be faster than machining a single layer with 3D features. Material Quality: material quality can be improved by eliminating layer boundaries. Layer boundaries can be sources of defects due to incomplete bonding or the presence of foreign particles. This is not as much of an issue in Mold SDM since the final part is cast monolithically, however this will affect the mold quality. Accuracy: using fewer layers generally leads to increased accuracy because each material deposition event causes some distortion due to thermal effects and material shrinkage. For accuracy reasons it is also desirable to minimize the amount of inclined surfaces. These must be machined using ball endmills (unless 5-axis cutting is used) which leave a finely scalloped surface finish. Because the various Mold SDM build strategies are applied in very part specific ways the difficulty of building any part depends to a great deal on the details of the geometry. However there are a number of features which generally increase the build difficulty: Undercuts: because undercuts force the part to be split they increase the number of layers required. Sharp corners: achieving sharp corners may necessitate the use of techniques such as the overcut-fill-trim-backfill method which generally increase the process complexity and the number of deposit and machine cycles required. In some cases sharp corners may not be manufacturable, even when using these techniques. Non-planar / inclined surfaces: unless 5-axis cutting is used, these surfaces must be machined by contouring with ball endmills. This also frequently results in unmachinable fillets which must be eliminated using more time consuming build strategies, such as the overcut-fill-trim-backfill method. There is also no guarantee that all of these fillets can be eliminated. 123

Inclined edges: as explained above in Section 5.3 inclined edges are not always manufacturable. Unmanufacturable edges must be avoided. Build height: taller parts generally require more layers and so take longer to build. Fine features: fine features often require the use of smaller endmills and since smaller endmills are also shorter, this forces thinner layers to be used. Split surfaces: surfaces that must be split between two or more layers should be avoided if possible due to the discontinuity present at layer boundaries. These can sometimes be avoided using one time surface machining techniques. Of all of these, one of the most important goals is to find a purely 2.5D orientation for the part. This is an orientation without inclined surfaces, as in the third orientation of the manufacturability analysis. Eliminating inclined surfaces will in general greatly reduce machining time and increase surface accuracy. 5.5 Decomposition Strategies There are three main areas addressed by the decomposition strategies presented here: improved general decomposition decomposition for implementing the new build techniques decomposition strategies for multiple build directions Commercial RP processes do not have much flexibility in terms of decomposition because the build processes can only build planar uniform thickness layers. SDM and Mold SDM use machining which is very flexible in terms of the geometries which can be produced and this allows much greater flexibility in the decomposition step. Because of the increased range of options however, the complexity is also greater. The decomposition strategies presented here are a first step in taking advantage of this increased flexibility to enhance the SDM and Mold SDM processes. 5.5.1 Improved General Decomposition SDM decomposition has traditionally used horizontal planes to split parts into variable 124

thickness layers. While this approach works it is frequently not optimal. Because SDM uses shaping methods capable of producing complex geometries this type of decomposition will usually result in parts being decomposed into more elementary units than is necessary. This in turn increases the build time. A natural extension of the process is therefore to remove the restriction that layer boundaries must be horizontal and planar. Instead layer boundaries should be able to have freeform geometries shaped to conform with the part. Figure 5.14 illustrates how a sample geometry would be decomposed using planar layer boundaries. This decomposition results in eleven compacts, shown on the right, but since some of these can be processed together (4a and 4b, 9a and 9b), only nine deposit and shape cycles are actually required. 9a 8 9b 7 6 5 4a 3 4b 2 1 Figure 5.14: Decomposition using planar layers Figure 5.15 illustrates how the same part can be decomposed using non-planar layer boundaries. In this case the part is decomposed into four compacts which require three deposit and machine cycles to build (3a and 3b can be processed together). 3a 2 3b 1 Figure 5.15: Decomposition using non-planar layers 125

While the geometry shown in this example is fairly simple, in general layers should be able to have freeform geometries. In practice process limitations will impose some restrictions on what layer geometries are manufacturable. The main planning issue with freeform layer geometries is how to pick the optimum layer geometry since there are now far more options. One relatively simple approach is to use horizontal planes to decompose the part and then identify which compacts can be consolidated. In general two compacts can be combined if they are made of the same material and are deposited one after the other. This reduces the number of layers and in general will produce freeform layer geometries. Figure 5.16 illustrates this approach applied to the part shown in the previous example. Layer 4 Layer 3 Layer 2 Layer 1 8a 5 4a 1a 6 3 2 8b 7 4b 1b III Ia Va IV II Vb Ib Layer 3 Layer 2 Layer 1 0 0 a b c d Figure 5.16: Compact consolidation Compact consolidation is readily automated using the Compact Adjacency Graph (CAG) [Pinilla 98]. The CAG is a graph representation of the connectivity between compacts and the manufacturing sequence. By analyzing the connectivity and material information provided in the CAG one can consolidate same material compacts [Cooper 99]. This process is illustrated in Figure 5.17. The connectivity between compacts, shown in Figure 5.17a, leads to the CAG shown in Figure 5.17b. The minimal precedence graph, shown in Figure 5.17c, is obtained from the CAG by eliminating redundant links. For example, the link between compacts 5 and 8a is redundant because the path via compact 6 enforces this requirement. Furthermore, compact 8a can t be built immediately after com- 126

pact 5 because compact 6 must be built first. Compact consolidation is then performed by analyzing the minimal precedence graph. Same material compacts which are fabricated sequentially, such as compacts 2 and 3, can be combined. Compacts 4b, 7 and 8b can all be consolidated because there is no sequence restriction between compacts 4b and 6. 8a 8b 8a 8b 8a 8b 7 5 6 4a 4b 3 1a 2 1b 0 5 4a 1a 6 3 2 0 7 4b 1b 5 4a 1a 6 3 2 0 7 4b 1b III Ia Va IV II 0 Vb Ib a b c d Figure 5.17: Compact consolidation using the CAG Reducing the number of material segments by compact consolidation is beneficial for two reasons. First, because fewer deposit and machine cycles are required to build the part the build time is reduced. In the case shown in Figure 5.14 and Figure 5.15 the number of operations was reduced from nine to three. Second, part quality is improved because there are fewer layer boundaries in the parts produced. Layer boundaries are often sources of defects due to incomplete bonding or the presence of foreign particles. In Figure 5.14 the part is built as five different compacts and contains four layer boundaries. In Figure 5.15 the part is built as one compact and contains no layer boundaries. Note that in Mold SDM this is only an issue for the mold since the final part is cast monolithically. The use of freeform layer geometries may also make previously unmanufacturable features manufacturable. An example of this, involving the fabrication of inclined sharp edges, is illustrated in Section 3.3.1 of Chapter 3. 127

5.5.2 Decomposition For The New Build Techniques A number of new build strategies were presented in Chapter 3 that require changes in the general process planning approach. Compacts that can be built without the need for any of these new strategies can be planned in the conventional fashion: for each compact one generates a deposition and a machining operation. However, many compacts may require the use of one of these new build techniques because: they may not be exactly manufacturable without the use of one of the new build techniques, often the overcut-fill-trim-backfill technique for sharp corners. they may be split between two or more layers and require the use of the one time surface machining technique to eliminate surface discontinuities. they may be most efficiently machined using cut through machining. All of these techniques involve machining geometries which are different from the actual compact geometry. In some cases the compact geometry must be modified slightly. In overcut-fill-trim-backfill, for example, some surfaces must be machined further out than they actually are to produce the overcut. In some cases new geometries must be created. For example during one time surface machining surfaces from separate compacts must be combined to produce the entire surface for the final machining operation. There are two ways that these new build techniques can be accommodated in the overall planner process: decomposition is preformed in the conventional fashion, without regard to the new build techniques, and then the operation generation module identifies the special cases and generates appropriate deposition and machining operations. As part of this process the operation generation module must create and keep track of all the necessary geometry modifications. the decomposition module identifies all the special cases where the new build techniques are required. Then as it decomposes the part it creates and inserts the required additional geometries into the build sequence. The operation generation module then generates the deposition and cutting files for these geometries without any awareness that some of the geometries are not in fact real compacts. 128

The basic issue here is where to incorporate the process specific knowledge required to implement the build techniques. In both cases the decomposition module will have to contain process specific knowledge, however in the second case the operation generation module needs only a minimum amount of process knowledge which makes it more amenable to automation using commercially available CAM packages such as Unigraphics. Cutting path generation algorithms, for example, generally take a set of surfaces and a boundary as input and then generate cutting paths to machine all parts of the surfaces that lie within the specified boundary. In the second approach the decomposition module can generate these surfaces and boundaries and then pass them to the operation generation module. Another advantage of the second approach is related to context. In the first approach the operation generation module receives a sequence of geometries but these do not have any context information associated with them. In cutting path generation, for example, context is important because features that are near the area to be machined must not be cut into. In the second approach the context information is available in the decomposition module, since it has access to the original part geometry. This can be used to generate geometries that take context into account so that the operation module does not need to consider context. 5.5.3 Decomposition Strategies For Multiple Build Directions Once multiple build directions are allowed the range of options for decomposition strategies increase enormously. One option is a divide and conquer approach where a complex part is divided into simpler sections. In examining a part it will often be found that certain sections of the part have clear optimal build directions which are different from other sections of the part. The most common example of this are sections that are 2.5D relative to a particular build orientation. The part can then be divided up into sections and each section can be built using its optimal build orientation. An example of this approach is presented below in Section 5.6. Here a relatively complex 3D part is split into three essentially 2.5D sections which are much easier to build. 129

A more complex approach is to divide the part into two: a core structure and a number of detail features. The goal is to identify a core structure which is an underlying frame for the whole part. Ideally this core structure can be built in a single deposit and machine operation. Once the core structure has been built then the detail features are built onto it. The goal of this approach is to fabricate most of the part in one operation. This will have three key benefits: no layer boundaries that might cause discontinuities reduced number of large deposition operations which will reduce warping and distortion. The detail feature addition operations will typically involve only small amounts of material deposition and so distortion and warping should be minimal reduced build time Note that the detail features can be considered to be either positive or negative. Positive features are added on to the core structure by additional material deposition. Negative features are machined out of the existing core structure. In many cases features could be either positive or negative. For example, tabs and slots can either be viewed as being either positive or negative features depending on the situation. The choice of which is optimal will usually be part dependent. Identification of the core and detail features could be performed using the Medial Axis Transform (MAT) [Blum 67]. The MAT is the loci of the centers of locally maximal spheres inside an object. The MAT can be thought of as the skeleton of the part and can be used to identify underlying geometries and symmetries in parts. 5.6 Pitch Shaft Planning Example The pitch shaft, shown in Figure 5.18, is a real part from a missile guidance system. It is interesting from a planning point of view because it is a relatively complex part that features both planar and curved surfaces. All of the part s edges are sharp, with no fillets, which means that the part probably can t be built exactly without the use of build strategies such as the overcut-fill-trim-backfill technique. 130

Figure 5.18: Pitch shaft geometry The pitch shaft has been built using three different approaches. The following sections outline some of the considerations involved in selecting the build orientations and show the resulting parts to illustrate the effects of build strategy on final part quality. 5.6.1 Horizontal Build Orientation The horizontal build orientation, shown in Figure 5.15, was the first build orientation selected. It was chosen because it minimizes the build height. The pitch shaft can be split into three compacts as shown in Figure 5.20a. However in practice, it must be split into five compacts as shown in Figure 5.20b. This is because some of the overcut-fill-trimbackfill machining operations required for the base would cut into the features of the uprights and cylindrical sections. Figure 5.19: Horizontal build orientation Because of the sharp corners, none of the five compacts can be built in single steps. Figure 131

Figure 5.20: Horizontal orientation decomposition 5.21 illustrates, in cross-section, the sequence of steps required to build the compact containing the lower section of the upright and the triple stepped cylindrical section. The first step is to machine a pocket for the upright and the first cylindrical section (2). The upright is machined using a flat endmill and the cylindrical section is machined using a ball endmill to obtain a smooth surface. As shown in the figure the outside (left) edge of the cylindrical section is overcut because the centerline of the endmill must reach up to the edge of the final surface. After filling the cavity with support material (3) the end of the cylindrical section is trimmed (4) and the slot is backfilled with mold material (5). The next cylindrical section can then be machined, again overcutting the outside edge (6). The sequence then continues as before (7-10). When making the last cylindrical section the outside surface is not trimmed back immediately. Instead, in order to get a continuous surface across the lower and upper halves of the end, the outside surface is not trimmed until the upper half has also been built, then both are trimmed together. This build orientation required 18 machine and deposit cycles to build and it would take approximately 540 hours to build 17 sintered silicon nitride parts. Section 7.2.3 in Chapter 7 contains a more detailed analysis of the part build time for this orientation. Figure 5.22 shows two views of a pitch shaft built in the horizontal orientation. From the pictures it is immediately apparent that there is a significant discontinuity in the cylindrical sections and sides of the uprights at the layer boundary. However, as a result of the combined trim operations the ends of the cylindrical sections do not show any discontinua b 132

