RAPID PROTOTYPING AS AN INTEGRATED PRODUCT/PROCESS DEVELOPMENT TOOL AN OVERVIEW OF ISSUES AND ECONOMICS

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1 240 Journal of the Chinese Institute of Industrial Engineers, Vol. 20, No. 3, pp (2003) RAPID PROTOTYPING AS AN INTEGRATED PRODUCT/PROCESS DEVELOPMENT TOOL AN OVERVIEW OF ISSUES AND ECONOMICS Matthew Frank *, Drs. Sanjay B. Joshi and Richard A. Wysk Department of Industrial and Manufacturing Engineering The Pennsylvania State University University Park, Pennsylvania ABSTRACT An overview of issues concerning current methods of rapid prototyping (RP) processes is presented. This paper establishes a basis for categorizing the models developed by both commercial RP systems and conventional means, in particular, CNC machining. Model accuracy in both representation and testing capability is described based on a set of three defining levels, Form, Fit, and Function. This classification system divides models into distinct categories based mainly on dimensional control and the build material used for model construction. The implication is that the accuracy of the model will determine its viability for a specific application. The current popular RP systems and CNC machining are reviewed with respect to these categories. Furthermore, an economic overview provides a second basis for deciding on a particular system for prototype development. Keywords: rapid prototyping, stereolithography, selective laser sintering, 3-dimensional printing, fused deposition modeling, laminated object manufacturing, cnc machining of Solid Freeform Fabrication (SFF) to include CNC 1.INTRODUCTION Rapid prototyping (RP) has received much attention in recent years and has been embraced as a powerful tool for the product development process. RP has been described as a technology for producing accurate parts directly from CAD models (typically in a few hours), with little need for human intervention [1]. However, it is reasonable to establish what we mean by accurate or even part for that matter since material and geometric fidelity can be critical for product development. According to Webster s dictionary, a prototype is a first full-scale and usually functional form of a new type or design of a construction (as an airplane) [2]. The question must be posed as to whether a rapid prototyping method is producing just that, or if the correct term is Rapid Modeling. Do the current RP technologies produce rapid prototypes or rapid models? Are the methods rapid? The key here lies in the use of rapid and prototyping. The current use of rapid is to time scale (hopefully a reduction) for the evolution of the first item. A Prototype has come to be a physical approximation of some or all of a product. In this paper, we attempt to overview and characterize some popular methods for generating rapid prototypes. Our vision of RP expands the notion * Corresponding author: mcf113@psu.edu machining as a suitable application. We will investigate the capabilities of current methods of producing the first-time functional part with respect to time, cost, and accuracy. RP methods will be classified with respect to their ability to produce a prototype for what will be defined as Form, Fit, and Function. A critical part of RP is the process engineering that goes into the production of a first item. As traditional methods such as CNC and EDM become more efficient and more refined, the time constraint for RP methods will be reduced. When both process engineering and process time are considered, many of the freeform RP methods begin to look less rapid. Cost will also be a greater contributing factor as we move further in development of these technologies. Currently, model materials, RP equipment, and maintenance of these machines are all very expensive. This paper will focus on 6 of the most popular prototyping techniques: Stereolithography (SLA), Fused Deposition Modeling (FDM), Selective Laser Sintering (SLS), Laminated Object Manufacturing (LOM), 3-D printing (3DP), and Computer Numerical Controlled machining (CNC). Of all the techniques described, CNC is the only subtractive method while the others represent additive techniques. The argument persists as to whether CNC can be used as an RP technique [3]. Using our definition of RP, it is

2 Matthew Frank et al,:rapid Prototyping as an Integrated Product/Process Development Tool 241 appropriate to include CNC in this analysis. Although there are other processes available, this discussion will focus on the most widely used machines offered today and are the most viable ones at this time for commercial use. Finally, we will limit ourselves to defining the model as that which is produced directly by the RP process as defined. In other words, we will ignore for now the notion of using the RP models as patterns for further processing. As an example, many of the current RP processes detail methods of using the models for investment casting, metal plating, and/or Silicone mold making. These post-processing steps are employed in an attempt to transfer the model into a part that is built of more suitable material. Although some RP manufacturers describe processes specific to their own unique modeling system, it should be noted that any of the physical models created by these systems could be used in such a manner. Therefore we will limit ourselves to comparing the models as produced from the given RP process directly. 