21st Century Manufacturing Episode 1 Part 8 ppt

134 Solid Freeform Fabrication (SFF) and Rapid Prototyping Chap. 4 Fipre 4.2 Stereolithography (SLA), based on the commercially published brochures of 3D Systems Inc. The helium-cadmium laser in the SLA-250 cures and fuses successive layers of resin. These descend on top of each other on the elevator until the whole part is formed. at which point the object is lifted from the vat. TABLE 4.1 History of SLA Date Person(s) Cnmpany and location Activity 19705 A. Herbert 1970s H.Kodama 1970:; C.Hull 1986 C.Hul! andRFreed November 3D Systems 1987 3M.Minneapolis R&D NagoyaPrefecture Research,Japan R&D UItTaViolet Products,California R&D 3D SystemsInc., formed from Patent secured UltraViolet Products, California 3D Systems Demonstration of the SLA-l (which became the SLA_2S0) at the Autofact show in Detroit leg, forming a solid shape. Then, the use of a helium-cadmium laser created more energy and more focused solidification patterns than a simple VV arc lamp. Finally, the rapidly decreasing costs of microprocessors during the early 1980spaved the way for the tessellation routines during CAD and the control of the lasers in the actual SLA machine. Formed object X-¥movabt, laser Blade position Laser curable liquid Liquid surface 4.2 Stereolithography: A General Overview '35 One can also imagine the excitement these first inventors felt, as they saw the first layer of SLA material solidifying on the surface of a vat of resin: viewed at an angle it resembles the first layers of ice solidifying on a pond in early winter. In production, once this first layer is cured, the elevator type stage lowers by 50 to 200 microns (0.002 to 0.008 inch) depending on the desired accuracy, and further layers are cured and connected by self-fusing to the previous ones.At the end of the process, the elevator rises and the component is lifted out and cured in its entirety. Postcuring is needed, probably overnight, before the prototype isready for use.Hand sanding may be required to mitigate the stair-stepping effect described later. Note that the object in Figure 4.2 has overhanging areas about halfway down its height dimension. During the actual process these need to be supported by slender sacrificial columns. Without these, the horizontal part of the component sags. Additional hand finishing is needed to snap out these slender sacrificial columns and sand any small stubs away from the surface. 4.2.2 StereoUthography Details: The" .STL" File Format Introduced by 3D Systems Inc. in 1987, the ".STL" file format has become the de facto standard, even though other "direct slice" methods have been tried. The" .STL" method tessellates the CAD model with triangles just like the hexagons and pen- tagons on the surface of a soccer ball. The ".STL" file is (a) a header, (b) the number of triangles, and (c) a list of the triangle description by vertices and the normal vector to the triangle. Table 4.2 shows the layout. The size of the ".STL" file is (50 x number of triangles) + 84. Thus a 1O,OOO-triangleobject needs 500,084 bytes. TABLE4.2 The N .5Tl N File Format Entity Described by The header The number of triangles For each tessellation triangle (50 bytes of information) Normal vector I Normalvector J Normal vector K First vertex X First vertex Y Fint vertex Z Second vertex X Second vertex Y Second vertex Z Third vertex X Third vertex Y Third vertex Z Attribute 80 bytes Unsigned long integer (4 bytes) See below Floating point integer (4 bytes) Floating point integer (4 bytes) Floating point integer (4 bytes) Floating point integer (4 bytes) Ploanng polnt integer te bytes) Flontingpo;ot integer {4bytes) Floating point integer (4 bytes) Floating point integer (4 bytes) Floating point integer (4 bytes) Floating point integer (4 bytes) FLoating point integer (4 bytes) Floating point integer (4 bytes) Unsigned inleger (2 bytes) 13. Solid Freeform Fabrication (SFF) and Rapid Prototyping Chap.4 Two rules govern the triangle descriptions (Figures 4.3 and 4.4). 1. The right-hand counterclockwise rule, or "ccw rule," is a corkscrew acting out- ward on the soccer ball, to order the vertices and the normal vector. 