1 2 3 4 5 6 7 8 9 10 Figure 5.21: Cylindrical section build sequence ities. Figure 5.22: Pitch shaft built in a horizontal orientation The ripples on the sides of the uprights are caused by slumping of the soldermask during bulk wax deposition. The tops of the uprights are also significantly distorted, again due to soldermask slumping. Conclusions from this build orientation are as follows. The surface quality of the cylindrical sections is poor due to the layer boundary that divides their top and bottom sections. The build time is very long because of the number of deposit and machine cycles required and the amount of inclined surfaces that need to be machined. 133

5.6.2 Vertical Build Orientation To eliminate the discontinuities on the cylindrical sections the pitch shaft must be oriented such that they are aligned with the build direction. This results in the vertical build orientation shown in Figure 5.23. The other advantage of this orientation is that the cylindrical sections also no longer need to be built using the Overcut-fill-trim-backfill technique. A flat endmill can be used to produce the sharp steps between the different diameter sections. Not needing to produce the curved sections by contouring with a ball endmill also saves time. Figure 5.23: Vertical build orientation The pitch shaft can be split into two compacts by dividing it at the level of the center of the hole in the base, as shown in Figure 5.24a. However, as in the horizontal build orientation, both compacts must be further divided because of the need to produce sharp corners. This results in four compacts as shown in Figure 5.24b. The vertical build orientation requires 13 deposit and machine cycles, which is 5 less than for the horizontal build orientation. In addition, the layers contain far fewer 3D features, primarily because most of the cylindrical sections are now 2.5D features. This reduces build time by significantly reducing the amount of machining time. The disadvantages with this orientation are that the build height is much greater than previously and the circular feature in the base is now no longer aligned with the build direc- 134

a b Figure 5.24: Vertical orientation decomposition tion. The build height in the horizontal orientation was 24 mm and now it is 55 mm. Greater build height generally correlates with increased build difficulty. Figure 5.25 shows two views of a pitch shaft built using the vertical orientation. The quality of the cylindrical sections is much improved due to the build orientation. There is also very little surface distortion due to the use of incremental casting which reduces slumping in the soldermask. As expected there is a slight discontinuity in the base but due to the use of cut-through machining this is only seen on the curved surfaces. Figure 5.25: Pitch shaft built in a vertical orientation 135

This second build orientation has improved the part quality over the first orientation, however the build time is not much shorter. 5.6.3 Multiple Build Orientations The vertical build orientation was a clear improvement over the horizontal build orientation in terms of part quality but in both cases the build time was long because of the large number of deposit and machine cycles required. One of the main reasons for the large number of cycles was the need to use the overcut-fill-trim-backfill technique to obtain all the required sharp corners. Examination of the pitch shaft geometry shows that the part is made up of three essentially 2.5D sections, as shown in Figure 5.26. Section 2 is in fact purely 2.5D, section 1 has five inclined surfaces (two chamfers on the side and three chamfers at the ends), and section 3 has two. If each of these sections could be built as a 2.5D section then the pitch shaft could be built in six deposit and machine cycles (one for support material and one for covering mold material for each section). To do this one must change the build direction during the build process. This was achieved using 5-axis machining. 2 3 1 Figure 5.26: Pitch shaft divided into 2.5D sections Instead of building the part in the orientation shown, it is best to orient the part upside down such that it looks similar to an upside down U. This orientation allows maximum machining access to all three sections of the part because they can be reached from the outside of the U. This allows the part to be built by machining into three sides of a block of 136

mold material, depositing support material and then covering the three sides with a layer of mold material. This approach minimizes the amount of mold material that needs to be deposited because the section between the uprights is part of the substrate and does not need to be deposited. In this orientation the part is theoretically a one layer part since all three sections can be machined during the same machine and deposit cycle. However, because of the need to produce sharp corners, the right side upright (section 3) must be built before the base (section 1). Figure 5.27 illustrates this issue. In order to obtain sharp corners on the shaded face, particularly on the edges indicated by the arrows, the face must be overcut and then trimmed back. Doing this will cut into the base where it meets the upright. Building the upright first will require machining into the space where the base will be built, but during the subsequent fabrication of base these overcuts will be machined away and won t affect the final part geometry. Figure 5.27: Interaction between sections 1 and 3 This issue is not present with the other upright, because its side surface matches that of the base where they meet and so both surfaces can be trimmed at the same time. Sections 1 and 2 can therefore be combined and built together. Figure 5.28 illustrates the build sequence for the part using multiple build directions. The first step is to machine the geometry for section 3 into the substrate, overcutting the sides (1). This is done by machining from the side using a horizontal tool axis. The support 137

material is then deposited and machined to shape (2). Depositing mold material finishes the construction of section 3 (3). Sections 1 and 2 are then machined into the substrate (4). Section 1 is machined from above and section 2 is machined from the side opposite to that used to machine section 3. The sides of the upright are overcut. Support material for sections 1 and 2 is deposited and machined (5) and mold material is deposited over them to close the mold. Machining casting holes and vents completes the mold construction (6). 1 2 3 4 5 6 Figure 5.28: Multiple build orientation build sequence This build strategy requires only 4 deposit and machine cycles which is much fewer than either of the first two build orientations. In addition there are very few inclined surfaces so the machining time is greatly reduced. Figure 5.29 shows two views of a pitch shaft made using multiple build directions. These parts do not have any discontinuities on the curved surfaces and there are no surface ripples or distortion due to soldermask slumping. The only visible defect is where the side of one of the uprights does not match perfectly with the corresponding surface of the base. In terms of part quality and build speed the multiple build direction technique is clearly 138

Figure 5.29: Pitch shaft built using multiple build directions superior. However since the parts must be machined from three directions multiple parts being built simultaneously can't be built in a grid and must instead be built in lines. Although this reduces the number of parts that can be built simultaneously, it will frequently be faster to build parts in more smaller batches using multiple build directions than to build parts in fewer larger batches using single build directions. 139

6 Process Automation The preceding three chapters have described the Mold SDM process without consideration for how it might be implemented in a commercial environment. This chapter explains the reasons for automation and describes some of the challenges that must be overcome in automating the process. A brief description of the current experimental automated Mold SDM machine is also included. 6.1 Reasons for Automation There are three primary reasons for process automation: 1. To increase part quality 2. To increase build rate 3. To reduce the need for skilled operators Part quality improvement results from two effects: fewer operator errors and increased repeatability of the process. Increasing the repeatability of the process also makes it easier to identify process issues which need to be addressed. 140

Increased build rate results from the fact that many of the part building operations can be performed faster by machines than by humans. Machining is an example of such a process. CNC machining is much faster than manual machining and in complex cases, such as 5-axis milling of freeform features, manual control is not even practical. Automation reduces the amount of process knowledge required to produce parts. Ultimately the goal is to have a system where one can submit a part model, press start and have the machine build the part with no further intervention on the part of the user. This has advantages in minimizing the number of skilled technicians required and in maximizing the number of people who could use the process. 6.2 Automation of the Mold SDM Process Mold SDM automation consists of both hardware and software automation. Hardware automation concerns the process steps, the materials and the construction and operation of the machine itself. Software automation consists of the entire process planning task, starting from a CAD model of the part to be built and ending with the generation of a complete set of machine commands which can be used to build the part. Software automation is not addressed here although some of the issues are discussed in Chapter 5 and in the literature [Pinilla 98, Kao 98]. For the purposes of hardware automation the Mold SDM process can be divided into a number of stages: Mold fabrication: this stage requires by far the most operator intervention and is also the most difficult to perform with high repeatability. It also accounts for a large fraction of the total process time. Together these factors make it the main candidate for automation. Support material removal: the support material is removed by dissolving it in a solvent. Placing the molds in an agitated solvent for a period of time requires minimal manual intervention but it may be possible to improve the dissolution rate with better 141

agitation or gentle heating. A way of determining when all the support material has been removed would be very useful. Casting: the casting process would be difficult to automate because it varies depending on the configuration of the mold and the part material being used. When producing small numbers of parts the preparation of the materials to be cast is also not well suited to automation. For example, automatic polyurethane mixing and dispensing systems are available but after each use the systems must be purged and cleaned and this is usually a lengthy and awkward process. Curing: there is little benefit to be derived from automating this step because parts are cured by letting them stand in ambient conditions or by placing them in an oven for a given amount of time. No intervention is required and there are no time-temperature requirements that would require temperature control. Demolding: this is a relatively simple manual process which involves a combination of melting and solvent dissolution. This task could be automated using commercially available part cleaning machines which operate with heated solvent baths. Post processing: there is a wide range of possible post processing operations because these vary by part and material type. Ceramics parts, for example, must be dried, burnt out and sintered. Polymer parts usually require minimal finishing. Automation of this step would probably only be beneficial in a high volume production setting. For the reasons outlined above, automation efforts to date have focused on the mold fabrication stage of the process. An experimental Mold SDM machine has been built to automate this stage of the process and it is described below. 6.3 The Experimental Mold SDM Machine 6.3.1 Hardware Description Figure 6.1 shows a picture of the Mold SDM machine that was built to automate the mold building phase of the process. The machine is based on a commercially available Haas VF- 142

0E 3-axis CNC milling machine (see Table 6.1 for milling machine specifications) which was modified by the addition of material deposition and curing hardware. The Mold SDM machine is controlled and operated from a computer. Figure 6.1: Mold SDM machine Parameter Value Work envelope X 762 mm Y 406 mm Z 508 mm Feed rates cutting 7600 mm/min rapid 18000 mm/min Spindle speed 0-7500 rpm Tool changer capacity 20 tools time 6.0 s chip to chip Maximum tool weight 6 kg Table 6.1: Milling machine specifications A pallet system is used to allow parts to be easily taken out of and put back in the machine without loss of reference. This enables greater process flexibility by allowing parts to be 143

removed from the machine for examination during the build process, or for several parts to be built at the same time by switching from one part to the other between operations. Material Deposition and Curing Hardware Material deposition and curing is accomplished by additional equipment that was retrofitted to the mill. Figure 6.2 illustrates this additional equipment in schematic form. Table 6.2 lists some specifications for the individual pieces of equipment. Soldermask Reservoir Wax Melt Units Ram (Z) Linear Actuator for Lamps Tool Changer (20 tools) UV and IR Lamps Dispense Valves (2 wax, 1 soldermask) Table (XY) Substrate Pallet Pallet Receiver Figure 6.2: Mold SDM machine schematic Function Wax deposition Soldermask deposition Soldermask curing Preheating Device Slautterback KB10 hot melt unit with E100-XT dispense valve. EFD 725DA-SS pneumatically operated valve Uvexs OCU-12C ultraviolet line source. Power intensity is 300 W/in over a length of 12 inches. Research Inc. 4453-A-10-06 infra red area heater. Covers an area of 10 by 6 inches using six 1000 W bulbs. Table 6.2: Material deposition and curing equipment specifications 144

Materials can be deposited using either of the two wax dispensers or the pneumatically operated dispense valve. All three dispensers are mounted on pneumatically actuated linear slides such that they can be held in either an up or a down position. The dispensers are individually lowered for deposition operations and are then retracted to keep them out of the way during other operations. Because the dispensers must be able to accurately track the part surface they are mounted to the mill s ram such that the machine s accurate Z positioning capability can be used to move them. If the dispensers were positioned in Z using an add-on actuator it would be necessary to coordinate the movement of this actuator with the XY motion of mill which would be difficult to do. The only drawback to mounting equipment on the ram is that the extra weight may alter the dynamics of the ram and affect its positioning accuracy. For this reason the.dispensers should be kept as light as possible (note that the maximum tool weight specified in Table 6.1 can be used as a rough indicator of how much weight is acceptable). The pneumatically operated dispense valve is used to deposit soldermask. The soldermask is stored in a pressurized reservoir. This dispenser deposits the soldermask as a bead, with a typical width of about 2 mm. Two wax dispensers were specified because the original build strategy involved using two different waxes. One wax would be used as the mold material and the second would be used to build containment walls around the areas into which the mold wax would be deposited. Recent success with using water soluble waxes to replace the soldermask may lead to the need for a third wax dispenser although at this point it appears that the water soluble wax can also be used to build the containment walls, in which case only two wax dispensers are required. Two lights are also included in the Mold SDM machine. The first, a UV line source, is used to cure the soldermask. The second, an IR flood lamp, is used to preheat the top surface before material deposition operations. Since neither light covers the entire build area the part must be scanned back and forth in X and Y to obtain full coverage. This XY motion is performed by the mill. Because the lights are too heavy to attach to the ram their height is controlled by an add-on linear actuator. Since the lights do not need to move up and down as the part is scanned below them there is no need to synchronize their Z motion 145

with the mill s XY motion. The lights can be moved into position first and then the part can be scanned back and forth below. Machine Control The mill and add-on hardware is all operated from a process control program running on a computer. Figure 6.3 shows the electrical configuration of the computer, mill, add-on hardware and controller electronics. Computer RS232 Serial RS232 Serial CNC Mill Relay Closure I/O Electronics Digital and/or Analog Deposition Equipment and Lights Figure 6.3: Machine control schematic The final output of the Mold SDM process planner is an operation sequence file and a set of files that specify all the parameters required for each operation. The process controller program executes this operation sequence. A typical operation will consist of a program for the mill which will specify a number of XYZ moves. The program is downloaded to the mill via a serial communication line. As the moves are being executed, commands in the program will trigger the mill's output channels. The controller monitors these signals via the control electronics and uses them to determine when certain actions must occur, for example the start of deposition or the switching on or off of one of the lights. The control- 146

