ANALYSIS OF MASS-TRANSFER PROCESS IN THE PORE FREE TECHNIQUE

Air entrainment is one of the most significant problems in pressure di e casting. A possible solution is to use the Pore Free technique. In t his technique an oxygen is introduced into the die to react with the l iquid metal. The vacuum created by the reaction reduces the porosity. The life span of the mold is augmented by the reduction of the pressur e during the process. In some cases the Pore Free technique yields acc eptable results, i.e., low porosity, while in other cases the results are not satisfactory. These differing results can be explained by an a nalysis of the mechanisms involved. A simple model is proposed based o n conservation laws that describes the relationship between mass trans fer mechanisms and pressure deviation. The model indicates that there is a critical dimensionless parameter above which the pressure is decr eased and below which the pressure is increased. In the first case, wh en 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 analysi s demonstrates that the Pore Free and vacuum venting techniques accoun t for two different extremes. The first case is when the flow is order ly throughout the chamber in which vacuum venting can produce acceptab le results. The second case is when the flow is turbulent and the Pore Free technique is the better choice.