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More rapid prototyping systems reach commercialization.

Rapid prototyping, a collection of computer-generated technologies used to fabricate complex 3-D plastic models directly and automotically from computer-aided design (CAD) drawings, was highlighted at the recent National Engineering Show in Chicago.

The field is less than four years old, and the technology is growing at a brisk pace. The news at the show included both the commercial debut of yet another rapid prototyping technique, and several improvements to previously announced systems.



A new laser modeling system capable of building three-dimensional solid prototype parts from CAD drawings was unveiled by Helisys Inc., Torrance, Calif. The new unit, known as the Laminated Object Manufacturing (LOM) system, Model LOM-1015, includes a 25-watt CO2 laser controlled by a computer-controlled, x-y axis positioning system. Michael Feygin, president and system inventor, describes the unit as being equivalent to a "three-dimensional laser printer," transforming CAD drawings into solid models. Feygin developed the system over the last three years at his previous firm known as Hydronetics Inc. in Chicago (see PT, Feb. '89, p. 80).

Precoated sheets of various modeling materials (paper, composites, thermoplastics) are fed into the unit, as parts are "built" from bottom to top. The laser cuts out each cross-sectional "slice" of the solid model from the two-dimensional laminate layer. The laminates are sequentially stacked stop one another in advancing order, with a hot roller sealing each succeeding layer of material on top of the last.

The unit is operated by a 386-type personal computer. CAD data are down-loaded to the PC through a STL input format, which then develops the laser's cutting pattern via a proprietary software package. Once completed, the laminated models can be use as patterns on which to cast tooling and then make near-net-shape prototypes.

Priced at around $100,000, the unit is capable of making models in a 13 x 10 x 15 in. envelope. Feygin says various "beta sites" for first field application of the system will be selected before year end. Future developments for the LOM technology at Helisys will include a desktop version of the modeling system, as well as an expanded version for larger models. (CIRCLE 5)


Selective Laser Sintering (SLS), another rapid-prototyping technology which was commercially introduced in late 1989, is almost into its second generation (PT, Jan. '90, p. 23; March '90, p. 17). SLS is a development of DTM Corp., Austin, Texas, which is now a unit of BFGoodrich.

DTM officials describe the SLS process as being an additive-layer technology. A thin layer of powdered thermoplastic material--such as polycarbonate, PVC, other thermoplastics, or investment-casting wax--is placed on a flat platform in a cylindrical container. A laser beam traces a prescribed pattern across this layer--again based on a 2-D slice of a CAD solid model--melting and sintering the material the beam touches. The process is repeated by lowering the platform, adding a new layer of powder over the previously sintered layer, and thus building up the model from successive layers of sintered material.

Once sintered, the prototypes need no post-curing process. Unsintered powder, which eventually is recycled, remains in the container during the process, supporting the sintered part. The system is designed for unattended powder feeding. The system uses a 386 PC, which accepts CAD information through a STL file. One virtue of the system is the flexibility of using various powdered theermoplastic materials, allowing for limited part function testing.

The build envelope for the SLS system is 12 x 12 x 15 in. DTM currently uses first-generation SLS Model 125 units at its own service bureaus in Austin and at BFGoodrich's R&D lab in Brecksville, Ohio. A larger second-generation version of the SLS system will be introduced by year end, with beta sites expected to be established by the fourth quarter. The new version is expected to use a 40-watt [CO.sub.2] laser in place of the 25-watt laser on current first-generation models. Pricing of the second-generation system will range between $350,000 and $400,000, according to the company. (CIRCLE 6)



A new advance in stereolithography, the first of the new generation of rapid-prototyping systems to hit the market in 1987, is designed to increase the accuracy and dimensional stability of plastic prototypes by as much as 20 times over previous stereolithography results. The new "Weave" system was introduced at the show by 3D Systems Inc., Valencia, Calif., the first firm to commercialize a stereolithography apparatus (SLA).

Like other rapid prototyping systems, the SLA translates CAD data into horizontal slices of a three-dimensional model. The difference is that the SLA builds up the model by scanning a laser beam over a liquid bath of photopolymer, curing successive layers of the part.

3D says the Weave system reduces warpage by solidifying about 96% of the part in the SLA vat before subjecting it to post-curing. The previous method could leave 40-60% of a just-built part as uncured liquid trapped within walls of cured resin. The Weave system increases the surface percentage of the part exposed to the SLA laser during formation. By making two separate laser passes perpendicular to each other, the exposure achieves very tight crosshatch spacing in each layer of the prototype, resulting in near-total solidification. With more complete solidification, there is reduced post-cure shrinkage and enhanced surface finish. Long-term dimensional stability also is improved as a result of fewer built-in stresses.

Another stereolithography system on the market from Quadrax Laser Technologies Inc., Portsmouth, R.I., was introduced at this show last year. It's said to achieve 60-90% curing by means of a higher powered laser (see PT, April '90, p. 13).

Besides the new Weave system, 3D System also is introducing a new software release, known as 3.81, which supports its SLA-250 and SLA-190 units. The company says the 3.81 software release can reduce prototype build time by as much as 50%, as it optimizes resin leveling and laser drawing routines. With the new 3.81 software, the laser moves continually, rather than pausing while the computer processes prototype vector data. It also provides for automatic, real-time measurement and adjustment of laser power, more accurate control of cure depths, and an improved user-friendly menu system with windows. (CIRCLE 7)
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Author:Gabriele, Michael C.
Publication:Plastics Technology
Date:Jun 1, 1991
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