2. The vertex-to-vertex rule, which insists that the vertices on an adjacent triangle link to the neighbor and that no vertices meet a neighboring edge. 4.2.3 Stereoli1hography Details: C-Slice Processing When the" .STL" file arrives at the rapid prototyping bureau, the slicing begins as follows: • Sort the" .STL" triangles into "z values" (this establishes the layers). •Find the boundary segments (gives contiguous internal and external pocket/shape contours). • Create boundary polylines. • Apply edge compensations (based on operator's knowledge of laser physics). • Compare with adjacent layers to minimize stair-stepping on chamfered sides. • Smooth boundaries. Fipre4.3 Rules for tessellation. 6 2" r , ," F'iJtlre4.4 Vertex-to-vertex rule means that a vertex cannot join to a random point somewhere un an "ull'" Each v"rt"x hllllto meet another vertex 00 the neighboring triangle. Normal 4.2 Stereolithography: AGeneral Overview 137 • Output boundary data. •Treat next cross section. 4.2.4 Stereolithography Details: The Resin The photocurable liquid was developed for printing and for furniture lacquer/sealant. To avoid liquid that had carcinogenic solvents, the UV curing process was developed. Lasers provide more direct energy and allowed the invention of SLA once com- puters were powerful enough to create a tessellation. SLA is a low-energy curing process compared with SLS (using a CO 2 laser). Photopolymerization is defined as linking small molecules (monomers) into larger molecules (polymers) comprised of many monomer units. Vinyl monomers have a carbon-carbon (C=C) double bond attached to complex groups donated by "R." In the original resin, the monomer groups are only weakly connected to their neighbors by van der Waals bonds. As the laser acts on the bonds, the C=C bonds break. The broken monomer groups connect to each other, forming long chains (see Table 4.3). TABLE 4.3 Polymerization Weak: van der waals bonds between the adjacent chains StrongcovalentbondsalOllgchains The bonding between such chains then creates three key effects: • The liquid gels into a solid. • The density increases. • The shear strength increases. Although the original vinyl monomers are already cross-linked, they get much more strength from the formation of the covalent bonds in the long chains. 4.2.5 Stereolithography Details: The SLA Manufacturing Process To create any individual layer, the laser traces out the boundaries of alayer first. This is called bordering; imagine a large elastic band or loop lying on the surface. Second, a hatching or weaving pattern crosses the entire area. Third, the hatched areas are filled in, causing the final gelling and solidification (Figure 4.5). After each layer is formed, the laser scanning moves to the next layer. How- ever, some careful process planning is needed to create the accuracy of only a few thousandths of an inch. Details that control accuracy are presented after Figure 4.5. HlC=CH H,C ~ T" R 138 Solid Freeform Fabrication (SFF) and Rapid Prototyping Chap. 4 '\ I 1/ Figure 4.5 Establishing the border, then hatching and filling (filling is shown on just one square) Note: These steps are for an SLA-500 machine at the time a/writing. The details for the SLA-250 are slightly different, and, in addition, new refinements are constantly taking place. 4.2.5.1 Step I. Preparation afthe Script The part building needs instructions on the desired accuracyTypically, a layer thick- ness of 100 microns (0.004 inch) is the average build layer. However, it may range from 50 to 200 microns (0.002 to 0.008 inch) depending on the desired accuracy. Also the Zephyr blade sweeping times, and the" z-wait" times, need to be programmed in. These are described later. 4.2.5.2 Step 2. Leveling and Laser Calibration SLA resins undergo 5% to 7% total volume shrinkage, and of this amount, 50% to 70%, occurs in the vat during polymerization (Jacobs, 1992). Since the liquid level is always shrinking down, a sensor must be installed to follow the level. If the vat is not at the desired height for the beginning of the run, a plunger mechanism adjusts the 4.2 Stereolithography: A General Overview 139 fluid level by fluid displacement. Also, it is crucial to adjust the laser position with reflective "eyes" at the corners of the machine's stage. In fact, this check of laser posi- tion occurs just before each layer is done. 4.2.5.3 Step 3. Making the Initial Supports The first few runs with the laser are not for the part itself but for small supports that the actual part will rest upon. The supports can be viewed