Corvette carbon fiber hood approved for production
By Staff | July 01, 2003
Working with General Motors, MacLean Vehicle Systems (MVS, Wheeling, Ill., U.S.A.) has developed a carbon fiber prepreg hood for GM's 2004 Chevrolet Corvette Z06 Commemorative Edition. Between 2,000 and 3,000 Corvettes will be fitted with the carbon fiber hood for the 2004 model year. This is the first time continuous carbon fiber has been used for a painted exterior original equipment panel on a North American production vehicle. The hoods are produced by MacLean Quality Composites (West Jordan, Utah, U.S.A., part of MVS) and shipped to GM's Bowling Green, Ky. assembly plant where they will be handled identically to current production fiberglass body panels.
Source: GM
"This application of carbon fiber is a major innovation," says Dave Hill, vehicle line executive and chief engineer for the Chevrolet Corvette. "It is a very appropriate upgrade to the Z06, the Corvette for the extreme performance enthusiast."
Like the existing fiberglass SMC hood, the carbon hood is made with an inner and outer panel. It weighs only 9.3 kg/20.5 lb, 4.8 kg/10.6 lb lighter than the existing hood. The painted, class-A outer skin panel is fabricated from six plies of carbon fiber/epoxy prepreg, which meets GM's stringent strength, stiffness and durability criteria as well as the cosmetic class-A surface finish requirements and the high temperatures of the prime and paint ovens. The outer panel is only 1.2 mm /0.048 inch thick, similar to stamped sheet metal and half the thickness of fiberglass SMC, and is the thinnest polymer composite skin on a street-legal production vehicle. The inner panel, required to stabilize the thin outer skin, is a compression-molded combination of carbon fiber SMC supplied by Quantum Composites (Bay City, Mich., U.S.A., select 292) and low-density fiberglass SMC. The two panels are bonded together with Pliogrip urethane adhesive, from Ashland Specialty Chemical Co., Specialty Polymers & Adhesives Div. (Columbus Ohio, U.S.A., select 293).
The challenge was to develop a manufacturing method capable of quickly producing thousands of parts per year while keeping the overall investment low. Toray Composites America Inc. (TCA, Tacoma, Wash., U.S.A., select 294) developed a 1m/39-inch wide unidirectional carbon/epoxy P3831C prepreg for GM with a proprietary formulation of G83C quick-cure resin and Torayca T600SC standard-modulus carbon fiber that can be fully cured in an autoclave in only 10 minutes at 149°C/300°F. Prepreg pattern cutting, ply orientation, post-mold routing and adhesive application are all automated.
To achieve the necessary volumes and provide tooling durable enough for years of aftermarket production, Invar, a nickel/iron alloy, was selected for the molds, a first for serial automobile components. However, the mold material choice required a unique mold release that would allow easy release of the epoxy laminate from the Invar, yet be soluble in GM's power washing cycle prior to painting, without the need for scuff sanding or wiping with solvent. The release, supplied by Chemlease (Boca Raton, Fla., U.S.A., select 295), took several months to develop.
"We expect this project is the first of many automotive applications for our carbon fiber technology, from body panels to structural components," emphasizes Jeff Keller, VP and general manager, plastics and composites, for MVS. The hood will be available through dealers for aftermarket replacement of existing fiberglass hoods.
