Reaction injection molding (RIM) involves the injection of low viscosity polymers into mold cavities at relatively high injection speeds. The short cycle times, coupled with low injection pressures and clamping forces make RIM well suited for the rapid production of complex parts. High injection speeds, low viscosities and narrow cavities lead to high Reynolds' number flows during mold filling. The presence of high Reynolds' number flows is a notable difference between RIM and conventional thermoplastic injection molding (TIM). Unfortunately, this type of flow with significant inertia effects can lead to reduced part quality. The material properties and characteristics of RIM parts are influenced by flow patterns, weld lines, air entrapment as bubbles or pockets, especially as the material flows around inserts. These defects and characteristics, coupled with a growing demand for high quality RIM parts, is responsible for the increased interest in the flow during mold filling. This paper presents the development and test of a model to stimulate high Reynolds' number flows in thin cavities present during reaction injection mold filling.