Lightweight carbon-fiber and SMC body panels under development

  • 19-Jul-2013 04:46 EDT
CF SMC Hood Inner Finished 1.jpg

Carbon-fiber-reinforced SMC hood inner structure adds strength to CSP's two-part composite hood prototype.

Carbon-fiber composite car structures are in vogue this year. General Motors’ Corvette Stingray sports a carbon-fiber roof and hood. Other recent autos that feature carbon-fiber-reinforced polymer (CFRP) components include the Audi R8, the BMW M6, and the Dodge Viper. Most of these models, however, are high-end, low-volume vehicles that are mainly assembled manually because use of composites in low- and medium-priced cars still awaits the development of cost-effective mass-production processes.

The engineering appeal is clear, though. CFRP composites can have about a fifth of the density but all the strength and stiffness of steel. They also provide engineers with greater design flexibility and the opportunity to cut parts counts. But the drawbacks of CFRPs—high costs and slow manufacturing throughput—are tough challenges: not only are carbon fibers very expensive to make, forming them into auto components takes much longer than the few seconds it takes to stamp out metal sheet.

“It’s no secret that reducing a vehicle’s weight can deliver major benefits for fuel consumption, but a process for fast and affordable production of carbon-fiber automotive parts in large numbers has never been available,” said Inga Wehmeyer, an advanced materials and processes research engineer at Ford’s European Research Center in Aachen, Germany.

To make the most familiar type of CFRP parts, technicians typically lay-up net-shape prepregs—woven preforms of carbon fibers that are impregnated with thermoset epoxy resin—in clamshell-like molds. The workers then place the molds in autoclaves to cure them at high temperatures for an hour or two. But several ways to make composite parts are now under development, including various resin transfer molding (RTM) and sheet molding compound (SMC) methods as well as long-fiber-reinforced thermoplastic processes. Each has its benefits and shortcomings.

Despite the obstacles, car makers are gradually introducing CFRPs into conventional vehicles. The business consulting firm Frost & Sullivan predicts that the automotive carbon fiber composites market will grow by a third annually during the next four years. Observers expect that most early commercial applications will be exterior body panels because they can be replaced without altering the basic car architecture. Recent efforts by Ford, Toyota, the United States Council for Automotive Research (USCAR), and Tier 1 automotive supplier Continental Structural Plastics (CSP) of Troy, MI, illustrate this trend.

Automakers’ CFRP research

Ford, for example, unveiled last fall a version of its Focus compact with a carbon-fiber hood that weighs half that of its steel counterpart. A research collaboration with several partners fabricated the part using a high-speed manufacturing process that could be suitable for high-volume production, according to the automaker. To develop the technology, Ford engineers in Aachen worked with Dow Automotive and Hightech.NRW, a collaborative research project in Germany that includes the Institute of Automotive Engineering and the Institute of Plastics Processing (IKV) at RWTH Aachen University, Henkel, Evonik, Composite Impulse, and Toho Tenax.

The Focus hood is made of a special polymer-foam core sandwiched between two layers of epoxy-infused CFRP fabric. The part is produced using a “gap-impregnation process” developed by IKV that works by injecting resin over a carbon-fiber preform in a slightly open tool, which is then closed to fully wet the fibers before curing. Low-volume production trials at IKV are reportedly targeting a total cycle time of 15 min.

Elsewhere, Toyota announced in January that it had installed a carbon-fiber roof on its Mark X G Sport, a Camry-class car sold only in Japan. The polymer composite panel saves some 6 kg (13 lb) compared to a standard steel version.

Meanwhile, the Automotive Composites Consortium (ACC) of USCAR completed a five-year project to develop a composite floor panel structure for sedans. It also recently began a four-year effort to use composite materials in lightweight but crashworthy front-end structures.

Composites R&D by suppliers

Another company that is pioneering lightweight CFRP body panel technology is CSP, a leading composite products design and manufacturing firm that fabricates both the Corvette C6 and C7 bodies for GM. The company also boasts BMW, Chrysler, Ford, and Toyota as customers.

CSP engineers have developed a prototype two-part hood design that comprises an outer face of low-mass glass-fiber-reinforced SMC sheet filled together with an inner structural panel of carbon-fiber-reinforced SMC. The company plans to offer the concept to an automotive OEM for evaluation soon, reported Frank Macher, CSP’s Chief Executive Officer and chairman, on a recent episode of the car industry TV talk show Autoline.

“CSP decided to focus on structural lightweighting technology a couple of years ago,” said Probir Guha, Vice President for Advanced Research and Development. “But we recognized that we needed expert help, so we hired specialists in surface chemistry and chemical engineering.”

The company developed its chops, for example, working with Zoltek, Huntsman, and the National Composites Center on an ACC-supported research project to develop cost-effective carbon-fiber-reinforced SMC materials and the associated processing techniques for high-volume production. “Through our work with USCAR,” Guha said, “we decided to look into other fabrication methods along with partners such as Toray and SGL.”

“About 18 months ago,” he continued, “we decided to design our own composite hood.” The prototype hood, he said, has an outer SMC body panel that is substantially less dense than the company’s standard SMC formulation because CSP engineers replaced the conventional calcium carbonate filler with mass-shaving glass microspheres. The high-stiffness carbon-fiber-reinforced SMC inner structural panel uses a patented hybrid matrix system of polyurea and vinyl ester.

“The two-part combination saves significant weight, from 20% to 35%,” Guha explained. “We’re using a mid-density SMC now as a developmental step toward producing hood designs made with low-density SMC that would save 50% or more in weight over standard SMC hoods.”

“Initially, we’re using 1-inch chopped carbon fibers with loadings up to 45% by volume,” said Mike Siwajek, Director of Research and Development at CSP. He described the fibers as “standard grade with some modifications including special sizing to make the surfaces more compatible with our chemistry.”

Alternative technologies

“We’re also looking at other processing techniques for the inner component such as RTM thermosets, which traditionally have longer cycle times,” Siwajek said. “We’re looking at ways we could shorten the cycle times, but that work is not as far along as the carbon-fiber-reinforced SMC approach.”

Guha noted that CSP is also collaborating with MIT-RCF in Lake City, SC, to use cheap, recycled aerospace-grade carbon fibers left over from aircraft manufacturing. Another in-house research effort is investigating the use of coconut shell fibers as a lightweight filler, which is renewable, inexpensive, and quite effective.

“So far,” he concluded, “we’ve gotten a very positive response from the OEMs. In fact, when we started out it seemed like our goal was very far away, but from what I’m seeing today, I think carbon fiber is close to becoming a mainstream material for a lot of car programs.”

Other recent AEI articles related to composites:

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