2. Form, Fit, or Function? We will discuss the concepts of Form, Fit, and Function with respect to the requirements for the model. When choosing an RP method, one must ensure that the type of model that needs to be produced justifies the investment. Economics cannot be overlooked since the cost in producing rapid models is significant. We will define the quality of a model with three levels of model accuracy. Form, as we define it, is the ability of the model to convey the design appearance and/or general size and shape of the part. A part that satisfies form would be capable of presenting the physical geometric appearance and could possibly lend itself to a preliminary discussion on part manufacturability. It is not necessary that a form model be constructed in the finished part material, nor would it even have to be a solid object. For instance in the case of Stereolithography, the user has the choice of a hollow or solid part. It is assumed that a form model would not be used for testing physical part interactions or clearance with any other mating parts; therefore, dimensiona control and surface finish is not critical. A model with multiple parts in an assembly would not necessarily need to function. (i.e., a model of a crescent wrench would not have to be able to tighten its jaws). Furthermore, it is not expected that any tests would be done, such as failure analysis. With all the aforementioned assumptions, it would be reasonable to choose an RP method that produces a form model quickly and in a cost effective manner. At this level, we argue that the prototype is merely a physical model and therefore the method should be described as Rapid Modeling. A model that is built for fit will require additional accuracy in feature representation and in dimensional control. This type of model will allow testing of the tolerance specifications of the design. It is assumed that this type of part could be assembled for fit testing. The model would not have to endure the operational stress of a final part, but assembly is expected with reasonable handling and interfacing with existing components or other models. It is not simple to establish a qualifying level of dimensional control, which would characterize a model as a fit model. The level of dimensional control and surface finish requirements for the finished part being modeled must be considered. One could certainly make an argument that for a fit model that might be used in the construction arena would be on the order of a quarter of an inch. For interacting assemblies like watches, the required tolerance would be on the order of 10-5 inches. For this paper, we will somewhat arbitrarily pick qualifying an RP process as the ability to produce fit models accurate to within a few thousandths of an inch. The ability to create a model for function places the most formidable requirements for the system. In order for the model to be truly considered a prototype, it will need to satisfy the constraints for function, as we will define. That is, the part must be tested for performance. Although advances in simulation and finite element analysis (FEA) have enabled a designer to test a design in the CAD arena, physical testing and failure analysis is often necessary. A basic requirement is that the part should be constructed in the material specified for the part being designed. Even though the actual product may be produced by a superior manufacturing technique, which may cause some uncertainty in the test results, a prototype made in the proper material should perform quite similarly. As an example, a part made using sintering of metal powder will have a somewhat different performance as that of a part which is machined, or cast out from the same material. The material constraint represents the greatest hurdle for most RP techniques with respect to function requirements, since many of the RP systems are enabled only by their use of a specific type of material. 3.STATE-OF-THE-ART 3.1 Stereolithography SLA is one of the most popular forms of rapid prototyping and is one of RP s founding technologies. Currently, 3-D Systems, Inc. manufactures the most widely used machine. SLA builds models using a laser to selectively cure layers of a photosensitive resin. As each layer is built, the build table lowers into a vat of resin and a new layer is added. Supportive

3 242 Journal of the Chinese Institute of Industrial Engineers, Vol. 20, No. 3 (2003) webbing is automatically built along overhanging boundaries of the part and then is removed before post-curing. SLA models are limited to photosensitive resins for a build material. This represents one of the greatest downfalls of SLA, but with respect to Form and Fit, this is not an issue. The resins are durable enough for handling, some machining, finishing, and can be assembled with other mating parts. SLA parts are reasonably accurate, typically providing dimensional control of +/-.005 to +/-.010 and a surface finish of roughly µin. Build rates are quite slow with SLA at approximately 1/8 vertically per hour. An option in the 3-D System s machines allow for a Quickcast part to be produced [4]. This part is mostly hollow, but the internal support structures must remain inside. The build time is reduced, along with the amount of resin used. This type of model is a good candidate for a form model. SLA requires post-processing, which begins with the removal of external support structures that must be scraped and/or cut off of the part. A