Polymer-melt interactions during casting formation in the lost foam process

Abstract

The lost foam casting process utilizes injection modeled polymeric foam patterns for the production of metallic components. Foamed polymer patterns of the desired shape are coated with a water-based refractory slurry, dried and embedded in unbonded sand. Molten metal is poured directly on the coated polymer. The polymer undergoes thermal degradation and is gradually replaced by the liquid metal to yield the casting after solidification. Expanded polystyrene (EPS) is the most common pattern material used in commercial practice. The use of EPS patterns with ferrous castings may result in the formation of carbonaceous defects in the casting. Consequently, polymethylmethacrylate (PMMA) and copolymers of EPS and PMMA have been developed for ferrous castings. The thermal degradation of the foamed pattern results in the formation of gaseous degradation products and of a partially depolymerized viscous residue. The fraction of viscous residue increased with temperature and is essentially constant above about 650 C. During the filling of EPS patterns, nearly 60% of the polymer is converted to the viscous residue and 40% is transformed to gaseous products. In the case of PMM, almost 60% of the polymer undergoing degradation at the metal front is transformed to gaseous products. The melt flow velocity duringmore » the filling of the mold generally increases with temperature.« less