A molded plastic transmission accumulator piston is replacing a cast and machined aluminum part, saving Chrysler more than $2 million annually. Testing shows outstanding wear performance.
The piston is made from Ryton polyphenylene sulfide, a high-performance thermoplastic that has a maximum service temperature of 218 °C (424 °F). PPS has not been found to dissolve in any solvent at temperatures below about 200 °C (392 °F).
“This part is being used in almost all Chrysler automatic transmissions,” says Rory Pawl, innovation and trend research manager for the Americas at the Freudenberg-NOK General Partnership (Plymouth, MI). That amounts to be about 7.5 million pistons per year at current build rates.
The PPS is 65 percent filled: half glass and half mineral filler.
Generally speaking, accumulators store hydraulic energy and then return energy when required. In a transmission, the piston moves as automatic transmission fluid pressure builds and then releases at a shift in gears. One accumulator is used per gear. The result is a smoother feeling shift. Weight reduction is about 30 percent per piston.
The parts are molded on 10 single-cavity Minco molds in six 270-ton Arburg injection molding machines. Pawl said the company prefers the quality control of several single cavity tools versus tools with multiple cavities. “If we have a problem, we just pull a tool out,” he says. The parts are molded at Freudenberg-NOK’s Global Fluid Power Division Automotive Lead Center in Findlay, Ohio.
Pawl said that use of plastics provided significantly more design flexibility compared to aluminum. The direct injection area, for example, is optimized for zero waste and dimensional stability. A chamfer near the top of the piston is designed for molding and assembly efficiencies. Chamfers often allow easier part ejection, for example. The groove design was optimized for seal function and piston durability.
The part is a finalist in the 2011 Automotive Innovation Awards program conducted by the Society of Plastics Engineers. Winners will be announced at a dinner in Livonia, MI Nov. 9.