How To Solve The Shrinkage Defects of Low Pressure Casting?

During the solidification process of the metal liquid, due to the liquid shrinkage and solidification shrinkage of the alloy, that is, the volume loss caused by the volume shrinkage cannot be compensated, that is, the shrinkage cannot be obtained, and holes often appear in the solidified part of the casting. Unlike general gravity casting, low pressure casting is filled from the bottom up, and the gate is at the lower part. In order to obtain sufficient shrinkage of the casting, it must form a solidification from the top to the bottom, that is, solidify away from the runner first, and solidify at the runner, otherwise defects such as shrinkage and shrinkage will occur. So how can manufacturers solve this problem?

Because low pressure casting is anti-gravity casting, gravity is always hindering the filling. Therefore, whether for sand casting or metal casting, whether for simultaneous solidification or sequential solidification, the quality of the liquid pressure control system is determined. The key link of casting compactness. Especially for thin-walled metal mold casting, the solidification time is not long. When filling to the top of the mold, the solid phase fraction in the liquid metal already occupies a considerable proportion. At this time, the pressure should be increased rapidly in order to overcome the negative effect of gravity and perform shrinkage. At this time, the compactness of the casting is a critical moment.

At present, some liquid pressure control systems are still slowly pressurizing at the filling rate at critical moments, and some control systems are even worse. They can increase pressure normally when the pressure is low, but the higher the pressure, the slower the speed This is the so-called parabolic filling with downward opening.

When the solidification of the liquid metal has basically ended, the control system raises the pressure of the supercharging and filling. It is obviously too late, which will not have a good effect on the density of the casting. Sometimes the shrinkage pressure is already high during production, but the casting still has shrinkage defects, which results in too high a leakage rate. When the replenishment channel is reasonable, this is mainly because the timing of the control system's pressurization is not well controlled, rather than the wrong statement that the amount of replenishment pressure has little effect on the compactness of the casting.