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It is difficult to predict foaming flow processes of polyurethane resin because this resin has a peculiar property whereby the density of the resin is decreased by heat generation of the twoliquid mixture of polyol and polyisocyanate. Therefore, simulating the precise behavior of polyurethane foaming is very effective for reducing the development period and production costs of plastic foamed products. In the present study, the temperature and specific volume of polyurethane resin are measured by a visualization. The resin temperature at the thickness center increases with the advance of foaming flow and is subsequently cooled by the mold. Thus, when the thickness is large, the resin temperature is high, but the time before reaching the highest temperature becomes longer. On the other hand, experimental results indicate that the specific volume of the polyurethane resin changes with time as the wall thickness and mold temperature vary. For example, the specific volume of polyurethane resin becomes large when the wall thickness is large and the mold temperature is high. However, by non-dimensionalizing the specific volume and foaming flow termination time, all values could be arranged along one line. This allows the derivation of a density formula that is a function of wall thickness, mold temperature and time. A foaming flow simulation was then developed by applying the density formula for a foaming flow process of polyurethane resin to a 3 D flow simulation program. This foaming flow simulation gives precise information, such as changes in specific volume with the progress of polyurethane resin, for a foaming flow process.