PP/PS alloy gets nod for Jeep hardtop
The search for the right material from which to mold hardtops for the DaimlerChrysler Jeep TJ/Wrangler has apparently been settled in favor of a grade of Montell's Hivalloy resin. While the project is...
The search for the right material from which to mold hardtops for the DaimlerChrysler Jeep TJ/Wrangler has apparently been settled in favor of a grade of Montell’s Hivalloy resin. While the project is still in development, the automotive company has cut tools to match the material shrinkage properties of the Montell resin, a decision which makes it the odds-on favorite to be the material used once the part goes into production in the 2001 model year. The resin, a polypropylene/polystyrene grafted alloy co-polymer, has edged out a glass-filled PET supplied by Ticona, mainly because its processability and material properties better match DaimlerChrysler’s requirements.
“In the beginning we were only working with Ticona on this application,” says Russ Fielding, senior manager, advanced body concepts. “Montell became involved later, and since that time, their material seems to better meet the requirements for this part.”
Fielding says one of the primary considerations for selection of the material was robust processability. “The processing window has to be fairly large for this application, otherwise you’d never hit a home run on a daily basis.”
According to Rob Olivero, Montell Hivalloy program manager, the hardtop application requires a material with a good balance between strength, impact and ductility. The material that was initially chosen, called Hivalloy G7155W, has a flexural modulus of 650,000 psi and a six percent elongation. By comparison, the PET has slightly lower values for both properties. Olivero says Hivalloy also has better UV resistance, an important property for the hardtop application. Since then, DaimlerChrysler has decided to also look at a grade of Hivalloy with slightly improved strength, having a modulus of about one million psi.
The single-piece hardtop is being injection molded on a Husky 8800 ton press at Husky Injection Molding System’s technical centre in Novi, MI. The Hivalloy hardtop weighs between 42 and 47 lb. and is displacing a five-piece part made of SMC that weighs 80 lb.
Fielding says the Jeep TJ/Wrangler sells about 55,000 to 60,000 units with hardtops in three colors. Plans are to initially mold white hardtops, which account for two to five thousand parts per year. Expanding production beyond a single color would require either relocating the current press to a facility zoned for production, or the purchase of a new press by DaimlerChrysler’s molding partner, Decoma, says Fielding.