Rapid Prototyping and. Reverse Engineering

Transcription

1 Rapid Prototyping and Reverse Engineering

2 A rapid prototyping primer Part 1: Stereolithography...4 Rapid Prototyping...4 Stereolithography...4 The process...4 Supports...5 Post-processing...6 Speed...6 Materials...7 Machine manufacturers...8 When to use stereolithography...8 Factors to consider when ordering...8 A rapid prototyping primer Part 2: laser sintering...9 The process...9 Supports...10 Post-processing...10 Machine makers...10 Materials...11 Metal...12 Speed...12 When to use laser sintering...13 Factors to consider when ordering...13 A rapid prototyping primer Part 3: fused deposition modeling...13 Fused deposition modeling...14 Equipment manufacturers...15 Supports...16 Accuracy and surface finish...16 Materials...16 Speed...17 When to use fused deposition modeling...17 Factors to consider when ordering...17 A rapid prototyping primer Part 4: Three-dimensional printing...17 Three-dimensional printing...18 Speed...18 Materials...19 Post processing...20 Accuracy and surface finish...20 Color...20 When to use three-dimensional printing...21 Factors to consider when ordering...22 Rapid prototyping primer -- Part 5: Multijet modeling...23 Supports...24 Material...24 Accuracy and surface finish...24 Speed...25 Post processing...25 When to use multijet modeling...25 On the horizon -- Invision...26 Preparing data for rapid prototyping...26 Service-bureau recommendations...28 CATIA version four

3 Pro/Engineer...28 SDRC I-DEAS...29 SolidEdge...29 SolidWorks...29 Rapid prototyping large objects -- from parts to art...30 The Materialise Mammoths...30 Virtual medical modeling and rapid surgical templates...32 Reverse Engineering...34 Capturing scan data...34 Read scanned data...34 Align point clouds...34 Simplify the data...36 Polygon meshing...36 Polygon editing...37 Defining surface boundaries...37 Applying NURBS...39 Exporting the data...39 God is in the details...39 Reverse-engineering update...39 Much improved...40 Picking the right mix...41 Software...41 Caveat emptor...44 UNDERSTANDING THREE-D SCANNERS AND DIGITIZERS Part Coordinate-Measuring Machines...45 Scanners...46 Laser Scanners...47 White-Light Scanners...47 Finicky About Finishes...48 Computed Tomography...48 Destructive Cross Sectioning

4 A rapid prototyping primer Part 1: Stereolithography Rapid Prototyping Rapid prototyping is helping many companies to shave days, weeks, or even months off the time required to bring new products from concept to market. Moreover, rapid prototyping technologies have improved greatly in recent years. God is in the details, and rapid prototyping equipment makers have worked hard to get the details right. Today stereolithography, selective laser sintering, inkjet printing, and fused deposition modeling all produce good parts with relatively little hassle. Each technology has strengths and weaknesses, and it s a mistake to think that all rapid prototypes are created equal. Some are more durable than others. Some are more accurate. Some are more resistant to heat. Some are cheaper. In order to choose the rapid prototyping system best suited for each application, it s necessary to understand the basics of each technology. Over the coming weeks, we ll present primers on stereolithography, laser sintering, inkjet, and fused deposition modeling, and discuss how the mechanics of each affects characteristics of the parts it produces. Stereolithography Stereolithography is the process most people picture when they think of rapid prototyping. Perhaps the most complex of all rapid prototyping technologies, it was the first to emerge as a commercial enterprise and is still the most widely used. Stereolithography builds three-dimensional objects by exposing and hardening thin layers of photocurable resin. It produces parts with good surface finish and detail but with limited physical properties. Parts won t withstand excessive heat or moisture and tend to be brittle. They are, however, durable enough to be used as patterns for duplication processes such as silicon-rubber molding. Solid stereolithography parts don t work well as investment-casting patterns because they expand during burnout and crack investmentcasting shells. But parts built with a lattice-like internal structure work well as long as they contain no thin walls or delicate features. The process The basic process of stereolithography begins by creating an STL file -- the standard input format for all rapid prototyping. (See Preparing Data for Rapid Prototyping) The STL file consists of a closed mesh of triangles generated from the surfaces of a CAD model. Software in the stereolithography system slices the STL file into a series of horizontal cross sections. These sections, solidified one on top of another, form a stereolithography prototype. Stereolithography machines build each layer by tracing the pattern of a cross-section on the surface of a vat of liquid resin. Wherever the laser touches, the resin absorbs light energy and polymerizes from liquid to solid. A platform suspended in the vat supports the part. After each layer is built, a mechanism lowers the platform into the vat by one layer s thickness -- typically to inches (0.025mm to 0.125mm) to allow the next layer to be scanned. Between layers, a recoater blade sweeps across the surface of the vat to smooth liquid resin across the top of the growing part. This process is repeated -- solidifying a layer, lowering the platform, smoothing the surface -- until the entire part is built. 4

5 Supports Because stereolithography builds parts by drawing on the surface of the liquid resin, overhanging areas that aren t directly above a previous layer must be connected with support structures either to the part below or directly to the bottom of the building platform so that they won t sag or float away. Stereolithography builds both a part and any necessary supports from the same material. After a part is finished the supports must be cut away. A large stereolithography part built by a 3D Systems SLA-7000 stereolithography system. Note the supports connecting the bottom of the part to the building platform. 5

