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Corvette Sheet Molded Composite

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Not a Lightweight at All:  New Composite from GM Is Stiff Enough for Structure, Resistant to Heat

WARREN, Mich., July 24, 1998/PRNewswire/ -- The automotive consumer may not even be aware of GM's latest product innovation.  But a new lightweight composite material called "SMC 3374" is revolutionizing the structure of the car.  It is a plastic material stiff enough to be a structural part, and tough enough to withstand the heat of an engine compartment.  The latest in a long line of materials known generically as "sheet molded composite", or "SMC", this new product from the Materials and Processes Laboratory at the General Motors Technical Center reduces the weight and number of parts in a car.  The result:  better mileage, increased durability, and in some cases, added features for the consumer.

The weight savings and superior performance of an internal structural part by itself doesn't have much effect on the driving experience, notes Hamid Kia, a senior staff research engineer who led GM's effort to develop the lightweight structural composite.  But what SMC 3374 does is free GM's engineers and designers to use those weight- and part-savings elsewhere in the car.  For example, after an earlier GM-developed SMC called "Dbl-Lite" shaved 41 pounds from the 1997 Chevrolet Corvette; designers were able to beef up other components because of the reduced bulk of the car's doors, hoods and wheelhousings.  A 2-pound savings in the hood of a Buick LeSabre may not sound like much, but "it allows the engineer to use that 2 pounds wisely somewhere where the consumer will see it," Kia said.

After the success with the Corvette -- and its demanding customers -- designers and engineers told the materials group they'd like to use more of the structural composite, but needed a material to withstand the demanding environment around the engine.  Kia and GM research scientist Harry Mitchell spent the next two years tinkering with the known properties of all the materials that go into SMC to come up with a formulation that would be more heat resistant.  "We kind of knew which keys to turn and which changes to make," Mitchell said. "But it's not straightforward."  Finding the right recipe and then scaling it for production takes a lot of adjustment. "It's fairly easy to make the formulation in the lab, but in order to make it so that any supplier in any factory could reproduce it is more difficult," Kia said.  The result is SMC 3374, so named for the technical specification GM wrote for its properties.  In recognition of their innovation, Kia and Mitchell were awarded GM's prestigious "Boss" Kettering Award in May.  While plastic body parts have become quite common, most still require a steel inner frame and fasteners or adhesives to be stiff enough.  SMC 3374 can stand by itself as a structural part.  SMC 3374 was used to replace steel in the front-end support of the 2000 Buick LeSabre, 2000 Pontiac Bonneville and 2001 Oldsmobile Aurora, and the savings were significant.

"The mass of that part, which supports the headlights and spans the front of the radiator, was reduced by about 25 percent", said Jim Schoenow of the GM Car Group's engineering and development operations in Flint.  Reduced mass helps improves federal fuel economy.  A piece of SMC 3374 is rigid and hard, like the old-fashioned Bakelite plastic, but it doesn't shatter and splinter like its forebear.  The plastic itself is tougher than Bakelite and a layer of embedded glass fibers ensures that it stays together even after a sharp impact.  "Where it breaks, you'd see the glass fibers.  You'd have to bend it back and forth several times to really pull it apart," Kia said.

GM is using almost 2 million pounds of SMC 3374 in its 2000 and 2001 vehicles, Kia said.  And engineers throughout the company are talking about new ways to use it in various brands.  The next recipe Kia and Mitchell will work on is a cosmetic-grade lightweight SMC that could be used side-by-side with steel on the exterior of a vehicle.  The injection-molded plastics now found on the exterior of some vehicles aren't firm enough to be weight bearing, so they always need a metal understructure with fasteners or adhesive.  If lightweight structural composite can be made attractive enough for the exterior, it would revolutionize the structure of the car body, Kia said.

How It's Made

SMC 3374 is a special re-formulation of plastic materials that have been used in vehicles since the 1970s.  But unlike its predecessors, SMC 3374 is lightweight and yet rigid enough to be a structural part, and can withstand high heat without softening.  It starts out as a dough-like material called resin paste, which is rolled out thinly in a continuous sheet.  Glass fibers, each about an inch long, are sprinkled across the sheet.  A cover sheet of polyethylene or nylon plastic with more resin paste on it is laid over the fibers, and the entire composite sandwich is run through several rollers to "knead" the dough and ensure that the fibers are thoroughly wetted.  The sheet is then either rolled into a 1,000-pound spool, or "festooned" in a large box, like taffy.  The polyethylene or nylon sheet ensures that the material, which is about as sticky as toothpaste, doesn't cling to itself.  These spools or boxes of material are then allowed to sit for two or three days to firm slightly.  At the end of this "maturation" the SMC becomes as firm as modeling clay. The protective sheet is peeled off and the SMC is cut into sections, called charges, which may be several layers of SMC thick, and is ready to be made into a vehicle part.  A charge is then inserted into a two-sided, heated die, where it is pressed at 1,000 pounds per square inch at about 300 degrees Fahrenheit for about two minutes to mold the part.  A lubricant built into the material helps to de-mold the formed, hardened part.  It is then ready to be trimmed and painted.