Paul E. Sandin - Robot Mechanisms and Mechanical Devices Illustrated (779750), страница 5
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The hot filamentbonds to the layer below it and hardens. This laserless process can be used to form thinwalled, contoured objects for use as concept models or molds for investment casting. Thecompleted object is removed and smoothed to improve its finish.elastomer. These materials melt at temperatures between 180 and 220ºF(82 and 104ºC).FDM is a proprietary process developed by Stratasys, Eden Prairie,Minnesota. The company offers four different systems.
Its Genisysbenchtop 3D printer has a build volume as large as 8 × 8 × 8 in. (20 × 20× 20 cm), and it prints models from square polyester wafers that arestacked in cassettes. The material is heated and extruded through a 0.01in. (0.25-mm)–diameter hole at a controlled rate. The models are built ona metallic substrate that rests on a table. Stratasys also offers four systems that use spooled material. The FDM2000, another benchtop system, builds parts up to 10 in3 (164 cm3) while the FDM3000, a floorstanding system, builds parts up to 10 × 10 × 16 in. (26 × 26 × 41 cm).Two other floor-standing systems are the FDM 8000, which buildsmodels up to 18 × 18 × 24 in.
(46 × 46 × 61 cm), and the FDM Quantumsystem, which builds models up to 24 × 20 × 24 in. (61 × 51 × 61 cm).All of these systems can be used in an office environment.Stratasys offers two options for forming and removing supports: abreakaway support system and a water-soluble support system. TheIntroductionwater-soluble supports are formed by a separate extrusion head, and theycan be washed away after the model is complete.Three-Dimensional Printing (3DP)The Three-Dimensional Printing (3DP) or inkjet printing process, diagrammed in Figure 6, is similar to Selective Laser Sintering (SLS)except that a multichannel inkjet head and liquid adhesive supplyreplaces the laser. The powder supply cylinder is filled with starch andcellulose powder, which is delivered to the work platform by elevating adelivery piston.
A roller rolls a single layer of powder from the powdercylinder to the upper surface of a piston within a build cylinder. A multichannel inkjet head sprays a water-based liquid adhesive onto the surfaceof the powder to bond it in the shape of a horizontal layer of the model.In successive steps, the build piston is lowered a distance equal to thethickness of one layer while the powder delivery piston pushes up freshpowder, which the roller spreads over the previous layer on the build pis-Figure 6 Three-Dimensional Printing (3DP): Plastic powder from a reservoir is spreadacross a work surface by roller onto a piston of the build cylinder recessed below a tableto a depth equal to one layer thickness in the 3DP process.
Liquid adhesive is thensprayed on the powder to form the contours of the layer. The piston is lowered again,another layer of powder is applied, and more adhesive is sprayed, bonding that layer tothe previous one. This procedure is repeated until the 3D model is complete. It is thenremoved and finished.xxvxxviIntroductionton. This process is repeated until the 3D model is complete.
Any looseexcess powder is brushed away, and wax is coated on the inner and outersurfaces of the model to improve its strength.The 3DP process was developed at the Three-Dimensional PrintingLaboratory at the Massachusetts Institute of Technology, and it has beenlicensed to several companies. One of those firms, the Z Corporation ofSomerville, Massachusetts, uses the original MIT process to form 3Dmodels. It also offers the Z402 3D modeler. Soligen Technologies hasmodified the 3DP process to make ceramic molds for investment casting.Other companies are using the process to manufacture implantabledrugs, make metal tools, and manufacture ceramic filters.Direct-Shell Production Casting (DSPC)The Direct Shell Production Casting (DSPC) process, diagrammed inFigure 7, is similar to the 3DP process except that it is focused on forming molds or shells rather than 3D models.
Consequently, the actual 3Dmodel or prototype must be produced by a later casting process. As in the3DP process, DSPC begins with a CAD file of the desired prototype.Figure 7 Direct Shell Production Casting (DSPC): Ceramic molds rather than 3D modelsare made by DSPC in a layering process similar to other RP methods.
Ceramic powder isspread by roller over the surface of a movable piston that is recessed to the depth of a single layer. Then a binder is sprayed on the ceramic powder under computer control. Thenext layer is bonded to the first by the binder. When all of the layers are complete, thebonded ceramic shell is removed and fired to form a durable mold suitable for use in metalcasting. The mold can be used to cast a prototype.
The DSPC process is considered to bean RP method because it can make molds faster and cheaper than conventional methods.IntroductionTwo specialized kinds of equipment are needed for DSPC: a dedicatedcomputer called a shell-design unit (SDU) and a shell- or moldprocessing unit (SPU). The CAD file is loaded into the SDU to generatethe data needed to define the mold. SDU software also modifies the original design dimensions in the CAD file to compensate for ceramicshrinkage. This software can also add fillets and delete such features asholes or keyways that must be machined after the prototype is cast.The movable platform in DSPC is the piston within the build cylinder.It is lowered to a depth below the rim of the build cylinder equal to thethickness of each layer.
Then a thin layer of fine aluminum oxide (alumina) powder is spread by roller over the platform, and a fine jet of colloidal silica is sprayed precisely onto the powder surface to bond it in theshape of a single mold layer. The piston is then lowered for the next layerand the complete process is repeated until all layers have been formed,completing the entire 3D shell. The excess powder is then removed, andthe mold is fired to convert the bonded powder to monolithic ceramic.After the mold has cooled, it is strong enough to withstand moltenmetal and can function like a conventional investment-casting mold.After the molten metal has cooled, the ceramic shell and any cores orgating are broken away from the prototype.
The casting can then be finished by any of the methods usually used on metal castings.DSPC is a proprietary process of Soligen Technologies, Northridge,California. The company also offers a custom mold manufacturing service.Ballistic Particle Manufacturing (BPM)There are several different names for the Ballistic Particle Manufacturing (BPM) process, diagrammed in Figure 8. Variations of it arealso called inkjet methods. The molten plastic used to form the modeland the hot wax for supporting overhangs or indentations are kept inheated tanks above the build station and delivered to computercontrolled jet heads through thermally insulated tubing.
The jet headssquirt tiny droplets of the materials on the work platform as it is movedby an X-Y table in the pattern needed to form each layer of the 3Dobject. The droplets are deposited only where directed, and they hardenrapidly as they leave the jet heads. A milling cutter is passed over thelayer to mill it to a uniform thickness. Particles that are removed by thecutter are vacuumed away and deposited in a collector.Nozzle operation is monitored carefully by a separate fault-detectionsystem.
After each layer has been deposited, a stripe of each material isdeposited on a narrow strip of paper for thickness measurement by opti-xxviixxviii IntroductionFigure 8 Ballistic Particle Manufacturing (BPM): Heated plastic and wax are depositedon a movable work platform by a computer-controlled X-Y table to form each layer. Aftereach layer is deposited, it is milled to a precise thickness. The platform is lowered and thenext layer is applied. This procedure is repeated until the 3D model is completed. A faultdetection system determines the quality and thickness of the wax and plastic layers anddirects rework if a fault is found. The supporting wax is removed from the 3D model byheating or immersion in a hot liquid bath.cal detectors.
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