144 Sofid Freetcrm Fabrication {SFFj and Rapid Prototyping Chap. 4 ~ / Platform Materialsupply roll Figure 4 8 Laminated object modeling (LOM), based on commercially published brochures ot Helisys Ine. fixtureless base plate. In terms of motion control, FDM is more similar to CNe machining than SLA or SLS. For simple Parts. there is no need for fixturing, and material can be built up layer by layer. The creation of more complex parts with inner cavities, unusual sculptured surfaces, and overhanging features does require a sup- port base, but the supporting material can be broken away by hand, thus requiring minimal finishing work. Thus,despite the similarities with the CNe machine from the point of view of control. the resulting parts that can be made are more in the SFF family.A similar deposition machine, the Mndel-Mater 3-D plotter, has been devel- Part block Optics X"Ypositioning device Layer outline and cross hatch Laser " Laminarlng rollet Sheet material 'Iake-up rc New layer Bonding Cutting 4.2 Stereolithoqrephv: AGenera! Overview '45 f'OM b.Synem J1lpre 4.9 Fused deposition modeling (FDM). based on published brochures of Stratesys Inc, oped by Sanders Inc. In the Sanders machine, nozzles are used to deposit viscous polymers. It is difficult to control the flow of the viscous polymer and obtain an evenly distributed layer,just as it would be to control the flow of toothpaste onto a flat surface to obtain an even thickness. Each formed surface layer is thus machined. (or planarized) with a milling cutter prior to the application of the next layer. 4.2.10 3·DPlotting and Printing Process •• Several types of 3-D printing processes have been developed in recent years and are constantly being updated at the time of this writing. Some of these are aimed. at the educational CAD/CAM market where students are invited to obtain quick models of an emerging design. At the same time. such machines might be useful in an industrial design studio, where artists might want to generate and regenerate a quick succession of prototypes for the "look and feel" of an emerging design. Examples include: •3-D printing of cornstarch, followed by layer-by-layer binder hardening, is the basic principle behind the Z-eorporation machine. The first step inFIgure 4.10a is to spread a thin layer of powder of the desired material across the top of the bed. The next step hardens the desired geometry into this layer of powder. The hardening is not done with a laser (like SLS) but with a biDder pbase. FIne Molten material Solidifying material Solidified material Part Filament Heated I:DMhead I ;,:~~u~:!~~:~ Step 1: collect powder Solid Freeform Fabrication (SFF) and Rapid Prototyping Chap.4, 148 Step 3: spread powder [a) '"' Step 4: deposit bind ~' (b) Step 5: source piston up, build piston down .:·:r~i I Steps 1-5 are repeated until build is complete Step 6; build complete ~'~i S~d" I ~ II ~ till USI layer priflltd Finished part Ftpp:e4.10 (a) 3-D printing (based on commercially published brochures of Z- Corporation Inc.) and (b) the method developed by Sachs and colleagues (2000) Step 2: spread powder Spread powder Primb'{tl Dmpplslon In!,mn(;'dinlestage 4.2 Stereolithography: A General Overview '47 droplets of the binder stream are printed down through a continuous-jet nozzle carried by the print head. Since material isbuilt up layer by layer in anx/y plane, the process resembles the motions of the ink-jet printing heads on a conven- tional word processing printer . • A more accurate 3-D printing process, developed by Sachs and colleagues at MIT, was the forerunner to this technology (Figure 4.10b). This process is being used to build the ceramic molds for metal castings and powder-metal tooling for injection molding dies. Commercial applications of this process are growing (Smith. 2000;Sachs et al., 1992,2000). 