6 Post-processing When a part is complete, the building platform is raised from the vat and the excess resin allowed to drain. At this point the part is not fully cured. After a hand-wiping to remove any of the viscous liquid resin that might still be on the surface, the part is placed in an agitated bath of solvent or wiped by hand with solvent to remove any additional uncured resin. It s particularly important to make sure all such resin is removed from narrow passages. Otherwise it could solidify during post-curing. Technicians who clean the parts must wear gloves, masks, and eye protection to avoid contact with the uncured resin, which can cause allergic reactions. (See Cleaning stereolithography parts.) Once a part is completed, it is carefully removed from the stereolithography system, washed with solvent to remove any excess resin, and then post-cured with light and heat. After cleaning, the so-called green part must be post-cured to complete polymerization of the resin and fully harden it. In a special chamber called the post-curing apparatus (PCA), the part is exposed to light from ultraviolet lamps and sometimes, depending upon the resin, heat from infrared lamps. Post-curing takes between one and ten hours depending upon the part size. After post-curing, any supports can be cut away and the surface of the part finished by sanding or bead blasting. Parts built with thinner layers require less sanding to remove the stairsteps between layers. However, thinner layers increase part building time. Speed Since a stereolithography machine s laser solidifies each layer one point at a time, stereolithography builds parts more slowly than some rapid prototyping technologies, particularly those that deposit materials with multiple jets or build parts from sheets of material. Between each layer, the stereolithography machine must pause in order to let the liquid resin settle before building the next layer. The time needed to build a particular stereolithography part, therefore, is dependant upon the height of the part (the number of layers), the complexity of the part (the time required to scan each layer), and the wait time 6

7 needed between layers to level the liquid resin (determined by the resin s viscosity). Postcuring time is generally determined by the overall part size. Materials The resins used in stereolithography -- exotic materials that harden when exposed to light energy -- both make stereolithography possible and also limit it. Perhaps the most impressive advancement in stereolithography technology over the last few years has been the improvement in the physical properties of resins. Unlike users of other rapid prototyping systems who are required to buy proprietary materials supplied by the machine manufacturers, stereolithography users may purchase resins from competing vendors including 3D Systems, DSM Somos, Vantico, and others. While the properties of proprietary materials used by other rapid prototyping technologies have remained fairly static over the years, competition among stereolithography resin manufacturers has resulted in a vast improvement in resin properties. Early stereolithography parts were so brittle that they rarely survived a design review meeting, and they often warped badly. But resins now deliver properties that approach those of such popular plastics as polypropylene, polystyrene, and polyethylene. Most resins build opaque parts, but some now make it possible to build parts that can be finished to optical clarity. (See 2002 Rapid Prototyping Materials Roundup.) Resin manufacturers often offer incentives to entice customers into switching resins. While stereolithography resins are still some of the most expensive materials used in rapid prototyping, competition has helped keep prices down. 3D Systems SLA-7000 stereolithography system 7

8 Machine manufacturers 3D Systems is the largest and most successful manufacturer of stereolithography equipment, although Japanese companies such as Teijin Seiki and Sony/D-MEC also make stereolithography systems, some of which soon may be available in the United States. (See 3D Systems settles with justice department.) 3D Systems machines range in capacity and price from the SLA Viper Si2, which starts at $179,000 and can build a part as large as a ten-inch cube (254mm), to the SLA-7000, which costs $799,000 and can build parts as large as 20 inches long, 20 inches wide, and 23.6 inches tall (508mm by 508mm by 600mm). (See 2002 U.S. Rapid Prototyping Equipment Roundup and Rapid Prototyping Outside the U.S.) When to use stereolithography Stereolithography isn t the fastest rapid prototyping technology, so it is not the best choice if you want the fastest possible delivery. Stereolithography uses expensive materials that are difficult to handle, so parts tend to be more costly. Photopolymers have limited physical properties. If you need high strength or high temperature resistance or you plan to submerge parts in water, stereolithography must be combined with secondary processes such as silicon-rubber molding or investment casting using 3D Systems Quickcast building style. Stereolithography remains the technology of choice for engineers seeking high accuracy and detail. For the most accurate, best looking parts that don t need to survive rigorous physical testing, stereolithography is ideal. Factors to consider when ordering When placing an order for stereolithography parts, here are factors to consider: Part size -- Can your part be built as a single piece in your service provider s stereolithography machine? Large parts can be built in pieces and then glued together but this takes extra time and can degrade accuracy. Feature size -- Does your part have any features that are smaller than the layer thickness. If so, be sure and tell your service provider so that the part can be built with thinner layers. Type of resin -- There are a variety of resins with varying mechanical, waterresistant, and optical properties. See the 2002 Rapid Prototyping Materials Roundup. Support removal -- Be sure part supports can be positioned for removal and that stubs are accessible for sanding if necessary. Layer thickness -- Thinner layers can eliminate the need for hand-finishing, but they require more machine time, which is costly. Secondary processes -- Silicon-rubber molding or investment casting using 3D Systems Quickcast building style add cost and take more time but can provide very good mechanical or chemical properties. Each rapid prototyping service company has unique policies and practices. Discuss your requirements and available materials with your rapid prototyping supplier before placing an order. 8

