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July 16, 1999

3 Min Read
Polymer substitution saves cost, adds ductility

Every designer has opinions about specific materials. They go with the territory. But if your opinion of styrenics rules them out when you need impact resistance, you may be passing up a valid, economical alternative to higher-end engineering thermoplastics. During a recent project, Camoplast Thermoplastic Group, a major supplier to Xerox, found that keeping an open mind can pay off.

Design engineers at Camoplast switched from PC/ABS to high-impact polystyrene (HIPS) in a commercial copier door panel for Xerox. After rigorous testing, they proved that the new resin not only reduced cost but also improved ductility and met requirements for rigidity and surface appearance. Surprised? So were design engineers at Xerox.

“We associated the material with clamshell-type molded products, and didn’t feel it would have enough impact strength,” recalls Eke J. Okeke, plastics technical manager for Xerox. “We were proven wrong.”

Hefty Requirements

Camoplast Thermoplastic Group, one of five divisions, operates two plants in Quebec with a total of 16 presses that range from 165 to 2700 tons. In addition to its role as a major supplier to Xerox, the group also molds parts for automotive and recreational vehicle customers.

Omar Khennache, R&D director for Camoplast, explains the situation designers faced when making the switch to HIPS. “Xerox has high standards, especially for the 3-by-3-ft door panels used on the high-end 5090 family of copiers,” he says. “The units fluctuate between high and low temperature extremes, so the doors need to resist impact at the low end and withstand heat at the high end.”

Unlike smaller models, the 5090 copiers are high-cost, high-volume systems subject to a good deal of wear and tear. One model, for example, has a speed of 135 copies per minute and weighs over 2936 lb. Typical end users are commercial printers and government agencies.

The panel doors for the copiers came with a laundry list of requirements, according to Okeke. “They cover most internal electrical, developing, fusing, and software subassemblies. The doors must also be aesthetically pleasing, without blemishes or flow lines. We’re looking for a Class A finish,” explains Okeke. For flammability resistance, the doors must also meet UL 94-5VB at 3-mm wall thickness.

Avoiding Tradeoffs

Previously, Camoplast had used PC/ABS to mold the panel doors, but because the material is hygroscopic, it had to be dried prior to molding. Besides the additional time and energy requirements for this step, the drying process often caused brittleness in the molded parts. “These panels encounter physical abuse and temperature extremes, making brittleness unacceptable,” says Khennache.

Camoplast’s design team evaluated several alternative materials before selecting HIPS. “We needed to avoid warpage by controlling the process, and found that polystyrene was easier to mold,” Khenache notes. “It processes at lower temperatures, but is still just as rigid as the PC/ABS blend that we used previously.”

Polystyrene cleared another major hurdle—impact resistance—with flying colors. Panels molded from Avantra 8130 and 8550 polystyrene (BASF) passed Xerox’s impact resistance tests, which included tests at both high and low temperatures and side impact. These materials are also nondecabrominated flame-retardant grades, which helped the parts pass UL requirements.

Xerox also found the material met specs for dimensional tolerances and passed its gap analysis test. “All parts have to match,” Okeke says. “Plastic parts have to match perfectly with the steel parts used on a portion of the exterior.”

As for the economy of switching to HIPS, several factors added to the savings. First, the raw material costs are lower. The savings in resin cost were reportedly 25 to 30 percent. Second, because PS has a shrinkage factor similar to PC/ABS, Camoplast was also able to use existing molds.

Third, eliminating the drying step saved time and energy costs. Fourth, the quality of the PS surface finish was better than PC/ABS, allowing for the elimination of a sanding/finishing operation and saving more time and energy. Finally, Camoplast is able to use PS regrind. If the material is “clean,” up to 20 percent regrind can be used in the mix.

Contact information
BASF Corp.—Polystyrene
Mount Olive, NJ
Jeff Viola
Phone: (973) 426-3963
Fax: (973) 426-3962
Web: www.basf.com

Camoplastic Thermoplastic Group
Sherbrooke, Quebec
Michel Truax
Phone: (819) 565-3737
Fax: (819) 565-7117
Web: www.camoplast.com

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