Quick Tools Development through Stereolithography (SLA) for Sheet Metal Forming An Evaluation Study

Transcription

1 Quick Tools Development through Stereolithography (SLA) for Sheet Metal Forming An Evaluation Study Ismet P. Ilyas * Design Engineering Department POLITEKNIK MANUFAKTUR BANDUNG Jl. Ir. H. Juanda, Kompl. Kanayakan, Dago Tromol POS 851, Bandung 40008, INDONESIA ABSTRACT This paper evaluates the rapid prototyping and tooling (RP&T) techniques for developing punch & die for Press Metal Forming Die. In this evaluation, one type of prototype tool called Quick tool has been manufactured and tested. This type of tool is being a stereolithography Quick Cast pattern infiltrated with aluminum filled epoxy. While this type of tool may be indeed rapidly manufactured it can be only a prototype tool, as the material it is made of does not render much durability. Further, the developed tool has been evaluated in press metal forming of by producing components with 0.8 mm aluminum sheets. Keywords: rapid prototype manufacturing, rapid tooling, Stereolithography, sheet metal forming INTRODUCTION AND BACKGROUND The present day industry trend is to use short production runs in the manufacture of products. As a result, the product development cycle has to be shortened. This is where Rapid Prototyping (RP) comes to the aid of industry. In recent years, advancements in RP technologies have led to considerable amount of research activities in the area of tooling for which Rapid Tooling (RT) term was coined RP techniques are employed to make prototype tools. The basic idea of RT is to produce prototype parts by using prototype tools so the part truly represents the future production. RP is a process by which a tangible prototype of a product is made in a relatively short time span. This prototype can then be used in analyzing the product s properties making necessary modifications. Tooling is an important area in the manufacturing process. This aspect can be extremely expensive as well as time consuming. The increase in the complexity of tooling for any operation results in a corresponding increase in the time and costs required to develop such tooling. The ideal candidate operations for RT have been those for which it is difficult to develop tooling by the usual methods. RT has found applications in a wide variety of areas. RT techniques are of two types soft tooling and hard tooling. For both types, they could be manufactured through direct or indirect method (Fig. 1). Soft tooling, is associated with low costs, used for low volume production and uses materials that have low hardness levels such as silicones, epoxies, low melting point alloys [1,2]. Aluminum filled epoxy resins can be cast into a polished release coated RP model to manufacture injection moulding tools that can be used to produce low-volume parts or prototypes [3]. Spray metal tooling is a very popular and common method for producing soft tooling. A thin metal shell (approximately 2 mm) is sprayed onto a pattern. This shell is then backed with epoxy [4]. Metal spray moulds have been used successfully for low pressure processes such as vacuum forming, rotational moulding and resin injection moulding (RIM). Plaster mould casting used as a prototype manufacturing process for simulated die casting. A silicone rubber mould is made and then plaster is poured over it to obtain a cavity. This serves as a die casting tool. Spin casting is yet another area where RT has great potential. High temperature vulcanizing (HTV) silicone rubber has been used in the process of mould making. The mould cavities are hand carved and then cured under high temperatures and pressures to form the finished rubbed moulds. Finally, in-gates and vents are cut out and the moulds are mounted on the spin casting machines, ready for use. They offer greater resistance to heat and chemical action than room temperature vulcanizing (RTV) silicone rubber [5]. * ismetpi@attglobal.net or polman@melsa.net.id 1