4.2.11 Solid Ground Curing (SGCI Solid ground curing was introduced by Cubital Inc. A schematic diagram of the process isshown in Figure 4.11.Thc quickest wayto understand the sketch is to begin with the operation at the "cross roads," where the mask is being used to photocure the uppermost layer of liquid of the block. Solid ground curing uses exactly the same physical process as SLA to photocure the polymer liquid. The key difference is that SLA does it by using a laser point source, whereas SGC does it by exposing a com- plete plane at once through the mask. Cubital's machine is one integrated unit. How- ever, two interlaced processes occur simultaneously. On the left of the schematic the following steps are shown: CAD files are sliced, a mask plate is prepared for a single layer, and the finished mask rotates into the exposure area. On the right of the schematic, a thin layer of photopolymer is spread over the surface of a block. This moves into the exposure area to be hardened. Postprocessing steps on the right include wiping off the residual photopolymer, using supporting wax to fill in any F1pre 4.11 Solid ground curing process, based on commercially publiahed brocbures of Cubital America, Inc. 4. Remove uncured resin 5.Fillcav\!ies with wax 6.Chi!lan, harden 7. Mill and Bnlldlng al.yet Creatinglhe- photomask 'l'ranstnit'lllyerdatlt Prepare the data Dewaxlllepllrts , Solid Freeform Fabrication (SFF) and Rapid Prototyping Chap.4 hollow areas, cooling, and planarization before returning to the first position again for more spreading of the photocurable liquid. Meanwhile, as shown on the left, the first pattern on the mask plate is erased and the next pattern is applied. 4.2.12 Shape Deposition Manufacturing 150M) In some of the two processes described earlier, for example, 3~Dprinting by Sanders and SGC by Cubital, a combination of material additive and material removal takes place. Shape deposition manufacturing (SDM) also exploits this paradigm (Weiss et al.,1990; Weiss and Prinz, 1995; Weiss et aI 1997;Weiss and Prinz, 1998).The goals for SDM are to combine the advantages of SFF {i.e., easy to plan, does not require special fixturing, arbitrarily complex shapes, and heterogeneous structures) with the advantages of machining (i.e., high accuracy, good surface finish, and wide-scale availability of existing CNC machines and infrastructure). In 80M, a CAD model is again sliced into 3-0 layered structures. Layered seg- ments are deposited as near-net shapes and then machined to net shapes before addi- tional material is deposited. The sequence for depositing and shaping the primary and support materials is dependent upon the local geometry (Figure 4.12).The idea is to decompose shapes into layered segments such that undercut features need not be machined but are fonned by previously shaped segments. 80M can use alterna- tive deposition sources from welding to extrusion. Producing smooth surface transi- tions between layers, however, remains a challenge, due in part to the layer-by-layer accumulation of residual stresses. SOM can therefore combine complex surfaces and high accuracy. In the future it also promises to fill a niche for creating "wearable computer" products with mul- tiple materials and even with embedded electronics (8mailagic and 8iewiorek, 1993). Deposit Remove P~ll material 4.3 Comparisons between Prototyping Processes 149 4.3 COMPARISONS BETWEEN PROTOTYPING PROCESSES 4.3.1 Materials That Can Se Formed with the Various Processe. The SLA process uses photocured polymers that do not exhibit great strength or toughness. Nevertheless, in the SFF family, the SLA process is the most accurate and has emerged as the industry standard for creating a master pattern that might then be used as the basis for a casting or injection mold. On the other hand, if a single prototype needs to be tested to destruction, or car- ried around for a while, it really has to be made from metal or a structural