Finally We Get Some Truth on Plastic Body Panels

Doug Smock

June 28, 2011

3 Min Read
Finally We Get Some Truth on Plastic Body Panels

Automotive body panels have been the Holy Grail for plastics for more than 50 years. And it seems as if I've been writing about it for almost that long. The whole idea took off with a bang in 1953 when GM engineers sheathed the first Corvettes with an FRP (fiberglass) body. It was light; it didn't corrode; it resisted dings; it was new.

The major producers of engineering plastics all set up development and sales offices in Detroit and went after the market with a full-court press. There were big successes, such as intake manifolds, fuel tanks, fascia, and instrument panels. But body panels were never a hit. FRP processes are slow and are still only suited for low-production cars. The injection molding of thermoplastics is speedier, but has not been widely used.

The reason? I haven't got one. But I still see the big plastics companies sponsoring concept cars with plastic body panels.

A new book by Detroit auto guy Bob Lutz, however, does get to the issues. Lutz has held executive roles, mostly in product development, with Chrysler, BMW, Ford, and General Motors. His book is called Car Guys vs. Bean Counters: The Battle for the Soul of American Business.

He relates the story of how GM launched the Saturn in 1990 with thermoplastic body panels. The Saturn cars were the first to use all vertical body panels molded in thermoplastics. Saturn commercials used to show shopping carts bouncing off a Saturn without leaving any marks.

Saturns used Pulse alloy for the door skins, and GTX for the quarter panels. Pulse, sold by Dow Plastics, was a blend of polycarbonate and ABS (PC/ABS). GTX was a blend of polyphenylene oxide and polyamide (PPO/PA) by GE Plastics (now Sabic Innovative Plastics).

After reviewing the reasons for using plastic in the Saturn, Lutz states:

"In practice, however, the plastic panels were finicky. They took longer to produce than conventional stamped steel, and they grew and shrank when the temperature changed, requiring the cars to have wide, unappealing gaps around the doors, hood and trunk for clearance."

Plastics have a higher coefficient of linear thermal expansion (CLTE) than steel, and require more space to grow and shrink. Efforts to improve CLTE properties of plastic compounds with fillers and backbone tweaking haven't resolved the problem.

GM moved back to steel for body panels when it launched the Saturn Sky roadster, Aura sedan, and Outlook cross/utility vehicle in 2006, and the Vue CUV in 2007.

Interestingly, when GM first showed the Volt concept car in January 2007, it featured a thermoplastic hood developed by GE Plastics. The production car uses steel for the body panels.

It's also interesting that a lot of the concepts for electric vehicles use plastic body panels in an effort to reduce weight.

This will be fun to watch in the next two years. It will be a battle between perfect fit and finish and lighter weight.

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