'Materials revolution': plastic wheels may be just around the corner

If you think plastic means cheap, look again. Not only is the Pontiac Fiero clad in an all-plastic skin, but the pricey front-drive Cadillac Fleetwood Seventy Five limousine has a plastic hood and rear doors, the latter a first. The brand-new limo is 26 inches shorter and 1, 200 pounds lighter - thanks in part to the plastics - than the car it succeeds.

The Chevrolet Corvette and the new General Motors Astro minivan use fiber-reinforced composite front and rear leaf springs. In the 1987-model year, Pontiac may use plastic wheels in some of its sporty cars.

Indeed, the automotive industry on three continents is in the midst of a ''materials revolution'' that will completely revamp the cars produced in the 1990s and beyond.

''There is nothing mystical about composite materials,'' reports John D. Withrow Jr., executive vice-president for product development at Chrysler.

''We know a lot about them in the United States,'' he adds. ''They're being used in the aircraft industry extensively, while the entire military-industrial complex is using composites in one form or another because cost is not a factor.

''But when you're building 10 million something or other a year and trying to compete in the economic world, that's something else.''

Far from being the raw-look material of the early days, when anything built of plastic was the same as shouting ''cheap,'' today's plastics are hard to tell apart from the ''real thing.''

The plastics-development process over the years, however, was fraught with problems.

In the past it was hard to get a good surface finish on the plastic SMC (sheet-molding compound), according to Ward's Auto World magazine. Designers turned away from fiber-glass-reinforced SMC because of waviness and graininess in the product.

Several utility vehicles - Dodge Ramcharger, Ford Bronco II, and Jeep Wagoneer - now use SMC liftgates in the rear. SMC liftgates are also used in Oldsmobile and Buick station wagons.

Meanwhile, the battle goes on between plastics, aluminum, and high-strength steel as the auto industry forges ahead in its battle to reduce weight, rust, and corrosion. This struggle is leading to the use of new coated metals.

''The most significant lightweight substitutions for '85,'' says Ward's, ''are plastic-composite driveshafts on Ford Econoline vans and aluminum driveshafts on (Ford) Aerostar (minivans). Both are a worldwide first.'' The Ford minivan isn't expected on the road until sometime next spring.

By 1992 the average US car will weigh no more than 2,350 pounds, predicts a study by the University of Michigan. The Delphi Forecast and Analysis of the US Automotive Industry Through 1992, the third in a series that began in 1979, points out that between 1978 and 1983, average car weight fell from 3,500 pounds to 3,000 pounds.

Chrysler's Mr. Withrow agrees: ''I think we'll continue to see a reduction in car weight, which gets more involved in materials sciences, such as better use of materials and new structural concepts.''

Up to 15 percent of the weight of an average passenger car on the road today is polymeric materials, according to Dr. T. J. Mao, head of the polymers department of the GM Research Laboratories.

''By volume, because polymers weigh much less than steel, 40 percent of the volume of the average car is already plastics.''

Reinforced plastics (RP) also are finding new opportunities in the '85-model cars.

GM is debating a fiber-glass-reinforced car frame or body pan for one car line, according to Automotive News, the trade weekly.

What it would do is consolidate the parts and eliminate fasteners and dozens of stamped components.

''The process calls for bonding plastic body panels, as in the Fiero, to the frame,'' the paper concludes. It would get rid of welders and welding machinery, improve quality control, provide better fatigue characteristics, and eliminate corrosion.

GM and the entire automotive industry are looking for less costly ways to assemble cars in order to become more competitive with the imports, especially the Japanese.

Besides plastics, carmakers also are using more aluminum these days. A decade ago the typical American car had about 80 pounds of aluminum, but by 1990 the average car will contain at least 200 pounds of the metal, predicts William O. Bourke, president of Reynolds Metals Company and former executive vice-president of Ford Motor Company.

Going out on a limb, Mr. Bourke predicts that in five years all cars and light trucks will have aluminum radiators.

Mr. Bourke's forecast doesn't jibe with the University of Michigan Delphi Forecast, which suggests that only 35 percent of all cars will have aluminum radiators by 1992, up from 20 percent in 1987. Nonetheless, Ford uses aluminum radiators in about half its cars and light trucks in 1985, up from a scant 20 percent in '84. Chevrolet uses aluminum radiators in its '85-model Chevrolet Camaros equipped with V-8 engines.

Until now the major drawback to wider use of aluminum in cars has been cost.

Today, according to Nissan Motor Company of Japan, cars contain about 80 percent steel, 3 percent aluminum, and just under 5 percent plastic.

By 1990 the proportion of steel is expected to fall to about 60 percent while both plastic and aluminum climb to 10 percent each. Ceramics will account for about 2 percent by 1990.

A Battelle Laboratory process allows the metal plating of plastics at a sharply lower cost than had been possible up to now. Use of the process in functional parts, not just decorative, may be down the road.