Typical reaction injection molding oligomers gel in less than 5s, making it very difficult to measure their viscosity rise in a rheometer. The parameters of the viscosity function for two thermoset polyurethane (PU) systems made from modified 4,4′-diphenylmethane diisocyanates and polyether polyols were obtained through nonlinear regression of a mold filling simulation model using pressure rise data. The pressure rises were measured at the inlet point of an end gated rectangular mold. Adiabatic temperature rise experiments were carried out to obtain reaction kinetic equations of the PU systems. A second order Arrhenius type reaction kinetic equation described the adiabatic temperature data well. Viscosity change of a low catalyst PU system during isothermal polymerization was measured using a parallel plate rheometer, and a viscosity function was estimated from these data and the kinetic data. The viscosity function determined through regression of pressure rise data using the mold filling simulation model compared reasonably well to the viscosity function obtained from the rheometer data.