plastic such as AB5. If only one or two prototypes are needed, the FDM process is an ideal choice. FDM can extrude ABS polymers and create prototypes that are between 50% and 80% of full ABS strength. For full strength plastic or metal prototypes, the standard machining process is the preferred choice, despite the more limited range of geometric shapes that can be made by machining. CNC machining is also the most likely prototyping process for the small batch manufacturing of 2 to 10 components. If CNC machining is out of the question because of geometric complexity, SLS metal-powder parts might be the best choice. Beyond batch sizes of 10, it is worth considering the use of small batch casting methods. This decision will be influenced by desired accuracy, machining being better than casting. Some developments in shape deposition manufacturing and 3-D printing are leading to direct mold making (e.g.,Weiss et al., 1990;Sachs et al., 2000). 4.3.2 Accuracy Accuracy is perhaps the next key feature that distinguishes the various prototyping processes. The list that follows gives some very general values for a variety of SFF processes and other more traditional processes that can be used to make one or two components. 1 • Hot, open die forging: +1- 1,250 microns (0.05 inch) • Laminated object modeling: +1- 250 microns (0.010 inch) • Investment (lost-wax) casting: +1- 75 microns (0.003 inch) • Selective laser sintering: +1- 75 to 125 microns (+1- 0.003 to 0.005 inch)- depends on part geometry • Stereolithography: + 1- 25 to 125microns (+ 1- 0.001 to 0.005inch)-depends on part geometry • Plastic injection molding from a machined mold (prototyping version): +1- 50 lu 100 microns (+1- 0.002 to 0.004 inch) • Rough machining: +1- 50 microns (0.002 inch) •Finish machining: +1- 12.5 microns (0.0005 inch) lThe rust entry corresponds to the age-old vtuege blacksmith's prototyping shop. See Wright and associates (1982) for the CNC controlled version 150 Solid Freeform Fabrication (SFF) and Rapid Prototyping Chap. 4 • Electrodischarge machining: +/- 2.5 microns (0.0001 inch) • Lapping and polishing: +1- 0.25 microns (0.00001 inch) When comparing the everyday prototyping methods, the most accurate remains machining, with easily achieved accuracies of +/- 25 microns (0.001 inch) and even half this with a good craftsperson. The next most accurate is prototyping by plastic molding from a machined mold, with an accuracy of +/- 50 microns (0.002inch). After that the SLA and SLS processes are listed. For a typical component, selective laser sintering and stereolithography average out at +/- 50 to 125 microns (0.002 to 0.005 inch). This is different from the accuracies of 25 microns (0.001 inch) quoted by the suppliers of SLA equipment, and confrontational e-mails will prob- ably be a result of the obvious differences used in this text. However, these adver- tised accuracies of 25 microns (0.001) are for simple linear objects. Some users have to make complicated computer casings and medical monitors where open shell struc- tures "warp and shrink all over the place," to quote one user. In some cases this warping and shrinking worsens the SLA accuracy to as much as +/- 375 microns (0.015 inch). For SFF,the other accuracy consideration is stair-stepping. Mentally picture the soccer ball again, but this time with perfectly smooth surfaces. Now approximate the soccer ball by representing it as a stack of thin slices.The largest diameter slice is at the equator; the smallest slice is at the poles. Figure 4.13 from Jacobs (1996) shows that the approximation to the soccer ball becomes worse as the bounding curve comes up around the object toward the poles. In addition, the loss in accuracy/fidelity is related to layer thickness. Since SLA processes are improving all the time, layers down to 25 to 50 microns (0.001 to 0.002 inch) are now possible, therefore giving better and