A model is developed for heat transfer, polymer vaporization, and gas diffusion at the interface between the advancing liquid metal and the receding foam pattern during mold filling in lost foam casting of aluminum. Most of the pattern interior decomposes by ablation, but the boundary cells decompose by a collapse mechanism, which creates an undercut in the pattern next to the coating. By regulating how much of the pattern coating is exposed to gas diffusion, the undercut controls the overall filling speed of the metal through the mold. Computed values for the foam decomposition energy from this model compare very well with experimental data on foam pyrolysis, and predicted filling speeds are consistent with observations in published experiments. In addition, the model explains several unusual observations about mold filling that until now have not been understood. (c) 2005 Elsevier Ltd. All rights reserved.