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New Molding Compound Cuts Weight 20 Percent

Navistar hood features use of nanoclay reinforcements

Doug Smock, Contributing Editor -- Design News, October 29, 2008

Navistar is using nanoclay fillers and other advanced materials in truck hoods in place of glass microspheres to reduce weight by 21 lbs, or by 20 percent of its original weight.

"The weight savings translates to $2 million per year in greater revenue productivity," says Edward Zenk, senior development engineer at Navistar's International Trucks and Engine group in Warrenville, IL. "Twenty one more bulk products can be loaded per truck per trip without exceeding over-the-road limits."

Navistar refers to the new technology as "tough low-mass SMC hoods," with SMC standing for sheet molding compound, a compression molding process using polyester resins and fillers.

"To reduce weight, we had to take out inorganic fillers," says Helena Twardowska, a senior staff scientist at Ashland Specialty Chemical Co. in Ashland, OH. Nanoclays are a very efficient filler, she says, due to their large surface area of 740 m²/g, allowing filler ratios to drop from a range of 150 to 200 parts by weight to 25 to 50 parts by weight.

The nanoclays have additional favorable properties, including improved flexural modulus and surface smoothness. At high temperatures, modulus of the materials increases by 25 to 20 percent. Glass microspheres, the previous material of choice for low-mass SM\C, can create a surface that is difficult to paint after sanding.

The raw material cost of low-mass SMC is about 15 percent higher than standard SMC, which has a specific gravity of 1.9. For comparison, aluminum could produce the same-sized part at a slightly higher weight savings (26 versus 21 lbs) but would cost more than double what standard SMC costs.

The new material from Ashland also uses proprietary additive technology.

The low-mass SMC hoods are a finalist in the Society of Plastics Engineers Innovation Awards. Winners will be announced at a banquet in Detroit Nov. 20.

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