The appeal of plastics
Advocates of the revolutionary approach, however, stress the advantages of plastics as a more radical lightweight alternative to steel. Plastics are more than twice as light as aluminum and can be formed into a much wider variety of shapes. Moreover, the equipment used to manufacture plastics costs much less than the heavy stamping equipment required to make metal parts. These qualities have attracted automakers' interest since the 1960s.
Today the industry has incorporated plastics in a variety of uses; they form the interior components of most cars, for example, as well as bumper covers and fenders. Manufacturers and designers have also used polymeric composites--plastics reinforced with either glass or carbon fibers--in the bodies of race cars and some commercially produced vehicles. In the 1980s, as automakers looked for new ways to reduce vehicle mass, many in the industry began to investigate the use of polymeric composites to substitute for steel in automobile bodies.
Like aluminum, composite materials have their disadvantages. For one thing, they are more expensive than other automotive materials. The plastic resin mixture costs between $1 and $10 per pound and glass fiber prices start around $1 per pound. Glass fiber polymeric composites are price competitive with aluminum or steel only when used in small quantities or in complex shapes that are prohibitively expensive to form from metal.
In addition, ordinary plastics are between one-thirtieth and one-sixtieth as stiff as steel, while reinforced plastics are about one-fifteenth as stiff as steel. The traditional uses of plastics in automobile interiors capture the advantages of light weight and ease of formation without requiring a high degree of stiffness. Unibodies, however, have to be stiff to perform effectively. Structural panels composed of reinforced plastics must therefore be much thicker than their metal counterparts, offsetting the reduced weight and raising costs even further.
Carbon fiber composites have drawn the industry's interest as an alternative to glass fiber composites because they are stiffer. Panels composed of these materials can be made thinner--and thus lighter--than their glass-reinforced counterparts. However, carbon fiber composites are prohibitively expensive: carbon fiber prices start at $20 per pound and rise dramatically with increases in fiber strength and stiffness.
Polymer-based unibodies are also difficult to manufacture. Although bodies made of reinforced composites would require only one-third as many parts as conventional metal bodies, these parts would have to be made to fit together exactly--something that is beyond the state of assembly art today. Since plastic resin and carbon fibers contract at different rates as they cool, the parts are bound to warp and shrink slightly in ways that vary unpredictably from piece to piece. That's not unusual--steel changes shape as it cools, too--but materials like steel can be bent and twisted into shape. For instance, assembly-line workers use wooden mallets and two-by-fours to make sure steel car doors hang properly and seal when closed. Reinforced plastic components cannot be deformed in this fashion--plastic will break sooner than bend--so there is no easy way to compensate for slight imperfections in the way parts fit.
Finally, producing an affordable vehicle requires large-scale production, with volumes of at least 30,000 units per year and possibly an order of magnitude higher. While nonstructural plastic components can easily be manufactured on this scale, processing technologies for reinforced plastics are better suited to lot sizes of hundreds or thousands rather than hundreds of thousands. The cheapest way to shift to mass production of polymeric materials would be to speed up the process, making many more parts with the same equipment. But the processes involved in manufacturing and shaping reinforced polymer-based materials are not particularly amenable to this kind of straightforward scale-up.
The critical problem is that processing these kinds of plastics is inherently slow. The parts are formed by preparing a mixture of ingredients and waiting for them to cool or react chemically. For parts the size of automobile body panels, this process can take a minute or more. By comparison, steel parts can be stamped in less than 10 seconds. It is hard to find ways to increase the rate of chemical reactions or the rate of heat transfer--if plastic cools too rapidly it becomes brittle, and if chemical reactions are sped up they become difficult to control.
To make a large number of plastic parts, then, automakers would need to buy multiple machines and set up parallel production lines--steps that would more than offset the capital advantage of plastic production and increase administrative overhead. While parallel production lines may sound feasible in theory, they are very difficult to coordinate in practice. As a result, automakers have tended to avoid processes that require more than two parallel production lines.