final curing step in a UV oven is then needed to finish the part. The part is placed in the oven for ½ to a few hours depending on part volume. SLA in some cases could be useful to test for fit, provided that the dimensional tolerance for the part is within the capabilities of the machine. With respect to function, however, SLA will be quite limited. Unless the part being designed is to be made of a similar resin, further processing would be needed to convert the model to a suitable material. SLA systems can cost $100,000 to $800,000, and the resin is very expensive. SLA is capable of producing parts that represent design form, and often fit. In many cases, SLA is a good choice for producing a fit model. Obviously, a part that is suitable for fit testing is assumed to qualify as a form model, but it may not be the most cost-effective method. High machine and material costs, along with an often lengthy process time may preclude a designer from using SLA for simple form models. If desired, it would be most reasonable to reduce cost and build time by producing a hollow part. It may be inappropriate to qualify SLA models as function prototypes due to the process-specific photosensitive resins used for construction. 3.2 Selective Laser Sintering Selective laser sintering (SLS) is a process in which powdered material is fused via a 25 to 100 watt CO 2 laser to produce parts. Successive layers of powder are deposited and fused. As each layer is fused, the preceding layers are lowered and new powder is applied. SLS has some advantages over other processes, especially with its increased flexibility in material choice. Parts can be built out of metal, plastic and ceramic. Another attractive property of SLS is that the support structures of overhanging parts consist of the unsintered powders of each layer. In this fashion, the supporting powder can simply be brushed, or vacuumed away and can be recycled after sifting in most cases. This reduces a post-processing step. The materials available for SLS range from a rubber-like elastomer, an investment-casting polystyrene, up to stainless steel powders. For metal powder, the models produced will not be at full density, which is usually achieved by infiltration of another metal, such as copper or bronze. Accuracy of SLS parts is similar, but often less than that of SLA. It is practical to expect dimensions on the order of +/-.005 to +/ The surface finishes of SLS models fall in the range of 300 to 400 µin. Build rates are quite slow with SLS, and are similar to SLA at approximately 1/8 vertical inches per hour. SLS models can be handled readily, finished, or machined to allow for assembly. As stated before, depending on the part requirements, SLS can produce models to fit. The greater benefit is the increased material choices. The ability to build with ceramic, durable nylons, and metal in particular lends to the possibility of having a model with function characteristics. As defined in this paper, SLS has the possibility of producing a true prototype. Obviously, a model for form could be built, but as stated before for SLA, the high cost of SLS (a typical machine will cost $250,000 - $400,000), might preclude one from using it in that manner D Printing Three-Dimensional Printing (3DP) is quite similar in process and model performance as that of SLS. This is due to the fact that the process shares the characteristic of a powder-based system. In 3DP, in lieu of a laser as in SLS, the powder layers are created through the spraying of a binder material in an ink-jet like fashion. The binder selectively adheres particles layer by layer, leaving unbound powder to act as a support structure. There are currently small versions of 3DP machines available that lend themselves to very attractive form model construction. Z-Corp manufactures a 3DP machine that is limited to starch/cellulose and plaster powders [5]. The machine is quite inexpensive (about $60,000) and has a fast build rate (approx. 1 vertical inch per hour). The machine is small and uses no toxic substances, which makes it very suitable for use in an office environment. This example of 3DP is by far one of the most suitable systems for form model construction. After a short curing time, the models need to be handled carefully until a sealant is applied which gives the parts added durability. Even after the sealant application, the parts are quite fragile and do not lend

4 Matthew Frank et al,:rapid Prototyping as an Integrated Product/Process Development Tool 243 themselves well to fit testing, even if the accuracy would be sufficient. Typical accuracy is limited to +/-.020, which makes the process unsuitable in most cases for fit models. 3DP has the ability to bind numerous materials available in a powder form including metal and ceramic, although the increased accuracy of the SLS process gives it an advantage over 3DP for powder-based models. The ability to produce fit and function models is therefore limited, yet it is the most promising form modeling process currently available. 