2 RP systems RP model Direct Method Tool (Prototype/Production) Master / Pattern Associate Techniques Indirect Method Fig. 1. RT Manufacturing Methods Hard tooling, on the other hand, is associated with higher volume of production, and use of materials of higher hardness such as hardened steel. Most hardened steel tools are manufactured by conventional metal cutting processes or by electric discharge machining. The ability to produce intricate details is common to rapid prototyping and investment casting. Hence rapid prototypes can be used instead of wax, in the investment casting process (as lost pattern) [3]. Investment casting involves making a wax pattern, around which ceramic slurry is coated to form a shell. Once the shell is hard enough, the wax is melted away and the ceramic shell is used to cast the required shape, by pouring molten metal into the shell. Since wax is melted away, the process is also called the lost wax process. The replacement of precision machined wax with RP, results in saving in cost of tooling and the lead time [6]. Electric discharge machining (EDM) seems to be an interesting area in which RT finds a potential application. Early research work [9] has shown the possibility of using Stereolithography (SL) pattern in the manufacture of EDM electrodes. Some methods of making EDM electrodes have been arrive at 3-D System Inc. that used a powder compaction technique to produced a sintered copper electrode. A metal powder/binder mixture with a solvent is poured around a silicone RTV cast pattern which was in turn produced from an SL negative pattern. After evaporation of solvent, the green compact was sintered to burn off binder. The porous structure was then infiltrated with copper. The tool showed good definition and surface finish. The only disadvantages are that the part undergoes shrinkage during sintering and proper allowance has to be made and the process is limited to small tools. Metal spraying and electroplating of SL pattern is another area that is being researched upon. The purpose is to give the SL pattern a thin coating cooper, and using this coated pattern directly as an EDM electrode. The pattern may be repeatedly used with subsequent coating of copper. LITERATURE REVIEW AND RT CURRENT STATUS From the beginning of 1990 s, several researchers [1-14] applied RP technique in various fields of manufacturing. In their work, Jacob [2] classified the various types of RP systems as liquid polymerization, fused deposition manufacturing, laminated object manufacturing, selective laser sintering and point-to point solidification based on material creation and shape building techniques. William et. al. [10] presented the results of their investigation into the effect of build styles (ACES, WEAVE and Quick CAST) and built parameters on the performance measures of dimensional accuracy, surface roughness and build time. They built all prototypes on SLA-250 by using SL-5170 photo polymer resin. Thomas [14] attempted to use SL tooling for FFR parts is a significant reduction in tooling requirement. By using the RT techniques, he reduced the tools required to produce the FFR parts to one, where as in the traditional manufacturing technique five different tools were required to produce the same part. Dickens [1] studied the SL photo-polymerization process in order to improve its efficiency. They were examining the behaviour of the STARWAVE build style technique using acrylate-based resin from Zeneca. Their results showed that the build parameter such as retraction, cure depth and staggered hatching are not consistent. According to the author, it could be due to the variation in machine, process, or material parameters such as laser power, re-coating mechanism, and resin parameter. Cobb and Reeve [12] studied the factors, which are associated with poor surface finish in using SL process. A methodology for surface finishing using additive processes, abrasive finishing and chemical etching has been discussed. The epoxy primer (Jaxotate) has been shown to be an excellent coating for SL components, either as a base for other coating or as a surface finishing. Finally, the dual additive and abrasive finishing have significantly reduced the surface deviation by up to 95%, with only minor changes to the part geometry. Murakami et. al. [13] developed Refrigerative Stereolothography for rapid product development. In his work, he described * ismetpi@attglobal.net or polman@melsa.net.id 2

3 the basic concept of Refrigerative SL and a refrigerative SL machine produced by embedding of a newly developed unit for supplying and cooling the liquid resin into a conventional SL machine. Bocking et. al. [11] discussed the recent work carried out to implement the rapid production of injection mould cavities and spark erosion tooling using electroforming and electroplating technology. One of the main problems he presented in the production of the injection mould cavities was electroforming. The distribution of the metal, particularly within the deep cavities, recesses, bores and blind holes were inadequate. From the above review, it is clear that many researchers made attempts in different directions to use RP technology for manufacturing of prototyping models or the products. Technologists and researchers involved in the