21st Century Manufacturing Episode 1 Part 9 potx

154 Solid Freeform Fabrication (SFF) and Rapid Prototvptns Chap. 4 ComparisoD of Approxlnule AeeurllCY of Rapid PwlutypinK PIon:_ Sinterstation 2000 (SLS) Sinterstation 2500 (SLS) SOC 4600 (SGC) 0.005 0.005 0.006 13~' SOC 5600 (SOC) ~ l5. LOM-2030H (LOM) ~~ ~ FDM 2000 (FOM) 0.006 0.01 0.005 0.003 SLA-350(SLA) 0.003 SLA-500(SLA) OJXl3 o 0.001 0.002 0.003 0.004 0.(lO5 0.006 0.007 0.008 OJXl9 0.01 Accuracy (inches per inch) Note that the machine suppliers quote in "thou' and that one "thou' '" 25 microns Figure 4.15 Comparison of accuracy (as of March 2000). 4.4 CASTING METHODS FOR RAPID PROTOTVPING 4.4.1 Introduction The classic manufacturing texts by DeGarmo and associates (1997), Kalpakjian (1997), Schey (1999), and Groover (1999) are remarkably comprehensive in their coverage of the casting process. The several methods of casting include: •Lost-wax investment casting • Ceramic-mold investment casting • Shell molding • Conventional sand molding • Die casting Rather than duplicate the material found in other books, this section focuses on casting as it is done by rapid prototyping companies. Batch sizes from 50 to 500 are typical. The key market strategy is that casting is cheap and fast. However, it may not be the choice for the final product because of its tolerances. Depending on the type of casting chosen, the tolerances vary from +/- 75microns (0.003 inch) for lost- SLA·250(SLA) 4.4 Casting Methods for Rapid Prototyping 155 wax processes to +/- 375 microns (0.015 inch) for standard sand castings (also see Chapter 2). 4.4.2 Lost-Wax Investment Casting As mentioned in Chapter 1, the fundamentals of casting were invented by Korean and Egyptian artists many centuries ago. The following steps are known as the lost- wax investment casting process (Figure 4.16): (a-c) a master pattern of an engi- neering or art object is first carved from wax; (d-f) it is surrounded by a ceramic slurry that soon sets into solid around the wax; (g) the wax is melted out through a hole in the bottom, leaving a hollow cavity; (h) this hole is plugged, and liquid metal is poured into the open cavity from the top; (i) after a while, the metal solidifies and the ceramic shell can be broken away to get the part; (j) some cleaning, deburring, and polishing are needed before the object is finished. The process was greatly improved and made more accurate during World War II for aeroengine components. Today it isused for products such asjet engine turbine blades and golf club heads. On the top line of Figure 4.16, wax patterns are formed from injection molds, assembled on treelike forms, and then treated with the slurry. Alternate layers of fine refractory slip (zircon flour at 250 sieve or mesh size) are applied, followed by a thicker stucco layer (sillimanite at 30 sieve or mesh size). The coated components are dipped in fluidized beds that contain isopropyl silicate and liquid acid hardener. Drying takes place in ammonia gas.The next step is to elim- inate the wax in a steam autoclave at 150°C,fire the mold for 2 hours at 950 "C,then pour in the liquid steel or aluminum. In summary, the modern lost-wax method has one of the best tolerances in the casting family because the original wax patterns are made in nicely machined molds. Today,tolerances of +/- 75microns (0.003inch) are readily obtainable. Also the as-cast surface is relatively smooth and usable for the same reason. Other advantages include: • No parting lines if the wax original is hand finished. • Waxes with surface texture can give direct features such as the dimples on a golf club. •Automation of the slurry dipping is possible using robots, thereby reducing costs. • Products such as turbine blades can be unidirectionally solidified, giving good mechanical properties in the growing direction. 4.4.3 Ceramic-Mold Investment Casting Procedures The snag about the previous method is that the wax pattern is destroyed. The ceramic-mold investment casting technique therefore employs reusable submaster patterns in place of the expendable wax patterns. This version of investment casting ideally involves five steps to make it efficient and to retain, as much as possible, the fine care and expense that go into creating the original master positive in Step 1 .The steps are as follows: • Step L Positive: make an original master pattern with stereolithography or machining. 156 Solid Freeform Fabrication (SFFl and Rapid Prototyping Chap. 4 (a) Mold to make pattern / (bJ'" CIW~. Injectingwax pattern or plastic pattern Ejecting pattern (c) ~~ Pattern assembly (tree) Slurry coating Stucco coating Completed mold (g) (h) (i) Autoclaved "~Jt. <, / Molten -s-c-wax or plastic W~.'.CW7 ,~.t ilf .