3.4 Fused Deposition Modeling Fused Deposition Modeling (FDM) is a process by which the build material is extruded through a nozzle, and each layer is drawn by the deposition of the molten material. Similar to SLA, the FDM process requires support structures to be added beneath overhanging features, which is later removed [6]. Like SLA these support structures will be most likely be left trapped inside when creating hollow parts. This can be alleviated by the recent introduction of water-soluble support materials, which are simply washed away. The FDM process is one that is difficult to classify with respect to form fit, and function. FDM can easily produce a model that conveys form, yet it is not the most cost efficient or rapid method. Accuracy is typically +/-.010 so it is limited in its ability to be used for fit modeling. FDM is typically twice as fast as SLS and SLA and can be approximated at ¼ vertically per hour. Its best quality by far is that it can create parts in ABS plastic, which is frequently used in manufacturing. Therefore, in some cases it could be a great choice for a truly functional prototype, if the modeled parts do not require tight dimensional control. The durability of ABS models lend themselves well to assembly and testing in real applications. 3.5 Laminated Object Manufacturing Laminated Object Manufacturing (LOM) is a process that typically uses paper sheets to construct the layers of the model. To create each layer, the paper is cut using a CO 2 laser that traces the profile of the particular model layer. Paper outside of the boundaries is cut in a crosshatched pattern that creates cubes, which are later chipped away. As each layer is completed, the construction deck is lowered and a new layer of paper is added. A hot roller activates an adhesive on the paper that bonds the sheets to each other. LOM is typically as fast as FDM and can be approximated at ¼ vertically per hour and the machines cost approximately $200,000. LOM is capable of producing form models but has some limitations. Since the crosshatched sections of layers have to be removed, LOM does not have the ability to readily create hollow parts or undercuts. The parts are quite durable and resemble similar properties to wood, and are therefore often sanded and painted to improve appearance. Paper parts made from LOM are very susceptible to moisture and therefore the parts are likely to swell. This is especially a problem in a direction perpendicular to the face of the paper layers, which leads to poor model accuracy for fit qualification. Typically, parts are accurate to +/-.015 to +/ LOM is not appropriate for creating function models due to the ease of delaminating the sheets. Furthermore, paper sheets are not suitable for modeling any typical manufacturing materials. 3.6 CNC Machining CNC machining, which includes both turning and milling operations, is typically not considered in most RP literature. In previous sections we see how certain processes lend themselves to some niche situations, ones where a specific quality is needed. The ability of CNC machining to create parts in a wide array of the most widely used materials to a very high accuracy is a unique combination not readily seen in any other RP process. On the other hand, CNC machining is limited to a considerably simpler set of part geometries. CNC is a subtractive process and therefore always requires a line of sight for the cutting tool. Hollow parts are impossible and undercuts require specialized tools and/or numerous re-fixturing operations. With respect to form, CNC is an unlikely candidate. For one, the limited part complexity leaves CNC incapable of producing some parts. Another obvious problem is that CNC is not typically a rapid or inexpensive method and would be uneconomical for just a form model. CNC does however represent the highest degree of accuracy in construction of any RP process. Rough machining to +/-.002 is typical, while finish machining can achieve +/ For many RP processes dimensional control greater than +/-.005 is highly unlikely [7]. Therefore fit models are well suited to CNC machining when such tight tolerances are necessary. The real benefit of CNC lies in its ability to create models that can be fully functional, particularly due to the wide choice of materials. When the part being modeled requires rigorous performance and failure testing, creating a part in the appropriate material is critical. By far, the extensive amount of set up and process planning needed for CNC is the greatest weakness that limits its acceptance as an RP process. Machining requires a highly skilled planner who can choose proper tools, fixturing and cutting parameters for material removal. Advances in process planning

5 244 Journal of the Chinese Institute of Industrial Engineers, Vol. 20, No. 3 (2003) and the use of 4 th and 5 th axis machine tools could bring CNC closer to an RP status in the near future. Table 1 summarizes the capabilities of the above RP processes to produce models that satisfy the qualifications for Form, Fit and/or Function and highlights some of the characteristics of each process. 