as small feet, rather like those on a heavy sofa or piano: they are needed on the bottom of the part to lift the lowest layer off the floor of the elevator platform. In particular, the supports are needed: • So that the Zephyr blade will not hit the platform •To compensate for platform distortion • So that it is easier to remove the finished part • Internal supports are also needed for any "overhanging" structures When making the supports, after the first laser cured layer is formed, the stage needs to be pulled down about 12 mm (0.5 inch) for the SLA·5oo(Jacobs, 1992).This "deep dip" allows the viscous, honeylike fluid to more easily flow over the surface of the first layer of the supports. The elevator then rises up to be positioned 100microns (0.004 inch) below the surface. It is usual to wait about 5 seconds and then do the laser curing again. This creates the second layer -but still, this is concerned with the supports, not the part itself. This procedure repeats until the supporting stubs are large enough. The operator usually makes these decisions. 4.2.5.4 Step 4. Creating the Actual Parr The procedure to make the actual part (not the supports) is somewhat different. Once the supports are finalized, the first bottom surface of the part is generated by the "bordering + hatching + filling" described earlier. The elevator descends by 100microns (0.004 inch) and then waits typically for 45 seconds. This time is programmed in by the operator. It is a recommendation from the SLA fluid supplier as the time needed for the full curing to occur of a part layer. Note that although the laser has begun the polymerization process, it still takes up to 45 seconds for the full effect of polymerization to occur and to harden the layer enough to build subsequent layers on top of it. After the 45-second wait, the first layer is hardened enough for the Zephyr blade to sweep over the surface and pre~ cisely set the 100 micron (0.004 inch) layer of liquid for the second polymerization. 4.2.5.5 Step 5. Sweeping Using the Zephyr Blade At first glance, the Zephyr blade looks like a "hard squeegee" used to clean a car window. In fact, it has a long, hollow cavity between two adjacent blades, and this cavity isunder the influence of a slight vacuum pump. This draws SLA liquid into the bottom of the blade. Thus, as the blade sweeps over the surface, it is "charged" with liquid and more easily and uniformly deposits the next liquid layer onto the first. At the same time the sweeping blade distributes the SLA liquid evenly. Note that the 140 Solid Freeform Fabrication (SFF) and Rapid Prototyping Chap. 4 honeylike SLA fluid is very viscous, and it needs the distribution of the vacuumized Zephyr blade to get an even surface. As the Zephyr blade traverses the whole vat, it removes excess resin in some areas, and yet because it is"charged" with resin, it distributes and fills any areas that lack resin. The sweep takes about 5 seconds (Jacobs, 1992) unless a hollowlike part is being made where the viscous fluid inside the hollow takes longer to follow the blade. The sweep gives a uniform thin layer, but given the viscosity of the fluid, there is a tendency for resin to adhere to the blade, followed by separation and a "bulge" just downstream from the part's leading edge. 4.2.5.6 Step 6. "Z-Wait" of about 15 Seconds Even after all the adjustments and sweeping, a "crease" exists around the edge of the part at the solid-liquid interface. The "z-wait" allows a relaxation of this effect to a flatter, smoother resin surface. 4.2.5.7 Step 7. Extra Skin Filling At the very end of the process, more intense hatching may be desirable on the top surface of the part. Very closely spaced line vectors cause more intense solidification structures on the up-facing surfaces. It is likely that similar patterns would have been done on the down-facing outer skin in Step 4. 