ler then performs these operations through the control electronics. 6.3.2 Issues Containment of Liquid Materials During Deposition In the manual process a number of custom made teflon casting frames (Figure 6.4) are used to contain molten wax during casting operations. Once the wax has solidified these frames are removed until the next casting operation. Because different casting frame geometries are needed for different sized parts this is not a flexible approach for general use. An option is to have an automated system that produces the required frame geometry and then automatically places and removes it as necessary during the build process. However the removal process can be difficult, particularly with low shrinkage waxes, because the frame does not release easily from the newly cast material. Figure 6.4: Teflon casting frame The current approach is to first deposit containment walls and then cast material inside the walled areas. The sequence of operations is shown in Figure 6.5, where the dashed outline in Step 1 indicates the volume of material that must be deposited in this operation. The approach involves using a higher melting temperature wax than the mold material as the 147

wall building material. The wall wax is deposited at a temperature just above its melting point so that it will not flow much before solidifying. This enables the fabrication of high aspect ratio walls (2). Since the wall wax has a higher melting point than the mold wax the walls are able to withstand the mold wax casting operation (3). 1 2 3 Figure 6.5: Casting using containment walls Rate of Material Deposition Wax casting in the manual process is usually done by pouring molten wax from a beaker by hand. This approach allows for a very rapid rate of material deposition which is desirable in many cases. If material is deposited too slowly it may cool and solidify before it has time to flow and accurately replicate the surface it is deposited over. Automated hot melt dispensers tend to deposit material at a much lower rate but with much higher precision. Most casting operations in mold building involve mass casting which does not require high precision. Since higher flow rate dispense valves are not readily available the current approach is to use preheating to reduce the cooling rate of the wax and give it enough time to flow and faithfully replicate surfaces. Degassing After Deposition When depositing materials by casting there is always a possibility of voids due to trapped air or incomplete filling. Degassing the material after casting is usually an effective way to reduce voids but it is difficult to implement in the current machine because of the need for a pressure chamber that seals around the part. Sealing surfaces must be kept clean and free from machining debris, for example. Casting voids are not usually a problem with the waxes, unless the deposition rate is too 148

slow or insufficient preheating is used, as explained above. Soldermask deposition can result in voids and since degassing is not practical, the emphasis has been placed on careful deposition procedures which minimize trapped air. 3-Axis Milling Since the machine is based on a 3-axis milling machine some of the more sophisticated machining and building strategies can't be used. Inclined and freeform surfaces can usually be machined much more efficiently using 5-axis milling. For example, in cases where 3-axis milling has to use a ball endmill and a multitude of passes with a very small stepover, 5-axis milling can often cut the same geometry in one pass and produce a scallop free surface. The multiple build direction strategy also can't be used. The obvious solution is to construct a machine around a 5-axis milling machine. While this would have advantages, these machines are much more expensive. Build Envelope In the current machine the build area is limited to a 240 mm square area due to the size of the pallets used. So far all parts have been smaller than this size, so this has not been an issue. In future it may be necessary to base the machine on a larger milling machine with a larger workspace. Building larger parts will also be limited by materials issues, particularly shrinkage, which will cause distortion and cracking. 149

7 Process Characterization This chapter presents initial process characterization data in terms of feature size capabilities, achievable accuracy and surface roughness, build rates and final part properties. Common part defects and their causes are explained and methods of avoiding them are described. The main factors that drive the process accuracy and build time are presented and compared in terms of their levels of importance. The process characteristics are compared to those of competing ceramic manufacturing processes and rapid prototyping techniques. 7.1 Objectives The goal of process characterization is to quantify the capabilities of the process. The principal quantities of interest are: Tolerances: How accurate will a part be geometrically? How close will dimensional tolerances, such as lengths and angles, be to the specified values, and how accurate will geometrical relationships, such as parallelism or concentricity, be? 150

Surface quality: How smooth will the part surface be, or if a textured surface is required, how well can the required texture be produced? Feature size range: What are the largest and smallest features that can be built using the process? What is the largest part that can be built? Build rate: How fast can parts be built? How does this scale with part dimensions, part volume, or the number of parts being made? Part properties: What are the properties of the finished parts? This might include mechanical, thermal, electrical, optical and other properties depending on the particular part and application. At this point the Mold SDM process has not been comprehensively characterized. This is due in large part to the fact that the process is under constant development and materials and processing conditions change frequently. During this stage of development it is more important to perform relative characterization to quickly identify which alternatives are better or worse. As the process matures and the rate of change decreases it will be possible to perform more rigorous and quantitative characterization. The following sections describe the current state of the Mold SDM process characterization. 7.2 Characterization 7.2.1 Tolerances and Surface Quality For the purposes of tolerance and surface quality quantification, the Mold SDM process can be divided into two phases. The first phase is the mold building phase which does not depend on the part material. All parts made using Mold SDM share the same mold building process. The second phase consists of everything after the mold has been built. This includes casting, curing, demolding and any post-processing operations such as sintering. The second phase is material dependent because in general each part material has its own specific processing requirements. Tolerance characterization must therefore consider the contributions from both the mold building and the post-processing operations. 151

Measurement of the mold dimensions is not easy for two reasons. First, the internal features of the mold are not accessible without cutting open the mold. Once the mold has been cut open it can not be used to make a part and thus the part dimensions can not be compared with those of the mold used to make it. Second, the molds are made of soft waxes which makes measurement difficult because any contact with the measuring instrument may damage the surface and produce an incorrect result. The potential for damage caused by contact measurements can be avoided by using non-contact measurement techniques such as laser scanning, however even in those cases the accessibility of complex shapes can make measurement difficult. Measurement of final part dimensions is relatively easy, unless the dimensions of internal features are required, in which case parts may have to be cut open. For initial characterization, the mold dimensions were measured by measuring polymer parts after demolding. Because the polymer part materials exhibit low shrinkage the polymer parts produced will very closely match the mold geometry. The manufacturer s shrinkage values for the three main polymer part materials are shown in Table 7.1. For part dimensions in the 30 mm range the worst case inaccuracy due to shrinkage would be about 0.07 mm. Material Shrinkage (m/m) Adtech LUC 4180 polyurethane 0.002 Ciba TDT 205-3 polyurethane 0.0022 Adtech 501/530 epoxy 0.000546 Table 7.1: Polymer part material shrinkage values A number of impellers and turbine rotors were measured to estimate the process accuracy. These parts were chosen because they were made in large quantities and also because they are relatively easy to measure. A number of these parts that were made at different times using different mold materials were measured. Although different materials and processing parameters were used to make the parts, the results provide a worst case estimate of the process accuracy. The outside diameters (ODs) of the top and bottom discs were measured as well as the 152

total height of the parts. For the discs the maximum and minimum outside diameters were averaged, and this value is reported together with the deviation. For example, if the maximum diameter was 30.06 and the minimum diameter was 29.98, then the reported value will be 30.02 ± 0.04. The target values are 30 mm for the diameters and 10 mm for the heights. The results are shown in Table 7.2. The height for the first impeller is not reported because its top surface was machined flat after demolding and therefore its height is not representative. Part Bottom Disc OD (mm) Top Disc OD (mm) Height (mm) Impellers #1 30.00 ± 0.03 29.85 ± 0.03 - #2 29.73 ± 0.02 29.68 ± 0.01 9.87 ± 0.01 #3 29.59 ± 0.03 29.43 ± 0.03 10.04 ± 0.02 #4 29.59 ± 0.09 29.42 ± 0.03 10.09 ± 0.02 #5 29.55 ± 0.03 29.46 ± 0.02 10.04 ± 0.02 #6 29.78 ± 0.04 29.79 ± 0.02 9.88 ± 0.01 Turbine rotors #1 29.89 ± 0.04 29.69 ± 0.02 10.07 ± 0.03 #2 29.81 ± 0.01 29.69 ± 0.01 9.99 ± 0.02 #3 29.94 ± 0.04 29.92 ± 0.04 10.17 ± 0.01 #4 29.95 ± 0.06 29.90 ± 0.03 10.18 ± 0.02 #5 29.90 ± 0.02 29.84 ± 0.05 10.03 ± 0.06 #6 29.78 ± 0.05 29.71 ± 0.06 9.97 ± 0.04 Table 7.2: Part size measurements The average bottom disc diameter was 29.79 ± 0.15 mm, the average top disc diameter was 29.70 ± 0.18 mm and the average height was 10.03 ± 0.10 mm. The error due to material shrinkage was calculated above to be on the order of 0.07 mm in the worst case, and this represents about half of the standard deviation in these values. Note that the disc dimensions are about 0.25 mm below the target value. This is likely to be due to the shrinkage of the wax causing the mold features to pull in during mold building. It is interesting to note that the top discs are smaller on average than the lower discs because one would expect the lower discs to have experienced more inward shrinkage since more material was deposited above them. However the values are within each other s standard deviations, so the difference is not necessarily significant. The variation may also 153

be due to the configuration of subsequent layers. If dimensions are consistently off, as they are here, then it is possible to compensate in the original part model to minimize the final error. In this case more accurate parts would be produced if the discs were built with a diameter of 30.25 mm. A number of inlet nozzles were also built and since these were built using a more optimized process it is interesting to see if they are more accurate than the impellers and turbines. Only one of the inlet nozzle molds was used to make a polymer part, using the TDT 205-3 polyurethane, and its diameter was measured to be 42.42 ± 0.04 mm, which is very close to the target value of 42.367 mm. Unfortunately with only one data point it is not possible to draw any general conclusions. The measurements presented so far give an estimate for the process accuracy for polymeric parts, as well as for the mold accuracy itself by inference. For ceramic parts the final part accuracy also depends on the sintering shrinkage. The sintering shrinkage in turn depends of the solids loading of the slurry used to make the part and on the completeness of the densification process. Measurements made on several inlet nozzles are shown in Table 7.3. Each of these nozzles was sintered using a different time-temperature profile and so some of the deviation can be attributed to differences in the sintering. Despite this the shrinkage values are within 1% of each other, although the first two parts have a higher shrinkage of about 17.8%, whereas the last two parts have a shrinkage of about 16.9%. It is therefore possible that the shrinkage could be controlled to within a fraction of a percent with consistent processing conditions and slurry compositions. Inlet Nozzle Diameter (mm) Shrinkage (%) #1 34.90 ± 0.04 17.7 #2 34.83 ± 0.05 17.9 #3 35.23 ± 0.11 16.9 #4 35.23 ± 0.26 16.9 Table 7.3: Inlet nozzle size measurements Surface roughness measurements were made on several ceramic parts by Sangkyun Kang. 154

A profilometer was used to measure the roughness on different sections of center seal and inlet nozzle parts. Measurements were made over a distance of 4 mm on horizontal as well as inclined part surfaces. The results are reported in Table 7.4. Surface Part, Surface Configuration Roughness (µm RMS) Center seal, top horizontal surface 1.5 Center seal, bottom horizontal surface 0.5 Inlet nozzle, top inclined surface 1.7 Inlet nozzle, bottom inclined surface 0.45 Table 7.4: Summary of surface roughness measurements In both cases the roughness of top surfaces is about three times as high as for the bottom surfaces. This is because of the way each surface was built. The bottom surfaces were machined directly into wax and since the wax machines well the surface roughness is low. The top surface of the inlet nozzle was made by replication from a machined soldermask surface. The higher surface roughness value is a result of the poor machinability of the soldermask which resulted in a finely pitted surface. The top surface of the center seal part was made by green machining of the gelcast part. In this case the higher surface roughness is a result of non-optimal machining conditions. Green machining should be able to produce very smooth surfaces, but if the machining parameters are not right the material can tear because it is slightly rubbery. This will produce a surface with small pits where material has been torn out by the cutting tool. These results indicate that a surface roughness of about 0.5 µm RMS can be achieved in Mold SDM parts. This is a good level of surface finish for a sintered part and is sufficient for use in flow path applications such as gas turbine engine parts. Improvements in the soldermask material should be able to increase its machinability such that the 0.5 µm RMS surface finish can be achieved everywhere. Table 7.5 shows estimates for the tolerance ranges resulting from various processing factors. The estimates are fairly rough since they depend so much on the specific conditions but the values shown are typical maximum values. The largest geometrical errors are caused by distortion that occurs during demolding and 155

Factor Wax shrinkage Material machinability Machining precision Heat distortion during material deposition Casting voids Curing shrinkage Distortion due to softening during demolding Distortion due to slumping during sintering Sintering shrinkage variation Tolerance Range <50 µm 50-500 µm >500 µm x x x x x x x x x Table 7.5: Factors driving process accuracy sintering. During demolding most parts soften due to the high temperatures required for mold removal. While the parts are soft and flexible it is very easy for them to slump due to gravity or handling forces. During sintering the parts are again susceptible to slumping due to gravity, particularly if they contain delicate features with high aspect ratios. Both of these types of distortion can be reduced by careful handling and by using support fixtures to prevent slumping. Casting voids can also be a significant source of defects. Voids usually result from poor mold filling or poor mold design which makes it difficult to remove trapped air after casting. In future vacuum casting will be used to eliminate this type of defect. Wax shrinkage and heat distortion are the two principal causes of inaccuracy during the mold building process. Inaccuracy caused by wax shrinkage can not be eliminated unless the shrinkage can be eliminated. Build techniques that minimize this effect, such as one time surface machining, and better part decomposition to minimize the numbers of layers can reduce this effect in most cases. Heat distortion is also difficult to eliminate but careful selection of material melting points and deposition temperatures, as well as techniques such as incremental casting can greatly reduce this source of error. Inaccuracy due to poor machinability can be eliminated by the correct choice of machining parameters to avoid clogging and the associated surface quality degradation. Curing shrinkage effects can be reduced or eliminated by curing under pressure. 156