3.7 ECONOMIC PERSPECTIVE Although RP provides a significant reduction in product and process development time and cost, one cannot neglect the cost of the prototype itself. Therefore the decision to use a particular RP technique involves selecting the lowest cost process that satisfies the physical requirements of the desired model, as outlined above. A cost model for an RP process will involve pre-processing, build rate, material cost, and post-processing as follows: C p = T pr *CR mo + (V p /R b )*CB mo + V p *C m + T po *CP mo Where: C p = Cost of a prototype T pr = Time for pre-processing (hours) CR mo = Hourly rate for pre-processing ($/hour) V p = Prototype volume (in 3 ) R b = Build speed (in 3 /hour) CB mo = Hourly rate for build ($/hour) C m = Cost of material ($/in 3 ) T po = Time for post-processing (hours) CP mo = Hourly rate for post processing ($/hour) Table 2 represents an example for comparing costs of producing one model from a given RP process. In this example, final costs are calculated for a part of 100 in 3. Table 1. Categorization of RP Methods RP Process Form Fit Function SLA Excellent complexity Expensive Support removal needed Sanding appropriate +/-.005 to +/-.010 Often suitable Durable Can be assembled Not typically, except for resin based final products SLS Good complexity Expensive No post-processing +/-.005 to +/-.010 Limited Durable Can be assembled Good as long as accuracy is sufficient 3DP Good complexity Economical Very fast Little post processing +/-.020 Not durable Poor accuracy and durability N/A FDM Good complexity Marginally expensive Marginal speed Support removal often needed +/-.010 Limited applications Durable Excellent material but limited accuracy LOM Limited complexity Marginal speed Extensive support removal step +/-.015 to +/-.025 Very limited Poor durability N/A CNC Limited complexity Expensive May be slow +/-.002 to +/ Excellent accuracy Can be most durable Excellent capability

6 Matthew Frank et al,:rapid Prototyping as an Integrated Product/Process Development Tool 245 RP Pre-pro cessing time ($/hour) Table 2. Example Cost Comparison for RP Processes Pre-proces s rate ($/hour) Build speed (in 3 /hour) Build rate ($/hour) Material cost ($/in 3 ) Post- Post-proc processing ess rate time (hour) ($/hour) Cost for one model SLA 0 n/a $1230 SLS 0 n/a $1170 3DP 0 n/a $125 FDM 0 n/a $386 LOM 0 n/a $220 CNC varies 0 n/a $830-$950 As can be seen from this example, there is a wide range of costs associated with producing rapid prototypes. Choosing a particular prototyping process should be done in two steps; 1) Determine the level of accuracy needed with respect to Form, Fit, and Function, and 2) Choose the lowest cost process that satisfies the accuracy requirement. Likewise, one may only want to purchase a machine that satisfies the typical requirements for prototyping, and use an RP service bureau or other outside contractor to create more accurate models. For Form models, it is obvious that 3-D Printing has a clear advantage with respect to cost. Only the LOM would be a reasonable substitute, with all other systems being much too costly for a model that only needs to survive reasonably limited handling. Basing a choice on economics for choosing a Fit modeling process is less intuitive. As explained earlier, many of the modeling systems would be suitable for fit testing depending on the tolerances specified for the model. FDM is a clear choice if tolerances are relaxed, due to its lower cost. For more critical dimensions SLS, and SLA will be necessary. For extreme accuracy, one would only be left with CNC, of course the limitations for part complexity may be a problem. A part suitable for Function testing is quite difficult to decide upon with respect to economics. Matching material properties will most likely be the prevailing constraint in the decision process for Function prototypes. 4.CONCLUSIONS Although there has been much excitement over RP technology, one must compare the requirements for their model with the capabilities of the processes. RP technology can be very cost effective or very cost prohibitive based on the prototyping quantities and requirements.the production economic considerations for conducting an economic decision analysis were provided in this paper. Before blindly choosing an RP method or system, careful economic decisions must be made. A major advantage of RP processes is that the manufacturing preparation process is extremely straightforward. A product engineer simply presses the MAKE button, or a close facsimile of it. The process engineering process for RP consists of generating an stl file from a CAD file, and then loading the file onto an RP machine and pressing the START button. The development process is extremely simple. However, production on an RP machine is anything but rapid. The trade-off is that the process engineering development time is reduced to almost nothing. For batch quantities of 1-3, the trade-off of increased production time for reduced set-up seems reasonable. Unfortunately, this analysis only deals with time to prototype. Other significant considerations such as form, fit and function need to also be addressed. An overview of form, fit and functional characterization of RP technologies included CNC machining as a proposed process was presented in this paper. CNC machining should not be overlooked, especially with the limitations in accuracy inherent with current commercial RP processes. Conversely, the limitations in complexity achievable with CNC will limit its use. For all the processes described in this paper, it is difficult to rank them in any general sense. Choosing an RP method will continue to be done on a per-model basis (especially if function is critical). Deciding to purchase an RP system may be even more difficult unless the same level of model accuracy will often be desired. Further acceptance of RP technology will hinge on at least one of two major hurdles: dimensional capability and material selection. In order for RP to truly accomplish the ideals of a prototype, it will have to be able to produce a first-run, functional model suitable for testing, which to date has not been fully realizable.

