A theoretical model is proposed for curing in polymer reaction molding operations like casting, potting, lamination, thermoset molding, or reaction injection molding. In many cases, convection and mass diffusion can be neglected. The resulting unsteady heat equation with temperature dependent generation is solved numerically. Temperature and property profiles, gel point, sol fraction, and modulus are presented for a typical urethane system under several conditions. Two dimensionless groups, the adiabatic temperature rise and the ratio of polymerization rate to heat conduction rate, are most significant. Process design criteria are discussed.