better accuracies. As with all manufacturing processes, the process then does take longer and more cost is involved. Investment (lost-wax) casting is listed at +/- 75 microns (0.003 inch). Thus if the casting process is used to make a short-run prototyping mold and then the part is injected in plastic, it would seem to offer about +/- 125 microns (0.005 inch), but some hand finishing and some cosmetic work on the mold will give as good a plastic part as the cast mold. Large layer thickness Medium layer thickness Fine layer thickness F'iple 4.13 The stair-stepping approximation in SFF processes CAD~esign CAp design C.t(DdeSign 4.3 Comparisons between Prototyping Processes 151 4.3.3 Lead TIme of Prototypes With an in-bouse dedicated FDM machine, a part can be produced within a 24-hour period. For an ongoing design activity where a design team needs a series of proto- types-for the look and fit of subcomponents and subassemblies-the FDM process is ideal. An in-house integrated CAD/CAM system for machining can generate a simple part in a "morning's work," whereas more complex parts will take two to three days. An in-house stereolithography machine willalso create the same parts in two or three days, measuring the time from receiving the" .STL" file to a fully cured product. The curing time, incidentally, is an added time factor, often overlooked when rival companies develop their advertising literature and compare their partic- ular process with others. If an in-house machine is not available, it should be realized that the SLA service bureaus are swamped with business in today's economy. Unless a special cus- tomer relationship exists, turnaround time of one to three weeks is more probable. Given the need for some negotiation with a client, and the need to check incoming computer files,the actual turnaround time may be longer still. Nevertheless, the rapid prototyping shops are selling "service and speed" rather than "fidelity." For small batches (10 to 5(0) of injection molded plastic parts, customers can expect a three- to six-week turnaround time. The steps might be (a) an SDRCflDEAS or Pro-Engineer CAD file is received from the Internet, (b) files are checked, (c) an SLA master is made, (d) an aluminum mold is cast, and (e) the fin- ished batch of 10 to 100 is injection-molded in ABS plastic. 4.3.4 Batch Size Chapter 2 describes the influence of batch size. For just one component, SFF processes-such as stereollthography.fused deposition modeling, and selective laser sintering-or machining is the obvious choice. Small-batch casting in metal, or small- batch injection molding in plastic, is used for batch runs between 50 and 500. 4.3.5 Cost In general, cost increases with fidelity and accuracy needed, fur all the rapid proro- typing processes. Figure 4.14shows why this is so. Specifically, if the designer desires more accuracy, the" .STL" files will need to be of finer resolution, the slicing will also be thinner, the laser will make more scanning paths, and the time and hence costs will increase. Also, all prototyping processes (SFF or machining) require some hand fin- ishing, sanding, and deburring. Obviously, costs increase if tbe designer prefers a smoother surface finish. In all prototyping processes there is also a relationship between complexity, surface finish, accuracy, and cost. For SFF, Figure 4.14 shows that overhanging features require explicit support especially for SLA. For the arrangement on the right of Figure 4.14,the support columns have to be broken off by hand after manufacture. This usually leaves small stubs on the surface, which must then be sanded away. Sacrificial material Solid Freetorrn Fabrication (SFFland Rapid Prototvptna Chap. 4 152 ~ O'~~; •."""",,g~. feature __ Forms • cavity feature a. Complementary support JIIpre 4.14 Supporting structures for SLS and SLA (courtesy of Lee Weiss). b.Explicit support Rapid