Effect of Regrind on SMA Copolymer Boss/Joint Performance

To minimize warranty costs, due to squeak and rattle from ill fitting joints, automotive OEMs are requiring increased durability of thermoplastic attachments. There are several evaluation techniques for determining thermoplastic joint durability performance such as: strip-to-drive torque, screw pull-out force, and clamp load fall-off. A thermoplastic attachment (i.e. boss) which experiences clamp load fall-off will result in a loose fitting joint and subsequently lead to squeaks and rattles. In conjunction with the boss's performance the type of captured material between the screw and the joint can also contribute significantly to the overall retention qualities of the attachment.

The purpose of this paper is to evaluate: 1.) strip-to-drive ratios for thermoplastic bosses, and 2.) changes in clamp load with respect to environmental effects (i.e. thermal exposure) on thermoplastic bosses. Injection molded styrene-maleic anhydride (SMA) bosses were tested using three different levels of regrind content, five different boss ID's, and eleven different screw types.