Automotive Plastics 2000

Plastics Engineering, Sept, 2000 by Victor Wigotsky

They are coming closer, faster. The automotive OEMs are realizing in more depth the benefits that plastics can provide in car design--the flexibility, the functionality, the potential for weight and cost savings. Watch for more modularity, more style, and more satisfaction of consumer desires and needs as the new models emerge. Plastics are stimulating new turns on the imagination that sells cars.

Automotive Plastics 2000

Research at GM

Ongoing research at the General Motors Technical Center's Materials and Processes Laboratory is advancing the technologies of sheet molding composites (SMC) and nanocomposites. For SMC, the property envelope has now been broadened to provide a lighter-weight structural material that is also more heat resistant. Senior staff research engineer Hamid Kia and staff scientist Harry Mitchell say that the SMC 3374 is stiff enough to be a structural part and tough enough to withstand the heat of an engine compartment. Applications envisioned for the new formulation, currently intended primarily for under the hood, at nominal thicknesses of 2.5 to 3 mm, now include the radiator support panel and the firewall.

SMC 3374 replaced steel in the front-end support of the 2000 Buick LeSabre, 2000 Pontiac Bonneville, and the 2001 Oldsmobile Aurora. The mass of the part, which supports the headlights and spans the front end of radiator, was reduced about 25% from the previous steel design. Compared with conventional SMC, with a specific gravity of 1.8g/cc, 3374 is significantly lighter at 1.25 to 1.3. The material also has a 20[degrees]C higher temperature capability for equivalent thickness than the earlier GM-developed SMC called Dbl-Lite.

Kia and Mitchell say understanding the role of the concentration of the different ingredients in the formulation is the key to the project's success. The new formulation, the result of a few years of research, consists of a significant amount of hollow-glass microspheres to lower the density; a greater amount of glass fiber to increase stiffness; and an improved mixture of polyester resin. Other selectively added resins also provide a synergistic effect.

When being processed, the SMC 3374 special reformulation starts out as a resin paste that is rolled thinly into a continuous sheet, followed by a sprinkling of glass fibers, each about an inch long, across the sheet. After application of a protective film of polyethylene or nylon, the material is kneaded through rollers to thoroughly wet the fibers. It is then rolled into a 1000-lb spool or "festooned" in a large box. After setting for two to three days, the protective film is peeled off, the SMC is cut into sections, creating a "charge," which may be several layers, and inserted into a heated die. The material is formed in about two minutes at 1000 psi and 300[degrees]F.

Kia and Mitchell add that the next effort will be development of a lightweight cosmetic-grade structural SMC that can be used side-by-side with steel on the exterior of a vehicle. Kia comments that "if lightweight structural composite can be made attractive enough for the exterior, without requiring supporting metal under-structure with fasteners or adhesive, it would revolutionize the design of the car body."

Robert Ottaviani, laboratory group manager at the GM Materials and Processes Laboratory, reports that molding trials are now in progress with two nanocomposite products targeted for cladding applications: a molded-in color grade and a paintable grade. Developed in conjunction with Montell and Southern Clay Products, the products contain a level of only 2.5% inorganic filler, compared with a typical 20% to 25% talc-filled material. Focusing on polypropylene-based systems, further development points to a range of interior and exterior uses, including the possibility of vertical body panels.

Normally, the specialty organo-clay materials are composed of many layers (or sheets), each about one nanometer in thickness. Currently, work continues on delamination (or exfoliation) and dispersion of the layers and improving miscibility to provide optimal polymer reinforcement. Primary goals are providing moduli (of about 100,000 to 150,000 psi for cladding and 250,000 to 325,000 psi for body panels), while maintaining ductility and low-temperature impact capability; dimensional stability to control shrinkage and fit and finish; and retaining the quality of molded-in color. Ottaviani foresees interesting developments with the nanocomposites in the near future as they focus on weight reduction while delivering equivalent or better functional properties.

Large body panels

The 8000 metric ton injection molding machine at the Husky Technical Center in Novi, Michigan, is being kept very busy. Leased exclusively by DaimlerChrysler, the machine is being used in the automaker's continuing program to develop largepart plastic body technology. One effort is the Chrysler Concept Vehicle (CCV) program. Another project is the production of two-part (inner and outer) molded-in color (tan, white, or black) reinforced polypropylene Jeep hardtops, in cycle times of less than three minutes, that are robotic-assembled next to the press. Part thickness ranges from 3.0 mm to 5.0 mm. By contrast, the existing five-piece SMC top is glue-bonded and painted.