Automotive brackets

In the automotive industry, cost and weight reduction are the driving force behind engineering design. Volvo Trucks recently identified two types of brackets as cost-cutting candidates. They selected RTP 4700 series glass-reinforced polytrimethylene terephthalate compound to replace metal in these brackets. PTT semicrystalline polyester is closely related to the more common thermoplastic polyesters, PET and PBT, by typically less expensive.

The new injection-molded brackets include L-shaped faring brackets — used to fasten the cab’s aluminum grab handle mechanically — and brackets designed to hold a roof-mounted satellite dish. During testing, the grab handle was subjected to a 250 lb load, which required a tensile strength of at least 20,000 psi. Its prominent placement required the brackets to exhibit a quality surface, which allows the brackets to be topcoat painted (requiring no primer) to control costs. Further, shrinkage rates of any replacement material would have to correspond to those of the existing tool. 

The satellite bracket called for a flexural modulus of at least 1.2 million psi. Since this bracket did not require painting, both performance and finish would have to withstand the rigors of the open road — mandating broad chemical and UV resistance. The RTP 4700 series compound contains both UV protection package and fiberglass reinforcement. With a tensile strength of 23,000 psi and a flexural modulus of 1.5 million psi, the material passed all load testing. Though reinforced, the part exhibits a smooth, resin-rich surface acceptable as a stand-alone finish and suitable for single-coat painting. 

Jason Spence, Volvo product design engineer, said of the PTT compound, “It represented the best combination of performance and value of all the materials we evaluated. It was less costly than both glass-filled PBT and nylon 6/6. Long fiber reinforced polypropylene was competitively priced, but it fell short of PTT’s performance in terms of UV exposure.” Spence adds that consolidation into a single, less expensive material also helped attain their goal of “producing more value in the parts.” He continues, “The RTP 4700 series material is a viable and economical alternative to other polyesters, and it was available from a supplier who offered the professionalism and technical skills to make it work.”

RTP Co, Winona, MN.
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