9 A rapid prototyping primer Part 2: laser sintering Choosing the best rapid prototyping technology for a job can be confusing. Rapid prototyping methods are lumped together as a class because they build complex parts without either tools or the complex programs and machine setups required for numerically controlled milling. But various rapid prototyping technologies employ different methods and materials to fabricate parts. Stereolithography, the most widely used rapid prototyping technology, excels at building accurate parts with good surface finish and detail. However, stereolithography parts have limited physical properties. When more durable parts are required, laser sintering often can deliver better results. The process Laser sintering is one of the most straightforward rapid prototyping technologies. Laser sintering builds three-dimensional objects by heating and fusing together grains of powder. Like most rapid prototyping processes, laser sintering builds parts in thin layers stacked on top of each other. Laser-sintering machines build each layer by first spreading a shallow layer of powder onto a building platform and then scanning a cross section of the part on the powder with a CO 2 laser. Everywhere the laser touches, its energy heats the powder grains, causing them to adhere to their neighbors. Once a layer is complete, the building platform, which is mounted on a moveable piston, lowers by a single layer s thickness inches ( mm) and a new layer of loose powder is spread across the top of the growing part. As in stereolithography, the part cross-sections are derived from an STL file -- the standard input format for all rapid prototyping. (See Preparing Data for Rapid Prototyping.) The STL file consists of a closed mesh of triangles generated from the surfaces of a CAD model. Software in the laser-sintering system slices the STL file into a series of horizontal cross sections that define the laser s pattern for each layer. In order to produce accurate parts, it s critical to control the amount of energy transferred as the laser scans the powder. If the powder grains soften too much and begin to flow like liquid, shrinkage may become unpredictable. If the powder isn t heated enough, coherent layers won t form. The building chamber is heated to just below the melting temperature of 9

10 the powdered material so that only a slight amount of extra energy from the laser will sinter the powder grains. Heating the chamber also reduces the thermal shrinkage of the layers during fabrication. The chamber is filled with nitrogen to reduce the hazard of explosion that is present when handling flammable powder in air. Supports Parts built by laser sintering don t require supports. The unsintered powder surrounding previous layers supports isolated or overhanging areas of a layer as it is built. When finished, a sintered part is encased in loose powder. Post-processing Sintered parts require no special post-processing. Once a sintered part is complete, the piston supporting the build platform rises and the unsintered powder surrounding the part can be scraped and brushed off and put back into the powder-supply bin for reuse. The warm part is then removed from the sintering system and transferred to a rack or table where it must be allowed to cool for several hours. After cooling, any remaining loose powder can be removed using compressed air, brushes, or other tools. Parts can be used as soon as they have cooled and the excess powder has been removed. Because they are constructed of a lattice of fused powder grains, however, sintered parts are porous and have a relatively rough surface finish. Parts can be infiltrated to improve strength and reduce surface roughness. Sanding and painting also can improve the smoothness of the surface of a sintered part. Laser-sintered part. Note the rough surface. finish. Machine makers Two companies manufacture laser-sintering equipment. In the United States, laser sintering was developed by the DTM Corporation, which was purchased by 3D Systems in D Systems sells the Vanguard laser-sintering system, which is capable of building parts as large as 15 inches by 13 inches by 18 inches (375mm by 330mm by 450mm) for $320,000 to $370,000. Electro Optical Systems GmbH (EOS) of Germany 10

11 also sells several models of sintering machines. The smallest is the EOSINT P380, capable of building parts as large as 13 inches by 13 inches by 24 inches (340mm by 340mm by 620mm). It costs approximately $300,000. The largest is the dual-laser EOSINT P700, which can build parts as large as 28 inches by 15 inches by 23 inches (700mm by 380mm by 580mm) and costs $750,000 to $850,000. Recently EOS said it was planning to sell systems in the United States. To date, however, we have not heard that this is occurring. 3D Systems Vanguard system. Materials Perhaps the greatest advantage of laser sintering is the broad range of potential materials. (See 2002 rapid prototyping materials roundup. ) Theoretically, any material that can be powdered and melted might be used. Commercially available materials include polycarbonate, nylon, glass-filled nylon, polyamide, acrylic-based polymer, elastomeric polymer, zirconium and silica sands, and polymer-coated steel and metal-alloy powders. In practice, the most commonly used materials are nylon and glass-filled nylon powders. While sintered nylon parts don t have the same physical properties as those made by injection molding, they are sufficiently durable to be used for physical testing and some end-use applications. NASA, for example, has used sintered parts in spacecraft production (See Selective laser sintering parts used directly in spacecraft production ), and Siemens is currently using laser sintering to manufacture custom hearing aids. (See Laser Sintering makes custom hearing aids. ) Most laser-sintering materials are available only from the equipment manufacturers. Companies who have attempted to use second-source materials have been sued. Over the years, this has inhibited innovation in materials development and kept prices high. Laser- sintering system manufacturers have developed some innovative materials but have pursued only the most popular. Nevertheless, some specialty materials do exist. DSM Somos, the maker of popular stereolithography resins, offers a flexible sponge-like material for laser sintering. 3D Systems sells a polystyrene material called CastForm for 11

