Characterization of microcellular plastics for weight reduction in automotive interior parts

Abstract

The present PhD thesis is framed within the Industrial Doctorate Plan promoted by the Generalitat de Catalunya and has been developed in cooperation between the Universitat Politecnica de Catalunya-BarcelonaTech, the Centre Catala del Plastic, SEAT SA and Volkswagen AG. The research project has as main objective the characterization of microcellular plastics obtained by injection molding, motivated by a concern to reduce weight, cost and carbon footprint in automotive plastic parts. First, cylindrical bars and square plates made of Acrylonitrile-Butadiene-Styrene (ABS) and 20% Glass Fiber reinforced-Polypropylene (PP 20GF) were injection molded and foamed through the MuCell® technology. Shot volume was found as the most influencing parameter on cell structure and tensile and flexural properties. The effect of mold temperature and injection speed was secondary and not statistically significant for the mechanical performance. Tensile and flexural properties decreased almost linearly with the apparent density, whereas impact resistance was strongly reduced during foaming. Glass fibers contributed to partially overcome the loss of properties due to the reduction in density. Cells act as crack arrestors by blunting the crack tip. However, once the crack is propagating, cells acting as stress concentrators lead to a decrease in fracture toughness. Because of the low amount of blowing agent injected during the foaming process, no significant changes in the thermal properties were determined as compared to that of the solid counterpart. Simulation of the microcellular injection molding process with Moldex 3D® software and prediction models of the mechanical properties based on the apparent density and morphological characteristics provided a good approach to the experimental results. On the other hand, the Core Back tool technology was also employed in this study. By pulling the core and increasing the final thickness of the part, the apparent density decreased but the bending stiffness was greatly enhanced. Finally, a new alternative foaming technology, called IQ Foam® and developed by Volkswagen AG, was used to produce rectangular plates and compare their properties to that of the obtained by MuCell® process. By using a minimum amount of blowing agent, foamed plastic parts through IQ Foam® obtained through this process exhibited thicker solid skins and lower cell densities, but consequently higher mechanical properties. Additional benefits such as cost-effectiveness, easy-to-use and machine-independence are also offered by this new emerging technology.