Auto composites quest: One-minute cycle time?
Faced with high fuel prices and ever-more stringent restrictions on tailpipe emissions, automakers are taking composites int
Posted on: 8/1/2012
Source: Composites Technology
Due out in 2013, BMW’s all-electric i3 commuter car will feature a resin transfer molded carbon fiber composite passenger cell (shown here on display at a recent trade show). Source: CT Photo: Jeff Sloan
CAFE standards are used by the U.S. government to establish vehicle fuel efficiency standards (measured in miles per gallon or mpg) for all cars and trucks sold into the U.S. The 2017-2025 proposed standard (summarized here) calls for substantial increases.
Teijin (Tokyo, Japan) used its 60-second thermoplastic composite press-forming process to mold the passenger cell on this demonstrator vehicle. Teijin now is working with General Motors (Detroit, Mich.) to integrate this molding technology into passenger-vehicle production. Source: Teijin
Automotive composites veteran Antony Dodworth says the biggest barriers to integration of carbon composites into cars and trucks are encountered in the design and engineering stages. A car door, for example, can be designed and manufactured with composites to use far fewer parts (above) than required when using metals (see photo below). Source: Antony Dodworth
This car door, in metal, requires a larger number of separately manufactured parts than are necessary in the same door designed optimally for composites (compare with previous image, above). But Antony Dodworth assumes that there will be a period in which car parts are designed as “black metal” before automakers start optimizing structures for composites use. Source: Antony Dodworth
Regulation of tail-pipe emissions.
As the world’s automakers emerged from the recent recession, shaken to the core and, in some cases, recapitalized with public funds, almost everything had changed but this. In the U.S., the National Highway Traffic Safety Admin. (NHTSA) and the Environmental Protection Agency (EPA) are in the midst of updating Corporate Average Fuel Economy (CAFE) standards for the 2017 to 2025 time frame. CAFE standards are used by the U.S. government to establish vehicle fuel efficiency standards for all cars and trucks sold into the U.S. The proposed 2017-2025 standard calls for a substantial increase in automotive fuel efficiency. Table 1 highlights what’s in store.
In the European Union (EU), the European Commission, which develops and promulgates most of the regulations that govern EU industry, is focusing on direct reduction of emissions in cars and trucks. The current emissions limit in passenger cars is 130g of CO2/km, but by 2020 that figure will drop to 95g CO2/km. Regulations in the works for the post-2020 era promise more of the same.
Whether a carmaker is trying to reduce emissions or improve fuel efficiency, one of the most effective strategies is weight reduction, and few inside the auto industry dispute that the best tools in the lightweighting toolbox are composites — in particular, carbon fiber-reinforced polymers (CFRPs), which offer a strength-per weight ratio superior to any other materials, whether metal or polymer based. But historically automakers could not act on that knowledge in a significant way because they faced a threefold challenge: cycle time, cost and availability. How can carbon fiber composite structures be made at a cost and manufacturing speed conducive to high-volume automotive production? And, some automakers have pointed out, even if carbon fiber were cheaper, how could the industry commit to carbon fiber composite structures with the fiber supply so volatile and unreliable?
In the prerecession auto world, auto OEMs asked these questions and appeared to be waiting for the composites industry to provide the answers. But in the glare of postrecession realities — the continuing high price of fossil fuels and a recognition that to dismiss concerns about greenhouse gas effects on the environment is, at best, politically indefensible — auto OEMs are now taking the initiative.
In the area of carbon fiber availability, for example, one way to address this challenge is to develop a partnership with a carbon fiber manufacturer to create a carbon fiber supply chain designed exclusively for your vehicles. In 2010 carmaker BMW Group (Munich, Germany) and carbon fiber manufacturer SGL Group (Wiesbaden, Germany) did exactly that, creating SGL Automotive Carbon Fibers, which recently commissioned a carbon fiber plant, with a capacity of 3,000 metric tonnes (6.614 million lb), in Moses Lake, Wash. BMW will need that much because those fibers will see significant use in large parts, including the passenger cell and other chassis structures and, possibly, body panels on the 2013 all-electric i3 passenger car and the 2014 hybrid-electric i8 sports car — the first time carbon fiber composites will have been used in chassis structures on a production passenger car.
The BMW/SGL venture also attacks the issue of cycle time. The manufacturing pace required to meet the high-volume requirements of car or truck production is generally acknowledged to be one part per minute, a cycle duration maintained for decades by auto OEMs in their metal-stamping operations. Ideally, an automotive manufacturer would prefer a true 60-second process, something the composites industry has been unable to promise, particularly with regard to thermoset composites, which necessarily consume some time to crosslink sufficiently to cure. But composites proponents have always maintained that because tooling for composite molding can be built for a small fraction of the cost of metal-stamping molds, the part-per-minute expectation could be met with lengthier processes by using multiple tools and presses. And mold cycle times have been reduced incrementally as innovative molding processes have proliferated over the past few years.
Although the cycle time for the BMW process is unknown, what is known is that it is based on resin transfer molding (RTM) of woven carbon fiber fabrics, a process much faster than the hand layup/autoclave-cure methods used on CFRP in low-rate supercars. At Momentive Specialty Chemicals’ (Columbus, Ohio) Cedric Ball, market development manager — automotive, says the company has developed several fast-cure epoxies for use in high-pressure RTM (HP-RTM) processes (up to 200 bar/2,900 psi and 200 g/s injection rate). Ball says fast-cure RTM resins have been used by BMW since 2009 to mold the carbon fiber roof on the M3 and M6 models, and by Audi since 2011 on the B-pillar side blades for the R8. Further, Ball reports that Momentive has a 5-minute cure epoxy system going into production on several unidentified vehicles, and a two-minute cure system in trial phase. The five-minute technology combines Momentive’s EPIKOTE Resin 05475 and EPIKURE Curing Agent 05443. On the process side, Momentive has worked the most with Italian machinery manufacturer Cannon (Borromeo, Italy) and KraussMaffei/Dieffenbacher (Munich, Germany and Eppingen, Germany). “We believe that one-minute cycle times are not out of the question for RTM with the right combination of resin system, process and tool design,” Ball says. “While the resin system is a key component, Momentive understands that all three elements have to come together to make a successful product.”
I have inside sources from the equipment project.
