A Large Helping of Carbon: The High-Fiber Diet
As stringent fuel economy and CO2 emissions requirements loom, the auto industry is on a binge to cut weight
“I have worked with more [automotive] carbon fiber reinforced plastics [CFRP] in the last four years than I have in my entire career,” said Carl Howarth of Howarth Development Associates (Camp Hill, PA). His career spans 24 years in various engineering roles as a designer and CAE analyst. “Historically, in the automotive industry, no one wanted to pay any piece price increase to save mass.”
Now, with CAFE standards going up and fuel economy generally becoming more important, “that changes how they view material expense for mass savings,” he said.
Though it is easy to focus on the cost of carbon fiber alone, Howarth points out there are really three elements that shape the economics of CFRP for a particular part: cost of carbon fiber, labor to create the part, and the cost and amount of tooling required to form and cure the parts.
All three matter, though Howarth maintains the cost of carbon fiber remains the highest single material cost. Resin cost is secondary, in most cases. “However, there are process-specific drivers of cost, depending on the tooling and type of resin you choose,” he said. In particular, resin setup time dictates your capital expenditure for tooling for high-volume parts. “A setup time of five minutes will mean buying many more tools than a setup time of one minute in high-volume production,” he said. “Faster resin setup is always better.”
Different Types for Different Applications
Auto engineers use different types of composites, including sheet molding compound (SMC) or bulk molding compound (BMC) formed in fast compression molding, pre-impregnated fibers (prepregs) that are cut and stacked into preforms for compression molding, and woven or stitched fibers (fabrics) that are used in resin-infusion methods. A popular infusion method for composites is resin transfer molding (RTM). SMC uses randomly oriented chopped fibers. SMC tends to be a faster and cheaper method with which to produce auto parts than an RTM process, but it’s usually less strong than the long fibers in RTM that are oriented or woven.
While SMC is readily available with glass fiber, now it also comes in carbon fiber. Magna Exteriors and Interiors (MEI, Concord, ON) and Zoltek (St. Louis), the carbon fiber producer, introduced an example in 2012. In March 2013, MEI said this best-in-class low-cost carbon fiber SMC material uses Zoltek’s Panex 35 commercial carbon fiber combined with Magna’s EpicBlendSMC thermoset resin formulations. They want auto and commercial truck makers to use it in lightweight parts and semi-structural applications. MEI also offers the carbon fiber SMC directly to third-party molders.
Zoltek believes the automotive market will eventually be the largest destination for its carbon fiber, eclipsing current wind energy usage, according to George Husman, Zoltek’s chief technology officer. He said Zoltek is predicting a potential automotive market of 20,000 t per year by 2017. “There is going to be a heavy push towards adapting carbon fiber parts for the heavier luxury vehicles, especially to meet the initial 2017 CAFE requirements,” he said. “There is willingness to pay a small premium for weight saved, depending on class of vehicle and location of the part you are making lighter. Parts above the center of gravity or [in] the front of the vehicle matter more.”
He believes an ideal application is using the SMC molding process to fabricate a large single complicated part, replacing several steel parts that need to be fastened or welded together. The SMC developed with MEI uses chopped fibers, about an inch long, and the material is flow-molded into complex shapes, similar to injection molding processes using pure plastics. “These are good for semistructural parts, but if you need a part with greater structural capability, you can combine SMC with localized continuous fiber reinforcements exactly where you need it in the part.”
A further consideration in future development of CFRP is the choice of thermoset versus thermoplastic resins. Heat cures thermosets. Once fixed, they set for all time. Thermoset resins used in automotive applications provide strong, stiff CFRP parts, but are difficult to recycle. Thermoplastics (think heat, squeeze, and freeze) are tougher and easier to form into complicated parts. “We are developing both thermoset and thermoplastic applications,” said Husman. In general, thermosets represent less risk to manufacturers as they are tried-and-true, producing parts with better heat resistance. “Thermoplastic technology offers some unique growth opportunities for very complex parts, using injection molding and compression molding with cycle times of less than 60 seconds,” he said. “There are some aggressive technology programs going in thermoplastic right now.”
