Automotive plastics

Plastics Engineering, Sept, 1997 by Victor Wigotsky

Under the hood

Increased efficiency, including the need to more tightly pack the engine compartments, are driving evaluation of high-temperature plastics for underhood applications. Ray Dugas, automotive sales director, Ticona North America (formerly Hoechst Technical Polymers), sees growing potential for the company's current automotive entries, including Forton polyphenylene sulfide (PPS); Vectra liquid-crystal polymers; and Celcon acetal copolymer.

Ticona says the first thermoplastic internal load-bearing part for automotive transmissions saved General Motors about 30% in weight and more than 10% in cost, compared with aluminum. An accumulator piston, the part is molded to 0.02 mm tolerances in linear PPS for the Series 4L60E and 4T60E transmissions. (The material has a UL continous-use rating of 428 [degrees] F). Costly machining and cleaning, typically necessary with die-cast aluminum components, are eliminated by the switch to the injection molded plastic. The pistons, 45 to 50 mm in diameter and 19 to 51 mm long, depending on the transmission model, feed collected transmission fluid to different ports for smooth gear shifting. The glass-and mineral-filled Fortron 6165 accumulator pistons provide a tensile strength of 19,000 psi and a flexural modulus of 2.8 million psi, and withstand internal pressures of about 220 psi while in contact with transmission fluid at temperatures up to 300 [degrees] F. The plastic alternative to the previous aluminum design also must retain the tight tolerances and dimensional stability to avoid softening or bowing and maintain effective seals to prevent blow-through of the fluid. The as-molded tolerances, with no need for secondary operations, can be matched in metal only with multiple machining steps, according to Webster Plastics, the precision molder

While PPS is more costly than aluminum, the higher initial material cost is more than counterbalanced by the savings in total production costs. Demonstration of the viability of thermoplastic parts in the demanding load-bearing automotive environment is stimulating consideration of other possible demanding underhood applications, says senior project engineer Ken Mehl at GM's Power Train Division, in Ypsilanti, Mich.

Increasing recognition of the ability of selected thermoplastics to withstand demanding combinations of high mechanical loads, wide temperature variations, and aggressive chemicals is also demonstrated by another new application by Delphi Chassis Systems, in which linear PPS replaced cold-rolled steel for a break-system air valve. Specifications included ability to withstand a 15,900 psi compressive load from-40 [degrees] F to 250 [degrees] F in the harsh brake fluid. The 132 grams of the original machined steel valve is reduced to only 18 grams with Ticona's Fortron 6165 A4, a 65% mineral/glass fiber-filled PPS, with the overall comparative cost being matched despite the more expensive plastic raw material. The safety-focused brake-system air valve, planned to be produced in six sizes totaling about four million parts per year, exhibits no more than 0.20% expansion and 0.35% shrink after 96 hours at 250 [degrees] F, eight hours in boiling water, and 30 days in brake fluid at 200 [degrees] F.

Underhood stresses are also managed with an up to 100% recycled-content polyester cover - molded of Ticona's Impet 610R glass-and mineral-filled PET - that protects the throttle linkage, pressure sensor, and air resonator sensor in the Ford F150 pickup. Believed to be the first underhood use of resin with up to 100% post-consumer recycled content derived from beverage bottles, the cover holds its shape at temperatures to 350 [degrees] F and withstands contact with gasoline, oil, coolant, and wiper fluid.

Both Ticona's Dugas and Alan Wilson, sales manager Automotive Exterior/Interior, attest to a growing interest of the OEMs and Tier 1 suppliers in evaluating thermoplastics for higher-performance, cost-effective operation. They say that Ticona's focus on smaller-volume, batch-type processes enhances the company's ability to deliver products specially developed to meet required specifications in a range of formulations for high-temperature underhood electrical/electronic and mechanical applications.

Integrated air-fuel units

Siemens Automotive's air cleaner and resonator induction and composite intake manifold products are examples of the trend toward increased integration of components, They include a V-8 manifold currently in production for Jaguar, which features two integrated fuel passages that replace self-standing fuel rails. Also, a product currently beginning production for Volkswagen is considered to be the first "hybrid" composite intake manifold that uses both the lost core and vibration welded "shell" plastic processing technologies.

David Geran, director, Business Development, Integrated Air-Fuel Systems, anticipates that this type of design approach will grow in the future. The VW manifold, made of nylon 6, will be delivered to the OEM engine plant with the fuel system, throttle, and associated components assembled as a complete fuel-air handling module.