Paul E. Sandin - Robot Mechanisms and Mechanical Devices Illustrated, страница 3

Thermoplastic systems are based on procedures for successively melting and fusing solidfilaments or beads of wax or plastic in layers, which harden in the air toform the finished object. Some systems form layers by applying adhesives or binders to materials such as paper, plastic powder, or coatedceramic beads to bond them.The first commercial RP process introduced was stereolithography in1987, followed by a succession of others.

Most of the commercial RPprocesses are now available in Europe and Japan as well as the UnitedStates. They have become multinational businesses through branchoffices, affiliates, and franchises.Each of the RP processes focuses on specific market segments, takinginto account their requirements for model size, durability, fabricationspeed, and finish in the light of anticipated economic benefits and cost.Some processes are not effective in making large models, and eachprocess results in a model with a different finish.

This introduces an economic tradeoff of higher price for smoother surfaces versus additionalcost and labor of manual or machine finishing by sanding or polishing.xvxviIntroductionRapid prototyping is now also seen as an integral part of the evenlarger but not well defined rapid tooling (RT) market.

Concept modelingaddresses the early stages of the design process, whereas RT concentrates on production tooling or mold making.Some concept modeling equipment, also called 3D or office printers,are self-contained desktop or benchtop manufacturing units smallenough and inexpensive enough to permit prototype fabrication to bedone in an office environment. These units include provision for the containment or venting of any smoke or noxious chemical vapors that willbe released during the model’s fabrication.Computer-Aided Design PreparationThe RP process begins when the object is drawn on the screen of a CADworkstation or personal computer to provide the digital data base.

Then,in a post-design data processing step, computer software slices the objectmathematically into a finite number of horizontal layers in generating anSTL (Solid Transfer Language) file. The thickness of the “slices” canrange from 0.0025 to 0.5 in. (0.06 to 13 mm) depending on the RPprocess selected. The STL file is then converted to a file that is compatible with the specific 3D “printer” or processor that will construct themodel.The digitized data then guides a laser, X-Y table, optics, or otherapparatus that actually builds the model in a process comparable tobuilding a high-rise building one story at a time. Slice thickness mighthave to be modified in some RP processes during model building tocompensate for material shrinkage.Prototyping ChoicesAll of the commercial RP methods depend on computers, but four ofthem depend on laser beams to cut or fuse each lamination, or provideenough heat to sinter or melt certain kinds of materials.

The fourprocesses that make use of lasers are Directed-Light Fabrication (DLF),Laminated-Object Manufacturing (LOM), Selective Laser Sintering(SLS), and Stereolithography (SL); the five processes that do not requirelasers are Ballistic Particle Manufacturing (BPM), Direct-ShellProduction Casting (DSPC), Fused-Deposition Modeling (FDM), SolidGround Curing (SGC), and 3D Printing (3DP).IntroductionxviiStereolithography (SL)The stereolithographic (SL) process is performed on the equipmentshown in Figure 1. The movable platform on which the 3D model isformed is initially immersed in a vat of liquid photopolymer resin to alevel just below its surface so that a thin layer of the resin covers it. TheSL equipment is located in a sealed chamber to prevent the escape offumes from the resin vat.The resin changes from a liquid to a solid when exposed to the ultraviolet (UV) light from a low-power, highly focused laser.

The UV laserbeam is focused on an X-Y mirror in a computer-controlled beam-shaping and scanning system so that it draws the outline of the lowest crosssection layer of the object being built on the film of photopolymer resin.After the first layer is completely traced, the laser is then directed toscan the traced areas of resin to solidify the model’s first cross section.The laser beam can harden the layer down to a depth of 0.0025 to 0.0300in. (0.06 to 0.8 mm). The laser beam scans at speeds up to 350 in./s (890cm/s).

The photopolymer not scanned by the laser beam remains a liquid. In general, the thinner the resin film (slice thickness), the higher theresolution or more refined the finish of the completed model. Whenmodel surface finish is important, layer thicknesses are set for 0.0050 in.(0.13 mm) or less.The table is then submerged under computer control to the specifieddepth so that the next layer of liquid polymer flows over the first hardened layer.

The tracing, hardening, and recoating steps are repeated,layer-by-layer, until the complete 3D model is built on the platformwithin the resin vat.Figure 1 Stereolithography (SL):A computer-controlledneon–helium ultraviolet light(UV)–emitting laser outlines eachlayer of a 3D model in a thin liquid film of UV-curable photopolymer on a platform submerged avat of the resin. The laser thenscans the outlined area to solidifythe layer, or “slice.” The platformis then lowered into the liquid toa depth equal to layer thickness,and the process is repeated foreach layer until the 3D model iscomplete.

Photopolymer notexposed to UV remains liquid.The model is them removed forfinishing.xviiiIntroductionBecause the photopolymer used in the SL process tends to curl or sagas it cures, models with overhangs or unsupported horizontal sectionsmust be reinforced with supporting structures: walls, gussets, orcolumns. Without support, parts of the model can sag or break off beforethe polymer has fully set. Provision for forming these supports isincluded in the digitized fabrication data.

Each scan of the laser formssupport layers where necessary while forming the layers of the model.When model fabrication is complete, it is raised from the polymer vatand resin is allowed to drain off; any excess can be removed manuallyfrom the model’s surfaces. The SL process leaves the model only partially polymerized, with only about half of its fully cured strength. Themodel is then finally cured by exposing it to intense UV light in theenclosed chamber of post-curing apparatus (PCA). The UV completesthe hardening or curing of the liquid polymer by linking its molecules inchainlike formations.

As a final step, any supports that were required areremoved, and the model’s surfaces are sanded or polished. Polymerssuch as urethane acrylate resins can be milled, drilled, bored, and tapped,and their outer surfaces can be polished, painted, or coated with sprayedon metal.The liquid SL photopolymers are similar to the photosensitive UVcurable polymers used to form masks on semiconductor wafers for etching and plating features on integrated circuits. Resins can be formulatedto solidify under either UV or visible light.The SL process was the first to gain commercial acceptance, and itstill accounts for the largest base of installed RP systems. 3D Systems ofValencia, California, is a company that manufactures stereolithographyequipment for its proprietary SLA process.

It offers the ThermoJet SolidObject Printer. The SLA process can build a model within a volumemeasuring 10 × 7.5 × 8 in. (25 × 19 × 20 cm). It also offers the SLA 7000system, which can form objects within a volume of 20 × 20 × 23.62 in.(51 × 51 × 60 cm). Aaroflex, Inc.

of Fairfax, Virginia, manufactures theAacura 22 solid-state SL system and operates AIM, an RP manufacturing service.Solid Ground Curing (SGC)Solid ground curing (SGC) (or the “solider process”) is a multistep inline process that is diagrammed in Figure 2. It begins when a photomaskfor the first layer of the 3D model is generated by the equipment shownat the far left. An electron gun writes a charge pattern of the photomaskon a clear glass plate, and opaque toner is transferred electrostatically tothe plate to form the photolithographic pattern in a xerographic process.IntroductionFigure 2 Solid Ground Curing (SGC): First, a photomask is generated on a glass plate bya xerographic process. Liquid photopolymer is applied to the work platform to form alayer, and the platform is moved under the photomask and a strong UV source thatdefines and hardens the layer.

The platform then moves to a station for excess polymerremoval before wax is applied over the hardened layer to fill in margins and spaces. Afterthe wax is cooled, excess polymer and wax are milled off to form the first “slice.” The firstphotomask is erased, and a second mask is formed on the same glass plate.