CFRP Debuting on Body Panels
Plasan Carbon Composites (Wixom, MI), is an emerging supplier of CFRP exterior body panel parts. The company currently supplies the Ford Shelby GT500KR, the Chevrolet Corvette Stingray, and the Chrysler SRT Viper. “I do not think anyone took carbon fiber seriously until we announced we were doing all of the exterior body panels [and closures] on the 2013 SRT Viper program,” said James Staargaard, CEO of the company. What is interesting for those monitoring CFRP are the Corvette Stingray volumes, expected to reach over 25,000 per year. Plasan is providing a base model hood, a premium exposed-weave roof and a painted roof in CFRP for the 2014 Chevrolet sports car.
The company invested heavily in developing a reasonably fast process for large exterior body panels, such as doors, fenders, hoods, and roofs. Plasan uses this process for the Corvette program. The core of this system is a specially designed pressure press that optimizes an autoclave process, curing parts in 17 minutes compared to the 90 minutes needed in an autoclave. “Instead of using convection heat as in autoclave, we precisely heat the surface of a nickel-shell tool. This provides instantaneous heat transfer, to bring the resin to flow temperature, then to cure temperature, then we cool it rapidly,” said Staargaard. Weber Manufacturing (Midland, ON) supplies the nickel-shell tool.
In the Plasan process, unidirectional prepreg is cut and layered in a kitting process, trimmed, and then laid on the nickel-shell tool. They use thermoset prepreg sheets provided by Toray Composites America Inc. said Staargaard. A flexible, elastomer, formed top tool is then placed over the CFRP. The part plus tooling is placed inside the press chamber and a vacuum pulled while moderate pressure is applied to the flexible top during the heating and cooling cycle. The part is then finished and primed before shipping to the assembly plant.
“What our team, led by Gary Lownsdale our R&D director, figured out is that curing an epoxy follows a nonlinear curve,” Staargaard said. Lownsdale used Design of Experiments in a years’ long quest to map out that curve to guide developing the process—an expensive, painstaking process according to Staargaard.
Why thermosets? “One of the difficulties with thermoplastics for body panels is heat resistance,” said Staargaard. “[Composite] body panels have to go through the existing assembly plants, especially the paint ovens. There is not a thermoplastic resin that I know of today that can be processed into these large Class-A body panels, that will survive the heat in those paint shops, exceeding 400°,” he said.
Wheels on the Horizon
Another especially important part where weight matters is wheels, since these contribute to the unsprung mass of a vehicle—the weight not carried by springs and shock absorbers. Cars with lighter wheels accelerate faster, handle and steer better. Lighter wheels made with CFRP reduce noise, vibration and harshness, according to Carbon Revolution (CR; Melbourne, Australia). The company is focusing its entire business plan on delivering CFRP wheels. The company has data indicating CFRP wheels are up to 50% lighter than comparable aluminum wheels. After spending about 10 years engineering a wheel design and manufacturing process, they are ramping up capability to deliver such wheels in mass-production quantities.
The first wheel in the marketplace is the CR-9, a nine-spoke, 19″ (483-mm), one-piece wheel that comes in widths from 8.5 to 12″ (215–305 mm), according to Brett Gass, CR’s engineering director. These wheels are designed to mount directly on a Porsche 911 as an aftermarket option. Carbon Revolution also offers wheels for a range of high-performance cars including the BMW M3 and Audi R8. More fitments are being developed. The eventual goal is to supply wheels to OEMs, though cost is an issue, he pointed out. “A set of four CR-9s for the Porsche 911 retails for $15,000,” he said.