4.2.5.8 Step 8. Final Steps The final steps include: • Draining excess resin from any inner or depressed cavities • Cleaning and rinsing with solvents • Snapping out bridgeworks • Hand sanding and polishing •Postcuring in a broad spectrum UV light source 4.2.6 Stereolithography Details: Laser-Based Manufacturing and Prototyping During stereolithography, selective laser sintering, or any laser-based process, many details of the "laser energy delivered" to the resin (or powder for SLS) control solid- ification and the accuracy that can be achieved. First consider penetration depth. Note that the bottom of each SLA layer has to adhere to the previous layer, and so the topic of main interest is the "energy at depth z" of the laser. Lasers give much more energy (i.e., are able to cause more "polymerization by irradlence") than reg- ular arc lamps. But as they travel down through the resin or powder they do never- theless decay exponentially by the Beer-Lambert exponential Jawof absorption: H(x,y.d = H(x.y.OJ ex p(- ~) (4.1) A critical exposure H(c) isneeded to "gel" the resin. D p isa resin constant defined by the depth of a particular resin that results in a reduction of irtadiance level to lie (= 112.718)of the H, level on the surface (Figure 4.6).That is,at a depth ot r = D p the 4.2 Stereolithography: A General Overview '4' x Radius F1gure4.6 Gaussian decay of laser across the surface. irradiance is - 37% of R o . For the SLA-250, typical values are given by Jacobs (1992) as follows, and it is of interest to relate laser behavior to resin solidification: Nominal laser power = (P L ) = 15 milliwatts Central spot size = (2W o ) = 0.25 millimeters For the whole spot the Gaussian irradiance curve controls the basic physics, and like any point source of light it decays from the center. Across the surface, as opposed to down through the surface (Equation 4.1), the laser decays as follows: ( (2)") H(x,y,o) = H(r,o) = Roexp - -wI where Wo is the ~GaUSSian half width (Figure 4.6). Thus, at r = W o , (4.2) H = Hoe- 2 = O.135R o It can also be shown that (4.3) = 30.56 watts per cm 2 (4.4) If the scan speed is 200 mm per second, the scanning laser's exposure time on a given area is i, = 2~o = 1.25 milliseconds (4.5) 142 Solid Freeform Fabrication (SFFJand Rapid Prototyping Chap. 4 The laser exposure's average energy density is then E(average) = Haverage X t, = 38.2 mJ/cm 2 (4.6) The analysis now proceeds to calculate the polymerization ability from the laser's photon flux. It is necessary to use Planck's equation to find the photon energy first: E ) ~!!E _ ~6~_ ;~_}!r3~ ~stx J3 ~J:0~o~m~) (4.7) (photon, ~ 11. ~ 3.25 X 10-5 em E(pho,on<) = 6.1 X 10 -19 Joules per photon (4.8) A is the laser wavelength c is the speed of light h is Planck's constant Denoting that N ph = number of photons per square centimeter hitting the resin surface: (4.9) This flux ofphotons penetrates into the resin and acts on the polymer chains to cause polymerization. Even if the photochemical efficiency is only 50%, the {C=C! bonds will polymerize to {C-c q. 4.2.7 Selective Laser Sintering (SLS) Another very popular method is selective laser sintertng (SLS), commercialized by the DTM Corporation. In many respects SLS is similar to SLA except that the laser is used to sinter and fuse powder rather than phorocure a polymeric liquid. The first step is to prepare the ".sTUSLI" files as described earlier. Inside the SLS machine, a thin layer of fusible powder is laid down and heated to just below its melting point by infrared heating panels at the side of the chamber. Then a laser sin- ters and fuses the desired pattern of the first slice of the object in the powder. Next, this first fused slice descends, the roller spreads out another layer of powder, and the process repeats (Figure 4.7). In comparison with SLA, this process can rely on the supporting strength of the unfused powder around the partially fused object. Therefore, support columns for any overhanging parts of the component are not needed. This allows the creation of rather delicate, lacelike objects. Nevertheless hand finishing is still needed to improve the inevitable stair-stepping. Also, SLS parts have a rough, grainy appear- ance from the sintering process, and it is often preferable to hand smooth the sur- faces. Another difficulty is maintaining the temperature of the powder at a few degrees below melting. This is done with the infrared panels, but maintaining an even temperature over a large mass of powder requires long periods of stabilization before sintenng by the laser can be started. 4.2 Stereolithography: A General Overview 143 i JIIpre 4.7 Selective laser sintering (SLS), based on commercially published brochures from the DTM Corporation. 