prototyplng maclrine (RP~) TABLE 4.4 Rapid Prototyping Machina Cost-Alao sea Section 4.3.6 for Installation and the Like (aa of March 2000) MachinecOlIt SLA-250 (SLA) SLA-3S0 (SLA) SLA-SOO(SLA) FDM 2000(FDM) LOM-2030H (LOM) SOC 4600 (SOC) SOC 5600 (SGC) Sinterstation2500(SLS) Sinterstation 25()()pIWI (faster than the above) (SLS) $210,000 400,000 500,000 120,000 275,500 275,000 450ilOO 200,000 310,000 4.3.6 Ancillary Cosu The costs shown in Table 4.4 are the base cost of the machine. It should be empha- sized that there are also additional miscellaneous costs of a warranty, installations. and so on. For example, the Helisys 2030H LOM machine has a base price (as of March 2000) of $275,500, which actually includes a first-year service warranty. Installation is estimated at $3,000; training at $2,000. Additional options include a chamber heating module at $4,499 and an initial supply package at $5,995.Thus the total for the complete package is $292,494. This example is not meant to endorse or criticize the LOM machine; rather it shows the real cost of doing business. All the machines in the table have such setup costs, which add 10% to 20% onto the base price. Some processes such as SLS also require a supplementary room for powder preparation and venting. Further data on cost comparisons (Table 4.4), materials (Table 4.5), part size (Table 4.6), and total part cost (Table 4.7) now follow. Figure 4.15 compares accuracy. 4.3 Comparisons between Prototyping Processes 4.3.7 Commercial Comparisons of Cost and Capability 153 Rapid prototyping TABLE 4.5 Modeling Material Comparison Liquid photocurable polymers Sintered ='" powder and Viscous Sheet Polymer solidifying materials spool polymers Stereolithography Selectivelaser sintering Laminated object modeling Fused deposition modeling Solid ground curing 3-D prtnnng followed by machining x x x x x x Machine TABLE 4.6 Maximum Part Size Comparison (as of March 2000) Company Partsi:;e capability (in.) SLA-250 SLA-350 SLA-500 FDM2000 LOM-2030H SGC5600 Sinlerstation2000 Sinterstation2500 3D Systems, Inc. 3D Systems, Inc. 3D Systems, Inc Stratasya Inc. Hellsys.Inc Cubiral America, Inc. DTMCorp DTMCorp. rox io« 10 13.8 X 13.8 x 15.7 20 X 20x 23 to x lOX 10 32 X 22x 20 20X 14 X 20 12x 15 15 X 13 x 16.7 TABLE 4.7 Rapid Prototyping Process, Speed and Cost Compariscn-c-Chrvsler Benchmarking Test Reported In "Rapid Prototyping Report," Vol. 1, No.6, June 1992. Note That This Comparison Was Done with 1992 Machines Such as the 3D Modeler by Stratesvs. Many Machines Such as the Sinterstatian 2500 P1u • Have Become Much Faster Since Then. Rapid prototyping process Total part cost Machine Total process time (hr:min) Stereolithography Stereclithography Fused deposition modeling Laminated object modeling Solid ground curing Selectlve laser simermg SLA-250 SLA-500 3-D Modeler LOM-I0IS Solider 5600 SinteTSlalioo2000 7:25 7:03 12:39 11:02 11:21 4:55 $133.94 187.95 344.94 109.40 88.70'" 199,23 "Assumes 35parts built simultaneously. [...]... International Mechanical Engineering Congress and Exposition, 8: 60 5 -61 0 Anaheim, CA: MED Jacobs, P F 1992 Rapid prototyping and manufacturing: Dearborn, MI: Society of Manufacturing Engineers Fundamentals Jacobs, P F 19 96 Stereolithography and other rapid prototyping gies Dearborn, MI: Society of Manufacturing Engineers of stereolithography and manufacturing technolo- Java, 1995, is a trademark of SUD Microsystems,... Thesis, Department of Computer Science, University of CalUornia, Berkeley of Science 4.8 References 167 Kochan, D 1993 Solid freeform Amsterdam: Elsevier Kruth, 1 P 1991 Manufacturing (2) :60 3 61 4 manufacturing: Advanced by rapid prototyping rapid prototyping techniques Annals New York and of the CIRP 40 Kumar, V., P Kulkarni, and D Dutta 199f\ Adaptive slicing of heterogeneous solid models for layered manufacturing. .. ComparisoD of Rapid Sinterstation 2000 0.005 (SLS) Sinterstation 2500 0.005 (SLS) SOC 13~' ~ 460 0 (SGC) 0.0 06 SOC 560 0 (SOC) 0.0 06 l5 LOM-2030H ~~ ~ Chap 4 of Approxlnule AeeurllCY PwlutypinK PIon:_ FDM 0.01 (LOM) 2000 (FOM) 0.005 SLA·250(SLA) 0.003 SLA-350(SLA) 0.003 SLA-500(SLA) OJXl3 o 0.001 0.002 0.003 0.004 0.