12 making investment-casting patterns. And EOS sells binder-coated sands for building casting patterns. Metal Both EOS and 3D Systems have developed methods to make metal parts with laser sintering. 3D Systems offers RapidSteel materials that consist of stainless-steel powders coated with binder. The binder coating on the powder grains melts during sinter to create a green part. After a part is built, the binder is burned away over several hours. With the binder gone, the part is delicate -- held together primarily by friction between the steel particles. In a second oven cycle, the part is placed in a crucible with bronze ingots. As the crucible is heated the bronze melts and flows into the part s pores via capillary action. EOS has developed a direct-metal sintering material that combines metal powders of different melting temperatures but no organic binders. During sintering the components with lower melting temperatures melt and flow into the pores of the part. EOS direct-steel parts don t require a separate infiltration step. An EOS DirectMetal part. Speed As in stereolithography, a sintering machine s laser solidifies each layer one point at a time and thus builds parts more slowly than rapid prototyping technologies that deposit materials with multiple jets or build parts from sheets of material. Between each layer, the sintering machine must pause while the build platform lowers and a new layer of powder is spread. Building speed also is determined by the melting temperature of the material being used. Materials with lower melting temperatures are processed more rapidly than those with higher melting temperatures. The time needed to build a particular part, therefore, is dependant upon the height of the part (the number of layers), the complexity of the part 12

13 (the time required to scan each layer), and the scanning speed of the laser (determined by the material s melting temperature). When to use laser sintering Laser sintering is best suited to applications requiring parts with superior material properties. Although laser sintering materials are more expensive than would seem necessary -- they are, after all, simply ground up plastic or metal -- sintered parts are strong and resist weather, chemicals, and heat. Because sintered parts don t require supports that must be removed after building, it s possible to create complex parts with inaccessible internal features that would be impossible to make with stereolithography. Laser sintering also is the only commercial rapid prototyping technology that can build complex metal parts, although the metals used are alloys that don t deliver the same properties as cast, forged, or milled steel. Sintered parts, because they are made of powders, tend to have a grainy surface finish and need extra sanding and perhaps infiltration or painting if a smooth finish is required. Factors to consider when ordering When placing an order for sintered parts, think about: Part size -- Can your part be built as a single piece in your service provider s machine? Large parts can be built in pieces and glued together but this takes extra time and can degrade accuracy. Feature size -- Does your part have any features whose size approaches the building-layer thickness? Sintering systems can have problems producing very small features. Type of material -- Available materials include: polycarbonate, nylon, glass-filled nylon, polyamide, acrylic-based polymer, elastomeric polymer, zirconium and silica sands, and polymer-coated steel powder. For details about these, see the 2002 Rapid Prototyping Materials Roundup. Layer thickness -- Thinner layers can reduce the need for hand-finishing, but they require more machine time, which is costly. At their best, laser-sintered parts won t match the surface finish of those produced by stereolithography. Finishing -- Be sure to specify what kind of finish you want or are willing to accept. If you require a smooth surface, you may need to have sintered parts painted. Each rapid prototyping service company has unique policies and practices. Discuss your requirements and available materials with your rapid prototyping supplier before placing an order. A rapid prototyping primer Part 3: fused deposition modeling Many companies see the advantages of bringing rapid prototyping technologies in house in order to produce parts for design review, fit testing, and functional prototypes. But as discussed in the first two parts of this series, stereolithography and laser sintering systems can t be operated in offices. Stereolithography machines use messy liquid chemicals that must be handled with caution. (See A rapid prototyping primer Part 1: stereolithography. ) Laser sintering employs fine-grained metal or plastic powders that can be messy. (See A rapid prototyping primer Part 2: laser sintering. ) Both processes employ powerful lasers that can cause eye damage. These characteristics dictate that stereolithography and lasersintering systems be installed in dedicated facilities apart from engineering offices. 13

14 Fused deposition modeling For companies that want to produce their own rapid prototypes but don t want the expense of setting up a dedicated rapid prototyping shop, fused deposition modeling (FDM) can provide a good alternative. Compared with stereolithography or laser sintering, fused deposition modeling seems downright low-tech. Instead of lasers and space-age materials, FDM builds by extruding molten plastic that hardens layer-by-layer to form a solid part. The building material, in the form of a thin solid filament, is fed from a spool to a movable head controlled by servomotors. When the filament reaches the head, heaters melt it. The molten material is then extruded through a nozzle onto the part surface. As the molten material is extruded, it is flattened by the nozzle the way a plumber or painter uses the tip of a caulking gun to spread the material. The width of the road (in FDM parlance) can range between and inches (0.193 and mm) and is determined by the size of the extrusion nozzle. Nozzles can t be changed during building, so the model resolution must be chosen in advance. As the molten material is dispensed, it quickly cools -- within a tenth of a second -- and solidifies. When a layer is completely covered, the building platform, mounted on a moveable stage, descends by one layer thickness, typically inches to inches (0.178mm to 0.356mm), and the process is repeated. As in stereolithography and laser sintering, FDM systems read STL files -- the standard input format for all rapid prototyping. (See Preparing Data for Rapid Prototyping.) The STL file consists of a closed mesh of triangles generated from the surfaces of a CAD model. Software in the FDM system slices the STL file into a series of horizontal cross sections that are traced by the material-extrusion head. In order to build accurate parts, it is critical to control the temperatures of both the building chamber and the growing part. The temperature of the chamber must be kept just below the melting temperature of the material so that only a small amount of heat is required to 14

