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Making models one layer at a time.

Based on part-building technology developed at the University of Texas at Austin by Carl R. Deckard and Joseph J. Beaman, DTM Corp. has the exclusive license for the selective laser sintering (SLS) process used in its SLS Model 125. The process involves using laser heating to fuse powders together into laminated parts.

The principal attraction of the SLS process is its ability to fabricate parts from several different materials including polycarbonate, polyvinyl chloride, investment casting wax, and soon, nylon and acrylonitrile butadiene styrene (ABS) plastic. In time, DTM researchers believe it will also be possible to make SLS composites from metal and ceramics. Selective laser sintering employs a 23-watt carbon dioxide laser to heat successive layers of different heat-fusable powders deposited by a roller mechanism. Maximum part dimensions are 12 inches in diameter and 15 inches deep.

The Model 125 is a prototype system that is now in the process of being redesigned for commercial production. Beta-test units will be out late this year and production units should be available early next year. The price is expected to be between $300,000 and $400,000. DTM Corp. currently has the prototype SLS 125 system operating at sales and service centers in Breeksville, Ohio, and near its Austin headquarters.

Sintering refers to a process in which powders are caused to adhere to each other by means of externally applied (usually thermal) energy, explained Beaman, who is a professor of mechanical engineering at the University of Texas at Austin as well as a member of the DTM Corp. board of directors. In general, sintering occurs when a particle's viscosity drops due to heating and allows surface tension to overcome viscosity.

In the SLS process, a thin layer of powder is deposited into a cylindrical part-build chamber by a mechanism consisting of a powder-feed piston and a leveling drum or roller. The piston meters a single layer of powder up to the work surface and the leveling roller spreads it into the part-build chamber. Infrared heaters even out thermal stresses in the powder. The chambers are filled with inert gases to control laser-induced powder combustion.

At this stage, the powder is raster scanned with a high-power laser beam, which fuses the particles together at temperatures of about 150 [degrees] C. This procedure is continued layer by layer, based on successive part cross sections from the CAD data. The laser power is modulated so that the volume inside the part is sintered and the remainder is not. The unsintered material remains in place to support the next layer of powder, which aids in limiting distortion.

Another layer of powder is laid down by the leveling drum and then the laser sinters the top layer to those underneath, heating down to a depth of 20 to 25 mils. When completed, the part is removed from the unsintered powder. Models are usually built in layers of 0.005 inch. Accuracies for the Model 125 are quoted at [+ or -] 0.0010 inch.

SLS parts are constructed out of three materials: polycarbonate for injection-molded parts (80 to 95 percent density), polyvinyl) chloride for templates in secondary molding operations (60 to 80 percent density), and investment casting wax (near full density). DTM will soon offer SLS components in nylon and ABS plastic.

One advantage of the DTM system is that the powder usually requires no support structures as the part forms. However, for large thin flat parts of polycarbonate an the casting wax, tie-down supports ed to stop the edges from curling.

The CAD data needed to construct a part by SLS are similar to those used in stereolithography processes, except that instead of tracing out the part, a raster scan approach is used. The Model 125 slices the CAD model on the fly with each layer. After slicing, the geometric information is reduced to on/off toggle points for the laser.

Beaman said that a prototype SLS machine capable of fusing metal and ceramic powders is now being designed at the University of Texas at Austin. He expects it to be completed by midyear. The machine will use a high-power laser and will require preheating to high temperatures. Researchers at the school are now studying the selective laser sintering of copper-tin alloys and an alumina-ammonium phosphate powder that forms a glassy ceramic composite which could find use in investment casting molds. Beaman indicated that future work will focus on aluminum and tungsten carbide-cobalt powders.

Fast Plastics

Scott Crump, president of Stratasys Inc. and inventor of the company's 3D Modeler rapid prototyping system, knows about product development delays from experience. In a prior business venture, he lost a substantial investment when Japanese competition beat his new product to market.

Crump's response was to spend two years designing and building the 3D Modeler, a rapid prototyping system based on the deposition of extruded thermoplastic materials in layers to construct parts up to 12 x 12 x 12 inches. Combining speed and nontoxic materials, Stratasys is targeting design engineers in an attempt to give them the opportunity to examine a plastic model of an intended design more quickly. Investment casting is another key market for the technology.

