E. I. du Pont de Nemours & Co. yesterday introduced a new family of plastics designed to eliminate exterior sheet metal on most cars produced after 1995.

Success in that endeavor would lower the weight and increase the fuel efficiency of new cars significantly. It also would cut auto makers' tooling costs by reducing the need for expensive metal presses -- many of which cost millions of dollars, but are only capable of stamping one kind of metal part in one specific way.

Du Pont officials said that their new nylon polymer resins, a group of seven that will be marketed under the name Bexloy C, could cut 20 percent from the current costs of producing exterior auto parts such as bumpers and door panels. An auto company with factories especially built to use the new plastics could realize even greater production-cost savings, Du Pont officials said.

"The mission of this family of engineering plastics simply is to replace steel on the outside of the car from front to rear," said Larry Gillespie, director of Du Pont's engineering plastics program.

"This new material will provide the public with a lighter-weight, corrosion-proof car; and it will give automotive engineers greater freedom of design," Gillespie said.

Plastic car bodies are not new; kit car companies have sold them for years. General Motors Corp.'s expensive sports machine, the Chevrolet Corvette, entered the U.S. market in 1953 as the nation's first "all-plastic" car. GM increased its use of plastics in 1983 with the introduction of the Pontiac Fiero, a small two-seater that has a high-strength steel frame covered with bolted-on plastic panels.

Ford Motor Co. and Chrysler Corp. also are using plastics in exterior applications, as are Japan's auto leaders, Toyota Motor Corp. and Honda Motor Co. Ltd. For example, Honda's new two-seater, the CRX, has plastic bumpers and lower-body panels.

But the Bexloy C plastics are a generation ahead of those now used in auto production, Du Pont officials said.

"The technology of the plastics found on the Fiero, for example, was developed in the mid-1970s, when the Fiero was still on the drawing boards," said Louis Collier, a Du Pont automotive sales business manager. (Du Pont created some of the Fiero's plastics.) He said the company's new group of Bexloy resins have been under development since 1982.

The new Du Pont resins are thermoplastics, which means they can withstand the kind of extremely high temperatures found in auto paint ovens, according to Du Pont and auto industry officials. The car parts made with Bexloy resins will be injection-molded -- essentially, a process of pumping melted plastic into a mold and then quick-freezing the mold to get the component shape.

According to latest available industry figures, plastics accounted for 230 pounds of the total material weight of a U.S.-produced car in 1984. Plastics will make up 233 pounds of the total this year.

By comparison, steels of various strengths accounted for 1,090 pounds of the material weight of the average U.S.-made car built last year, and will make up 1,106 pounds of the average domestic car produced in 1985.

Aluminum also is gaining use as a weight-saving material in auto production.