The effects of pattern molecular weight and density on casting formation in the lost foam process have been studied. The tensile properties of injection molded ASTM D638 tensile rods have been measured for various densities and molecular weights. Thermogravimetry, differential scanning calorimetry and other techniques have been used to study the thermal degradation characteristics of the pattern. A thermometric technique has been used to study the mold filling behavior with aluminum alloy A356. The results indicate that the tensile properties in the foam generally improve with increasing molecular weight and density. The structural characteristics of the foam may have a greater influence on mechanical properties than molecular weight. The rate of volatilization of the polymer increases as the weight average molecular weight, M-w, is lowered from 304,000 g/mol to 152,000 g/mol. The mold fill time increases with pattern density and M-w. The data suggest that pattern density and molecular weight may have a significant effect on the quality of the casting. (C) 2003 Kluwer Academic Publishers.