GM is First Automaker to Use Class A CFRP Parts From New Pressure-Press Technology
Source: High-Performance Composites, March 2013 pgs. 42-45
On Jan. 14, General Motors Co. (GM, Detroit, Mich.) introduced its 2014 Chevrolet Corvette Stingray reboot at the North American International Auto Show (NAIAS, Detroit, Mich.). Returning to the iconic Stingray name badge for the first time since it was retired in 1976, the new baseline Corvette is also the first production car to feature structural, Class A carbon fiber-reinforced plastic (CFRP) body panels produced via a new out-of-autoclave “pressure-press” technology. Invented by Plasan Carbon Composites (Bennington, Vt.), the process relies on equipment developed and built by Globe Machine Manufacturing Co. (Tacoma, Wash.).
The Stingray’s removable roof is available painted or in an exposed-weave/ clear-coated version. The hood is painted to match the body. Each is formed and cured by Plasan’s process 75 percent faster than previous autoclaved parts.
Mass-producing mass reduction
The commercial debut of the hood and roof is a significant milestone for automotive composites. Pound for pound, carbon composites are the lightest, strongest construction materials available to industry. Automakers today readily acknowledge that they offer an ideal means to reduce vehicle mass without sacrificing occupant safety or driving performance as OEMs seek to meet more stringent U.S. fuel economy standards and significantly more difficult European tailpipe emission limits. But the high cost of carbon fiber, the time and expense of autoclave processing and a lack of predictive-engineering tools have kept away all but a handful of supercar and highend sports car builders.
At their low volumes, autoclave cure is fast enough and the manufacturer’s suggested retail price (MSRP) is high enough to recoup the expense. However, when you move from a few thousand to tens of thousands of cars annually, autoclave cure becomes too slow and expensive — hence the scramble to find faster-curing resins and to develop new out-of-autoclave process derivatives.
“Corvette has pioneered the use of innovative, lightweight materials since its introduction in 1953 as the industry’s first production car with a fiberglass body,” notes Corvette chief engineer Tadge Juechter. “The new Stingray … will continue that tradition by becoming the industry leader in high-volume use of carbon fiber. For the standard Stingray coupé, we’ll use 18 lb [8.2 kg] of carbon fiber just on the roof and hood. Based on last year’s sales of 14,132 units, Corvette will account for at least 238,000 lb [107,955 kg] of carbon fiber annually — and we expect to sell significantly more Corvette Stingrays than that.”
Notably, the reborn Stingray and the pressure-press for its roof and hood were developed during the darkest days of the post-2008 recession. GM was in bankruptcy. Although Plasan had supplied autoclave- cured CFRP body panels for Corvettes since 2006, its new owners, who had purchased the automotive side of Vermont Composites Inc., were wondering why on earth they’d decided to diversify from their core defense business into the capricious world of cars. GM persevered, even renovating the Corvette Bowling Green, Ohio, assembly plant. Likewise, Plasan’s owners invested “tens of millions of dollars” to develop the new molding process, build a new technical center in Wixom, Mich., and then build a new production facility in Walker, Mich.
Hood and roof production will occupy only half of the facility’s available 170,000 ft2/15,794m2 manufacturing space, leaving room for growth. Gary Lownsdale, Plasan’s chief technology officer and coinventor of the process, sees the Walker facility’s mission as nothing less than an opportunity “to bring CFRP into mass production for cars.”
Minimizing the makeover variables
Although the new Stingray panels are similar in size to those used on the previous ZR1, they have different contours. Plasan’s president, James Staargaard says the company simplified approvals and minimized the number of variables during the changeover process by using the same resin, structural adhesive, clearcoat chemistry and laminate technology. Plasan did, however, make one significant change: the company is now hot bonding the hood’s inner and outer panels. The adhesive is heated while it is robotically applied, and parts are fixtured for cure at ambient temperature. This is 72 percent faster than the oven-cure method used on the ZR1.
The pressure presses are fitted with 6-ft by 6-ft (1.8m by 1.8m) platens. Lownsdale says the large hood inner and outer panels and smaller roof can be molded on the same size press. Plasan uses single-sided, thin-shell nickel vapor deposition (NVD) tools from Weber Manufacturing Technologies Inc. (Midland, Ontario, Canada). A reusable bladder/canopy closes off the part’s B-side. Plasan produces the bladder in-house using silicone heat-cured rubber (HCR) from an undisclosed source. The bladder’s smooth surface eliminates wrinkles, read-through and other blemishes typical of vacuum bag.
Each tool features built-in oil-type heating/ cooling elements and, according to Plasan, provides precise thermodynamic control without inductive mold heating. Roof and hood tools can run in any Globe press at the Walker and Wixom sites. Most notably, Plasan developed Coupling Recognition System (CRS): When a part’s tool is plugged into the press, the CRS not only automatically identifies what part the tool is designed to mold, but it also queues up the “cure recipe” (process settings) for optimum production, effectively providing plug-and-play functionality.
Maximizing the molding process
Each part is hand layed in its NVD tool on an indexing table outside the press, using prepreg from a robotically cut kit produced in another part of the facility. “Unfortunately, the technology doesn’t exist yet to do automated layup of very complex geometric parts with the compound sweeps that you find in automotive,” says Lownsdale. “We’ve successfully automated layup of simple parts, and the ability to translate that to more complex designs is one of the goals we’re chasing to continually decrease total cycle time.”
When layup is complete, the canopy is pulled down over the part and smoothed into place. Then the table indexes into the press chamber, and its front and rear doors close. A pressure box (slightly larger than the tool) descends from above, sealing off the top and sides of the tool and bladder. A vacuum is drawn, and a “very small column of air” completely surrounds the tool and bladder inside the box. Pressure is applied to the top of the box (and, therefore, the air column) by a hydraulic ram. In this sealed environment (which doesn’t require nitrogen because the process tightly controls exotherm) the tool surface rapidly heats, enabling resin to flow quickly before crosslinking begins. This accounts for a much-improved surface finish.
Using conventional resin and laminate technology, Plasan can achieve a 10-minute cycle time with a six-minute cure, says Staargaard. But in practice the press maintains a “balanced” 17-minute cycle to avoid overproducing parts and getting ahead of the bonding, trimming and finishing stations. The process eliminates the expense of vacuum bagging materials and uses a small fraction of the energy consumed in an autoclave cycle. And the part’s excellent as-molded surface has accelerated downstream operations by 80 percent. Although he is pleased, Lownsdale says the benchmark is injection- molded surface quality and speed. To that end, Plasan is working closely with major resin and prepreg producers.
Moving into the mainstream
“It’s absolutely possible to make CFRP components mass producible in the auto industry,” Lownsdale sums up. “Now, when we sit down and talk seriously about a CFRP component, our customer no longer says, Prove it to me and I’ll consider it, but rather says, How can we work together to make this happen?