Paul E. Sandin - Robot Mechanisms and Mechanical Devices Illustrated (779750), страница 4
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Masking andlayer formation are repeated with the platform being lowered and moved back and forthunder the stations until the 3D model is complete. The wax is then removed by heatingor immersion in a hot water bath to release the prototype.The photomask is then moved to the exposure station, where it is alignedover a work platform and under a collimated UV lamp.Model building begins when the work platform is moved to the rightto a resin application station where a thin layer of photopolymer resin isapplied to the top surface of the work platform and wiped to the desiredthickness. The platform is then moved left to the exposure station, wherethe UV lamp is then turned on and a shutter is opened for a few secondsto expose the resin layer to the mask pattern. Because the UV light is sointense, the layer is fully cured and no secondary curing is needed.The platform is then moved back to the right to the wiper station,where all of resin that was not exposed to UV is removed and discarded.The platform then moves right again to the wax application station,where melted wax is applied and spread into the cavities left by theremoval of the uncured resin.
The wax is hardened at the next station bypressing it against a cooling plate. After that, the platform is moved rightagain to the milling station, where the resin and wax layer are milled to aprecise thickness. The platform piece is then returned to the resin application station, where it is lowered a depth equal to the thickness of thenext layer and more resin is applied.xixxxIntroductionMeanwhile, the opaque toner has been removed from the glass maskand a new mask for the next layer is generated on the same plate. Thecomplete cycle is repeated, and this will continue until the 3D modelencased in the wax matrix is completed. This matrix supports any overhangs or undercuts, so extra support structures are not needed.After the prototype is removed from the process equipment, the wax iseither melted away or dissolved in a washing chamber similar to a dishwasher. The surface of the 3D model is then sanded or polished by othermethods.The SGC process is similar to drop on demand inkjet plotting, amethod that relies on a dual inkjet subsystem that travels on a precisionX-Y drive carriage and deposits both thermoplastic and wax materialsonto the build platform under CAD program control.
The drive carriagealso energizes a flatbed milling subsystem for obtaining the precise vertical height of each layer and the overall object by milling off the excessmaterial.Cubital America Inc., Troy, Michigan, offers the Solider 4600/5600equipment for building prototypes with the SGC process.Selective Laser Sintering (SLS)Selective laser sintering (SLS) is another RP process similar to stereolithography (SL). It creates 3D models from plastic, metal, or ceramicpowders with heat generated by a carbon dioxide infrared (IR)–emittinglaser, as shown in Figure 3. The prototype is fabricated in a cylinder witha piston, which acts as a moving platform, and it is positioned next to acylinder filled with preheated powder. A piston within the powder delivery system rises to eject powder, which is spread by a roller over the topof the build cylinder.
Just before it is applied, the powder is heated further until its temperature is just below its melting pointWhen the laser beam scans the thin layer of powder under the controlof the optical scanner system, it raises the temperature of the powdereven further until it melts or sinters and flows together to form a solidlayer in a pattern obtained from the CAD data.As in other RP processes, the piston or supporting platform is loweredupon completion of each layer and the roller spreads the next layer ofpowder over the previously deposited layer. The process is repeated, witheach layer being fused to the underlying layer, until the 3D prototype iscompleted.The unsintered powder is brushed away and the part removed. Nofinal curing is required, but because the objects are sintered they areporous.
Wax, for example, can be applied to the inner and outer porousIntroductionFigure 3 Selective Laser Sintering (SLS): Loose plastic powder from a reservoir is distributed by roller over the surface of piston in a build cylinder positioned at a depth belowthe table equal to the thickness of a single layer. The powder layer is then scanned by acomputer-controlled carbon dioxide infrared laser that defines the layer and melts thepowder to solidify it. The cylinder is again lowered, more powder is added, and theprocess is repeated so that each new layer bonds to the previous one until the 3D modelis completed.
It is then removed and finished. All unbonded plastic powder can bereused.surfaces, and it can be smoothed by various manual or machine grindingor melting processes. No supports are required in SLS because overhangs and undercuts are supported by the compressed unfused powderwithin the build cylinder.Many different powdered materials have been used in the SLSprocess, including polycarbonate, nylon, and investment casting wax.Polymer-coated metal powder is also being studied as an alternative.
Oneadvantage of the SLS process is that materials such as polycarbonate andnylon are strong and stable enough to permit the model to be used in limited functional and environmental testing. The prototypes can also serveas molds or patterns for casting parts.SLS process equipment is enclosed in a nitrogen-filled chamber that issealed and maintained at a temperature just below the melting point ofthe powder. The nitrogen prevents an explosion that could be caused bythe rapid oxidation of the powder.The SLS process was developed at the University of Texas at Austin,and it has been licensed by the DTM Corporation of Austin, Texas.
