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Improvements in clamp load retention of fastened joints in instrument panels are desired by automotive OEMs to minimize warranty claims due to squeak and rattle problems. The decrease in torque retention of these plastic boss and metal fastener joints over time and temperature cycling was described in a previous SAE technical paper.1 This loss in clamp load retention (which is another measure of joint durability), as measured by torque, was shown to be affected by differences in the thermal expansion rates of the captured materials. The purpose of this paper is to further quantify these differences by using strain gages to measure the thermal expansion rates and dimensional changes of the joint's various components: metal fastener, molded plastic boss, and captured material.

The use of Post Consumer Recyclate (PCR) or Post Industrial Regrind (PIR) to manufacture thermoplastic (or thermoset) automotive parts and components has significantly increased over the last 10 years. Due to this increase in use, automotive designers are continuously challenged with the question of how PCR or PIR material differ in performance from the virgin material? To compound the dilemma, automotive OEMs are requiring increased durability of thermoplastic attachments (joints), so that warranty costs associated with interior squeak and rattle (from ill-fitting joints) are minimized. To answer this question, there exist several techniques for finding thermoplastic joint durability performance. Some of them are: strip-to-drive torque ratios, screw pull-off force and clamp load fall-off. A thermoplastic attachment (i.e. boss) which experiences clamp load fall-off will lead to a loose fitting joint and subsequently result in squeaks and rattles.