PRESSURE DIE-CASTING - A MODEL OF VACUUM PUMPING

Multifunctional characteristics of die casting parts are significantly compromised by the presence of voids, which can result in a substanti al decrease in the elastic moduli and thermal conductivity. Gas/air po rosity constitutes a large part of the total porosity. To reduce the p orosity due to the gas/air entrainment a vacuum can be applied to remo ve the residual air in the die. In some cases vacuum castings have low porosity, while in other cases the results are not satisfactory. Thes e differing results can be explained in some instances by an analysis of the vent area. A simple model is proposed based on conservation law s which describes the relationship between vent area and pressure vari ation in the die. The analysis of vacuum venting indicates that there is a critical/optimum vent area below which the ventilation is poor an d above which the resistance to the air flow is minimal. The model yie lds a simple equation to select the optimum area which is a function o f the duct resistance, the evacuated volume, and the filling time. Thi s result should be useful to the design engineer. The result also prov ides a tool to ''measure'' the vent size for numerical simulations of the cavity filling, taking into account the compressibility of the gas .