Thecompany makes a Sinterstation 2500plus. Another company participating in SLS is EOS GmbH of Germany.xxixxiiIntroductionLaminated-Object Manufacturing (LOM)The Laminated-Object Manufacturing (LOM) process, diagrammed inFigure 4, forms 3D models by cutting, stacking, and bonding successivelayers of paper coated with heat-activated adhesive. The carbon-dioxidelaser beam, directed by an optical system under CAD data control, cutscross-sectional outlines of the prototype in the layers of paper, which arebonded to previous layers to become the prototype.The paper that forms the bottom layer is unwound from a supply rolland pulled across the movable platform.
The laser beam cuts the outlineof each lamination and cross-hatches the waste material within andaround the lamination to make it easier to remove after the prototype iscompleted. The outer waste material web from each lamination is continuously removed by a take-up roll. Finally, a heated roller applies pressure to bond the adhesive coating on each layer cut from the paper to theprevious layer.A new layer of paper is then pulled from a roll into position over theprevious layer, and the cutting, cross hatching, web removal, and bonding procedure is repeated until the model is completed.
When all the layers have been cut and bonded, the excess cross-hatched material in theFigure 4 Laminated Object Manufacturing (LOM): Adhesive-backed paper is fed acrossan elevator platform and a computer-controlled carbon dioxide infrared-emitting lasercuts the outline of a layer of the 3D model and cross-hatches the unused paper.
As morepaper is fed across the first layer, the laser cuts the outline and a heated roller bonds theadhesive of the second layer to the first layer. When all the layers have been cut andbonded, the cross-hatched material is removed to expose the finished model.
The complete model can then be sealed and finished.Introductionform of stacked segments is removed to reveal the finished 3D model.The models made by the LOM have woodlike finishes that can be sandedor polished before being sealed and painted.Using inexpensive, solid-sheet materials makes the 3D LOM modelsmore resistant to deformity and less expensive to produce than modelsmade by other processes, its developers say. These models can be useddirectly as patterns for investment and sand casting, and as forms for silicone molds. The objects made by LOM can be larger than those madeby most other RP processes—up to 30 × 20 × 20 in. (75 × 50 × 50 cm).The LOM process is limited by the ability of the laser to cut throughthe generally thicker lamination materials and the additional work thatmust be done to seal and finish the model’s inner and outer surfaces.Moreover, the laser cutting process burns the paper, forming smoke thatmust be removed from the equipment and room where the LOM processis performed.Helysys Corporation, Torrance, California, manufactures the LOM2030H LOM equipment.
Alternatives to paper including sheet plasticand ceramic and metal-powder-coated tapes have been developed.Other companies offering equipment for building prototypes frompaper laminations are the Schroff Development Corporation, Mission,Kansas, and CAM-LEM, Inc. Schroff manufactures the JP System 5 topermit desktop rapid prototyping.Fused Deposition Modeling (FDM)The Fused Deposition Modeling (FDM) process, diagrammed in Figure 5,forms prototypes from melted thermoplastic filament.
This filament,with a diameter of 0.070 in. (1.78 mm), is fed into a temperaturecontrolled FDM extrusion head where it is heated to a semi-liquid state.It is then extruded and deposited in ultrathin, precise layers on a fixtureless platform under X-Y computer control. Successive laminations ranging in thickness from 0.002 to 0.030 in. (0.05 to 0.76 mm) with wallthicknesses of 0.010 to 0.125 in.
(0.25 to 3.1 mm) adhere to each by thermal fusion to form the 3D model.Structures needed to support overhanging or fragile structures in FDMmodeling must be designed into the CAD data file and fabricated as partof the model. These supports can easily be removed in a later secondaryoperation.All components of FDM systems are contained within temperaturecontrolled enclosures. Four different kinds of inert, nontoxic filamentmaterials are being used in FDM: ABS polymer (acrylonitrile butadienestyrene), high-impact-strength ABS (ABSi), investment casting wax, andxxiiixxivIntroductionFigure 5 Fused Deposition Modeling (FDM): Filaments of thermoplastic are unwoundfrom a spool, passed through a heated extrusion nozzle mounted on a computercontrolled X-Y table, and deposited on the fixtureless platform. The 3D model is formedas the nozzle extruding the heated filament is moved over the platform.
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