articles made by the reaction injection molding process obtain a reduced residence time in the mold , hence increasing the production rate , by choosing processing and material parameters according to the inventive method . this method makes use of material parameters and processing conditions , including temperature and cavity shape for determining the viscosity of prepolymer material as it solidifies in the cavity . this information is used to optimize fluid injection rates , temperature of the fluid or cavity wall , choice of reactive fluid , or shape of the cavity . articles having more complex geometries than previously obtained can be produced when the cavity filling is mathematically simulated according to the present procedure , typically by means of a computer . this can ensure against blockage of critical passages in the cavity , polymer degradation , etc .