(lO5 0.0 06 0.007 0.008 OJXl9 0.01 Accuracy (inches per inch) Note that Figure 4.4 CASTING 4.4.1... machine tools part 1: Design principles, part 2: A real-time, Quintic spline interpolator.Jownef of Man· ufacturing Science and Engineering 120:417 432 Smailagic,A., and D P Siewiorek 1993.A case-study in embedded system design:The 2 Wearable Computer IEEE Design and Test of Computers, 56- fJ7 VuMan Smith, C, and P K Wright 19 96 CyberCut: A World Wide Web based design to fabrication tool Journal of Manufacturing. .. Methodologies for Solid Freefonn Fabrication, June 5 -6, Pittsburgh, PA II Design Weiss, L E., R Merz, F B Prinz, G Neplotnik, P Padmanabhan, L Schultz, and K Ramaswarni, 1997 Shape deposition manufacturing of heterogeneous structures SME Journal of Manufacturing Systems 16: 239-248 Weiss, L E., and F B Prinz 1998 Novel applications and implementations manufacturing Paper presented at the Naval Research... manufacturing University of Michigan Technical Report, UM-MEAM-98-02 Manufacturing Studies Board (National Research Council) 1990 Rapid prototyping in the U.S manufacturing research community Edited by T C Mahoney McMains, S 19 96 Rapid prototyping of solid three dimensional Thesis, Computer Science, University of California, Berkeley parts Master facilities of Science McMains, S., C.S Sequin and 1 Smith... Manufacturing Review 5 (2):117-1 26 Sachs, E., N Patrikalakis, D Boning, M Cima, T Jackson, and R Resnick 2000 The distributed design and fabrication of metal parts and tooling by three dimensional printing In Proceedings of the 2000 NSF Grantees Design and Manufacturing Conference Arlington, VA: University of British Columbia and National Science Foundation Sarma,S., and P K Wright 19 96. Algorithms for the minimization... the surface of the powder, first sintering the bottom slice of the desired object A roller spreads more powder and a second layer is sintered, also fusing to the one below  '66 4.8 References 4.7. 16 Shape Deposition Rapid across Manufacturing prototyping with alternative an object to build up complex ISDM) deposition prototypes 4.7.17 Solid Freeform Fabrication runs followed by machining runs ISFFI... Machine Theory 25 (3): 365 -381 DeGarmo, E P., 1.T Black, and R.A Kohser 8th ed New York: Prentice-Hall 1997 Materials and processes in manufacturing, DeMeter, E c., Q Sayeed, R E DeVor, and S G Kapoor 1995 An Internet model for technology integration and access part 2: Application to process modeling and fixture design MTAMRI Report 1995 University of Illinois Dutta, D 1995 Layered manufacturing in Project... Industrial Press Wright, P K., D.A Boume,J.A E Isasi, 0 C Schatz, and J 0 Colyer 1982 A flexible manufacturing cell for swaging Mechanical Engineering 104 (10): 76- 83 4.9 BIBLIOGRAPHY Benett, 0., ed 19 96 Developments in rapid prototyping lication Ltd London: Bury-Saint Edmunds and tooling Mechanical Koenig, D T 1987 Manufacturing engineering: Principles for optimization York, and London: Hemisphere Publishing . machining x x x x x x Machine TABLE 4 .6 Maximum Part Size Comparison (as of March 2000) Company Partsi:;e capability (in.) SLA-250 SLA-350 SLA-500 FDM2000 LOM-2030H SGC 560 0 Sinlerstation2000 Sinterstation2500 3D. PIon:_ Sinterstation 2000 (SLS) Sinterstation 2500 (SLS) SOC 460 0 (SGC) 0.005 0.005 0.0 06 13~' SOC 560 0 (SOC) ~ l5. LOM-2030H (LOM) ~~ ~ FDM 2000 (FOM) 0.0 06 0.01 0.005 0.003 SLA-350(SLA) 0.003 SLA-500(SLA) OJXl3 o 0.001. and venting. Further data on cost comparisons (Table 4.4), materials (Table 4.5), part size (Table 4 .6) , and total part cost (Table 4.7) now follow. Figure 4.15 compares accuracy. 4.3 Comparisons