Determination of Dimensional Changes in Injection Molded Bosses Using Strain Gages: Effects on Joint Durability

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.