15 melt the filament sufficiently for extrusion and the growing part won t sag or otherwise deform. The part must be kept cool enough that the molten material solidifies upon contact. 3 A close-up of the extrusion head in Stratasys' Maxum FDM machine. (Click image for a close-up.) Equipment manufacturers The only maker of fused deposition modeling equipment is Stratasys of Minneapolis, Minnesota. Stratasys offers a range of FDM systems. The affordable Dimension (sold by a distributor network) builds parts as large as eight inches by eight inches by 12 inches (200mm by 200mm by 300mm) and costs $29,900. The giant FDM Maxum, which has a building capacity of 23.6 inches by 19.7 inches by 23.6 inches (600mm by 500mm by 600mm), costs $250,000. (See the U.S. rapid prototyping equipment roundup ) Stratasys $29,900 Dimension, like all FDM systems, can be operated in an office environment. 15

16 Supports Overhanging or isolated features of FDM parts require temporary support during building. FDM machines use a second nozzle located next to the primary nozzle to extrude support material where necessary. Stratasys offers two types of supporting material: a wax-like material for its lower-priced machines and a water-soluble material for its more expensive ones. The wax supports can be broken away from a part but are difficult to remove from internal passages or small features. The water-soluble supports can be dissolved in an agitated bath. Other than support removal, FDM parts require no additional postprocessing. Accuracy and surface finish Stratasys claims its FDM systems deliver accuracy of +/ inches (0.127mm). The surface finish of FDM parts is not as good as that of parts made with stereolithography but ranks with those produced by laser sintering. Whereas sintered parts have a grainy texture, FDM parts have a ribbed appearance because both the horizontal layers and the roads that make up the layers remain visible. Materials Over the years, Stratasys has developed a number of materials for its FDM systems. The one that produces the highest-quality parts is acrylonitrile butadiene styrene (ABS). Other materials include wax for investment-casting patterns, polycarbonate for tough parts, and polyphenyl sulfones for high-temperature applications. FDM parts, because they are made of material that has been melted and solidified, exhibit physical properties close to those of parts made of similar materials by conventional methods. (See the 2002 rapid prototyping materials roundup. ) Vacuum cleaner-maker Oreck uses FDM parts for extended physical testing of new designs. 16

17 Speed Because FDM systems build parts by drawing a narrow bead of material, large or solid parts or those with thick walls can take a long time to complete. Small or thin-shelled parts can be built fairly rapidly. The time needed to make any part depends upon the deposition rate of the FDM system (determined by nozzle size), the height of the part (the number of layers), the gross size of the part (the time required to deposit each layer), and the number and complexity of supports required (supports for each layer are built in a separate step). When to use fused deposition modeling FDM parts are tougher and more durable than those produced by stereolithography, but they have an inferior surface finish and sharpness. Stratasys ABS, polycarbonate, and polyphenyl sulfone materials are sufficiently resistant to heat, chemicals, and moisture that FDM parts can be used for limited to extensive functional testing, depending upon the application. FDM parts aren t porous and don t require secondary infiltration. Unlike stereolithography or laser sintering, FDM machines can be used in an office environment and FDM materials don t require any special handling. Most FDM systems cost less than comparable stereolithography or sintering systems. For companies wanting to produce durable and accurate prototypes in house, FDM can be a good choice. Factors to consider when ordering When placing an order for fused deposition modeling parts, consider: Part size -- Can the part be built as a single piece in your service provider s machine? Large parts can be built in pieces and then glued together, but this takes extra time and can degrade accuracy. Support removal -- Will your part have any supports in inaccessible areas? If so, specify that water-soluble supports should be used if possible. Feature size -- Does your part have any features whose size approaches the building-layer thickness? FDM systems can have problems producing very small features. Layer thickness -- Thinner layers can reduce the need for hand-finishing, but they require more machine time, which is costly. Increasing bead size and layer thickness can speed the building process considerably but at the expense of part detail. Finishing -- Be sure to discuss with your service provider what kind of finish you want or are willing to accept. If you require a smooth surface, you may need to have FDM parts painted. Each rapid prototyping service company has unique policies and practices. Discuss your requirements and available materials with your rapid prototyping supplier before placing an order. A rapid prototyping primer Part 4: Three-dimensional printing October 31, In the first three articles of this series, we discussed the pros and cons of stereolithography, laser sintering, and fused deposition modeling. Each of these technologies has advantages and drawbacks: Stereolithography produces parts with high accuracy but limited durability. Sintering creates parts with good material properties but mediocre surface finish. Fused deposition modeling provides an economical way in an office environment to make parts that are durable but with limited accuracy. The ultimate aim of the aforementioned technologies is to make prototypes -- accurate parts that are 17

