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As automobile designs become more aerodynamic, the windscreens become larger and flatter, magnifying the intensity of solar radiation at the instrument panel surface. Temperatures of over 110°C (230°F) are not uncommon for IP surfaces in desert testing, with substrate temperatures approaching 94°C (200°F). Designers must anticipate the effects of such harsh service environments and seek ways to accommodate and control the thermal expansion of thermoplastic IP substrates and minimize the distortion that can result. Adding to the problem of greater solar effects from flatter windscreens are recent design trends toward more sweeping, cockpit-like automotive interiors. These configurations place additional structural demands on the large injection molded IP substrates that support many components. Passenger-side airbags too, have added to the size and strength requirements of these thermoplastic structures.

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.

A large percentage of the total number of instrument panel substrates in North American vehicles are injection molded from a fiberglass reinforced and rubber modified thermoplastic copolymer comprised of styrene and maleic anhydride (S/MA). Most of these instrument panels contain a number of molded-in attachment bosses, which are used in conjunction with a variety of metal screws. Previous SAE Technical Papers* have defined processing and design parameters for these thermoplastic materials and have suggested improved designs to optimize these structural attachment bosses. The purpose of this paper is to further describe the fastening characteristics of these molded S/MA bosses with a laboratory evaluation which includes standard torque measurements, evaluation of various bearing surfaces, and evaluation of insertional speed. Improvement of torque retention over time is of great interest in this automotive application and suggestions for its optimization will be made.

The intent of this paper is to demonstrate the benefits of using experimental design techniques in product formulations. In this example, an optimized product was developed using minimal experimental trials by the use of a simplex mixture design. The resultant product was tailored within mechanical property, weight, and cost constraints. The new formulation was developed, prototyped, and tested quickly in order to be available to substitute for a heavier, more expensive steel and aluminum part.