7 246 Journal of the Chinese Institute of Industrial Engineers, Vol. 20, No. 3 (2003) REFERENCES 1. Pham, D.T., R.S. Gault, A Comparison of Rapid Prototyping Technologies, International Journal of Machine Tools and Manufacture, October,38, (1997). 2. Merriam-Webster s Collegiate Dictionary, Tenth Addition, Merriam-Webster(1998) 3. Wang, F.C., L. Marchetti, and P. Wright, Rapid Prototyping Using Machining, Society of Manufacturing Engineers, Technical Paper, (1999) 4. 3-D Systems, Avenue Hall, Valencia CA, 91355, Available : Z-Corporation, 20 North Avenue, Burlington, MA, 01803, Available : Stratasys Inc., Martin Drive, Eden Prairie, MN, 55344, Available : Mueller, T., Accuracy in Rapid Tooling, Time-Compression Technologies, September, Communication Technologies, Inc.,38-45(2000). ABOUT THE AUTHORS Richard A. Wysk is the William E. Leonhard Chair in Engineering and Professor of Industrial Engineering at The Pennsylvania State University. He received his B.S.(1972) and M.S.(1973) from the University of Massachusetts and Ph.D.(1977) from Purdue University. He has also served on the faculties of Virginia Polytechnic Institute and State University and Texas A&M University where he held the Royce Wisenbaker Chair in Innovation. Dr. Wysk's research and teaching interests are in the general area of Computer Integrated Manufacturing (CIM). In particular, he is interested in Lean Manufacturing, Computer-Aided Process Planning and Flexible Manufacturing Systems (FMSs) planning, design and control. Dr. Wysk has coauthored six books including Computer-Aided Manufacturing, with T.C. Chang and H.P. Wang -- the 1991 IIE Book of the Year and the 1991 SME Eugene Merchant Book of the Year. He has also published more than a hundred technical papers in the open-literature in journals including the Transactions of ASME, the Transactions of IEEE and the IIE Transactions. Dr. Wysk is a IIE Fellow, a Fellow of SME, a member of Sigma Xi, and a member of Alpha Pi Mu and Tau Beta Pi. He is the recipient of the IIE Region III Award for Excellence (1982), the SME Outstanding Young Manufacturing Engineer Award (1981) and the David F. Baker IIE Distinguished Research Award (1993). He has held engineering positions with General Electric and Caterpillar Tractor Company. Matthew C. Frank is a PhD candidate in Industrial and Manufacturing Engineering at the Pennsylvania State University. He received his B.S. (1996) and M.S. (1998) in Mechanical Engineering from the Pennsylvania State University. His research interests are in the general area of Manufacturing Engineering. In particular, he is interested in CNC Machining and Fixture Design, CAD/CAM, Rapid Prototyping, and Integrated Product/Process Design. He is a member of IIE, SME and ASME. Dr. Sanjay Joshi is currently Professor of Industrial and Manufacturing Engineering at Penn State University. He received Ph.D in Industrial Engineering from Purdue University in 1987, MS from SUNY at Buffalo, B.S degree from University of Bombay, India. His research and teaching interests are in the areas of Computer Aided Design and Manufacturing (CAD/CAM) with specific focus on computer aided process planning, control of automated flexible manufacturing systems and Rapid Prototyping and Tooling. He is a recipient of several awards, including Presidential Young Investigator Award from NSF 1991, Outstanding Young Manufacturing Engineer Award from SME 1991, and Outstanding Young Industrial Engineer Award from IIE He has severed as the Department Editor for Process Planning - IIE Transactions on Design and Manufacturing, and currently serves on the editorial board of Journal of Manufacturing Systems, Journal of Intelligent Manufacturing, and Journal of Engineering Design and Automation, and International Journal of Computer Integrated Manufacturing.