development of RP process are now devoting much of their efforts to take the philosophy of reduced lead times and development cost in manufacturing of production tooling. Attempts are made by many researchers to manufacture production tooling for injection moulding of plastic products, press tool for metal products and EDM tooling for die sinking processes. Dover and others [9] attempted to use Electro-deposition technique to directly produce metal tools. They used copper sulfate electrolyte through a small nozzle. With the anode upstream from nozzle, the electro-deposit is limited to a small area on the cathode at the end of the nozzle. The nozzle is then moved in a vector path in a similar way to a SLA laser. In most of the manufacturing industries it is generally considered that the cost of traditional tooling (either prototype tooling or production tooling) has been a major constraint in launching a new product. It is also recognized that the long lead times encountered during the tool-making stage can seriously delay the product launch and so erode the market lead. However with the development of RP systems which can generate master tools in a matter of hours. Depending on the manufacturing process, material, component geometry, size and the number of components required, the tool techniques used in tool production can be subdivided into resin tooling, metal, faced tooling, investment casting and EDM electrode Manufacturing. MANUFACTURE OF TOOLING FOR SHEET METAL FORMING In this section the procedure used to manufacture of the tool starting from conceptual design stage to the tool's fabrication stage is presented. The various stages involved in the tool production and evaluation stage are shown in Fig. 2 in the form of manufacturing flow diagram. From Fig. 2 it is evident that there are four distinct activities namely concept development, virtual prototyping, physical prototyping and evaluation of the tool by making prototype parts are involved in production of tooling for sheet metal forming applications. Conceptual Design & Idea Part Design SL Model Model Building: SLA 3D Tool Solid Modeling Quick CAST style SL model Tool Assembly Die Design Infiltrated Al. Filled Epoxy Drawing Process Machining Process and Hand Working Sheet Metal Components Concept Development Virtual Prototyping Physical Prototyping Evaluation Fig. 2. 3D CAD SOLID MODELLING The three-dimensional solid models of the tools were designed using I-DEAS Master Series TM software from SDRC. First the 3-D solid model of the part has been created then both tools punch and die are developed from solid model of the part (Fig. 3). In this process the solid model of the part is used to develop a 3D CAD modeling that has been used to manufacture Tooling Manufacturing Stages punch and die (Fig. 4). While developing 3D CAD solid models and preparing STL files, two aspects need to be considered. Formerly, the resolution of the curved surfaces should be good enough to reproduce the part accurately. The CAD package estimates curved surfaces with polygons (usually triangles and rectangles in shape). The number of polygons used affects the smoothness of the resultant SL model (model resolution). I-DEAS CAD software allows to * ismetpi@attglobal.net or polman@melsa.net.id 3

4 set the number of polygons by setting 'Iso-line Density' and a 'Facet Deviation' for the CAD solid models. Settings for the Quick tool were as follows; the iso-line density was set to 4 (maximum) and the facet deviation to These produced 97,000 and 95,000 polygons for punch and die respectively. The second consideration is that the part should be scaled to fit the working space (envelope) of the SLA. This envelope represents the length of the sides of the cube on the machine. Based on the size of the tool, the SLA 500 machine was selected to build the model. Scaling can be difficult since the SLA cannot build any horizontal feature smaller than mm (due to the laser beam diameter). PART DESIGN "Left Hand Guard", part of the land mower (Fig.3), was selected as the trial part to evaluate the prototype tools. However, several modifications have been made due to varies of the tools required to produce the original part. The part modifications include: scaling the original part by 50%; redesigning some features on the original part by excluding cutting features such as trimming, piercing, and blanking; the part s material has been changed to aluminum. The geometry of the part indicates that the forming process will consist of stretch forming and bending. Therefore, the arrangement of punch, die and blank holder is similar to drawing operations. Since