The effects of coating thickness and sand fineness on the mold filling characteristics of aluminum alloy 319 have been investigated. Experiments have been conducted with strip patterns and transparent molds have been used to visually record the flow behavior. The results indicate that a higher fill time is observed in thicker coatings and finer sands. Typical flow velocities are on the order of 80 mm/s and 90 mm/s for sands with AFS numbers of 80 and 20 respectively. By comparison, when a completely impermeable mold is used, the flow velocity is reduced to about 20 mm/s. Under normal casting conditions, the gases formed at the metal front are eliminated rapidly into the sand, so that there is no substantial gas layer ahead of the flowing melt. A gas layer may build up at the metal front if the permeability of the mold medium is not adequate and eventually the gas bubbles may escape by penetration through the liquid metal. Factors affecting polymer degradation may have strong effect on mold filling while parameters associated with the elimination of degradation products may control defect formation. (C) 2003 Kluwer Academic Publishers.