To facilitate the study of new reaction injection molding (RIM) materials on a practical scale, a lab-scale mini-RIM machine was constructed. This machine is capable of delivering up to approximately 250 cc of material at rates of up to 125 cc/s. The RIM machine was first tested with conventional materials such as crosslinked and linear polyurethanes. The effect of varying the nozzle Reynolds number upon the adiabatic temperature rise corresponded to the results reported in previous works. Following this verification, two developmental materials, a polyurethane/polyester interpenetrating polymer network and a polyurea, were examined. The polyurea material showed a very strong dependency on mixing up to a Reynolds number above 400. The 75/25 PU/PEster reaction exotherms also showed a dependency upon mixing. However, this dependency is noticeable only in the polyester portion of the reaction, which can be attributed to the mixing sensitivity of the redox-type initiators used.