7.2.2 Feature Sizes A number of factors determine what the smallest and largest feature sizes are for parts made by Mold SDM: Machinability: the size of features that can be produced depends directly on the machinability of the material. If the material is too brittle fine features will break off due to the cutting forces. Softer materials will deflect due to the cutting forces and result in poor accuracy. Experience has also shown that as cutting tools get smaller the machinability tends to get worse. This is at least partly due to the geometry of the cutting tools because the smaller tools have relatively less clearance between the flutes for chip removal. Cutting tool sizes: since all features are either machined, or replicated from machined surfaces, cutting tool sizes have a direct influence on the minimum feature size. Large features can usually be machined in multiple passes using smaller endmills. Endmills with diameters as small as 0.13 mm (0.005 ) are available, although sizes down to about 0.79 mm (1/32 ) are more common. The tool aspect ratio determines the maximum depth of cut. As tool sizes become smaller the aspect ratios usually decrease to between 1 and 2. Thin, deep slots therefore can t usually be made by direct machining and must be built in a different orientation where they are no longer slots relative to the build direction. An example of this was presented in Section 3.3.1 of Chapter 3. Shrinkage: shrinkage may limit the maximum feature sizes achievable. Waxes shrink during solidification and this results in stress buildup. The tendency to distort or crack increases as the size of the area being cast into, the wax shrinkage and the number of layers increase. Parts with build areas up to about 150 mm square have been successfully built using the low shrinkage Kindt-Collins Master Protowax. Casting large volumes of wax in several smaller increments, instead of all at once, may help reduce this problem. Minimum layer thickness: there is a minimum layer thickness for most materials. For waxes the minimum layer thickness is typically around 5 mm and this is determined by the requirement that the newly deposited material remelt the previous layer 157

sufficiently that it bonds to it. Thinner layers do not contain enough heat to accomplish this and tend to delaminate easily. To build thin layers, material must be deposited as a thick layer and then machined to the desired thinness. Deposition into fine features: as features become smaller it becomes more difficult to deposit material such that it will completely replicate all the features. Low viscosity materials reduce this problem and techniques such as degassing and the application of pressure can be used to remove bubbles and force material into fine features. The use of pressure to force material into small spaces may cause damage to delicate features. Remelting damage: fine features, in particular high aspect ratio walls are most susceptible to damage from remelting. Softening will weaken these features and then the fluid forces exerted by the material being deposited will cause distortion. Machine size limitations: the limitations of the physical machinery used to implement the process will also affect the range of feature sizes. For small features, the machine precision may be important. The Haas VF-0E milling machine has a positioning accuracy specification of better than 1 µm. Large feature sizes are limited by the size of the machine. The workspace of the Haas is 762 by 406 by 508 mm (in X, Y and Z respectively), but not all of this range can be used because the deposition equipment can t reach the whole workspace. Part sizes are further limited by the size of the pallets used to build parts on. These are currently 240 mm square. The largest parts built using Mold SDM were the engine vanes shown in Figure 7.1 and the silicone artery models shown in Figure 7.2. The engine vanes are about 70 mm tall. The alumina vane shown in Figure 7.1a was built in three layers (one for the inner shroud, one for the blades and one for the outer shroud) using Kindt-Collins Master Protowax. The epoxy vane shown in Figure 7.1b was built in 11 layers using WI Green Rigidax as the mold material (this part was built by Sangkyun Kang). Both artery models were built using Protowax as the mold material. The branched model is 120 mm long and the other is 200 mm long. Both of these parts were built in two layers. The miniature alumina turbines built by Shelley Cheng are representative of the smallest features that can be built using Mold SDM. These parts, shown in Figure 7.3, are about 7 mm in diameter and have section thicknesses of 0.5 mm. The larger turbine parts were 158

a b Figure 7.1: Engine vanes a b Figure 7.2: Silicon artery models built with radial clearances as small as 200 µm. This required mold features of that size and so this is also representative of the small feature capabilities. Smaller features could probably be built successfully, based on the success in the 0.5 mm size range, but this has not been investigated yet. Table 7.6 provides a summary of estimated size capabilities of the Mold SDM process. 7.2.3 Build Rate Evaluating the build rate for Mold SDM is more complicated than for most other RP processes because of the flexibility of the Mold SDM process. Some of the issues that complicate build rate calculation are: 159

Figure 7.3: Miniature alumina turbines Variable layer thicknesses: most RP processes are limited to one layer thickness, or to one of several available thicknesses. Most RP processes also can't change layer thickness during the build. These limitations are due to factors such as nozzle sizes in FDM, feedstock material thicknesses in LOM or cure depth limitations in SLA. Layer thickness in Mold SDM is usually limited by the depth of cut of the available cutting tools. Layer thickness is not quantized, as it is in most RP processes, and can be varied from layer to layer, or even within a single layer. However, because smaller cutting tools have smaller depths of cut, the maximum layer thickness in Mold SDM is partly determined by feature sizes. Feature sizes will also usually vary between different sections of a part. Freeform layers: in addition to the flexibility in layer thickness, Mold SDM can in general use layers of almost any shape. Layers need not be planar and need not be of uniform thickness. These more general layer shapes have implications for the machining time. Size (mm) Machining: the time required to machine a layer depends greatly on the geometry of the layer and the machining technique used. When machining inclined surfaces, 5-axis 160 Limiting Factor(s) Maximum part size > 150x150x150 wax shrinkage, number of layers, machine workspace Minimum feature size < 0.5 available cutting tool sizes, machinability, heat resistance, ability of materials to fill fine features during deposition Table 7.6: Estimated part and feature size capabilities

milling will always be faster and produce better surfaces than 3-axis milling because the surfaces can be machined directly instead of having to approximate them by contouring with a ball endmill. When contouring with a ball endmill, using larger endmills will allow greater stepover distances to be used which will reduce machining time. However, because of feature sizes it may not be possible to use larger endmills. Once again build rate is affected by the part features. New build techniques: the new build techniques, presented in Chapter 3, also increase the difficulty of estimating build times. Most of these techniques are only used in special circumstances, again depending on part features. Despite these issues, the rate limiting steps in Mold SDM can be identified. During mold building the rate limiting steps are wax deposition, soldermask deposition and machining. Wax deposition is fairly rapid and even thick layers can be deposited in a matter of seconds, as long as a high flow rate can be obtained from the deposition system. After deposition the wax must cool and harden sufficiently that it can be machined. The cooling time depends on the wax being used and the layer thickness, but is typically about two hours, without forced cooling. Forced cooling can be used to reduce cooling times by half. Wax deposition is a serial process because each area that needs to be cast must be cast sequentially. Cooling is a parallel process because any number of cast areas can be cooled simultaneously. Since cooling is by far the more time consuming step, wax deposition is mostly a parallel step. Because of the cure depth limitations of the soldermask, thick layers must be deposited as a series of thinner layers. Each layer must be cured, and due to heat generation and absorption during the curing process, a cooling period may also be required. Curing a 1 mm thick layer of soldermask typically takes 2 minutes. The curing and cooling steps are parallel but the deposition step is not. Soldermask must currently be extruded as a bead, so the deposition time is proportional to the area. Currently the deposition step takes longer than the curing or cooling, however if a lower viscosity soldermask could be developed, then it could be deposited by casting which would be much more rapid. Soldermask deposition could therefore be considered as either a parallel or a serial step. 161

As explained above, the machining time is highly dependent on part geometry. Layers that consist of 2.5D geometries only can be machined relatively rapidly, typically in a matter of minutes. Layers with inclined surfaces can take hours to machine using 3-axis milling. Machining is also an entirely serial process. Once the molds have been built, the soldermask removal step is by far the longest. There are two main reasons for this. First, the soldermask dissolves fairly slowly and efforts to speed up the dissolution rate have been largely unsuccessful. Second, since one can't see inside the molds it is difficult to be sure that all of the soldermask has in fact been dissolved. Because of this the etching step is usually not hurried and typically takes days to a week to complete. One way to reduce this time would be to develop a chemical indicator that would detect dissolved soldermask. Measuring weight loss is another alternative, but unless this could be done in water, the molds would periodically have to be dried to be measured. Table 7.7 summarizes the main operations that drive process build times. The values shown cover a wide range of possible part configurations because most of these operations depend heavily on part features. The last column indicates whether each operation is parallelizable when fabricating multiple parts. Paralellizable operations take the same amount of time, irrespective of the number of parts being built, because many parts can be processed at the same time. The fabrication of silicon nitride pitch shafts will be used as an example to illustrate actual build times. For the horizontal pitch shaft build orientation, there were 18 machine and deposit cycles. Seventeen parts were built simultaneously. Manual soldermask deposition took approximately 3 hours per part and the total machining time was about 2.5 hours per part. Soldermask etching took 2 weeks, because it was a complex mold cavity with relatively restricted access. Given these times, Figure 7.4 shows the contributions of each of the operations to the total build time for the 17 pitch shafts. When building only one part the total build time would be about 450 hours, or a little under 19 days. When building 17 parts, only the non-parallelizable operations increase in time. These increases are shown in Figure 7.4 as the darker sections of the bars. Significant increases in operation times only occur for the soldermask deposition and machining operations. When building 17 parts the 162

Operation Time Parallel? Wax deposition (per layer) casting a few minutes no cooling 2 hours yes Soldermask deposition (per 1 mm thick layer) Machining deposition curing cooling 2.5D features 3D features a few minutes 2 minutes 2 minutes a few minutes tens of minutes to hours no yes yes no no Soldermask etching 1-2 weeks yes Mold drying 4 hours yes Part material casting casting curing 30 minutes 2-24 hours no yes Demolding 30 minutes yes Ceramic part postprocessing drying burnout sintering 25 hours 52 hours 8 hours yes yes yes Table 7.7: Summary of main build time drivers total build time is about 540 hours, or about 22.5 days. However when this figure is divided by the number of parts built, the per part build time is only about 32 hours. 350 300 250 200 150 100 50 Soldermask Etching Drying, Burnout and Sintering Soldermask Deposition Machining Wax Deposition Time (hours) Mold Drying Part Material Casting Demolding Figure 7.4: Pitch shaft build time breakdown 163

From the figure it is clear that the soldermask etching time is by far the largest contributor to build time, contributing between 60 and 75% of the total build time. Note that the etching time depends on two issues: the rate of etching and being able to determine the completeness of the etching. Without knowing when all the soldermask has been etched, the etching time has to be conservatively long in order to ensure a complete etch. Shortening this operation could tremendously increase the build rate. New water soluble waxes appear to have potential in this respect. Their dissolution rates may be up to an order of magnitude faster than the current soldermask. New soldermask formulations currently under development may also have higher dissolution rates. In terms of the mold building operations, all three of them are significant but their relative importance depends on the specifics of the situation. When building one or few parts the wax deposition time is very significant because of the 2 hour cooling time. However since this is a parallel operation, as more parts are built simultaneously its relative contribution to the build time decreases. In the case of the pitch shaft, when building one part the wax deposition time accounts for 18 hours, versus the 5.5 hours for the soldermask deposition and the machining combined. When building 17 parts, the wax deposition still accounts for about 18 hours but the soldermask deposition and the machining now take about 51 and 42 hours respectively. In this example the wax deposition goes from being 77% of the build time to only 16%. The machining time depends very strongly on the geometry being machined. In the case of the pitch shaft, and even more so in the case of the inlet nozzle, the part is built using relatively few layers but each layer contains a large fraction of 3D features. Therefore the wax deposition time is less significant and the machining time is more significant. In the case of the inlet nozzle, the two layers that form the top and bottom of the gas passageways each required 1-2 hours of machining per part. As a counter example, for the impeller parts, which feature exclusively 2.5D geometry, the total machining time is a matter of minutes but wax must be deposited twice. In this case the wax deposition time dominates the build time. The simplest way to reduce mold build times would be to reduce the cooling time required after wax casting. Currently this takes 2 hours, but with forced cooling this can be halved. 164