4.2.8 Leminated Object Modeling (LOM! Laminated object modeling (LOM) was developed by Helisys Inc., and like SLA and SLS, it was first offered commercially in the period from 1987 to 199(J. In LOM, the laser is used to cut the top slice of a stack of paper that is progressively glued together. After each profile has been cut by the laser (shown at the bottom right of Figure 4.8), the roll of paper is advanced, a new layer isglued onto the stack, and the process is repeated. After fabrication, some trimming, hand finishing, and curing are needed. For larger components, especially in the automobile industry, LOM is often preferred over the SLA or SLS processes. 4.2.9 Fused Deposition Modeling IFDM) Fused deposition modeling (FDM) was developed by Stratasys Inc. and is executed on machines called the FDM 1650, 2()(M), or 8()(M) series. Figure 4.9 shows that the material is supplied as a filament from spool. The overall geometry and system are reminiscent of icing a cake. The filament melts as it flows through a heated delivery head and emerges as a thin ribbon through an exit nozzle. The nozzle is guided around by CNC code. and the viscous ribbon of polymer is &fadualJ.y~,uiJtup from a I [...]... 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 12 5 microns ( +1- 0.003 to 0.005 inch)depends on part geometry • Stereolithography: + 1- 25 to 12 5 microns (+ 1- 0.0 01 to 0.005 inch)-depends on part geometry • Plastic... modeling Solid ground curing Selectlve laser simermg Machine SLA-250 SLA-500 3-D Modeler LOM-I0IS Solider 5600 SinteTSlalioo2000 "Assumes 35 parts built simultaneously (hr:min) 7:25 Total part cost $13 3.94 7:03 18 7.95 12 :39 344.94 11 :02 11 : 21 4:55 10 9.40 88 .70'" 19 9,23 ... deposition modeling Solid ground curing 3-D prtnnng followed by machining TABLE powder Much Many Faster 32 X 22x 20X 20 14 X 20 12 x 15 x 15 X 13 16 .7 Compariscn-c-Chrvsler Done Machines Since 15 .7 20 X 20x 23 to x lOX 10 Prototyping Was x 13 .8 X 13 .8 Report," with Such Vol 1, No.6, 19 92 Machines Such as the Sinterstatian Then Total process time Rapid prototyping process Stereolithography Stereclithography... pattern is applied 4.2 .12 Shape Deposition Manufacturing 15 0M) In some of the two processes described earlier, for example, 3~D printing 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. ,19 90; Weiss and Prinz, 19 95; Weiss et aI 19 97; Weiss and Prinz, 19 98) The goals for SDM... mold (prototyping version): +1- 50 lu 10 0 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 (19 82 ) for the CNC controlled version Solid Freeform Fabrication (SFF) and Rapid Prototyping 15 0 • Electrodischarge machining:... technology (Figure 4 .10 b) 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., 19 92, 2000) 4.2 .11 Solid Ground Curing (SGCI Solid ground curing was introduced by Cubital Inc A schematic diagram of the process is shown in Figure 4 .11 .Thc quickest way to... cosmetic work on the mold will give as good a plastic part as the cast mold CAp design CAD~esign Large layer thickness F'iple 4 .13 Medium layer thickness The stair-stepping approximation in SFF processes C.t(DdeSign Fine layer thickness 4.3 Comparisons between Prototyping Processes 15 1 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... 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 15 3 4.3.7 Commercial Comparisons... x x x x Part Size Comparison (as of March Partsi:;e capability (in.) ro x io« 10 3D Systems, Inc 3D Systems, Inc 3D Systems, Inc Stratasya Inc Hellsys.Inc Cubiral America, Inc DTMCorp DTMCorp SGC5600 Sinlerstation2000 Sinterstation2500 Rapid Prototyping Benchmarking June 2000) Company SLA-250 SLA-350 SLA-500 FDM2000 LOM-2030H 4.7 as the 3D Modeler 2500P1u• Have Process, Speed Test Reported 19 92 Note... inFIgure 4 .10 a 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 Solid Freeform Fabrication (SFF) and Rapid Prototyping 14 8 Chap.4, Step 5: source piston up, Step 1: collect powder build piston down :·:r~i I Steps 1- 5 are . Modeler LOM-I0IS Solider 5600 SinteTSlalioo2000 7:25 7:03 12 :39 11 :02 11 : 21 4:55 $13 3.94 18 7.95 344.94 10 9.40 88 .70'" 19 9,23 "Assumes 35parts built simultaneously. . 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 12 x 15 15 X 13 x 16 .7 TABLE 4.7 Rapid Prototyping Process, Speed and. laser sintering: +1- 75 to 12 5 microns ( +1- 0.003 to 0.005 inch)- depends on part geometry • Stereolithography: + 1- 25 to 12 5 microns (+ 1- 0.0 01 to 0.005 inch)-depends on part geometry • Plastic