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A numerical approach is taken to the analysis and simulation of structural reaction injection molding (SRIM). Formulation is based on anisotropic Darcy flow through a porous medium. Governing equations are developed for thin gap mold cavities with no gapwise flow. The reaction of the resin and its effect on the local viscosity are accounted for by using constitutive models. A numerical grid generation method is employed to handle irregularly shaped flow domains during the filling stage. A new finite difference mesh is generated at each time step to cover the flow domain as described by the mold boundaries and the moving resin front. The technique is shown to be capable of handling a wide range of irregularly shaped planar geometries in monitoring the instantaneous state of the cure along with the temperature and the flow fields in a detailed form. The results indicate a highly pronounced disturbance of the temperature field due to the exothermic nature of the chemical reaction. It is also shown that this temperature field and the state of cure significantly influence the flow field through fluid viscosity. The method reduces to the simulation of resin transfer molding (RTM) processes by dropping the reaction terms. The comparisons with limited experimental data for isothermal flows indicate high accuracy in both predicting the resin front locations and the fill pressures.