Machine Design

Desert heat's no sweat for overmolded trim

2005 Mustang convertibles sport the world's first loadbearing, overmolded plastic trim piece on their allplastic deck-lid closeout panels.

2005 Mustang convertibles are the first to use an overmolded plastic in a load-bearing trim piece. Overmolded Zeotherm TPV adheres to the glass-reinforced nylon even under the demanding sun and heat of the Desert Southwest where conventional thermoplastic elastomers often delaminate.

The overmolded design gives a smooth transition from sheet-metal panels as it tensions the convertible top against the Mustang body. Zeotherm thermoplastic vulcanizate (TPV) from Zeon Chemicals, Louisville, Ky. (, overmolds onto glassreinforced nylon and resists intense Southwest desert UV exposure without delamination.

Thanks to their chemical make-up, Zeotherm TPVs inherently resist UV, says Zeon spokesman Brian Cail, "The backbone of the rubber used in Zeotherm 100-80B TPV is fully saturated. There are no double bonds in the rubber that would make it susceptible to UV degradation. No additives are needed to boost its UV resistance," he says.

"Traditional touch-off molding approaches use thermoset rubber (EPDM) over steel," says Cail. "The overmolded TPV/reinforce nylon parts let designers reduce part cost without loss of structural rigidity. And the rubberlike TPV closely matches the clean lines and appearance of the Mustang's other weather/body seals."

Most important for this application, says Cail, is the adhesion of overmold to the nylon substrate. Zeotherm 100-80B TPV provides a complete cohesive, chemical bond to the glass-filled nylon substrate. It also has a tensile strength of 9 MPa, an 85 Shore-A hardness, and retains a tensile strength (at 100% elongation) of 6 MPa.

The gating system used for overmolding the TPV on to the nylon substrate resembles that used for conventional olefinic-based TPVs, says Cail. Molders must, however, make adjustments for Zeotherm's higher melt temperature and melt viscosity. Processing temperatures range between 260 to 280°C and no special treatment was done to the nylon surface prior to overmolding.

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