Founded in 1988, Stratasys has 3D Modeler machines at five beta sites. Having recently received venture funding from Battery Ventures (Boston), "We're now gearing up for volume manufacturing, with production versions available in the second quarter of 1991," Crump said. A turnkey system, including the modeler, slicing software, and a Silicon Graphics Unix workstation, is priced at $178,000.

In Stratasys' fused deposition modeling process, a spool of 50-mil thermoplastic filament feeds into the unit's heated extruding head like wire feeds into an automatic welder. Inside the flying extruder head, the filament is melted to liquid at 180 [degrees] F by a resistance heater. As the head moves in the x and y axes according to a numerical control tool path generated by the system's software, the thermoplastic material is extruded out a nozzle by a precision pump like toothpaste. An elevator controls the z direction.

After extrusion, the new layer is wiped, or sheared, on the fly. Layer thickness varies from 0.001 to 0.050 inch. Accuracy on the 3D Modeler is [+ or -] 0.005 inch. It produces layers rapidly, but speed is a function of the degree of precision required by the operator, so high-tolerance parts take longer to build.

All the extruded materials for the 3D Modeler are nontoxic, which is important for operating in the office environment, Crump said. He also stressed that special enclosure protect the operator from toxic gases or lasers are not required, nor are curing agents or ovens needed.

Materials currently available for use with the 3D Modeler are a machinable wax for presentation models, a smooth investment casting wax, and a tough nylon-like thermoplastic material with a proprietary formulation. Under development are a silicone rubber sealing material, an automotive body foam material, and a transparent version of the nylon-like material.

The Stratasys system produces models from three-dimensional wire-frame, surface, or solid CAD models. The slicing software, called StrataSlice, takes 3-D CAD data and processes it into successive cross-secotional layer on the fly. StrataSlice, which is based on nonuniform rational B-splines, produces an extruder head tool path in a form similar to numerical control code.

Laser Cookie Cutter

Helisys Inc. (formerly Hydronetics Inc.) of Torrance, Calif., uses a process called laminated object manufacturing (LOM) to produce parts from bonded paper, plastic, metal or composite sheet stock. Company president Michael Feygin described the LOM machine, which was introduced late last year, as a "three-dimensional printer that incrementally stacks two-dimensional images."

To manufacture objects, the LOM system sequentially bonds a layer of 0.002- to 0.010-inch rolled sheet material (plastic, paper, foil, or glass or fiber composites) to a stack of previously formed laminations and then cuts it to shape with a 40-watt carbon dioxide laser beam that is tuned to a depth of one lamination. The scanning beam follows the perimeters of CAD-generated part cross sections like a laser cookie cutter. Parts up to 15 x 10 x 15 inches can be constructed with accuracies to [+ or -] 0.005 inch. The use of sheet materials means that models do not shrink or distort.

When material is to be cut out of the center of a part, it can be removed immediately by an automated vacuum after each layer is formed or the material can be left inside to support subsequent laminations.

As with most other systems, the Helisys software slices 3-D CAD models into thin cross sections. The slices correspond to the sheet thickness to be used in the object. The system accepts .STL files as input.

Feygin claimed that the LOM process is fast, produces parts that are less fragile and more dimensionally precise, and uses cheaper materials than other model-building processes. Helisys is seeking organizations interested in participating in its beta-test program. Beta units cost $75,000. Future commercial versions will be priced at $100,000.

Free-Form Fabrication

A team of researchers at the Massachusetts Institute of Technology (Cambridge) is developing a free-form fabrication technology called three-dimensional printing, with the goal of direct production of tooling and functional prototypes from CAD models. The research project is funded by the National Science Foundation and an industrial consortium that includes General Motors Corp. (Detroit), United Technologies Corp. (Hartford, Conn.), and Howmet Corp. (Greenwich, Conn.).

MIT's three-dimensional printing system uses a continuous-jet inkjet printing head to selectively deposit binders on layers of powders, explained Emanuel Sachs, assistant professor of mechanical engineering. The result is a solid object composed of sequentially printed two-dimensional layers. "We believe that three-dimensional printing has a fundamentally lower cost structure than other technologies," he said.