[( J" .: • ::., . ~ Casting Patternmeltout Pouring Shakeout Pattern Figure 4.1f. The lost-wax investment casting process. Upper diagrams (a) through (c) lead to the tree of wax master patterns. Middle diagrams show the slurry and stucco being applied Lower diagram shows the casting (adapted from literature of the Steel Founders' Society of America). 4.4 Casting Methods for Rapid Prototyping 157 • Step 2. Negative: create a shell around the master with highly stable resin. A negative space is created around the original positive master pattern. This shell can be pulled apart to give a parting line. • Step 3. Positive: create reusable submaster rubbery molds from the shells. • Step 4. Negative: create the destroyable slurry/ceramic molds. •Step 5. Positive: pour metal into the ceramic molds, which are then broken apart to get the components, which must then be degated and deburred. SLA can be used to make the original master pattern, or a CNC machine can be used to mill the master from brass, bronze, or steel. Of course, the process can start at Step 3, but this might damage the original master, especially if it is SLA. Also, to get high productivity in the factory, it is preferable to have many molds at Step 3, all of which can be made from the stable resin negative in Step 2. Prototyping companies like to use the hard resin to fabricate the negative in Step 2, because the resin has good dimensional stability. Note that it is typical to have two resin molds, one for each side of the casting, separable by a parting line. Once the hard resin shells have set, they can be filled with a slurry gelthat solid- ifies to a hard "rubbery positive" for Step 3. This intermediate submaster mold can be stripped away from the resin shells while it is still "rubbery." The material is ideal for the rather rough handling environments of a foundry, and the rubbery properties mean that no draft angles are needed for stripping these submasters off the resin shells. The Step 4 negative mold is made from a graded aluminosilicate with a liquid binder (ethyl silicate) and isopropyl alcohol. This is poured around the subrnasters from Step 3. Once the slurry has set, the two ceramic halves are joined to create the inner cavity, the slurry is fired at 950 ~Cto give it strength, and the casting process, say with molten aluminum, can begin. After solidification, the component is broken out of the ceramic, cleaned up, and deburred. The parting line can cause problems, but in general, good accuracy is obtained: +f- 125 to 375 microns (+1-0.005 to 0.015 inch). 4.4.4 Shell Molding An alternative form of high-accuracy casting is shell molding. Metal pattern plates are first heated to 200°Cto 240°C.A thin wall of sand,S to 15 millimeters (0.25 to 0.75 inch) thick, is then sprayed over the plates. The sand is resin-coated to ensure adhesion to the metal plate. Phenolic resins, with hexamethylene-tetramine addi- tives, are combined with the silica to ensure rigid thermosetting of the sprayed sand. The next steps are to cure, strip, and dry the sand molds, which are comparably very accurate for casting. Once the excess sand is removed and casting is finished, accu- racies can be as low as +/- 75 microns (0.003 inch). 4.4.5 Conventional Sand Molding The cruder, cheaper version of casting starting with wooden or plaster patterns is called sand casting. A sand impression is made around the pattern with gates and 158 Solid Freeform Fabrication (SFFl and Rapid Prototyping Chap. 4 risers for the poured metal. This gives tolerances of +1- 375 microns (0.015 inch). Newer developments include: 1. A high-pressure jolt-and-squeeze method: Here mechanical plungers push the sand against the mold at a jolt of 400 psi. This gives a tighter fit of the sand against the pattern and hence better tolerances after casting. 2. Carbon dioxide block molding: Here the interfacing between the sand and tbe pattern is made up of a special material about 12 millimeters (0.5 inch) thick. It is a refractory mix of zircon or very fine silica, bonded with 6% sodium sili- cate, which is then hardened by the passage of carbon dioxide. 4.4.6 Die Casting Die casting is predominantly done by the high-pressure injection of bot zinc into a permanent steel die. Today, the die or mold for this type of casting is almost certain to be milled on a three- or five-axis machine tool. Die costs are relatively high, but smooth components are produced with accu- racies in the range of +1- 75 microns (0.003 inch). However, these high costs for the permanent molds mean that die casting does not really fit into the rapid prototyping family. It is mostly used for large-batch runs of small parts for automobiles or con- sumer products. Since low melting point materials such as zinc alloys are used in the process, component strengths are relatively modest. Today, the injection molding of plastics (Chapter 8) is often preferred over zinc die casting. 