If you think that carbon-fiber composites belong only in the air and on racetracks, think again. The 34th annual Society of Plastics Engineers' Innovation Awards, which honor the year's best automotive plastics applications, not only featured innovations in injection-molded thermoplastic parts (see sidebar on page 66) but also highlighted three interesting composites applications.
Two of them show the promise of carbon-fiber composites on low- to mid-volume production vehicles. The third involves a patented processing breakthrough that expands the design possibilities for compression-molded composites.
2005 Porsche Carrera Composite Engine Frame
With its 605 hp engine and a top speed greater than 200 mph, the 2005 Porsche Carrera GT has plenty in common with a racecar. And the similarities don't come down to power and speed alone. The car features an open-top, rolling-chassis design that's more LeMans than Livonia. At the same time, though, this car still takes to the road with safety performance that meets or beats international standards. "Think of it as a race car with a road license," says Walter Schaupensteiner, team leader for interior body engineering.
To build a car that combines racecar looks and performance with safety, Porsche engineers made liberal use of carbon-fiber composite technology throughout the car's monocoque chassis. For example, the Carrera features a carbon-fiber-reinforced epoxy passenger tub. And for the first time in a production car, it has a brand new composite engine frame, which won the SPE's Engineering Excellence Award.
Weighing in at 45 kg, the frame supports both the engine and gearbox. And along with the passenger tub, it acts as the car's chief structural member. Porsche engineers designed the frame in composites normally used in aerospace applications—toughened epoxy prepreg (Cytec 997), carbon fiber, and some Nomex. The company's composites supplier, ATR of Italy, makes the parts at a rate of three per day using hand-lay-up techniques and an autoclave process.
According to Schaupensteiner, the use of aerospace composites resulted in important weight and stiffness benefits. He estimates that the composite engine frame weighs about 40 percent less than designs based on steel tubes. At the same time, he says, these stiff composites contributed to improvements in the car's torsional and flexural stiffness, which "enhances the driving dynamics."
The composite parts also offered a huge parts integration advantage. Schaupensteiner notes that the engine frame has more than 200 attachment points to accommodate the drivetrain and crash structures. And from a quality standpoint, he adds that Porsche attaches components to the frame with a group tolerance of just ±4 mm. This need to precisely attach other components was one reason that Porsche engineers ruled out an aluminum space frame design. Schaupensteiner says that the aluminum design would have required many heavy brackets, which would have been tough to locate on the frame and would have offset any possible weight savings. What's more, aluminum had unacceptable CTE differences with some of the attached components, potentially threatening Porsche's tight manufacturing tolerances.
Engine Frame: Porsche's Carrera GT features a carbon-fiber-reinforced epoxy engine frame that acts as one of the vehicle's most important structural members.
Porsche engineers had to overcome three key challenges to reap the benefits of composites. First, they had to find a materials system that would work in difficult environmental conditions. The engine frame, which essentially forms the engine compartment, commonly experiences temperatures as high as 180C. And heat isn't the only environmental threat. "These materials are exposed to heat, oil, dust, moisture and salt for 20 years," Schaupensteiner says.
And with the structural importance of the engine frame, Porsche engineers finally had to worry about the possibility delamination of the composite sandwich in high-stress areas. Schaupensteiner reports that the engineering team used FEA tools to optimize the location of all the attachment points and the direction of the fiber reinforcements.
2005 Ford GT Carbon-Fiber Decklid
Porsche isn't the only company to adopt carbon-fiber-reinforced epoxy composites in a production vehicle. Ford's 2005 GT uses a composite system—in this case, one based on a Toray quick cure epoxy-prepreg—for the inner panel that forms the structural backbone of the car's decklid assembly. Here, too, weight savings, torsional rigidity, dimensional stability, and parts integration represented the drivers for composites use.
Adrian Elliot, senior technical specialist at the Ford Research Laboratory, reports that the 6.4 kg decklid weighs about 50 percent less than a comparable aluminum part and 75 percent less than steel. He adds that the carbon-fiber part, which was one of the Innovation Award finalists, helped Ford meet critical strength and stiffness goals. And the design freedom imparted by composites also helped enable creation of one large part that integrates hinges and closure hardware. After a hand lay-up and autoclave molding process, the decklid inner is trimmed with a CNC router and ultimately adhesively bonded and hemmed to the vehicle's aluminum outer panels.
The ability of composites to shave off the pounds while offering parts integration opportunities may not come as a surprise. What may is that carbon-fiber composites actually represented a low-cost alternative. For example, Ford considered the use of superformed aluminum for the decklid inner. But aside from weighing more, these aluminum-based designs would have required multiple parts-and thus multiple tools. Elliot estimates that the one-piece, one-tool carbon fiber decklid will cost about 32 percent less than aluminum over the life of the project. Mostly the savings come from avoiding tooling cost-by dropping from four tools for a multipart design to one Invar tool for the carbon fiber part. "Recognizing full amortization, carbon fiber is considerably less expensive than the alternatives," Elliot says.
Patented Combination: To combine its decklid aluminum outer panels with a carbon-fiber inner panel without triggering galvanic corrosion, Ford engineers came up with a patented method that joins the two materials without letting them touch.
Because the decklid structure combines carbon fiber composite with aluminum, Ford also had to find ways to avoid galvanic corrosion and CTE mismatches between the inner and outer materials. Elliot reports that the company developed a patented method for dealing with both problems. "We came up with an innovative way to isolate the carbon fiber from the aluminum," he says. Ford's method involves adhesively bonding the inner and outer panels so that they can move relative to one another. The adhesive also physically separates the two materials. For the hemmed portions of the decklid structure, Ford puts a glass scrim between the composite and rolled aluminum edge.
Cequent Compression-Molded Towing Tray
Changing gears from thermoset composites to those based on thermoplastics, an aftermarket towing tray made from a 25 percent glass-filled, compression-molded polypropylene took the top honors in the Performance and Customization category. These platforms extend from the back of a truck or SUV to provide extra space for hauling gear. Usually they're fabricated from steel or aluminum, but Cequent Towing instead took advantage of a new gas-assisted composite molding process developed by Composite Technologies and Alliance Gas Systems.
This patented compression molding process uses a reciprocal gas pin to inject air into a closed compression-molding cavity during the molding process—so that the air selectively hollows out wall sections. The technology is similar to the gas assist systems used in injection molding for years. "But this is the first time it's been applied to compression molding," says Maria Ciliberti, vice president of Composite Technologies.
Like gas assist for injection molding, the compression-molding version offers compelling manufacturing and design advantages. The partially hollow parts weigh less and cool faster. "Cycle time reduction is the primary driver for the technology," says Ciliberti. And from a design standpoint, she adds, hollow sections allow engineers to stiffen parts without increasing wall thickness.
In Cequent's case, all these advantages came into play. At just 14.5 lb, the two-piece plastic tray assembly weighs 25 percent less than a comparable steel grate and 15 percent less than aluminum. It still resists deflection, though, and has a capacity up to 500 lbs. Thanks to gas assist, the plastic part has 50 percent less wall stock in the lip that runs around its edge. And Ciliberti says this material removal reduced part weight by 5 percent compared to solid plastic. It also improved cycle times by 40 percent for a $1.00/part savings. Compared to the cost of metal parts, the gassed plastic cost 25 percent less than steel and 30 percent less than aluminum, according to Ciliberti.
Though this job for Cequent involved a thermoplastic composite, Ciliberti says the technology could be applied to thermoset composites, too. "Gas assist changes the economics of compression molding," she says.