The research has focused primarily on the use of ceramic powders to produce ceramic cores and shells for metal casting and on porous ceramic preforms for metal-ceramic composites produced by infiltrating pressurized molten metal.

"So far, small parts of alumina have been produced with colloidal silica as the binder," Sachs said. Some of these parts have been used as cores for the investment casting of nickel-superalloy components. "We plan on making parts of silica, zirconia, and silicon carbide soon, and we're also starting to investigate metal powders such as steel, stainless steel, and nickel-based superalloys."

In three-dimensional printing, each layer begins with a thin distribution of powder spread over a powder bed by a cylindrical rod, an arrangement similar to that employed in DTM's SLS system. From a CAD model of the desired part, a slicing algorithm creates the cross-sectional layers to be built. An inkjet printing head primed with binder material is raster-scanned over the powder bed by a stepper motor-driven x, y positioning device that is controlled by an 80386-based computer. The binder is distributed in very small droplets by means of continuous-jet printing heads. When they impact, the binder droplets join powder particles to form a solid.

In a continuous-jet head, a pressurized stream of liquid binder breaks into droplets when it is vibrated by a piezoelectric ceramic as it exits an orifice. The droplets are selectively charged as they pass through a capacitor and then deflected by an electric field. The inkjet nozzle is "on" until the droplet stream is deflected onto a catching device, whereupon the device is "off." Nozzle reliability--more specifically, freedom from clogging--is a primary issue in this technology, Sachs said.

After scanning, the piston that supports the powder bed and the part in progress lowers so that the next powder layer can be spread and selectively joined. This layer-by-layer process repeats until the part is completed. Following a heat treatment, the loose powder is removed, leaving the fabricated part. The current machine's build volume is about 3 cubic inches:

"We're designing a new system with 10 inkjet nozzles," Sachs said. The team's eventual goal is a scaled-up version with a 100-nozzle print head that is capable of building parts up to 12 x 12 x 24 inches.

Ballistic Manufacturing

Perception Systems Inc., a six-man firm in Easley, S.C., is developing a model-building process that resembles inkjet printing, called ballistic particle manufacturing (BPM). The BPM system interprets CAD model cross sections to spray their patterns onto a worksurface with fine jets of molten wax. Wax layers are deposited on one another to build the desired part.

Perception Systems plans to address two application niches in the rapid prototyping field: investment casters and design engineers. According to Kendrick Richardson, director of research and development, "We expect to have a working prototype in about 12 to 18 months, then we'll set up several prototypes with investment casters and run the machines for awhile. Further down the road, we plan to develop office systems that sell for under $50,000.

"Our philosophy is to use existing technology such as inkjet printing pumps and organic wax chemistry," he said. To date, the staff has modified drop-on-demand inkjet mechanisms from printers to spray 50-micron droplets of wax at a rate of 10,000 droplets per second. These piezoelectric pump mechanisms operate when an electric charge is applied to a piezoelectric cylinder behind the faceplate. The resultant mechanical impulse creates a shock wave that ejects a tiny droplet of molten wax from inside the cylinder and out the faceplate orifice.

Richardson and his staff are building an array of 32 inkjet pumps that will act in unison to rapidly lay down a large quantity of wax. The droplets will be precisely positioned in the x and y axes by robotic-type mechanisms actuated by servo and stepper motors.

Upon impact, the molten droplets bond to previously deposited wax layers. After each layer is deposited, the baseplate lowers a specified amount to maintain the distance between faceplate and substrate. Accuracy in the z-axis is maintained by a simple feedback loop involving an optical or ultrasonic proximity sensor. If the distance is less than specified, the material flow will be decreased to compensate, creating a slightly thinner layer. If the distance is greater than desired, the inkjet array will deposit a somewhat thicker layer.

As the wax layers are deposited, support structures for overhangs and voids will be built in tandem from polyethylene glycol, which is a water-soluble synthetic wax. After the part is completed, a warm water bath the supports away.
COPYRIGHT 1991 American Society of Mechanical Engineers
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Title Annotation:engineering models
Author:Ashley, Steven
Publication:Mechanical Engineering-CIME
Date:Apr 1, 1991
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