4.5 MACHINING METHODS FOR RAPID PROTOTYPING 4.5.1 Overview Chapter 7 deals with the generalized machining operation including the mechanics of the process. This chapter focuses on advances in CAD/CAM software that allow CNC machining to be more of a "turnkey rapid prototyping" process. One goal isto fully automate the links between CAD and fabrication. Another goal is to minimize the intensely hands-on craft operations (e.g., process planning and fixturing) that demand the services of a skilled machinist. CyberCut™ is an Internet-based experimental fabrication test bed for CNC machining. The service allows client designers on the Internet to create mechanical components and submit appropriate files to a remote server for process planning and fabrication on an open-architecture CNe machine tool. Rapid tool-path planning, novel fixturing devices, and sensor-based precision machining techniques allow the original designer to quickly obtain a high-strength, good-tolerance component (Smith and Wright, 1996). 4.5.2 WebCAD: Design for Machining "on the Internet" on the Client Side A key idea is to use a "process aware" CAD tool during the design of the part. This prototype system is called WebCAD (Kim et al., 1999). Sun Microsystems' Java™_ a portable, object-oriented, robust programming language similar to C+ +-is being 4.5 Machining Methods for Rapid Prototyping 159 used as a framework for serving mini-applications.The GUI isa 2.5D feature-based- design system that uses the destructive solidgeometry (DSG) idea introduced in the last chapter (Cutkosky and Tenenbaum, 1990;Sarma and Wright, 1996).Recall that the user starts out with a prismatic stock and removes primitives or "chunks" of material. By contrast, conventional constructive solid geometry (CSG) means building up a part incrementally from "nothingness." In the "destructive" paradigm, instead of allowing arbitrary removal, the user is also constrained to removing cer- tain shapes of material, referred to as features. These features take the form of pockets, blind holes, and through-holes. WebCAD also contains an expert system capturing rules for machinability.At the top of Figure 4.17, the designer is shown being guided by these rules. For example, a "forbidden zone" is imposed around a through-hole feature to prevent it from being designed too close to an edge. In the event that the designer violates a rule, a "pop-up" window advises on an appropriate remedy by moving the hole fur- ther into the block-typically by its radius dimension. WebCAD also uses a WYSIWYG ("what you see iswhat you get") environment, with explicit cutting tool selection and visible comer radii on pockets. At the time of this writing, further improvements also include Ireeform surface editing and selection ofdifferent cutting tool sets depending on final fabrication location (Kim, 2000). The rationale for imposing destructive features upon the designer is that each of these features can readily be mapped to a standard CNC milling process. The scheme thus resembles the interaction between a word processor and a printer regarding the "printability" of the document. It is easyto criticize that the restriction to DSG limits the set of parts that can be designed. However, the key advantage of this design environment is that the design-to-manufacture process is more deter- ministic than conventional methods, which rely on unconstrained design and on looser links between design, planning, and fabrication. Experience shows that designers are somewhat concerned at first that they are constrained; however, the opportunity to be provided with the correct part veryquicklyproves to be attractive. 4.5.3 Planning on the Server Side When the client's design is finished, the resulting geometry can be sent over the Internet to a process planner residing on a remote server. An automated software pipeline takes the geometry and determines in which order the features should be cut, the exact tool paths to traverse, cutting feeds,and spindle speeds for a machine tool. Macroplanning orders the individual features and creates the specific machining setups in fixtures.CyberCut's current macroplanner is a feature recogni- tion module that can reliably extract the volumetric features from 2.5V parts. The output of the module isnotjust a machining feature set but a rich data structure that also givesimportant connectivity information that relates one feature to another. A recent advance in the macroplanner is its ability to recognize and process features containing freefonn surfaces (Sundararajan and Wright, 2000). 