Plastic Displaces PlasticGM chooses RRIM Over SMC for GMT800 Panel

Body shop workers at General Motors Corp.'s Silverado/Sierra factories manually lift a big sport side fenderand fasten it to a pickup truck rolling down the assembly line.

It's worth noting because in the past workers needed hydraulic assist equipment to handle the sheet molding composite (SMC) fenders used on the GMT800's predecessor, the GMT400. The new sport side panels on the Silverado/Sierra pickups are made from a new class of reinforced reaction injection molded (RRIM) polyurea polymer. Each part weighs 20.5 lbs. (9.3 kg) and is 38% lighter than the rear fender on the GMT400.

It's just one of several manufacturing procedures simplified by using the largest production RRIM body panel ever, the automaker says.

GM, working with the Dow Chemical Co. and Decoma International Inc.'s Polyrim Div., decided to use RRIM material - featuring the first commercial application of Dow's new Spectrim HH 390 polymer - for GMT800 sport sides in order to meet various product requirements for the all-new pickups. Importantly, the fenders had to be processed through GM assembly plants like steel body panels. That means surviving plunges into hot anti-corrosion baths used to protect metal bodies (called ELPO dips) and even hotter trips through paint bake ovens.

"Previously, it (the SMC sport side fenders) could not stand the ELPO tanks and oven temperatures, which are nearing 4001/2F (1821/2C). So the assembly plants would have to have more space to route these panels around the ELPO tanks. We said that's unacceptable. We're going to have an efficient assembly process where these panels are going to go straight through the system like anything else," says Maryanne Combs, GM director of body structures.

"That was really the toughest challenge - getting a material that could withstand that 4001/2F and come through that and accept paint at the same time the rest of the vehicle is being painted without having any warping or degradation in the material properties."

Unlike the glass fiber-reinforced SMC, which is relatively brittle and would crack in high temperatures, the concern among GM engineers about the RRIM material was warping. "We didn't want it to turn back to almost liquid and soften. So those were the development stages with the supplier to ensure that they had the right types of filler in the material to keep it stable through that temperature," says Ms. Combs.

As a result of developing a composite material that could withstand the high temperatures, a new system to fasten the panel to the body also had to be developed. That's because the RRIM fenders expand by three-quarters of an inch from the time they arrive at the plant to when they roll out the door on a brand new GMT800 pickup. "If you had it (the RRIM fender) bolted on the way you bolt on steel, it'd buckle," says Randy Scott, marketing manager for body panels at Dow Automotive, which began working on the project in 1994.

The result is a new procedure that requires 53% fewer fasteners because holes and slots are molded into the fender, so secondary drilling and punching operations aren't needed. The sport side fender essentially is attached only where it meets the cab. "They bolt it on there. And that allows it to grow while it is going through the ELPO dip and oven. It grows that three-quarters of inch backward. Then at general assembly where the truck is finished off, they bolt it on at the other end," says Mr. Scott.

Dow and GM also were able to reduce piece costs by 4% and investment expenses by 20% because the new material uses existing molding equipment.

With the advances made by the RRIM fender is it possible that plastics could further displace steel in exterior body panels? "We'd love to see plastic on every vehicle, all over the place," says Mr. Scott. "I think you're going to see more and more smaller builds going to plastic modules. It depends on the program, but the breakeven point seems to be somewhere around 150,000 units. It's not going to beat out steel if it's over 200,000-a-year build because of the (tooling) economics."

GM, which is known to be preparing composite pickup boxes for production by July 2000 at its Ft. Wayne, IN, Silverado/Sierra facility, is just glad it has options. "From an engineering perspective, I like having choices," says Ms. Combs. "In the composite world we have a choice of RRIM in addition to the SMC. SMC still has a lot of value, especially if you were looking at a horizontal panel, like a hood. That just adds to the choices we already had with the different types of steel and alternative materials like aluminum or magnesium," she says. "Each have their place in the vehicle. Each of those materials brings a different trade-off and balance among the imperatives that get passed down to us from the vehicle team."