Evaluation and Modeling of Injection-Molded Rigid Polypropylene Integral Foam

Abstract

Employing a modified injection-molding technology, where the mold is opened a short stroke after injection of the polymer melt, it is possible to manufacture plastic parts with a density reduction of 50% and more. For this processing approach, standard injection-molding equipment can be used. Together with an appropriate selection for the chemical foaming agent, most thermoplastic materials are suitable. The resulting moldings are a real lightweight design with compact skin layers and a foamed core fraction. To estimate the lightweight performance, bending specimens were investigated, and the gain in stiffness was visualized. By varying the thickness and the expansion degree, hints on the optimum foaming conditions were obtained. For engineering purpose, it is important to predict the performance of the integral foam molding during part design. A three- and a five-layer modeling approach were employed to represent the integral foam structure. Both ways were suitable to predict the stiffness enhancement, where already the three-layer representation is sufficient for primary design calculations. It became obvious that the foaming of thermoplastics with a breathing mold enables weight reduction up to 40% in flexion-loaded applications. Since the processing is still highly integrated, as it is the case with standard injection molding, and the cycle times do not increase significantly, the extra cost for the gain in stiffness almost reduces to the cost for foaming agent.