18 sturdy enough to be used for at least limited functional testing. Although parts made by these technologies cost a fraction of similar prototypes made with traditional methods, they can still cost hundreds or thousands of dollars. At these prices, many companies don t bring rapid prototyping into play until a design is nearly complete. Concept modelers, on the other hand, produce relatively crude parts cheaply and quickly. Concept models are typically used early to evaluate a product s design. Whereas a prototype simulates how a product will function, a concept model is the equivalent of a three-dimensional sketch. One industrial designer tells us that his concept models often have a useful life of only minutes before they hit the trashcan -- just long enough to tell him whether or not he is on the right track. Three-dimensional printing The most popular concept-modeling systems are those based on three-dimensional printing (3DP), a process developed by the Massachusetts Institute of Technology and licensed to Z Corporation of Burlington, Massachusetts. Z Corporation offers a range of 3DP machines. The Z400, which costs $33,500 and can build parts as large as eight inches by 10 inches by eight inches (200mm by 250mm by 200mm) is the least expensive. The Z406, with the same build envelope, sells for $66,500 and offers greater speed. The Z810 with a maximum part size of 16 inches by 20 inches by 24 inches (400mm by 500mm by 600mm) and automated production features lists for $175,000. (See A closer look at the Z810 for details.) As with other rapid prototyping systems, the input for three-dimensional printing is a CAD file that has been converted into the tessellated STL format. (See Preparing data for rapid prototyping. ) Software in the 3DP system slices the STL model into a series of horizontal cross sections. 3DP machines build parts by printing the cross sections in liquid binder onto layers of powder. To start the process, a thin layer of powder -- between inches and inches thick ( mm)-- is spread by a roller onto a platform mounted on a piston in a building box. Multiple inkjets sweep across the surface of the layer and print the image of a single cross section onto the powder. Where the binder is printed, the powder is glued together. Loose powder surrounding the printed area remains in place and supports the part as it is built. Once a layer is complete, the piston lowers the build platform by one layer thickness and the process is repeated. The only time required between layers is the time needed to spread each new layer of powder. When a part is finished, the piston raises the build platform and any excess powder can be brushed or vacuumed off the part. Speed Because the printheads sweep across the platform in a raster pattern and dispense binder from multiple jets simultaneously, three-dimensional printing is very fast, as much as 40 times faster than other rapid prototyping technologies. (In raster printing the printheads sweep back and forth rather than drawing patterns.) Every layer, regardless of complexity, takes the same amount of time to print. Building speed is approximately one layer every 30 seconds. According to Z Corporation, the Z810 can build an automotive manifold in about four hours and a full-sized engine block in about 16 hours. The Z406 can build six parts the size and shape of a soda can in less than two hours. 18

19 Materials Z Corporation offers two material systems. The fastest and least expensive is a starchbased powder. Parts made of this material can be used as investment-casting patterns. More durable parts can be built using a plaster-based powder. (See 2002 rapid prototyping materials roundup. ) Total material cost is $0.30 to $1.65 per vertical inch of part. Once a 3DP part is finished, the build platform is raised and excess powder brushed and vacuumed away. 19

20 Post processing Because parts are surrounded by powder as they are built, no support structures are required. Starch parts, however, are very fragile and must be infiltrated with wax, cyanoacrylate, or two-part epoxy for added strength. Plaster parts are stronger but also require infiltration for extensive handling. The infiltration process can be messy. Parts are porous, and it is only necessary for the infiltrate to penetrate one-quarter inch to one-half inch in order to give the part a durable shell. Z Corporation sells an automated system for wax infiltration but cyanolacrylate and epoxy typically are applied with a cloth. Accuracy and surface finish The principal advantages of three-dimensional printing are speed and low-cost materials. Accuracy and surface finish are inferior to stereolithography and laser sintering. Because they are made of layers of powder, 3DP parts, like sintered parts, have a grainy texture. Once infiltrated, however, they can be sanded to achieve a smoother surface. 3DP parts have a grainy surface finish Color A unique capability of three-dimensional printing is the ability to produce multicolored parts. A part s color is determined by dyes added to the liquid binder. Almost any color is possible. Each layer, in essence, is like a full-color image printed on a flat sheet of paper by an ink-jet printer. 20

21 Multicolor 3DP parts can be built by mixing different colored inks in the liquid binder. When to use three-dimensional printing Three-dimensional printing is best used when speed and price are more critical than accuracy, surface finish, and material properties. 3DP parts are most often used as concept models but, when infiltrated, also can be finished and used as patterns for duplication. Parts built with starch-based material can be used directly as investmentcasting patterns. 3DP parts a have relatively poor surface finish. Small features and fine details are not replicated well. Thin walls can be problematic because all materials are brittle. A secondary use of three-dimensional printing is for making casting shells. Soligen Technologies of Northridge, California is a long-time M.I.T. licensee of 3DP for the production of ceramic investment-casting shells. Recently Soligen s license with M.I.T. became nonexclusive, and Z Corporation is now testing methods of building casting shells from its plaster material. One challenge Soligen has faced in the production of ceramic casting shells has been that the poor interior surface finish of molds produced by 3DP is transferred during casting to the cast metal parts. Instead of making single-piece shells as Soligen does, Z Corporation suggests making molds in pieces that are put together along with cores prior to casting, much like sand-casting molds are assembled. That way, the casting surface of each mold piece can be sanded or filled to remove stair steps. Z Corporation says shells built with 3DP deliver a surface finish and accuracy similar to traditional sand casting in a fraction of the time. 21

