Analysis of Mass Transfer Process in the Pore Free Technique

Abstract

Air entrainment is one of the most significant problems in pressure die casting. A possible solution is to use the Pore Free technique. In this technique an oxygen is introduced into the die to react with the liquid metal. The vacuum created by the reaction reduces the porosity. The life span of the mold is augmented by the reduction of the pressure during the process. In some cases the Pore Free technique yields acceptable results, i.e., low porosity, while in other cases the results are not satisfactory. These differing results can be explained by an analysis of the mechanisms involved. A simple model is proposed based on conservation laws that describes the relationship between mass transfer mechanisms and pressure deviation. The model indicates that there is a critical dimensionless parameter above which the pressure is decreased and below which the pressure is increased. In the first case, when the parameter exceeds the critical value, the technique is useful. In the second case, when the parameter is less than the critical value, introducing oxygen does not greatly reduce the porosity. The analysis demonstrates that the Pore Free and vacuum venting techniques account for two different extremes. The first case is when the flow is orderly throughout the chamber in which vacuum venting can produce acceptable results. The second case is when the flow is turbulent and the Pore Free technique is the better choice.