the geometry of the part is considerably irregular the required drawing forces and other tool parameters have been estimated using empirical data. In this Quick tool, the punch and the die were designed as inserts which was generated directly as the punch and die from 3D CAD solid models as shown in Fig. 4. Maestro is software that guides through the preparation stages that are required to build the Stereolithography pattern using SLA (Fig. 5). The solid model of the tool was converted into STL format and then sliced into individual layers. A compromise between accuracy and the build time often needs to be achieved. Also, when layer are too thin they tend to warp adding to inaccuracy in z direction. Taking the above issues into account the thickness of each layer was set to 0.15 mm. Any overhangs of the SL pattern need to be supported in the build process. This is done using Maestro package that pinpoints any area of the overhanging features of the part and incorporates a support structure underneath them. The support structure is then removed before the final curing. Die Punch Fig. 3. 'Left Hand Guard' CAD Package "View" Fig. 4. SL 'Quick CAST' Model CAD File Verification Output STL File and Verify STL File "3dverify" Model Orientation MAESTRO Slicing Building Style (ACES or QuickCast) Support Generation Die Punch Merge Part and Support Data Build File SLA Build SL Model Fig. 5. SL Model Building Procedures in SLA * ismetpi@attglobal.net or polman@melsa.net.id 4

5 GEOMETRY AND TOOLING ELEMENTS DESIGN CONSIDERATION Depending on the applications the scheme of the SL model is determined. In general, the greater the percentage of resin which is cured in the initial SLA build, the less overall shrinkage and warpage will occur in the post cure process. The model for the Quick Tool has been manufactured using Quick Cast style. The internal structure is similar to a honeycomb while the external surfaces are built in form of a tough thin shell made of three skin layers. Ciba Geigy SL 5170 photopolymer epoxy resin has been used for prototype development of the tool. No data on the compressive strength for this material could be found, hence appropriate tests have been conducted. Test specimens have been build using Quick Cast. The Quick Cast specimens were infiltrated with aluminum filled epoxy thus representing the properties of the Quick tool material. The ASTM D3410M - 95 test procedures have been followed. The ultimate compressive strength of SL 5170 was found to be MPa. PRODUCTION OF THE QUICK TOOL The tool consist of direct SL Quick Cast Model and a it was then infiltrated with high-strength aluminum filled epoxy as a backing (support) material, in order to increase the strength of the tooling elements, and; a standard press die set components of steels for containment. The tool production and its layout for the production of the required part are shown in the Fig. 6. The aluminum filled epoxy is an easily cast material which can fill the honey comb structure to provide a backing and uniform support to the structures. Both the punch and the dies are contained and aligned by the steel frame forming a very rigid structure. In the case of direct tooling Quick Cast technique was used to produce the punch and die for the press metal forming. The SL model is developed by implementing a "Quick CAST" (QC) building style. It looks like a honeycomb semi hollow part with a tough thin shell around it. In developing QC tool, an important consideration given regarding to model quality; Liquid resin inside the semi hollow must be drained out in order to overcome with model deformation during postcuring. After it has been postcured, the QC model then was back-filled with aluminum filled epoxy, under a vacuum, through the holes. In this stage, one important consideration is to get rid of any bubbles that may occur inside the tool because it may effect the tools in forming process. The resultant models were used as inserts (punch and die) of the press tool to make the aluminum part. This process of producing direct tooling is illustrated in the Fig. 6. The model produced by this process is have a tendency of distorting after post-curing. The deforming features of the distorted model are machined in order to bring back the original geometry and dimensional accuracy of the model. 1. Part / Product 2. SL "Quick Cast" Model 3. Mounting to Die Sets Punch Block Die Block Die Sets Fig. 6. Production of Quick Tool RESULTS AND DISCUSSIONS The Quick tool was developed by utilizing a direct RP&T technique. In other words, there was no secondary step involved. The tool utilizes a direct SL model, built in Quick Cast building styles, and back filled with the aluminum filled epoxy. The press dies for this Quick tool was designed and operated with blank holder. However, there was still limitation in the aspects of press die design, especially in spring construction and calculation. Since the shape of the part was considered irregular the calculation of spring * ismetpi@attglobal.net or polman@melsa.net.id 5