22 3DP casting shell produced by Z Corporation's Zcast process.. Factors to consider when ordering When placing an order for 3DP parts, consider: Part size -- Can the part be built as a single piece in your service provider s machine? Large parts can be built in pieces and then glued together, but this takes extra time. Moreover, the low accuracy of 3DP can prevent part sections from fitting together well. Feature size -- Does your part have any small features or thin walls? 3DP systems can have problems producing very small features, and thin walls are sometimes impossible to build. Accuracy -- Three-D printed parts have low accuracy. Consequently, it is difficult (although not impossible) to assemble parts made with 3DP. Threads are not replicated. Joining -- Because materials are brittle, 3DP parts are difficult to drill and tap for joining with conventional fasteners. Material -- Be sure to specify what material and infiltrate you want. Plaster powder is more expensive than starch but delivers better strength, detail, and surface finish. If you want to use your parts as investment-casting patterns or just want to get parts as cheaply and quickly as possible, go for the starch dipped in wax. Changing materials in a 3DP system is messy and takes time. Not all service providers will offer both starch and plaster. Finishing -- Be sure to discuss with your service provider the type of finish you want or are willing to accept. For concept models, it may not be necessary to spend the extra time required to get pretty parts. 22

23 Each rapid prototyping service company has unique policies and practices. Discuss your requirements and available materials with your rapid prototyping supplier before placing an order. Rapid prototyping primer -- Part 5: Multijet modeling Rapid prototyping systems such as stereolithography, laser sintering, and fused deposition modeling are designed to create accurate parts that are durable enough for at least limited functional testing. Concept modelers, on the other hand, produce relatively fragile parts cheaply and quickly. Where rapid prototypes are often used to evaluate how a part will perform, concept models typically are used early to evaluate a product s design. Just as there are multiple rapid prototyping technologies, there are also several processes for making concept models. The characteristics of concept models vary widely. To get the most benefit from a concept model, choose a process that makes parts matching your requirements. In our last installment, we discussed Z Corporation s three-dimensional printing (3DP). (See A rapid prototyping primer Part 4. ) Z Corporation parts are inexpensive but have relatively poor detail and surface finish. While 3DP is a good choice for building bulky parts, if you require small features or fine detail, 3D Systems multijet modeling might do a better job. 3D Systems ThermoJet multijet modeler builds parts as large as 10 inches by 7.5 inches by 8 inches (250mm by 190mm by 200mm). It costs $50,000. The ThermoJet builds parts by spraying a molten wax-like material in layers onto a platform using an eight-inch-wide print head containing 352 jets. Droplets from the jets measure just inches (0.025 mm) in diameter) so they cool rapidly and harden upon striking the model surface. When each layer is complete, the build platform lowers by one layer thickness -- typically inches (0.042mm) -- and the next layer is sprayed. 23

24 Supports Features of ThermoJet parts that are isolated or overhanging require supports. 3D s ThermoJet takes a unique approach to support-generation. Thin hair-like structures hold up all downward-facing surfaces. When a part is complete, the supports can be brushed away. Material The waxy materials used by the ThermoJet are not very strong. 3D doesn t provide much data on material properties, but parts generally are tough enough to withstand handling and shipping if properly packed. Because the materials are wax-based, ThermoJet parts can be used directly as investment-casting patterns. Accuracy and surface finish The real strength of ThermoJet parts is their detail and surface finish. 3D claims that resolution of the system is 300 by 400 dots per inch along the horizontal X- and Y-axes and 600 dots per inch in the Z (vertical) direction. The smallest feature or wall that can be built measures about inches (0.075mm) across. Surfaces where supports have been removed are quite rough but can be improved by polishing. 24

25 Thermojet parts have good surface finish and detail. The material is available in a number of colors including gray as pictured here. Speed Because the ThermoJet builds by dispensing material through hundreds of jets simultaneously, the process is relatively fast. Z Corporation s three-dimensional printers are faster, but ThermoJet systems can make even parts that fill the building volume in less than a day. Post processing Other than support removal, ThermoJet parts require no post processing and can be used as soon as they are built. Unlike porous parts produced by laser sintering or threedimensional printing, they can t be infiltrated with resins for additional strength. When to use multijet modeling MultiJet modeling is best for applications where part detail is more important than durability. It is also ideal for building parts to be used as investment-casting patterns. Because ThermoJets run cleanly and quietly in an office environment, the technology is ideal for small company use in-house. Material is loaded into the machine in cartridges. There is no messy powder or liquid. Support removal can be done over a wastebasket and requires no protective clothing. Factors to consider when ordering When placing an order for multijet modeling parts, consider: Part size -- Can the part be built as a single piece in your service provider s machine? It s not easy to glue waxy Thermojet parts together. 25

