SMC in the driver's seat.

Thermoset body panels, polyester sheet-molding compound (SMC) in particular, are currently on the inside track in Detroit, riding on evolutionary technology developments in materials and processing methods. As one example, four years of cooperative materials development between a Big Three OEM and a custom molder have opened up significant new SMC opportunity in vertical body panels.

This opening was a product not only of technical developments, but also of a broader change in Detroit's attitude toward plastics and toward how it evaluates materials choices for new parts. Plastics' benefits of light weight, parts consolidation, design flexibility, and corrosion and dent resistance have become accepted as a given. Plastics compete today with metals on an equal footing. The choice will be made on the basis of "total system cost." Tool cost, in particular, has become recognized as the critical factor that can tip the balance toward acceptance of SMC or any plastic in future body-panel applications.

Nowhere is all this more evident than at Ford Motor Co. in Dearborn, Mich. Ford will launch SMC fenders on its high-end 1995 Lincoln Continental, utilizing a new flexible resin formulation. Up to now, Ford has made very limited use of plastic vertical panels. (The Continental will also carry an SMC hood and deck lid.) Says Richard Kollar, Ford section supervisor for front-end and underbody engineering, "The real breakthrough is that until now, we've never had a plastic material we could use for this application." His criteria for this judgment include dimensional stability of parts (coeffecient of linear thermal expansion) and the ability of plastic panels to be treated "the same as steel" with regard to assembly, fixturing, and painting.

A FOUR-YEAR PROJECT

The new "Hi-Flex" SMC, which will see its first commercial use on the Continental's fenders, was developed by The Budd Co.'s Plastics Div., Madison Heights, Mich., in association with Ford. The two firms initiated a pilot program in May 1990 to commercialize the materials technology, which Budd first announced in 1988.

Late last year, Ford conducted field trials with the Hi-Flex material, evaluating it in fenders on various rental-fleet vehicles. The 20% glass-reinforced Hi-Flex material is now set to debut as the specified material for all '95 Lincoln Continental fenders. Ford anticipates selling over 80,000 Continentals in 1995.

Daniel H. Bergstrom, Budd executive sales engineer, says the key to the Hi-Flex technology is the addition of a thermoplastic-elastomer copolymer (which he declines to identify) as a low-profile agent Bergstrom notes that the TPE displaces some of the calcium carbonate filler in the resin mix. Hi-Flex also contains a proprietary polyurethane additive that increases the flexibility of the polyester matrix.

Bergstrom says Hi-Flex has about twice the flexibility and impact strength of standard SMC and provides a 10% weight savings because of its lower density (see table). Compared with steel, a pair of Hi-Flex fenders saves 5 lb per car.

HI-FLEX SMC FROM BUDD PLASTICS

Specific Gravity                                        1.5
Water Absorption, % (24 hr, 73 F)                     <0.45
Heat-Distortion Temp., F (264 psi)                     >450
Nothed Izod Impact Strength, ft-lb/in.                   21
Flexural Strength, psi                               15,800
Tensile Strength, psi                                  8500
Flexural Strength, psi                              650,000
Mold Shrinkage, in./in.
(cold piece)                                         +0.0006
CLTE, in./in./[degrees]F (75-300 F)        5.6 x [10.sup.-6]

Kollar says of Hi-Flex, "It does represent a `next step' in SMC material technology. It was one choice among several contenders for this application We were impressed with dimensional stability, its flexibility, and its surface quality."

The Hi-Flex fenders are assembled and painted on the car platform the same as any steel or SMC panel. Bergstrom says Hi-Flex has a CLTE that's virtually equivalent to steel. Bergstrom notes that this characteristic of Hi-Flex technology opens up the possibility of a future steel/SMC combination panel.

Budd will compression mold the Hi-Flex fenders at its Kendallville, Ind., plant using one press and one set of molds built by Tech Center Tooling, a unit of Spartan Molds Ltd. in Windsor, Ont. Budd also will mold the SMC hood and deck lid for the Continental in Kendallville. All these parts will be molded in a Budd process called "System 59." This refers to a 59-sec molding cycle, which employs a special vacuum system in the mold, eliminating trapped air that can cause "paint pop" surface porosity in SMC parts (see PT, May '88, P. 25).

All of the plastic body panels fabricated in Kendallville are delivered "just-in-time" to Ford's Wixom, Mich., assembly plant. The Hi-Flex fenders will be bolted onto the body structure, run through the E-Coat dip process at 365 F, then primed and painted along with the other panels.

SMC's WINNING POINTS

The primary factors in Ford's decision to employ this new SMC material technology rather than steel or thermoplastic were tooling cost design freedom, and manufacturing feasibility. Kollar says that compared with stamped steel, tooling costs for the SMC fenders on this car (with planned volume under 100,000 units) will save more than 40%. A related advantage will be a greater opportunity for "in-series" styling changes, using the existing tools.

Ford has all but closed the book on the use of injection molded thermoplastic body panels, owing to problems associated with high CLTE and consequent lack of dimensional stability (PT, April '93, P. 70).

Kollar points out that thermoplastics' higher CLTE imposes certain design constraints - specifically, wider spacing of body panels is required to accommodate thermal expansion. According to Kollar, Ford is unwilling to make such concessions in its body design. In fact he says the trend at Ford is in the opposite direction - toward even tighter margins between body panels.

Also, the existing production systems at Ford do not tolerate separate handling of body panels for painting or assembly. "We're looking for `bolt-on' body panels," Kollar says. Without getting specific, he says, We certainly will consider Hi-Flex for other applications in the future."