6 forces used for holding the blank were not accurate. The inaccuracy in the calculation leads to selection of springs which were not strong enough to hold and to flatten the wrinkle blank at deeper side. Therefore, the spring forces applied on this side of the part should be constructed stronger than the other side. Examination of the final products revealed that the product quality need to be improved as well as tooling parameters, such as radius, die ring radius, and punch - die clearance. Furthermore, the part geometry and shape also create a difficulty in determining the tooling parameters accurately. It was evaluated that the Quick tool performance shows an impressive result in terms of tooling development. In terms of the tool material, the tool prototype seems to become potential tool in sheet metal forming process because the Quick tool is economical. The process involved in this tool development and production is relatively simple and shorter. The Quick tool can be developed and built in SLA machine directly from 3D CAD model. CONCLUSIONS Using rapid prototyping the time and cost saving can be achieved in the production of tooling of sheet metal forming applications. It can be realized that the SL parts could be used as low scale production parts, significantly cutting time through by passing the detailed process of die design and die making. The results and limitations that were found in this research brought the other potential aspects which need to be not addressed. The aspects that need to be addressed in future study are: The first aspect is in relation to the product quality improvement and the verification of tool and die design, including the improvement in drawing parameters and operation. The second aspect concerns variables involved in evaluating these tools so that it can be utilized for steel sheet instead of aluminum. The outcome of this present research forms a sound basis for ongoing further development that will involve a large variety of RP&T technologies and techniques for tooling development. REFERENCES 1. Dickens, Philip M. (1996) Rapid Tooling: A Review of the Alternatives. Rapid News, 4 (5), Jacobs, P.,1996, Stereolithography and other rapid prototyping & manufacturing technologies, 3. Tromans, Graham and Wimpenny, David Ian. (1995) Rapid Tooling - The Future for Industry, Rapid News, 3 (3), Flint, R. and Ellis, D. (1994) Secondary Tooling Using Rapid Prototyping, Proceeding of the SME Rapid Prototyping & Manufacturing Conference, SME Dearborn, MI. * ismetpi@attglobal.net or polman@melsa.net.id 6 5. Seybert, Arlene. (1996) A Prototype and Short Run Casting Process for the 21 st Century, Rapid News, 4 (5), Sarkis, Bassam. (1996) Rapid Prototyping for Non-ferrous Investment Casting, Rapid News, 4 (3), Arthur, A., Dickens, P., and Cobb, C. (1996) "Using rapid prototyping to produce electrical discharge machining electrodes", Rapid prototyping journal, vol. 2, no.1, pp Prasad K.D.V. Yarlagadda, Periklis Christodoulou and Vijay Subramanian (1999) "Feasibility Studies on Production of Electro Discharge Machining Electrodes by Stereolithography and Electroforming Process", Journal of Materials Processing Technology, 89-90, pp S.J. Dover, C.E.Bocking and G. Bennett (1995) Proc. Of 1 st National Conference on Rapid Prototyping and Tooling Research, Ed. G. Bennett, Mechanical Engineering Publications Limited., Williams, R.E., Komaragiri, S.N., Melton, V.L., Bishu, R.R. (1996) Investigation of the Effect of Various Build Methods on the Performance of rapid prototyping, Journal of Materials Processing Technology, 61, pp Bocking, C., Bennett, G., Dover, S., Arthur, A., Cobb, C. and Dickens, P. (1997) "Electrical routes for engineering tool production", The GEC Journal of Technology, 14, pp Cobb, C. and Reeves, P. (1995) "Improvement in the surface finish of Stereolithography models for manufacturing applications", Proceeding of the 1 st national conference on rapid prototyping and tooling research, UK, pp Murakami, T., Kaminura, A. and Nakajima, N. (1997) "Refrigerative Stereolithography for rapid prototyping", Rapid product development, pp Thomas, S. (1992) "Stereolithography simplifies tooling for reinforced rubber parts", Mechanical engineering, 114, pp