Gas-assisted Reaction Injection Molding (GRIM): Application of the Gas Injection Technology to the Manufacturing of Hollow Polyurethane Parts

Abstract

This chapter describes the gas-assisted injection molding (GIM)—one of the most promising special injection molding technologies for processing thermoplastics. The first step in GIM process is a short shot with the reactive polyurethane (PU) mixture, characterized by the degree of filling. After a certain period of time, the so-called (gas) delay time that enables a pre-reaction of the PU mixture, the gas is injected into the core area of the PU mixture. During this process, the gas bubble propagates inside the center of the mixture, while because of the fountain flow at the flow front, the PU skin material is displaced from the inside to the boundary area of the cavity. One of the most important influencing parameters of stable gas bubble propagation is the viscosity. For thermoplastics, the change of viscosity during the molding process is mainly influenced by cooling effects of a hot melt in a cooled mold. This solidification behavior leads to a lower viscosity and flow resistance in the middle area of the cavity so that centered gas bubble propagation is preferred as desired. The molding of reactive PU systems is influenced by an exothermic chemical coupled with physical processes. Thus the rheological behavior of PU systems depends on temperature and the degree of conversion. This so-called rheokinetic material behavior leads to rather complex viscosity profiles inside the mold depending on time, mold temperature, and the reaction kinetics of the PU system.