Soldermask deposition should become easier as new soldermasks with lower viscosities and greater cure depths are developed. It may also be possible to reduce machining time by using high speed machining techniques. However the most effective method would be to use 5-axis milling because this will greatly speed up the cutting of inclined surfaces. 7.2.4 Part Properties Ceramic Part Materials Mechanical strength testing, using a four point bend configuration, has been performed on test bars made from several of the silicon nitride gelcasting formulations. This testing was mostly done by Dr. Jim Stampfl, with the assistance of Professor Reinhold Dauskardt, Dr. Ajay Bhatnagar and Kevin Ohashi of the Materials Science Department at Stanford University. The best average strengths reported to date were 414±70 MPa, with a Weibull Modulus of 9.6, for as made test bars. The highest strengths obtained were around 590 MPa. These bars were built using Mold SDM techniques and tested without any surface preparation such as grinding or polishing. They therefore represent the strength capabilities of the Mold SDM process because in general it will not be possible (or desirable) to do surface finishing on complex shaped parts. These strength values are significantly below the expected range for high quality silicon nitride parts and this is believed to be due to the use of a non-optimal sintering procedure as well as to the presence of defects, particularly surface flaws due to surface roughness. As the process is further optimized higher strength values should be achieved. Thermal shock resistance testing was also performed at M-DOT Aerospace (Phoenix, AZ). Two first generation silicon nitride inlet nozzle parts were tested in a burner test rig which simulates the environment that the part will be subjected to in service in their miniature engine. The parts were cyclically heated to and cooled from temperatures up to about 1250 C. One part was also operated continuously at 1250 C for 5 minutes. Both parts withstood the testing with minimal damage. Surface color changed slightly and some of the very fine tabs around the base broke off. Since these tabs were designed for a metal 165

part and were not optimal for a ceramic part this failure was not considered significant. Metallic Part Materials Katsu Sakamoto developed a metal gelcasting formulation for use with Mold SDM that was based on the formulations developed at Oak Ridge National Laboratory. He made a series of tensile test bars and obtained ultimate tensile strengths up to 776 MPa for a 17-4 PH stainless steel. The elongation to failure was up to 7%. These results are reproduced from his Masters thesis [Sakamoto 99]. Expected strength values for a 17-4 PH steel are closer to 1100 MPa with a 12% elongation to failure. The lower strengths obtained here are believed to be due to imperfect sintering procedures which resulted in a number of voids and cracks in the final sintered parts. Improved processing should enable the higher strength values to be obtained. Polymeric Part Materials In general, unless there is a chemical incompatibility between the part material and the mold material, Mold SDM should be able to produce parts with the full strength of the part material. There are no layer boundaries in the final part, because it is always cast monolithically, so part material self-adhesion strength, which is a major issue with polymer parts made using SDM, is not an issue [Kietzman 98]. The manufacturer s mechanical property specifications for the polymeric part materials used in Mold SDM are shown in Table 7.8. Material Ultimate Tensile Strength (MPa) Elongation to Failure (%) Adtech LUC-4180 polyurethane 55 15 Ciba TDT 205-3 polyurethane 23 9 Adtech 501/530 epoxy 42 1 Table 7.8: Properties of polymeric part materials used in Mold SDM 166

7.3 Defects and Methods for Eliminating Them The following sections describe the causes and remedies for typical defects found in parts produced using Mold SDM. The causes of these defects are related to process and material issues and so it is most logical to classify the defects based on the process step where they occur. Almost all of the work related to defect elimination has been experimental in nature. Analytical analysis of these effects is very difficult for a number of reasons. First, the material deposition processes are very difficult to model because of the interactions between mechanical, thermal and fluid effects. Second, good reliable material property data is not available for the materials used to build the molds in Mold SDM. Third, most of the materials exhibit a fairly wide batch to batch variability in their properties. 7.3.1 Defects Resulting From Mold Fabrication Temperature Related Defects All of the current mold and support material deposition processes expose the previously built layers to high temperatures at some point. Wax is deposited in the molten state and the soldermask cures exothermically. Because the materials have limited temperature resistance, excessive temperatures can cause deformation of the existing geometry. Figure 7.5 illustrates surface ripples caused by depositing excessively hot wax against a soldermask surface. The ripples are particularly pronounced if the soldermask is not fully cured. In many cases there is a thin poorly cured region at the boundaries between layers of soldermask. When slumping occurs these incompletely cured regions often become very apparent. Reducing the thickness of soldermask layers produces a more complete cure which helps to reduce this effect. Figure 7.6 shows how geometry can be significantly distorted. The step is supposed to be horizontal and has slumped at both ends from excessive heating during wax deposition. Excessive heat can also cause sharp corners to become rounded and fine features may become sufficiently soft and flexible that they deflect. 167

Figure 7.5: Surface ripples Figure 7.6: Slumped tops of uprights Another effect of excessive heating is seen in Figure 7.7. The figure shows identical sections from impeller molds made using Kindt-Collins Master File-a-wax (right) and a 25% Kindt-Collins Master File-a-wax / 75% Kindt-Collins Master Protowax mix (left), together with parts made from similar molds. In the wax section shown on the right a number of small pits can be seen in the surface (left arrow). These are believed to be due to small bubbles near the surface of the cured soldermask expanding as the temperature rises and the soldermask softens. The pits are not seen on the left mold section because the wax was deposited at a lower temperature. Notice also that the positions of the six blades (right arrow) can be seen in the right wax mold but not in the left wax mold. These defects can also be seen replicated in the parts. All of these defects can be reduced by either using more heat resistant materials or by minimizing the heat input to previously built geometry. For thermoplastic materials, such as 168

Figure 7.7: Impeller mold sections showing thermal distortion waxes, more heat resistance means a higher melting point, and this is not desirable because it will force higher deposition temperatures and consequently higher heat input during deposition to the other material. For this reason minimizing the heat input is the best strategy. For wax casting this is accomplished by using incremental casting, as explained in Chapter 3. Minimizing the heat generated during curing of soldermask is accomplished by depositing the soldermask in thinner layers. This reduces the amount of heat generated during curing and also makes it easier to cool the material as it cures. Thermal expansion is another thermal effect that is important in Mold SDM. As materials are heated and cooled during deposition processes they expand and cause residual stresses and warping. Ideally materials with zero coefficients of thermal expansion could be used, but in practice this is not realistic so the best solution is to minimize the process temperature range. 169

Shrinkage Related Defects There are three main types of defect caused by material shrinkage effects. In order of increasing severity these are: Christmas Tree effect Warping Delamination and cracking All of these defects result from the same fundamental process. New material is deposited on top of existing features and as the new material solidifies or cures, it undergoes volumetric shrinkage. Because of the bonding between the new material and the previous layer, this volumetric shrinkage causes residual stresses both at the interface and throughout the bulk. This is a greatly oversimplified explanation of the process. In practice there are many processes contributing to the overall effect. At higher temperatures materials soften and weaken, making it easier for them to yield or creep to relieve residual stresses. Because of the heat input during deposition thermal expansion effects also contribute. In extreme cases, with high shrinkage materials, there can be sufficient residual stress accumulation to crack the parts during the build process. If the interlayer bonding is poor then delamination will usually occur. Figure 7.8 shows a partially completed mold with two cracks caused by residual stress buildup. Figure 7.8: Mold with cracks due to residual stress buildup 170

Current Mold SDM materials and processing conditions have mostly eliminated cracking, although it can still be a problem in large parts. The larger the area of material being deposited, the worse the effect will be. In less extreme cases the stresses are not high enough to cause cracking, however they will still cause some degree of warping. The Christmas Tree effect manifests itself on the surface of parts, at the layer boundaries, as small steps. The name is from the sawtooth surface geometry which is similar to a Christmas Tree profile. An example of the Christmas Tree effect is shown in Figure 7.9. Figure 7.9: Christmas Tree surface profile The part build sequence shown in Figure 7.10 illustrates the development of the Christmas Tree effect. Although this is actually an SDM sequence it illustrates the effect very well. a b c d Figure 7.10: Christmas Tree effect experiment 171

In the first step a cavity is machined into Adtech LUC 4180 polyurethane (a). Kindt-Collins Master File-a-wax is then cast into the cavity (b) and it is machined down to its final height (c). In the last step more wax is cast to fill the cavity (d). The picture after the second step shows that the wax has shrunk significantly and has pulled away from one side of the cavity slightly. In this case the wax separated from the polyurethane, but in other cases it remains attached and in fact pulls in the material around it. Then when the next layer is machined a step is produced. This effect is best seen in the last picture (d). This simple experiment illustrates how one material s behavior during deposition can cause distortion which results in the Christmas Tree effect. In general this is a very complex effect which depends on the interactions between materials properties and processing conditions. The effect is also time dependent because factors such as process rates and cooling times affect the end result. Further research into the Christmas Tree effect is needed to better understand the key issues and how to minimize the severity of the discontinuities. Alex Nickel has investigated the effect in metal SDM and has compared analytical and experimental results [Nickel 99]. The metal SDM material deposition process is different from Mold SDM in that materials are deposited in a point by point fashion using a laser welding process instead of being deposited a whole layer at a time, as in casting for example. The differences in material properties between metallic materials and polymeric materials will also affect the results. Because of this the results are not directly applicable to Mold SDM but the general conclusions are useful. The research indicated that the amount of overall warping in parts depended on the deposition patterns but that the Christmas Tree effect was a local effect that only depended on conditions near the surface of the part. Machining Related Defects The most common machining defects are caused by overly aggressive machining. High feed rates and large depths of cut tend to result either in chipping on corners and fine features, or clogging of the cutter which causes smearing. See Chapter 4, Section 4.4.1 for a more detailed discussion of these effects. High feed rates also result in poor surface fin- 172

ishes because the endmill moves a significant distance between passes of the cutting edges. This is seen as fine ripples on surfaces cut with the sides of endmills and circular marks on surfaces machined with the ends of flat endmills. Using higher spindle speeds and endmills with more flutes helps, but endmills with more flutes tend to clog up more easily because there is less clearance for chip removal. In most cases chipping or clogging occurs before the surface finish is seriously degraded due to excessive feed rates. These defects can be reduced by increasing the machinability of the materials, by making them tougher and stronger, and by using less aggressive machining parameters. Using ball endmills to cut inclined surfaces will always result in a scalloped surface finish. Reducing the stepover distance will reduce the scallop height at the cost of increased machining time. An issue with the milling machine used in the automated Mold SDM machine is spindle growth. This machine does not have a temperature compensated spindle and as it heats up during operation the spindle extends by as much as 150 µm, such that the cutting tool cuts deeper. The only way to get around this effect is to warm up the spindle before beginning to machine. More sophisticated milling machines have temperature controlled spindles so this would not be an issue. Support Material Removal Related Defects Although the support material removal process can cause mold damage, in practice this is very rare and is only seen in cases where there are very fine features which might be damaged by the etch water as it flows around inside the mold. The most common problem is incomplete removal of the support material. Since one usually can t see inside the mold and there is currently no way to test for complete removal, there is no way to be certain that the removal process has been completed. Current practice involves etching for longer than necessary to ensure complete removal. The development of a method to test for complete removal would be most useful. 7.3.2 Defects Resulting From Casting The most common defect resulting from casting is voids due to incomplete mold filling. 173

Figure 7.11 shows an example. The likelihood of incomplete mold filling is dependent on both the mold design and the casting procedure. Figure 7.11: Voids resulting from incomplete mold filling A number of features should be avoided in molds because they are difficult to fill. Figure 7.12 illustrates some examples. Trapped volumes (a) can t be filled because air becomes trapped in them during casting and can t be removed. Large horizontal areas (b) are difficult to cast because bubbles will tend to accumulate there without moving horizontally towards the vents. Fine features (c) can be difficult to fill if the part material does not wet the mold well or if the part material is too viscous. Fine slots also tend to trap bubbles. Long and narrow gates and vents (d) make casting more difficult because they reduce the size of the exit path for bubbles. a b d c Figure 7.12: Mold features that are difficult to fill 174

Mold SDM mold design techniques are not currently very sophisticated. Casting orientations are selected to eliminate trapped volumes and minimize the amount of horizontal surfaces. Vents are added as appropriate and are made as large as possible. Using more larger vents increases the amount of postprocessing because these must be removed after casting. Vents are preferentially placed on non-critical surfaces and if possible on planar surfaces because these are easiest to finish after casting. The casting procedure is also critical for void elimination. There are two primary causes of voids in cast parts: air that is drawn into the part material during mixing prior to casting bubbles trapped in the mold during mold filling All part materials used in Mold SDM require mixing before casting, usually to combine the resin with the hardener or to add an initiator to cause curing. More viscous materials tend to trap more air during this operation because it is harder for the trapped bubbles to flow to a free surface and escape. To avoid trapped air the mixing process can be performed under vacuum or the mixed material can be vacuum degassed. Mixing under vacuum is preferred but it requires more complex equipment. Bubbles trapped in the mold during casting can be eliminated by casting under vacuum. Ideally the materials can be mixed under vacuum and then cast directly, without repressurizing. Currently parts are cast under ambient conditions and they are then degassed under vacuum several times. By manually shaking the molds and changing their orientations slightly during degassing, it is possible to minimize the chance of horizontal surfaces trapping bubbles in the molds. 7.3.3 Defects Resulting From Curing A number of curing effects can cause part defects: exothermic curing can soften and distort the mold shrinkage during curing can cause cracks mold surfaces can inhibit the curing reactions and cause surface defects The Ciba TDT 205-3 polyurethane is the only Mold SDM part material that has a signifi- 175

cant cure exotherm. It can be used with molds made using Kindt-Collins Protowax, which is the lowest melting point mold material, but fine features, such as thin walls, may be distorted. Faster curing polyurethanes generally exhibit higher cure exotherms which is the principal reason that faster curing materials are not used in Mold SDM. Most Mold SDM part materials do not display significant shrinkage on curing. The gelcasting slurries shrink the most and this can lead to cracking, but it is difficult to distinguish cracking that may have been caused by curing shrinkage from damage caused by the mold removal process. Shrinkage can also cause river marks on the part surface where it has pulled away from the mold during curing. Shrinkage effects can be minimized by curing under pressure. Pressures up to about 400 kpa have been used. The use of pressure also helps to crush any bubbles trapped during the casting process. Cure inhibition caused by the mold material has only been seen in gelcast parts and results in surface roughness. Since the cure is oxygen inhibited it was suspected that oxygen adsorbed on the mold surfaces was preventing the part surfaces from curing completely. Casting under nitrogen was found to eliminate this problem. Figure 7.13 shows a green silicon nitride pitch shaft that suffers from a number of the casting and curing problems described above. There are several voids caused by air trapped during casting (a). Most of the part surfaces show significant flaking from cure inhibition or shrinkage. This is particularly evident on outward facing surfaces (b), and less pronounced on inward facing surfaces. This would suggest that the effect is shrinkage related because the worst surfaces are the ones that would have shrunk away from the mold surfaces. This effect may also allow air to enter the mold and inhibit the cure. The base is also cracked. 7.3.4 Defects Resulting From Demolding The demolding process can cause defects via a number of mechanisms: cracking due to thermal expansion effects warping due to softening and relaxation of residual stresses warping due to the effects of gravity and handling of soft parts 176