2A 2.50 feature is a machinable feature with arbitrary outside peripheral contour and a uniform sweeping depth into the block being machined 160 Solid Freeform Fabrication (SFF)and Rapid Prototyping Chap. 4 Desip. (WebCAD) A novel JAVA-based design with WYSIWYG part features A design consultant: 'The hole is too dose to the edge" Freeforrn feature based manufacture Custom control algorithms for precision and flexibility Sensor integration FIgure 4.17 The CyberCul project: integrated design, planning, and f"bricotion. MicropJanning and tool-path planning decompose the DSG volumes into spe- cific tool motions. Colloquially speaking, this is the step that is like lawn mowing: each volume has to be carved out with a specific tool diameter, and the overlap between each strip has to be considered in relation to part tolerance and surface roughness. The comers of pockets Gust like lawns) might require special methods. Mill;l'opiaD • Make hole 1 before pocket 2 Change setup • Refixture Mkro_ • Use tool 1 Cutatl600rprn • Feedrate Jrnm/s • Perform one pass F_brlcatloll 4.6 Management of Technology 161 Freeform surfaces must be divided into flat and steep regions.The flat regions are machined with a projecting spiral tool-path pattern, and the steep regions are machined with a slicingtool-path pattern.This yieldsgood tool-path uniformity and only moderate computational complexity.The decomposition aims at minimizing machining time within the constraints of the specified surface roughness, tolerance, and machine tool safety.The time of individual operations can also be estimated, which can be sent back to the designer, providing an early estimation of the machining costs. 4.5.4 Fabrication by Milling on the Server Side Finally,a stream of NC commands performs the machining on an open-architecture millingmachine.(Bycontrast, ifit had been determined alongthe waythat the client would have been better served by SFF technology,CyberCut can connect to a fused deposition modeling [FDM] machine.) The particular milling machine being used is an open-architecture machine that can execute advanced tool-path trajectories. One example isamachine path interpolator that can traverse complicated freeform paths represented by NURBS. This ability brings a richer surface generation capability to the ostensibly traditional machining process.By doing so, it continues to compete with the SFFmethods from the point ofviewof geometric complexity (Hillaire et al., 1998). More details of the open-architecture machine tool itself are reserved for Chapter 7 on machining. 4.6 MANAGEMENT OF TECHNOLOGY 4.6.1 Summary Solid freeform fabrication (SFF) techniques and conventional rapid prototyping techniques such as machining and casting are key technologies for improving product realization cyclesand reducing time-to-market. The availabilityof lntemet- based software tools (Berners-Lee, 1989; Java, 1995) has accelerated the links between CAD and prototype creation. The Internet has also allowed accessto man- ufacturing sitesin many different countries (Smith and Wright, 1996;DeMeter et al., 1995;Mitsuishi et al., 1992;Frost and Cutkosky,1996;Finin et al.,1994).In summary: •SLA emerges as the most commercially accepted of the newer SFF proto- typing methods. •SLS emerges as a very useful, commercially accepted alternative to SLA for overhanging structures needing support and for stronger materials that can be sintered rather than photocured. •FDM is an excellent choice for an in-house machine that can be used by an industrial design team for an iterative series of prototypes. •LOM isexcellent for larger components. •3-D printing and planarization using the inexpensive Sanders and Z- Corporation machines are gaining commercial acceptance at the time of this writing. Very inexpensive 3-D digitizers coupled with miniature milling '62 Solid Freeform Fabrication (SFF) and Rapid Prototyping Chap.4 machines are also entering the market (see URL for Roland Digital Group at the end of the chapter} . • Machining and casting remain central to the rapid prototyping field, especially for high-strength prototypes and longer batch runs of several prototypes. 