Innovative molding cuts cost, weight of structural plastic parts.

Weight savings, cost reduction, and a wider emphasis on recycling were prominent themes at SPI's recent annual Structural Plastics conference in San Francisco. Two-shell overmolding was proposed as a lower cost alternative to lost-core injection. The weight-saving potential of gas-assist injection molding was demonstrated in new applications, and coinjection illustrated its ability to eliminate expensive finishing steps.

RECYCLING DURABLES

An industry R&D program to investigate the use of recycled plastics in computers and business equipment was announced by the American Plastics Council's Computers and Business Group, based in Washington, D.C. This consortium of resin producers and computer and business-machine OEMs has two projects under way. One is to collect plastic parts from Apple, AT&T Digital Equipment, Hewlett-Packard, IBM and Xerox, and then to sort, grind wash, and separate the recovered materials before recompounding them and molding them into test pieces. The program will examine the ability of mechanical size reduction to liberate the plastic from metals or coatings. It will also look at processing costs and at the resulting plastic properties as compared with virgin. A testing protocol is being developed with Underwriters Laboratories to develop future design specifications for use of recycled plastics.

A second project is to develop a design-for-recycling guide. The team is being led by Dr. Inder Wadhera of IBM. The team reportedly has already made strides in getting major OEMs to agree on universal material-identification codes. IBM has already released two commercial PS/2 PC models and three monitor products that are marked with material-identification codes. In accordance with IBM's Corporate Recycling Standard, these products use a single material for all major plastic components and snap fits for easy disassembly. This summer, IBM plans to begin making housings and other parts from a blend of recycled and virgin resin at its Engineering Center for Environmentally Conscious Products at Research Triangle Park, N.C.

As another example, Xerox Corp. has a goal of incorporating 25% by weight of recycled plastics in its products by 1995, and 50% recycle by 2000. Xerox uses the SPI voluntary materials-identification system where possible and follows the International Standards Organization (ISO) coding system from Europe for engineering resins not covered by the SPI coding system.

TWO-SHELL OVERMOLDING

Two-shell overmolding as a less expensive alternative to lost-core injection molding of complex hollow shapes was discussed by Miles Inc., Pittsburgh. What Miles says will be the first commercial application for the process is currently in development: an air-intake manifold for an 18-22 hp gas-powered engine for small-to-medium-sized industrial equipment. The current manifold of cast aluminum weighs 1.75 lb, vs.0.5 lb in plastic, and requires several costly secondary operations.

Miles says two-shell overmolding was first developed by its German parent, Bayer AG, in 1985. Miles says its process is protected by U.S. patent 4,544,588, though a similar approach has been promoted by Hettinga Equipment Inc., Des Moines, Iowa (see PT, Jan. 90, p. 72). The process involves first molding two mating shells in separate cavities. The shells are temporarily attached together by simple snap or press fits and installed in a second tool for overmolding.

For this air-intake manifold, 30% glass-reinforced nylon 6was chosen for its strength and rigidity in attachment areas as well as resistance to gasoline. All sealing surfaces, O-ring grooves, and attachment through-holes are molded in, and the only secondary operation is ultrasonic welding of metal inserts. Relative to cast aluminum, Miles reported that lost-core injection would have cost 20% more, while two-shell overmolding cost 40% less. (CIRCLE 85)

TWIN-SHEET FORMING

For production of large, flat parts in low volumes, twin-sheet thermoforming was described as a possible alternative to blow molding by Grimm Brothers Plastics Corp. of Wapello, Iowa. Thick-gauge twin-sheet forming can be done on modified conventional single-station thermoformers capable of pressure forming. Theoretically, any product that can be blow molded can be twin-sheet thermoformed. Twin-sheet forming is said to have lower tool costs than blow molding for non-critical applications such as dunnage or pallets with a simple compression flange. In cosmetic applications requiring precise tool alignment, twin-sheet tool costs approach those of blow molds.

Grimm Bros. say twin-sheet has certain advantages for making large, flat parts: Few blow molding machines extrude a parison larger than 30 in., and parisons are round; therefore wide, flat parts are difficult to achieve. Because twin-sheet starts with two thermoplastic sheets that cost 3(}50 /lb more than resin pellets, part cost can be a disadvantage in the beginning. But quick machine setups (15-45 min), plus the lower cost of processing equipment, is said to give twin-sheet a cost advantage over blow molding for low quantities.

FOAM MOLDING MODIFIED PET

Feasibility of foam molding modified PET was demonstrated in a paper from DuPont Co., Wilmington, Del. Historically, use of PET in foamed applications has been limited by the material's rheological properties, crystallization behavior, and inadequate toughness.

But with appropriate material and process modifications, small and large foamed parts with high strength and creep resistance at elevated temperatures were shown to be possible with low-pressure injection molding. Parts weighing over 50 lb could be readily molded at low pressure, DuPont said.

PET for DuPont's experiments was obtained from reclaimed post-consumer soft-drink bottles. The resin was impact modified with an ethylene copolymer and reinforced with short glass fibers. The fibers added stiffness and also served as a foam nucleating agent. The result was molded parts with a low-crystalline skin that was very tough and a more crystalline core that contributed higher stiffness and heat resistance. (CIRCLE 86)

LOWER WEIGHT AND COST

A number of parts exhibited at the conference demonstrated weight and cost savings through innovative molding techniques.

* The conference award winner was a bicycle wheel molded with the lost-core injection molding process by Co-Mack Technology, Dover, Ohio. Relatively low cost was achieved through automated manufacturing of the three-spoke support structure in long-carbon-fiber reinforced nylon.

* A coinjected electronic book for an IBM AS400 minicomputer, also molded by Co-Mack, is said to provide a 25% cost saving over the magnesium enclosure that it replaces. The EMI/RFI shielding is molded in, eliminating the need for expensive plating operations.

* The first application for gas injection of a large tv cabinet (31 in.) was molded by Munekata America, Dalton, Ga. A 43% reduction in weight was achieved, along with 51% reduction in cycle time, as well as a consolidation of 23 parts over the previous model using structural foam and gas counterpressure.

* A backhoe cab door RIM molded by Plastek Corp., Newburyport, Mass., consisted of a steel frame comprising all structural attachment points, which was encapsulated with polyurethane. The design cut the number of parts by a factor of 10 and also the number of molds. Thick sections exceeding 1.5 in. were achieved without sink marks.