Figure 7.13: Flaking and cracking of green part during curing surface damage due to solvent attack cracking caused by excessively rapid drying of ceramic green parts cracking caused by thermal shock The gelcasting slurries must be cured at elevated temperature, typically around 55 C. Mold removal is usually performed at temperatures up to about 120 C. If the mold is allowed to cool to ambient temperatures between curing and demolding it will experience a thermal contraction as it is cooled down, followed by a thermal expansion as it is heated up again. If the wax has a high coefficient of thermal expansion it is possible for the mold to change size sufficiently that it breaks the green part. To avoid this problem, parts are not cooled between curing and mold removal. The temperature is ramped directly from the curing temperature to the mold removal temperature. 177

Even using this technique it is possible for the mold to crack the green part if it expands sufficiently before softening during the heat up to the mold removal temperature. To avoid this, parts can be cured at temperatures closer to the mold removal temperature, to reduce the temperature differential, or a mold material with a lower coefficient of thermal expansion can be used. Another alternative is to make the mold walls thinner so that they can t apply as much force. In the limit the mold would be just a thin shell around the part. Most of the part materials do not have particularly high temperature resistances and so the parts will often soften significantly during the mold removal stage. If the parts are delicate they may warp or sag due to gravity or forces applied during handling. Support fixtures can be used to minimize this effect. In a few cases the BioAct 280 was found to attack the surfaces of gelcast parts and dissolve them slightly. This was almost always found to be a result of incomplete curing and is no longer a problem. 7.3.5 Defects Resulting From Drying, Burnout and Sintering Ceramic and metal parts made by sintering green parts produced by Mold SDM may suffer from a variety of problems associated with drying, burnout and sintering. These problems are not Mold SDM specific, they are common to all drying, burnout and sintering processes. The most common problem related to drying and burnout is cracking due to excessively fast processing. As green parts dry they shrink slightly and if the process occurs too rapidly differential shrinkage can cause cracking. During drying and burnout it is also possible for vapor or decomposition products to build up inside the pores of the part. If the gasses can t escape the pressure can build up sufficiently to cause cracking. Sintering is a complex process involving densification and microstructural change. As the powder particles of the green part fuse together, the grains in the material also tend to grow. In ceramic materials larger grains generally result in inferior mechanical properties. Both densification mechanisms and grain growth mechanisms increase in rate as the temperature increases, but a minimum densification rate is required to achieve full density. 178

There is therefore an optimum sintering time-temperature profile that depends on the ceramic material and the sintering aids used. Delicate structures need to be supported during sintering to prevent distortion due to gravity and sintering forces. The significant shrinkage experienced during sintering, typically between 15 and 20% linear, means that fixed supports can t be used in most cases. Support is frequently provided by a ceramic powder packed around the parts being sintered. Packing density is important because too loosely packed powder will not provide enough support, and too tightly packed powder may not allow the parts to shrink during sintering. Some Mold SDM parts, such as the pitch shaft, have been sintered using fixtures made of the same green material as the part. During sintering both the part and fixture shrink together and thus maintaining the part shape. 7.4 Comparison With Other Processes Mold SDM is based on solid freeform fabrication techniques, typically used in rapid prototyping processes, however it also uses CNC milling which is a common production process. Because of this combination Mold SDM's characteristics are intermediate between those of prototyping and production processes. 7.4.1 Materials Production processes use the widest variety of materials, partly because they are optimized for specific applications. Parts can be produced from a huge variety of ceramic, metal and polymer materials. Most RP processes are very limited in the range of materials that they can make parts from. This is often due to the process depending on a particular material property or combination of properties. For example, materials for SLA must be UV curable and have relatively low viscosities. Materials for FDM must be low temperature extrudable and able to be formed into filaments. Most of these materials are polymeric and are therefore unsuitable for many real world applications. 179

None of the commercial RP processes can build metal parts directly, however several of them can now produce short run metal tooling via indirect routes such as infiltration and sintering [Hejmadi 96, Sachs 97]. There are currently no commercial rapid prototyping processes for the fabrication of functional ceramic parts. A summary of the research into the fabrication of ceramic parts using commercial RP processes was presented in Chapter 2. Mold SDM is able to use a wide range of materials for two primary reasons. First, Mold SDM is based on casting so it can use a wide range of castable part materials, including thermoset polymers and ceramic gelcasting formulations. Metal gelcasting formulations are also under development [Janney 95, Janney 98]. Secondly, the mold fabrication process is largely independent of the properties of the part material. This means that the mold and support materials can for the most part be optimized independently from the part materials. Part materials only need to be compatible with the mold materials and able to withstand the mold removal process. Commercial RP processes build using the part material directly and this places restrictions on the part material properties, as noted above. Some commercial RP processes, for example Sanders and FDM, can be used to produce fugitive molds [Jamalabad 96]. This allows them to fabricate parts from a variety of materials in much the same way that Mold SDM does. However since these processes are limited in terms of achievable surface quality, and since internal mold surfaces can't be finished manually it will be difficult to produce high quality parts in this way. 7.4.2 Speed Production processes typically produce parts at a very high rate. Injection molding and pressing processes, for example, produce parts in seconds. Machining is a notable exception where parts usually take significantly longer to produce. The high production rates are achieved by using a tool, such as a mold or a die, to shape the entire part in one operation. However because tooling is expensive, different for each design and typically takes weeks or months to fabricate, production processes are not suitable for prototyping applications where small numbers of parts must be made at short notice. 180

Prototyping processes rely on more flexible manufacturing methods that build parts without the need for tooling. RP processes build up complex shapes from simpler elements but because they do not shape the entire part in one operation they produce parts much more slowly than production processes. However they produce the first part, or batch of parts, much more quickly because they eliminate the weeks or months of tooling lead time. Mold SDM is currently not as fast as commercial RP processes. Parts are usually made in days whereas RP processes typically build parts in hours. This speed difference should become smaller as better build strategies are developed and as Mold SDM process automation improves. The use of multiple build directions, for example, has the potential to greatly reduce build time. The dependence of build time on part geometry varies from process to process. For SLA, SLS and FDM/FDC the part build time scales with the part volume because material is added on a point by point basis. LOM and CAM-LEM scale with surface area because they deposit entire layers in one operation and then cut the perimeter of each layer. SDM and Mold SDM build time scaling depends on both the machining and the particular material deposition processes being used. Machining scales approximately with surface area since the entire surface of the part must be machined to shape. Deposition processes exhibit more variety. Some, such as extrusion deposition, scale with part volume, others such as mass casting scale with the part height since each layer takes about the same amount of time to cast. However SDM and Mold SDM can use variable layer heights and this affects build rate. In general, simpler shapes with fewer features can be built more quickly since layer thicknesses can be greater. But this again depends on the deposition process used. Mass casting can build thick layers almost as fast as thin ones whereas the time for extrusion deposition processes will scale directly with layer thickness since thick layers must be deposited as a series of thinner layers. 7.4.3 Accuracy Production processes range in accuracy from low accuracy processes, such as casting and forging, to very high accuracy processes such as machining and grinding. For traditional ceramic manufacturing processes the achievable tolerance is around 0.1 mm due to the 181

slight variability in the sintering process. Machining and grinding can then be used to increase the accuracy to better than 0.01 mm. RP processes can't achieve the tolerances required for typical mechanical applications. The most accurate RP processes are SLA and SLS which can achieve accuracies of approximately ±0.005 inches with an additional ±0.001 inches per inch of part size. Therefore at best these processes have accuracies on the order of ±125 µm. The accuracy of Mold SDM parts is comparable to those produced by RP processes. The measurements of impeller dimensions shown above indicate that the accuracy of Mold SDM parts is on the order of ±150 µm for parts in the 30 mm size range. Process improvements, particularly reductions in material shrinkage and new build strategies, should be able to improve accuracy. As with accuracy, production processes vary greatly in the level of surface finish. Machining can produce surface finishes below 1 µm RMS and grinding can reach below 0.01 µm RMS. Sintered ceramic parts can achieve surface roughnesses comparable to machined parts. Measurements made by Sangkyun Kang on a variety of RP parts indicate that at best RP processes achieve surface roughness values on the order of 3 µm RMS as shown in Table 7.9. Surface Process Roughness (µm RMS) Selective Laser Sintering 2.8-6.1 Three Dimensional Printing 6.4-12.7 Sanders Prototype 3.8 Table 7.9: Surface quality achieved using RP processes Mold SDM can achieve superior surface quality to RP processes because it is an additivesubtractive process. The machining step in Mold SDM produces smoother surfaces than can be made by near-net shape deposition alone. As reported above, surface roughness values of better than 2 µm have been measured and it should be possible to improve this to better than 0.5 µm by improving the machined surface quality of the soldermask. 182

7.4.4 Sintering Ceramic and metal parts made by Mold SDM must be sintered. Since the sintering process is not specific to Mold SDM, the issues are the same as those for other parts that must be sintered. Key issues are the uniformity of the green part density, repeatability of the sintering shrinkage and prevention of slumping or distortion during sintering. Green part uniformity is key to successful sintering. Variations in density will result in variations in sintering rates and this will lead to distortion and in extreme cases, cracking. One of the benefits of gelcasting is that the green parts typically have very uniform densities. Processes such as powder pressing often have density variations due to compaction stress variations due to die wall friction effects. Sintering shrinkage is usually 15-20% (linear) depending on the solids loading of the gelcasting slurry used. Any variability in this shrinkage will directly affect the achievable tolerances of parts made by sintering. Variability in shrinkage is typically 1-2%. Measurements presented above indicate that Mold SDM parts are well within this range and may in fact have much lower variability. Slumping during sintering due to gravity is a serious concern, particularly for delicate parts. A common solution to this is to pack a ceramic powder around the green parts to provide support during the sintering process. This approach has been tried but it is difficult to do well because the powder must be packed just right. If it is packed too tightly it will not allow the part to shrink during sintering, if it is packed too loosely then it will not provide enough support. A more promising approach has been to use Mold SDM techniques to build green support structures. These fit under the parts during sintering to provide support and shrink together with the parts. After sintering the support structure is discarded. In many cases the part orientation can be chosen to minimize distortion. Shafts, for example, can be sintered in vertical orientations so that they do not sag in the middle during sintering. 183

8 Applications This chapter explores some of the current and potential applications for Mold SDM. Characteristics of ideal Mold SDM applications are identified and used to evaluate potential applications. Applications in prototyping are described, but the main focus is on production applications because that is where there is the greatest commercial interest. A variety of ceramic, metal and polymer applications are covered. 8.1 Application Objectives The goal in developing Mold SDM is to develop a production manufacturing process to enable the fabrication of complex functional ceramic parts. The primary application is ceramic components for use in gas turbine engines, however other applications and potential applications have been identified during the course of this research. Ideally Mold SDM can be used as a production process, rather than a prototyping process. Prototyping processes are useful during design but there is always the issue of how the production parts will be made. Production processes are also much more interesting from 184

a commercial point of view because they have the most potential to enable change. Existing products might be made better or it might become possible to produce entirely new products which could not previously be made. 8.2 Mold SDM Process Characteristics The general characteristics of the Mold SDM process are: Shape complexity: Mold SDM is a layered manufacturing process which enables it to fabricate complex shapes which are difficult or even impossible to produce using conventional processes. Accuracy: The accuracy of Mold SDM parts is less than that achievable using production processes but it is comparable to that of RP processes. Surface finish: because of the machining step, the surface finish is superior to that achievable using commercial RP processes. Materials flexibility: Mold SDM can make parts from a much wider range of materials than RP processes. Ceramic, metal and polymer parts can all be made using Mold SDM. Speed: in terms of speed Mold SDM is close to RP processes, although in most cases RP processes will be faster. Part production rate is much less than that of production processes but the time to make the first parts is much shorter because no long lead time custom tooling is required. Special capabilities: Mold SDM can build unique parts, such as pre-assembled mechanisms and multi-material parts, which may not be manufacturable using other processes. Because the process is still under development there is room for significant improvement in many of these areas. Improvements in materials and process steps should be able to improve part accuracy and surface finish. Better automation should improve repeatability and reduce build times. The range of materials is potentially very large because of the large number of castable part materials that are available. These include a wide spectrum of thermosetting polymer resins and a variety of ceramic and metal gelcasting formula- 185

tions. An additional Mold SDM consideration is the sintering step required for the fabrication of ceramic and metallic parts. Because the green parts are only approximately 50% dense, linear shrinkage in the range of 15-20% occurs during the sintering process. The key issue is the repeatability of this shrinkage because that directly affects the dimensions of the final parts. Shrinkage variability is usually 1-2%, which means that tolerances better than 1-2% can not be achieved on the final parts without machining or grinding after sintering. 8.3 General Application Characteristics Ideal applications for Mold SDM will tend to have the following features or requirements: Functional materials High shape complexity Smooth surfaces Some applications may require one of the special capabilities of Mold SDM, for example the fabrication of pre-assembled mechanisms, and in cases such as these Mold SDM may be the only alternative. For ceramic and metallic materials that must be sintered there are the following additional application characteristics: Tolerances >2% sufficient for most surfaces. Minimum amount of high precision surfaces that need to be machined or ground after sintering. The following sections discuss applications for Mold SDM for a variety of ceramic, metallic and polymeric materials. Both current and potential applications are covered. 186