The company is designed to be a Tier One supplier. “We do not view this technology as solely aftermarket. Right now it is viewed as an exotic, lightweight performance item; however ultimately it is a technology for efficiency,” said Gass. Moving to the mainstream requires both cultivation and education. For example, currently Germany regulates wheels. “They do not allow CFRP wheels on the road because there is no way to certify them,” Gass said. He is working with German OEMs, regulatory bodies TÜV and the Fraunhofer Institutes to develop such standards. They are on schedule to have a standard published later this year. He also believes European OEMs may well be the first to adopt CFRP wheels as OEM offerings.
While careful in revealing CR’s manufacturing process, he provided some insight into these mass-production CFRP parts. “We are able to produce about 10,000 pieces per year in our manufacturing line, running three shifts,” he said. Carbon Revolution was deliberate in building a process that was scalable, in anticipation of greater demand. The company expects this capacity will increase significantly over the next two to three years as larger production facilities are constructed. Their process starts with spools of carbon fibers. A series of automated processes create the fiber preforms. A process “similar to a resin transfer molding [RTM] process” infuses them with a thermosetting resin system, according to Gass. The precise and automated approach to the preform manufacturing process is a key element in making their CFRP wheels. A post-process and low-energy heat-curing cycle is used ahead of the final coating process.
Faster Processing Requires Faster Cure
Three common thermoset resins are polyester, vinyl ester and epoxy. Epoxy resins are the popular choice for automotive composite applications when structural performance is needed. Henkel (Düsseldorf, Germany) now offers a polyurethane, Loctite MAX 2, for automotive composite applications. “Our polyurethane matrix resin has unique properties for manufacturing fiber-reinforced plastics. Compared to standard epoxy resins, it combines very short cycle times for RTM-processing and superior toughness,” said Frank Deutschländer, Global Market Manager Automotive for the company. Henkel also states that its product endures high fatigue loadings, ideal in certain automotive applications.
Deutschländer believes Henkel is the first to offer a polyurethane resin suitable for RTM mass production for auto applications. “Potential applications for Loctite MAX 2 are structural, complex parts, for example for body parts or roof systems,” he said. Demonstrating its potential speed, in late 2012, Henkel, working with machinery manufacturer KraussMaffei (Munich, Germany), announced reducing the cure time of Loctite MAX 2 to just one minute. This was in a resin transfer molding (RTM) process with a representative automotive part composed of four layers of carbon fiber, molded at 120°C. Loctite MAX 2 permits short cycle times (< 5 min) in composite component manufacture, according to the company. The first commercial application of Loctite MAX 2 is a glass fiber reinforced leaf spring, produced by Benteler-SGL.
Cutting Cost of Carbon
Reducing the cost of carbon fiber is one of the main tasks at Oak Ridge National Laboratory (ORNL, Oak Ridge, TN). “We are trying to create a more cost-effective carbon fiber with similar or acceptably reduced performance [to today’s carbon fiber],” said Cliff Eberle, composite materials technology development manager. While the aerospace industry can use carbon fiber at its present cost, it is more of a challenge for automotive, even as it is migrating onto premium platforms, he said. “As volumes go up, price is critical,” he remarked. Eberle noted ORNL believes $5–$7 per pound is a target price for carbon fiber that will bring about more widespread use in the auto industry.
“About half the cost of carbon fiber is in the precursor feedstock, and about half is in the energy-intensive processes used to convert it,” he said.
The cost-reducing efforts have been enhanced with a new facility. In March 2013, ORNL announced the start-up of its Carbon Fiber Technology Facility, a demonstration scale plant for testing new recipes of materials and conversion technologies.
A key price driver for carbon fiber is the quality. In most cases, aerospace grade is not required to meet the needs of automotive applications, which helps with cost. For example, Zoltek advertises its Panex 35 as more of an industrial grade of carbon fiber. Zoltek’s Husman said that the company sells Panex 35 for $8–$9 per pound, sometimes less, depending on the size of the order to a particular customer. He also said Zoltek and Weyerhauser, with support from the US Department of Energy, are collaborating on a lignin/PAN mix precursor technology, aimed at reducing cost while delivering the needed structural performance appropriate for the auto industry. ME