By using Reaction Injection Molding (RIM) in which two kinds of chemically active liquids are mixed instantaneously, an integral-skin foam that has a composite foamed structure consisting of an internal foamed layer enclosed by rigid surface layer is obtained by a single injection process. This paper is concerend with the optimization of mechanical properties of composite foamed structures. Assuming that the composite foamed structure made by rigid polyurethane RIM is a composite beam of sandwich structure, the basic equations for predicting the tensile and flexural properties of RIM moldings have been derived. The validity of these equations has been confirmed by comparing of theoretical results with experimental results obtained by tensile and flexural tests. And the influence of the layer composition on static mechanical properties of such moldings have been studied. Furthermore, the influence of the layer composition on dynamic mechanical properties of such moldings have been investigated by Charpy impact tests and flexural fatigue tests. As results, the method of optimization of the layer composition for the mechanical properties of composite foamed structures could be proposed by using the above-mentioned equations.