8.4 Applications For Ceramics 8.4.1 Heat Engines The primary goal of this research is to develop Mold SDM as a manufacturing process for the fabrication of ceramic components for use in gas turbine engines, particularly in the hot sections of the engines. Current engines use superalloy parts and their temperature resistance limits the maximum engine operating temperature. By using ceramic parts the operating temperature can be increased and this in turn increases the efficiency. The lower density of ceramic materials also helps to reduce engine weight. Silicon nitride is the material of choice in these applications because of its good mechanical properties and high thermal shock resistance. Parts such as blades and vanes are an ideal application for Mold SDM. Figure 8.1 shows several views of a CAD model of a vane doublet from a Rolls-Royce Pegasus gas turbine engine. These parts have high shape complexity and often include intricate internal cooling passages. Since they are used in the flow path they must have smooth surfaces but the tolerances on the surfaces are not particularly high because hot section parts typically use thermal barrier coatings which are quite rough. Only the mounting surfaces on the vane need to be accurately machined for fit. Figure 8.1: Vane doublet from Rolls-Royce Pegasus engine Because the commercial and military aircraft industries are very conservative initial efforts are being directed at the fabrication of ceramic engine components for miniature 187

engines for use in unmanned aircraft and portable power sources. These applications have a much lower risk to human life and are therefore much more suitable for initial proof of concept studies. The miniature gas turbine engine developed by M-DOT Inc. (Phoenix, AZ), shown in Figure 8.2, currently uses superalloy parts. Replacing these with silicon nitride parts will increase the thrust to weight ratio by both reducing the weight and increasing the operating temperature. Figure 8.2: M-DOT miniature gas turbine engine An example part from the M-DOT engine is the inlet nozzle shown in Figure 8.3. Its function is to direct the hot gasses from the combustion chamber into the rotor. It is a nonrotating part and it experiences the highest temperatures in the engine. By 1990 silicon nitride turbocharger rotors had been used in over 400,000 production automobile engines [Katano 93]. Silicon nitride is used because of its low density, mechanical strength and temperature resistance. The low density is important because it lowers the rotor's inertia and allows it to spin up faster, thus improving engine response. Current turbocharger rotors are made by processes such as slip casting and green machining. Because of the complex geometries involved this could also be an application for Mold SDM. 188

Figure 8.3: Inlet nozzle from the M-DOT engine 8.4.2 Structural Applications Ceramic materials can also be used in a number of applications where high temperature strength is not the key issue. Parts for gyroscopes and other instrumentation applications are potential applications for ceramics. Although these parts do not typically operate at high temperatures, they often require high specific stiffness and light weight because they are moving parts. An example of this type of part if the pitch shaft shown in Figure 8.4. This is a component from a missile guidance system which is currently made by CNC machining of titanium billets which is a very expensive process. Figure 8.4: Pitch shaft from a missile guidance system IC fabrication equipment often uses high purity alumina components because of their high 189

temperature resistance and chemical inertness. Wafer chucks, that are used to hold silicon wafers during processing, are an example of such an application. Wafer chucks typically contain passages for holding wafers by vacuum suction as well as cooling channels for temperature control. This would be an application for Mold SDM where the shape complexity capabilities could be used to produce the internal channel geometries. Ceramic materials are also used in high temperature heat exchanger applications, some of which involve fairly complex arrangements of pipes or flow channels. Mold SDM s shape complexity capabilities could be used to advantage to fabricate these types of devices and eliminate the need for potentially weak joints. Alumina and zirconia have been used in biomedical applications, such as hip prostheses, for over 20 years [Hench 91]. Alumina is used because of its excellent biocompatibility, low friction and low wear rate. For wear and friction reasons the bearing surface must be highly accurate and finely polished but the rest of the part does not need to be as accurate. Mold SDM could be used to rapidly produce custom implants where only the bearing surfaces would need to be ground and polished. 8.4.3 Ceramic Mechanisms A potential application for Mold SDM is the manufacture of mechanisms for use in harsh environments. Polymeric materials are usually fairly chemically resistant but their low strengths and stiffnesses, as well as their low temperature resistance, limit their use in structural applications. Metallic materials are much stronger and stiffer and can operate at higher temperatures but corrosion is an issue. Protective polymeric coatings might be practical in some low temperature applications, but in general as temperatures increase ceramics become the best option. One of the problems with mechanisms is how to assemble and hold the parts of the mechanism together. Traditional fasteners such as bolts can't be used in extreme environments. Ceramic joining processes are typically awkward and often produce joints with properties that are inferior to those of the bulk material. One option is to use Mold SDM to produce pre-assembled ceramic mechanisms and thus avoid the joining issue entirely. 190

A potential application for ceramic mechanisms would be for the fabrication of grippers or manipulators for use in harsh environments. Thes might be useful in chemical processing, hazardous waste cleanup or nuclear applications. 8.5 Applications For Metals As with ceramics, Mold SDM could be used to fabricate metallic blades and vanes for use in the hot sections of gas turbine engines. It could be used as the production process or as a prototyping process. Metallic vanes are currently made by investment casting which means that there is also potential for using Mold SDM to produce short runs of prototype parts for initial testing before committing to the fabrication of production tooling. A potentially large application area for Mold SDM would be for the on-demand rapid fabrication of spare parts. In many cases the tooling used to produce parts for old equipment is either no longer available or lost. Producing a new set of tooling to produce a few spare parts would be extremely expensive, and Mold SDM might be able to produce the part quickly and at far less cost. Mold SDM would be useful for the fabrication of parts made from materials which are difficult to form, for example because of the difficulty of machining them. Refractory metals and superalloys would fall into this category. 8.6 Applications For Polymers Mold SDM can be used to make parts from a variety of castable polymeric materials. Materials used to date include polyurethanes, epoxy and silicone. In contrast with ceramic and metallic parts, polymer parts tend to be much easier to produce (typically by injection molding) and far less expensive and so the use of a relatively expensive process such as Mold SDM is probably not realistic for mass production applications. Mold SDM might be useful if its unique capabilities, such as shape complexity and preassembled mechanism fabrication, are required. This would be particularly so if a rela- 191

tively small number of parts were required. Mold SDM might be used to produce a part with complex internal flow channels which could not be fabricated by injection molding. In this application area the competition from commercial RP processes would be an issue, although the superior surface finish achievable in Mold SDM parts could be a deciding factor. Fabricating models for fluid flow experiments would be a general type of application where the superior surface finish achievable using Mold SDM might be useful. Any surface roughness in the flow path may affect the flow properties in undesirable ways. With commercial RP processes it may not be possible to smooth all of the surfaces in the flow path because they may not all be reachable for manual finishing. An example of such an application is the use of Mold SDM to fabricate silicone artery models for flow measurement experiments being performed by Professor Charles Taylor of the Mechanical Engineering Department at Stanford University. The goal was to produce flexible, transparent artery models which could be used to perform flow experiments to verify the results obtained from computer simulations. By making the artery models from a transparent silicone it is possible to monitor the flow using tracer particles placed in the fluid. 8.7 Prototyping and Short Run Production Applications Mold SDM could be used in any of the preceding application examples as a prototyping method or short run production process. For ceramic or metallic engine vanes, for example, it may be impractical to use Mold SDM as the production technique because of its low production rate. However Mold SDM could be ideal as a means for producing prototype parts that could be used for testing under real operating conditions. If sintering is required for the production parts then Mold SDM could also be used to fabricate green parts for sintering optimization runs to determine the ideal green shape before fabricating production tooling. For general prototyping purposes Eyerer identified three categories of parts [Eyerer 95]: 192

Geometric: geometric prototypes have the right size and shape and are typically used for look and feel evaluation of new designs. Geometric prototypes may also be used as patterns in casting processes, for example. Functional: functional prototypes are intended to be used for actual testing. They must not only be geometrically correct and but must also have properties sufficiently close to the final production parts that they can be tested. Technical: technical prototypes are intended to exactly match production parts. They must be geometrically accurate and must be made of the same material, with the same properties, as the production parts. Commercial RP processes initially made geometric prototypes but due to process improvements most are now capable of making functional polymeric prototypes. In most cases these functional prototypes will require extensive hand finishing to produce acceptable surface quality. Hand finishing will generally lead to reduced accuracy which may be unacceptable in some applications. Technical prototypes can also be made using several of the commercial RP processes by using indirect approaches. These techniques use the RP process to fabricate rapid tooling, either directly [Hejmadi 96, Sachs 97] or from a master pattern made using the RP process which can then be used to make a silicone mold. This tooling can then be used to produce a short run of prototype parts. The number of parts that can be made varies depending on the process and the part material. Mold SDM could potentially be used for the fabrication of any of the three types of prototype. It could also be used to duplicate most of the rapid tooling approaches. The advantages of Mold SDM over RP processes lie primarily in the increased range of materials, particularly ceramics and metals, and the superior surface finish achievable. In most cases Mold SDM will be slower than commercial RP processes, both because the process is currently slower and because there are more process steps involved. Improvements to the Mold SDM process, in particular better automation, should be able to reduce the speed disparity. 193

9 Conclusions 9.1 Summary This research has resulted in two main achievements: the development of Mold SDM, a new manufacturing process based on SDM the development of improvements to the basic SDM manufacturing strategies Mold SDM was developed as an extension of the basic SDM process to enable the rapid fabrication of high quality, complex shaped, structural ceramic components. Mold SDM enables the fabrication of geometrically complex ceramic shapes that could not be fabricated using conventional manufacturing processes. It enables quick fabrication of parts because it does not use expensive, long lead time part specific tooling. It enables the fabrication of accurate parts with surface quality comparable to that achieved using conventional ceramic production processes and superior to that possible with rapid prototyping processes. Although Mold SDM was developed as a means to fabricate ceramic components for use in gas turbine engine applications, it is not limited to this application area. Mold SDM can be used to fabricate ceramic, polymeric and potentially metallic parts for a wide range of 194

applications. Mold SDM can also be used to fabricate pre-assembled mechanisms and multimaterial polymer parts. Parts have been built to demonstrate these capabilities. As part of the development process a number of new techniques were discovered to improve part quality and increase process capabilities. These include both enhancements to the process planning strategies as well as better materials combinations and processing techniques. The majority of these improvements can be used with the basic SDM process to improve the quality of metal and polymer parts produced using SDM. These achievements are significant because they: enable the fabrication of high quality, functional ceramic parts which could not previously be made increase the potential range of applications for ceramic materials make it possible to quickly produce functional parts for use in design iteration extend the capabilities of the basic SDM process The miniature gas turbine application is an example of an application that has the potential to be enabled by Mold SDM. The current engine has a very short operating lifetime due to durability issues with the current metallic components used in the hot section of the engine. If the Mold SDM parts can successfully replace these metal parts the engine will be able to operate for much longer periods of time and may become a viable power source for miniature aircraft. As part of this engine development effort, Mold SDM will also enable the rapid fabrication of ceramic parts for design iteration and testing. This will make the use of ceramic parts much easier to implement because it eliminates the limitations of high cost and long lead time traditionally associated with the fabrication of ceramic parts. 9.2 Contributions The main contributions of this thesis are summarized below: Development of the Mold SDM process concept: The Mold SDM process was conceived as a mold based variation of the basic SDM process. This extension of the SDM 195

process was necessary for the successful fabrication of geometrically complex, functional structural ceramic parts because the materials available for use with SDM were not capable of producing high quality parts. Development of the Mold SDM concept involved adapting SDM techniques to the fabrication of fugitive molds which could then be used to fabricate parts by a casting process, such as gelcasting. The initial process was based on materials previously used or tested for use in polymer SDM and the sum of process planning techniques developed up until then. Implementation of the process indicated that there were a number of shortcommings in the process that limited the quality of parts that could be fabricated. Materials selection for improved part quality: Initial efforts to improve part quality focused on improving the materials combination used. Materials issues leading to part defects and processing difficulties were identified. Properties that were found to be particularly critical were shrinkage, machinability and temperature resistance. A set of testing procedures were developed to quantitatively and qualitatively evaluate these properties. The initial materials were evaluated using these techniques and then compared with potential new materials to identify superior materials. As a result, several good material combinations were identified and used to build parts. However, most part defects could not be eliminated by improved material selection alone. Defects could only be reduced in severity. Enhanced process planning strategies and build techniques: Since better materials could not eliminate part defects new process planning strategies and build techniques were developed to avoid situations that tend to cause defects. Techniques such as incremental casting, one time surface machining and cut through machining effectively eliminate a number of common shrinkage and heat distortion related defects. Other defects, such as the inability to produce truly sharp corners, were caused by limitations in the process planning techniques rather than by material property limitations. The overcut-fill-trim-backfill technique was developed to allow the fabrication of sharp corners. Several decomposition techniques and planning strategies based on more general layer geometries were also proposed as ways to increas part quality and reduce build time by taking advantage of the flexibility of the Mold SDM process. 196

