Carbon Fiber, Light and Strong, Arrives Where It's Most Needed

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Unless you drive in a neighborhood frequented by millionaires, you may never have seen a car made of carbon-fiber composites, though they have been on the road for more than two decades.

The reason is simple: the models available with carbon-fiber structures are mostly exotic sports machines from makers like Ferrari and Lamborghini that carry price tags well into six figures. The situation will be different next year, when BMW's electric city car, the i3, goes on sale in the United States -- for roughly the price of the company's 3 Series models, which start at about $33,000.

Carbon fiber's high strength and low weight make it ideal for applications where the finished product needs to be as light and strong as possible. Jetliners and fighter planes, made in small numbers where the material's slow and complex production process is not such an impediment, use these composites extensively. Designers of racecars and high-end sports gear turn to carbon for the same properties.

Until recently, however, there was no way that cars with everyday price tags could contain substantial amounts of carbon fiber. Electric vehicles in particular would benefit, as the weight reduction would translate into longer driving distances on each battery charge.

That is how BMW ended up plunging far deeper into the lightweight materials world than executives might have expected a decade ago when the company started making carbon-fiber roof panels for the high performance M3 CSL. Now BMW is not only producing carbon-fiber body structures for the passenger cell of the i3 E.V. -- first shown as a design study in 2011 and due to be presented to the media in Germany this week -- but it will manufacture the basic material itself. This is something of a throwback to Ford making its own steel in the Model T days.

BMW took the initiative because it saw little progress from carbon-fiber suppliers in bringing the material's cost low enough for mass-production cars. The automaker says it can supply carbon fiber to the i3's highly automated assembly line in Leipzig, Germany, at about one-third the market price per pound. The sporty i8 plug-in hybrid will also take advantage of the technology.

BMW produces the fiber in a joint venture with another German company, the SGL Group, in Moses Lake, Wash. The plant uses low-cost electricity from nearby hydroelectric dams to turn fine white filaments of a precursor material known as polyacrilonitrile, or PAN, into shiny black filaments of resin-coated carbon fiber.

"I'm not telling you that BMW will still be in this business in 10 years, but at this time it had to happen," Joerg Pohlman, an SGL managing director, said.

In the production process, PAN filaments -- tens of thousands at a time -- are slowly pulled through a long oven. Following a serpentine path defined by guide rollers, the fibers turn amber, then black, in the oven's 460-degree heat. The temperature must be precisely controlled at this step: if it's too hot, the fibers catch fire.

Next, the fibers run through back-to-back carbonization ovens, where they are heated to 1,300 degrees and then 2,550 degrees in an inert nitrogen atmosphere that prevents them from burning.

At this point the fibers are 95 percent carbon and have the properties the i3 designers want, yet weigh half as much as the precursor material. A surface-etching step follows, and then the fibers are sprayed with an epoxy solution before being shipped to Wackersdorf, Germany, for processing into flexible sheets.

Designing the i3 body with a passenger cell of carbon fiber and a lower "drive module" of aluminum saved about 550 pounds compared with a steel structure, helping to wring the most miles from the battery. BMW says the car will have a range of 80 to 100 miles; a 2-cylinder range-extender engine will be optional.

The car's carbon upper frame is formed using a process called resin transfer molding. Multiple layers of the flexible carbon-fiber textile are placed into molds by robots. Resin and catalyst are injected under pressure, followed by a period of heat and pressure; the pieces harden into rigid structures in minutes.

Next, the carbon parts are joined to one another, and to metal parts used in some of the assemblies, by an adhesive bonding process. The showers of sparks and unrelenting din of a traditional steel bodymaking operation, which relies on thousands of robotic welds, are eliminated.

Even though the carbon-fiber material is still much more expensive than steel, differences in the overall bodymaking process yield cost savings that help to offset the cost. For starters, the i3 body structure uses just 130 carbon-fiber pieces, compared with about 400 for a steel body. The smaller number is partly explained by the ability of engineers to design very complex parts for the molding process that would not be feasible with the huge stamping presses and dies used to make steel parts. Often a single complex carbon part can replace four or five metal parts that would be welded together.

"We can produce an i3 in about 20 hours, versus about 40 hours for a 3 Series car and using just one-half the space needed for a steel body shop," said Daniel Schaefer, who oversaw development of the i3 production process.

Another important element in the time savings is the use of exterior body panels with molded-in color, eliminating the traditional paint shop -- one of the most costly sections of a car factory.

Carbon fiber's superior strength properties let structural engineers do things differently. For example, the i3 doesn't need the familiar B-pillar to support the middle of the roof; the structure is strong enough to meet crash safety standards without it. This can translate into aesthetic payoffs.

"Without the B-pillar we have a relatively open-feeling interior, and when you open the doors, you will see some carbon fiber in the body sides," says Benoit Jacob, who directed the car's styling. "This is interesting to look at. We left it unpainted, with just a clear coat."

autonews

This article originally appeared in The New York Times.


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