Application of flow simulation to injection condition optimization for diecasting

摘要

First, the effect of gate speed on diecast internal porosity (air entrapment) was investigated by changing the gate thickness of a simple stepwise test mold. The result was inspected by x-ray observation and vacuum remelt gas measurement, which showed that, when the gate speed exceeded the critical speed of 0.5 m/sec, much air entrapment was found in the casting. The worst case was 11.0 cc/100 g(Al), which caused a blister defect when the casting was heat-treated. Second, computer-aided flow simulation was applied to the test mold to analyze the flow phenomena. The simulated results coincided well with the actual casting defects (air entrapment). The results showed that, when the gate speed was 2.4 m/sec, the metal flow collided against the opposite mold side at the very early stage of filling, which then resulted in the largest amount of air entrapment, corresponding to the actual worst case. The entrapped air volume was directly calculated, based on the timing when the metal flow choked the outgoing air; then the calculated value was corrected, considering the internal pressure and the temperature. Though the calculated result of the remaining gas content did not show a very good correlation with the measured one, the proposed scheme could help better understand the air entrapment phenomena. Finally, the simulation was applied to find the optimum gate speed condition, which included an approaching speed before the gate, a slow speed below the critical speed and a final high speed.