26 Build orientation -- Downward facing surfaces on multijet modeling parts are uneven after supports have been removed. Though much of this roughnes can be smoothed with solvent, you should indicate which surfaces are most important so the operator can orient them facing upwards. Finishing -- Be sure to discuss with your service provider the type of finish you want or are willing to accept, especially on downward facing surfaces. For concept models, it may not be necessary to spend the extra time required to get perfect finishes on all surfaces. Each rapid prototyping service company has unique policies and practices. Discuss your requirements with suppliers before ordering. On the horizon -- Invision Earlier this year, 3D announced that is developing a new machine that combines multijet modeling with photopolymer technology. The Invision Si2 modeler will build parts by spraying acrylate photopolymer as a building material and using a wax similar to its current Thermojet material for supports. 3D says the new Invision system will include a separate finisher unit that will automatically remove the waxy supports. 3D has not yet announced a price for the Invision or when it will be available. 3D Systems demonstrated the new Invision at this summer's SIGGRAPH trade show. Preparing data for rapid prototyping Rapid prototyping machines are often described as three-dimensional printers. Just as inkjet or laser printers produce physical copies of electronic documents, rapid prototyping systems make physical copies of three-dimensional CAD models. But making rapid prototypes is not as easy as printing documents. Whereas most twodimensional printing is done through drivers installed with a computer s operating system, rapid prototyping machines require that CAD models be converted into a special format called STL, a name derived from the first rapid prototyping process, stereolithography. 26

27 Generating STL files is usually a fairly simple process. Virtually all modern CAD systems now include STL output as a standard feature and, for the most part, the files created are suitable for rapid prototyping. But the extra step of converting CAD models into STL format still presents a barrier to rapid prototyping novices or occasional users. Imagine how annoying it would be if you had to select parameters and convert word-processing documents or spreadsheets into a special file format before you could print them. Making STL files is further complicated by the fact that every CAD system uses different terms and parameters for defining the STL file s resolution, requiring users to interpret such mystifying terms as chord height, absolute facet deviation, angle control, and adjacency tolerance. Such terms are needlessly confusing. The STL file format is simple. It s not necessary to understand precisely what all the CAD system parameters mean in order to create useful files. An STL file is simply a mesh of triangles wrapped around a CAD model. CAD system settings specify how closely the STL mesh conforms to the actual surface geometry of a part. A mesh with triangles that are too large will create a small STL file, but the prototype made from it will have visible facets. A mesh with triangles approximately the size of the layers used by the rapid prototyping systems (typically about inches or mm) will produce a prototype with the best fidelity. A mesh with even smaller triangles will increase the size of the STL file and take longer to process, but it won t increase prototype accuracy or resolution. If an STL file is set with too loose a tolerance, facets will show on the prototype (left). Ideally, the triangle size should be close to the layer thickness used by the rapid prototyping system. It would make sense for the rapid prototyping machine manufacturers to work with the CAD software folks to develop a set of standard drivers that would eliminate the need for users to select STL output parameters. Whenever the idea has been suggested to 3D 27

28 Systems or Stratasys, however, executives have brushed it off, saying they ll get around to it sooner or later. Service-bureau recommendations We ve surveyed rapid prototyping services and find that they generally agree on the STL settings for various CAD systems that will produce the best rapid prototypes. Regardless of the parameters that any CAD system uses to define model resolution, service bureaus say the chord height of an STL file (the maximum distance a point on a triangle can deviate from the true surface of the part) should be between and inches (0.025 and mm). Whenever possible, STL files should be in binary format. (In the past, some CAD systems did such a poor job of STL creation that developers included an ASCII STL option as a debugging tool. ASCII STL files tend to be huge, but they can be edited with a text editor.) Some CAD systems will generate error messages during STL conversion saying that some part geometry is outside of the positive X, Y, Z quadrant or is in negative space. These messages can be ignored, as the STL file will be located in the rapid prototyping machine s build space and oriented for optimal surface finish and build speed by the system software. If you re using a solid-modeling CAD system listed below, the following settings should produce good STL files. If you re using a surface modeler or another system not listed, we d suggest discussing file preparation with your service provider or in-house prototyping shop. Surface-modeling CAD systems describe part geometry using mathematical patches with no thickness. In order to create good STL files, all of these surfaces must be joined so that there are no gaps or overlaps. This stitching or sewing, as it is frequently called, can be a tedious process, and it s easy to miss small flaws that can crash a rapid prototyping system. CATIA version four Select STL command. Set Maximum Sag to inches or mm. Pick the model to be converted to STL. Select Yes to generate triangles. Select Export, type a file name, and click OK to output an STL file. Note: CATIA has a utility for checking and analyzing STL files. It is a good practice to check the direction of normal vectors (they should point outward) and verify that the STL surface is closed. Parts created as CATIA solids should be no problem. Parts created as volumes from surfaces are more likely to have flaws. Pro/Engineer Open your model and from the File menu choose Export/Model. Select the file type STL. In the Export STL dialog box, set Format to Binary. Set the Chord Height to 0. The field will be replaced by a minimum acceptable value for the geometry of the model. Set Angle Control to 1. Name the file and click the OK button. Pro/Engineer will save your STL file. 28