4.6.2 Future Trends The accuracy of processes such as stereolithography and selective laser sintering is improving as time goes by. These processes are being used more and more in the cre- ation of the original, first master for casting and for plastic injection molding. As con- sumer products such asstereos, cellular phones, personal digital assistants (PDAs), and handheld computers (Richards and Brodersen, 1995) begin to look more aerody- namic, there is a need to create molds that have unusual curves and reentrant shapes; these are easy to create in SLA or S~ especially in comparison with machining. It has nonetheless been emphasized that SFF's accuracy is poor in comparison with machining. Overhanging structures may be hard to support during fabrication, and there are problems with component warping during curing. While simple shapes might have accuracies of .ct-: 25 to 75 microns (0.001 to 0.003 inch), the range for complex shapes might be as high as +1- 125 to 375 microns (0.005to 0.015 inch). While the strength of SFF parts is today less than machined parts, new trends are closing the gap.The FDM parts made by the Stratasys machine can be formed in near full strengthABS and similar polymers. Cheung and Ogale (1998),for example, have increased the strength of photopolymers by fiber reinforcement. Also, research at Sandia Laboratories on a process called laser engineered net shaping (LENS) is permitting direct fabrication of high-strength metal molds. This and similar projects are modified versions of DTM Inc.'s SLS process. At the same time, CA O/CA M techniques for machining are advancing rapidly. For example, the CyberCut freeform design tools linked to open-architecture milling machines will continue to expand machining's capability (Greenfeld et al., 1989; Schofield et al., 1998; Hillaire et al., 1998).There is a subtle point to be made here: much of the increased activity in the SFF prototyping methods was originally prompted by the poor communication between CAD and CNC machine tools. During the late 1980s, stereolithography's competitive edge over machining came from the fact that the CAD model could be instantly "sliced" and then turned into laser scanning paths for rapid part production. With the CyberCut methodology and open-architecture control, the conventional machining process can be equally com- petitive from an art-to-part speed standpoint, and it continues to give the high- accuracy and product integrity qualities that it always gave. The evidence is thus clear that the capabilities in both the machining and the SFF fields are constantly improving. It has also been noted that several of the methods such as 3-D printing with planarization, SGc, and SDM combine deposi- tion with machining "to get the best of both worlds." Perhaps in a similar way,the 3D Systems' QuickCast method uses "the best of SLA combined with the best of invest- ment casting." In QuickCast, disposable SLA patterns are fabricated with distinctly hollow internal structures. When the ceramic shells for casting are created around these hollow SLA patterns, the latter collapse inward, leaving the casting mold intact 4.7 Glossary 163 and ready for use.This process, described by Jacobs (1996, 183-252), is gaining rapid acceptance commercially. The issues mentioned are predominantly technical. As this chapter draws to a close,it is important to recall an earlier point from Chapter 2 that "prototypes structure the design process" (see Kamath and Liker, 1994).Physical prototypes focus the efforts of a distributed design team, especially if subcontracting is a big part of the process. Perhaps the most important conclusion is this: each manufacturing process will playa vital role at different points in the product development cycle. SFF techniques will be more evident at the front end, machining will be more evident partway through to create highly accurate molds, and plastic injection molding will be most evident in the final high-volume production method for the consumer's product. Once again it must be emphasized that "manufacturing in the large" is an integration of many software tools, physical processes, and market strategies. In summary, rapid prototyping dramatically accelerates time-to-market. • Psychologically, it focuses the attention of the members of the design team in a "learning organization" (see Chapter 2). • Physically, it reduces the time necessary to make a full production die from hardened steel and to launch into mass production. 4.7 GLOSSARY 4.7.1 01. Casting Low-pressure casting, often of liquid zinc, into a machined mold. 4.7.2 Electrodischarge Machining IEDM. The use of an electrode to melt and vaporize the surface of a hard metal. Usually restricted to low rates of metal removal of very hard metals. 4.7.3 G-Codes The standard low-end machine tool command set that gives motion, for example, G1 = linear feed. 4.7.4 Injection Molding Viscous polymer is extruded into a hollow mold (or die) to create a product. 4.7.5 Ink-Jet Printing in 3-0 Rapid prototyping by rolling down a layer of powder and hardening it in selected regions with a binder phase that is printed onto the powder layer. 4.7.6 Investment Casting The word investment is used when time and money are invested in a ceramic shell that is subsequently broken apart and destroyed. The original positive master that is [...]... 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