To evaluate the accuracy of simulated results of mold filling behaviors in the vacuum suction casting process, mold filling of aluminum casting alloy - AC4C was directly observed by X-ray radiography. A numerical method, in which evacuation of gas through vent hole is taken into account and backpressure of gas is coupled with the pressure field of melt, was applied to simulate the mold filling behaviors Backpressure of gas in the empty mold cavity was also measured by using a semiconductor pressure sensor. The experiments were carried out at two differnt reduced pressures (10 kPa and 20 kPa) and three different reducing pressure rates (1.2 kPa/s, 4.2 kPa/s and 80 kPa/s), which were controlled by the aperture of the valve (in between the gas pipe of the vacuum suction system). Experimental results showed that the effect of reduced pressures and reducing pressure rates on the mold filling behavior were significant. That is the higher the reduced pressures and the reduced pressure rates, the more turbulent the free surface of molten metal, and this resulted in earlier occurrence of the gas entrapments, larger volume of the entrapped gas regions and higher risks of misrun. By comparing simulated results with experimental ones, it was found that good agreement can be obtained in terms of mold filling patterns, positions of gas entrapments and mold filling time. It was also found that simulated results by the conventional method, in which gas evacuation and the effects of backpressure on mold filling are not taken into account, show poor agreement with the experimental ones.