Development of manufacturability analysis and orientation selection techniques: Previous work on process planning had focused on automation of the decomposition and operation generation steps. Issues such as orientation selection and manufacturability had not been addressed. Based on experienced gained during process development and part fabrication a number of rules of thumb were developed to guide in build orientation selection. These rules are based on an evaluation of the part features and how these contribute to build difficulty and build time. A set of manufacturability rules were also developed to help identify which part orientations are manufacturable and which are not. These rules can be used to identify a set of candidate orientations as input for the orientation selection step. Fabrication of parts to demonstrate process capabilities: Numerous ceramic and polymer parts were fabricated during process development to demonstrate and evaluate the process capabilities. Ceramic parts were made using alumina and silicon nitride. Prototype silicon nitride parts were built for use in a miniature gas turbine engine and some of these were successfully tested in a combustion test rig at temperatures in excess of 1250 C. The capability to fabricate pre-assembled ceramic mechanisms was also demonstrated as a special capability of the Mold SDM process. Polymer parts were fabricated from two types of polyurethane, one type of epoxy and also a silicone elastomer. Pre-assembled mechanisms and multimaterial mechanisms were also built. Metal gelcasting is being investigated as a means for producing metal parts using Mold SDM. Initial results are encouraging but further development is required. Identification of application characteristics and potential applications: Process characteristics were used to identify the characteristics of ideal applications. Based on these characteristics, a number of applications for the Mold SDM process were identified in addition to the orignial ceramic gas turbine parts application. Potential application areas for Mold SDM include ceramic parts for structural applications, for example silicon wafer processing equipment and biomedical implants, metal parts made from refractory alloys or alloys that are difficult to machine or cast. Mold SDM might also be used to produce on-demand spare parts to avoid having to maintain inventories of spare parts. Because of the short lead time, Mold SDM also has applications for proto- 197

typing applications where its capability to produce quality parts from a wide range of engineering materials may be required. 9.3 Future Work Now that the Mold SDM process concept has been validated by the construction of a variety of ceramic, polymer and metal parts, future work must focus on process optimization and the identification of additional applications. The primary goals of process optimization should be to: increase part accuracy and surface finish increase the build rate simplify the process planning task to make Mold SDM easier to use Improving part quality is key to the long term success of Mold SDM as a manufacturing process because parts made by Mold SDM do not yet match the quality of parts made by conventional manufacturing processes. Increasing part quality will require more extensive process characterization to obtain a better understanding of the main factors involved. Improvements in part quality will result from better material combinations, better build techniques and better automation. Key material properties include shrinkage, machinability, heat resistance and soldermask removability. Reduced wax shrinkage will probably have to be achieved by the use of fillers and this will affect the melt viscosity which will affect feature replication. Filler settling issues will also be critical to successeful automation. The use of fillers may also improve machinability and heat resistance. Machinability may be further improved by using different machining strategies or even different types of cutting tools. Improved soldermask removability, by optimization of its chemistry or solvent selection, has the potential to greatly increase the process rate. New build techniques will need to be developed to further improve the process capabilities and increase process efficiency. Further exploration of the tradeoffs between 3- and 5-axis machining, and single and multiple build orientations may uncover opportunities for sig- 198

nificant gains. As the process becomes more developed it must move away from the limitations of the planar layer based approach and become more flexible to accomodate part features and application requirements. Improved automation will increase the repeatability of the process by eliminating the possibility of operator variability or error and increase the process rate by reducing the amount of operator intervention and rework required. Better automation will also make the process easier to use by reducing the amount of process knowledge required to build parts using Mold SDM. A more automated process planning environment would contribute significantly to this. Although several applications for Mold SDM have already been identified there are many potential application areas that should be investigated more fully. The possibility of fabricating entirely new and innovative devices that take advantage of the unique capabilities of Mold SDM presents some exciting possibilities. One particular example is applications for pre-assembled ceramic mechanisms. 199

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Glossary Build Direction: rapid prototyping processes build parts in layers and the direction normal to the plane of the layers is called the build direction. The build direction is usually vertically upwards. Build Envelope: all rapid prototyping machines are limited in the sizes of the parts that they can build. The build envelope is the maximum volume available for building parts in. It is usually a box or cylindrical shape. Burnout: the process of removing the organic components from a ceramic green part prior to sintering. This is usually accomplished by oxidizing the organic material at elevated temperature, typically in the 300-600 C range. In most cases the temperature is increased as burnout proceeds. Christmas Tree Effect: a surface discontinuity effect in SDM and Mold SDM caused by material shrinkage. Distortion due to material shrinkage during deposition produces a small step on the surface between layers. Computer Aided Manufacturing of Laminated Engineering Materials (CAM-LEM): a rapid prototyping process that fabricates parts by laminating sheets of material together. For each layer the cross section of the part is cut from a sheet of part 210

material and is then placed on top of the already built layers. The complementary shape is then cut from support material and this is also placed on top of the previously built layers. Once all layers have been built pressure is applied to laminate the layers together after which the support material is removed to reveal the finished part. Computer Numeric Control (CNC): instead of controlling a machining process, such as milling or turning, by hand a computerized system is used to automatically control the motion of the cutting tool. This automation greatly reduces the labor required to produce machined parts and also improves repeatability and production rate. Compact: a section of a part that can be fabricated in one SDM machining and deposition cycle. A ray along the build direction will enter and leave a compact only once. Because of this compacts are defined relative to a particular build orientation. Changing the build orientation may invalidate the compacts. Compact Adjacency Graph (CAG): a graph representation for the connectivity between compacts. Each compact is a node in the graph. Each link in the graph represents contact between two compacts. Compact Precedence Graph: based on the Compact Adjacency Graph. This graph adds directionality to the links to indicate the fabrication sequence of the compacts. The graph is acyclic, that is there are no loops. The minimal Compact Precedence Graph is obtained by eliminating links which represent illegal build sequences. For example, if compacts 1 and 3 are adjacent but compact 2 must be built before compact 3, then there can not be a link between compacts 1 and 3. Decomposition: in layered manufacturing processes this is the step where the original part geometry is broken up into simpler manufacturable elements. These are typically 2.5-dimensional uniform thickness horizontal slices, although some processes use more complex geometries. Drying: the process of removing any liquid components from a ceramic green part before beginning burnout. In gelcasting for example a solvent occupies the pore spaces in the cured green part. Drying is usually accomplished at moderate temperatures 211

between 100 and 200 C, but in some cases may also be performed at ambient temperatures. Temperatures are usually increased as drying proceeds. Electric Discharge Machining (EDM): a machining process where material removal is accomplished by electrical discharges between a tool electrode and the workpiece. The tool and workpiece are immersed in a dielectric fluid which is also used to flush away the machining debris. Two types EDM are used: wire EDM and sinker EDM. In wire EDM the electrode is a wire which is moved through the workpiece to produce cuts. In sinker EDM a shaped electrode is used to produce a cavity that is the negative of the electrode shape. EDM can be used to produce sharp concave corners in situations where milling can t, however EDM only works with electrically conductive materials. Fused Deposition of Ceramics (FDC): a variation on Fused Deposition Modeling where a ceramic filled material is used as the part material to build ceramic green parts. Fused Deposition Modeling (FDM): a rapid prototyping process that builds parts using an extrusion system to deposit polymeric materials. Each layer is built by scanning the extrusion head to fill the area inside the part cross section. A second extrusion head deposits support material in a similar fashion where needed. The support structure is not 100% dense so that it is weaker and easier to break away after the part has been completed. Gelcasting: a ceramic forming process developed at Oak Ridge National Laboratory which involves suspending ceramic particles in a liquid monomer and solvent mixture to form a castable slurry. The slurry is cast into a mold and polymerized to form a strong green part that consists of ceramic particles held together by a polymer network with solvent occupying the pore spaces. Green Part: a porous part composed of fine ceramic particles held together by a binder. The binder is typically a polymeric material and its purpose is to provide some mechanical strength to the green part so that it maintains its shape and can be handled. Green parts typically contain 45-60% ceramic particles by volume, the rest is composed of binder materials and pore space. 212

Hot Isostatic Pressing (HIP): a densification process where a ceramic powder preform is encapsulated and then simultaneously subjected to high temperatures and pressures. The pressure enhances the densification such that it proceeds more rapidly and at lower temperatures. The pressure also has the effect of collapsing any voids in the part. This process typically produces the best mechanical properties but it is very expensive because of the equipment required. Laminated Object Manufacturing (LOM): a rapid prototyping method that builds parts by laminating together sheets of paper-like material. For each layer the outline of the part cross section is cut into the sheet and areas outside the part are crosshatched to form small squares. This material forms the support for the part as it is being built. Once the part has been built the cross-hatched areas are broken away to expose the finished part. Medial Axis Transform (MAT): the Medial Axis Transform is the loci of the centers of locally maximal spheres inside an object. The MAT can be thought of as the skeleton of the part. Mold Shape Deposition Manufacturing (Mold SDM): a variation on Shape Deposition Manufacturing which uses SDM techniques to fabricate fugitive molds which can then be used to make parts from a variety of castable materials. Rapid Prototyping (RP): the name for a broad category of processes that fabricate parts by building them up in a layerwise fashion. Three-dimensional parts are decomposed into simpler geometries, typically 2-dimensional cross sections, and are then built by sequentially fabricating each of the simpler geometries in sequence. These processes are capable of building very complex geometries in relatively short times, typically on the order of hours to a few days. Most processes are highly automated. The main drawbacks are the general lack of capability to produce parts from engineering materials and the poor surface quality of as built parts. Also sometimes referred to a Solid Freeform Fabrication processes. Sanders Model Maker: The Sanders rapid prototyping process is an inkjet process that uses two thermoplastic waxes as part and support material. The Sanders process 213

builds layers by printing part and support material using an inkjet printhead. After printing, each layer is planed flat. After completion of the part, the support material is removed using a heated organic solvent. The Sanders process has good accuracy but it is very slow. It is well suited to the fabrication of investment casting patterns for use in the jewlery industry. Selective Laser Sintering (SLS): a rapid prototyping process based on using a laser to fuse powder particles together. Layers of powder are first spread over the build envelope and then a scanning laser fuses the powder that lies within the part cross section for that layer. Layers are built up using the unfused powder as support. Once the part is finished the unfused powder is removed. Sintering: the high temperature process where the ceramic particles in the burnt out green part are fused together to form a dense ceramic object. Sintering is usually performed at temperatures exceeding 1500 C and may also involve the application of external pressure to assist in densification (see also Hot Isostatic Pressing). Sintering is also usually performed under controlled atmospheres to reduce minimize oxidation or decomposition of the material during the process. Shape Deposition Manufacturing (SDM): a rapid prototyping process that differs from others because it is an additive-subtractive process instead of being a purely additive process. Layers are built up by alternately depositing and shaping part and support materials. The shaping, or material subtraction step, allows greater variety in the range of materials that can be used because net shape deposition is no longer required. Solid Freeform Fabrication (SFF): the name for a wide range of processes that fabricate parts by gradually building them up from simpler elements of material. Parts are typically built up incrementally using droplets, filaments or sheets of material. Solid Ground Curing (SGC): a rapid prototyping process that builds parts from an ultraviolet curing resin. Layers are built by spreading a coat of resin over the build area. A mask is then used to selectively expose the resin to ultraviolet light from a flood lamp so that only the areas that correspond to part material are cured. Uncured 214

resin is then removed and wax is cast to take its place. The wax and cured resin are then planed to the correct layer thickness before continuing. Once the part has been built the wax support material is removed by melting and solvent etching. Stereolithography (SLA): a rapid prototyping process that builds parts by selectively curing layers of polymer resins using an ultraviolet laser. Each layer is built by coating a fresh layer of resin over the already built layers and then scanning the laser over the area inside the part cross section to cure the resin. Support is provided by building an open framework structure which can be broken away after the part is finished. Three Dimensional Printing (3DP): a rapid prototyping process that has two variants. The first uses an inkjet technology to selectively print binder into a powder bed. Each layer is built by spreading a new layer of powder over the build area and then printing binder. Once finished the part is removed from the lose powder which provides support during the build process. The second variant deposits a slurry, instead of a powder, over the build area and then selectively prints a binder into it. After all layers have been built the unprinted slurry is redispersed in water to free the part. 215