A two dimensional model is developed for the flow, extent of reaction, and temperature changes during filling of an irregular shaped mold during reaction injection molding (RIM) process. The model is based upon the solution of Navier Stokes equations for pressure and velocity with viscosity dependent on the temperature and concentration fields. The simulation are conducted in a simple two dimensional domain (A rectangular channel with a forward facing step) using a finite element analysis package FIDAP. Rectangular elements with moving nodes are employed to prevent element distortion, and a free surface is assumed at the liquid-air interface. The relevant dimensionless groups are identified.