Conference showcases specialty injection molding technologies.

Structural Plastics '92 revealed a widening circle of commercial uses for three specialized injection molding technologies. Held recently in Dallas, the annual SPI-sponsored conference highlighted the benefits of gas-assist injection molding as well as structural-web and coinjection molding.

Through technical papers and a parts competition, the show particularly demonstrated the ability of these injection molding variants to slash material costs and cycle times while providing good surface finishes.

GAS INJECTION SUCCESSES

In the design competition, Xerox Corp. displayed a new color copier made with gas injection. Sajar Plastics Inc. of Middlefield, Ohio, uses its own Nitrojection process to mold 12 of the machine's 16 exterior cover parts from an ABS/PC blend. Owing to the quality of the finish, the parts need only a single mist coat of water-based paint.

From a cost perspective, the process has allowed substantial material savings. According to Sajar chief engineer Richard Seman, these large, partially hollow parts have a total wall thickness of only 3.5 mm and a combined weight of 45 lb. By contrast, a comparable conventionally molded part would be more than 4-mm thick and weigh close to 60 lb.

Another entry in the design competition, a 5-lb PS television cabinet from Japan's Tohoku Munekata Co., also relied on gas-injection technology to achieve reductions in weight and materials. According to Hirojumi Tateyama, the cabinet's wall thickness averages 2.5 mm, compared with 6 mm for a similar structural-foam part. Perhaps more importantly, the process allowed the company to simplify the design, reducing assembly time significantly. Measured against structural foam, gas injection permitted a 30% saving in materials, 50% drop in assembly time and a 50% reduction in cycle times.

CUTTING COST OF CONDUCTIVITY

A paper from Kevin Quinn of LNP Engineering Plastics, Exton, Pa., demonstrated how gas injection can help cut the cost of electrically conductive plastic compounds for static dissipation or EMI shielding, chiefly in larger structural applications.

Comparing Cinpres II gas-assist with conventional and structural foam molding, LNP used the three process to make test bars from its EMI-X and Stat-Kon compounds. These ABS, polycarbonate, nylon 66, and polyethersulfone (PES) compounds each contained either carbon fibers, carbon powders or stainless-steel fibers. In addition, LNP molded samples from a new inherently antistatic ABS based on a conductive polymeric additive.

All three processes produced samples that retained their static-dissipation and EMI-shielding properties and "reasonable mechanical properties," Quinn says. Gas injection resulted in properties comparable to, and often better than, those obtained with structural foam. At the same time, the Cinpres II process brought about a 27% weight reduction while structural foam caused a 16-25% weight drop.

STRUCTURAL WEB GOES COMMERCIAL

The design competition also exhibited a commercial use for a less well known gas process. Plastic Research Corp. of Fenton, Mich., used the "structural web" process from Johnson Controls, Inc. of Manchester, Mich., to produce a chair base that took first place honors in the recreation/leisure category. Measuring roughly 22 x 22 x 3 in., the partially hollow 5-lb base was molded from GE Plastics' Noryl GTX PPO/nylon alloy and finished with three coats of polyurethane paint.

About 18 months ago, Plastic Research became the first custom molder to license the system, which was introduced in 1982 (see PT, May '82, p. 13; June '89, p. 72). Since then, the company has used structural web for the chair base, a basketball backboard, and shelves.

According to CEO Carl Schwartz, the structural-web system permitted a 30% reduction in cycle times and 20% resin savings compared with a solid part. He also says this low-pressure process produces parts without any molded-in stresses. "I'm a big believer in the process," Schwartz says. "It's never failed us." (CIRCLE 3)

Aside from Plastic Research, two large proprietary molders currently use the structural-web system, notes Ed Hunerberg, engineering manager for Johnson Controls' plastics division.

RECYCLING WITH COINJECTION

Another older process, coinjection or "sandwich" molding, uses a common nozzle to shoot two resin components into the mold, encapsulating one within the other. The resins' laminar flow keeps the two materials separate throughout the molding. In the design competition, Co-Mack Technology of Carlsbad, Calif., illustrated the process's capacity for putting regrind to use. The company's coinjected end cover for a Hewlett-Packard plotter has a foamed PC/ABS core made from 100% industrial scrap generated by Hewlett-Packard's own molding operations. The outer skin is virgin PC/ABS.

Co-Mack collects the core scrap material from Hewlett-Packard at a lower cost than buying virgin, according to Co-Mack president Joseph McRoskey. Other savings come from the cover's molded-in texture, which eliminates finishing operations.

McRoskey adds that every unit made with the regrind has exceeded Hewlett-Packard's physical-property requirements. And cosmetic appearance has not suffered because the skin layer remains unchanged.