Flexural properties of moldings made by RIM (Reaction Injection Molding), which are structural foams consisting of high density skin and low density core, were investigated by three-point bending tests. The following two breaking modes were observed in bending tests of the moldings, depending upon the density ratio of skin layer and core layer.
(1) The other side of the skin layer on which load was applied was broken by tensile stress.
(2) The same side of the skin layer on which load was applied was broken by compressive stress, causing Wrinkling buckling.
Then, the conventional composite beam theory was applied to the former breaking mode and Hoff's buckling to the latter, and equations were derived to predict the flexural properties of the structural foams which involve buckling from the flexural properties of solid construction.
In addition, it has been shown that there exists the best density distribution that maximizes the flexural strength of the moldings made by RIM with a given overall density. The results obtained here should be useful to optimum structural designing of moldings made by RIM.