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Automotive Industry

Making The Bed

Automotive Industries, Oct, 1999 by Gerry Kobe

Next fall, Chevrolet will introduce a high-volume, all-plastic pickup truck bed on its full-size, extended cab Silverado. If it's a hit with consumers, kiss steel -- and bedliners -- goodbye.

Talk about Holy Grails. The notion of a rust-flee, dent-free, composite pickup truck bed has been as attractive and elusive as perpetual motion. Every truck manufacturer in the world has been working on it, but a magnet still clings to the bed of anything you can now buy.

And while drop-in truck bedliners enjoy a 60% dealer penetration, they are at best a stop-gap measure that actually promote rust, are prone to warpage and shrinking, and have questionable resistance to gouging.

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In response, both Ford and General Motors are introducing composite cargo beds next year, although the approaches and implications are much different.

Mid-year, Ford will introduce a sheet molding composite (SMC) box on its Explorer SportTrac (see sidebar). But since it is only four feet long and built in low volume, it is less likely to have industry changing potential.

Chevrolet, on the other hand, is featuring a 6.5-foot structural reaction injection molded (SRIM) box as an option on its hotselling extended cab Silverado beginning next fall. With luck, it hopes this marks the dawn of a new era in truck making.

GM's foray into plastic beds actually goes back to the 1980s when it studied such a program for the S10. The research was halted in 1990 during the economic downturn, in favor of dealer-installed bedliners. But in 1993, customer clinics for the full-size GMT800 showed a strong interest for composite boxes. This prompted a new internal program, while at the same time GM offered the lessons it learned on the S10 project to a newly formed USCAR effort called the Automotive Composites Consortium (ACC). The ACC focus was to develop a process (see August 1998 AI, p.45) while GM's goal was to develop a product.

"The two ended up being similar, but our internal program moved faster," says Thomas Jensen, engineering group manager for GM truck box development. "On our SRIM box we worked with an Ohio-based molder called MFG and our tier two suppliers were Owens Corning and Bayer. MFG actually developed the process, but because they are a small, family-owned company they didn't want to take on the volume of actual production. Through competitive bidding, Cambridge will be doing the molding out of Huntington, Ind."

The process produces a complete box inner including floor, sidewalls, side rails and headboard as one molded part. A robot with a "chopper" on the end lays down glass strand on a perforated screen preform. Cycle time is about two minutes. The preform is then put in a double shuttle molding press that allows one mold to be prepared while another part is molding. Cycle time is about 4.5 minutes. The quick cure Bayer polyurethane resin is not ultraviolet stable and will "chalk" if not coated, so the part is grained during molding and is spatter painted in black to match the black substrate.

Although Bayer is developing UV stabilizers that will not impact the moldability of the resin, Jensen says the added paint makes repair an easy process. Gouged or damaged surfaces can be filled with new material and when spatter painted again the repair is invisible. It is also helpful that black is the only color, and that is not likely to change even if stabilizers allowed molding in color. Jensen says black beds carry an upscale image as established by today's bedliners.

GM's choice of materials is also a topic of hot debate, particularly for Ford. Jensen says GM did not dismiss the viability of SMC and researched high-glass content vinylester SMC before it decided on SRIM. He says the issues that tipped the scales were manufacturing requirements and product resistance to moisture.

For example, an SRIM mold capable of making an eight-foot cargo box (GM's next project) operates at just 200 psi. But an SMC mold would require five to six times the tonnage, making it among the largest in the world and expensive. It also is not proven out yet, whereas GM has eight-foot SRIM boxes in the field. Additionally, tests in humidity chambers over long periods show that vinylester SMC shows up to a 50% drop in some key material properties due to moisture absorption. In contrast, polyurethane maintains 90% or more of its original properties, according to GM.

The outer fenders on the GM trucks are reinforced reaction injection molded (RRIM) units supplied by Budd. GM actually switched from SMC to RRIM in 1999 (see January 1998 AI, p.45) and carries the technology over to the composite box trucks. Compared to steel, the composite box saves 20 pounds and the fenders save an additional 30 pounds.

The new bed will be a Chevrolet exclusive initially, brand named "Protec" and identified by a badge on the tailgate. Cost is not determined, but Jensen plans to be competitive in cost with a steel bed using a liner. Cost of a bedliner averages about $225.

Automotive Industry

Making The Bed

Automotive Industries, Oct, 1999 by Gerry Kobe

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