In the solidification process of liquid metal, due to the liquid shrinkage and solidification shrinkage of the alloy, that is, the volume loss caused by volume shrinkage can not be compensated, that is, feeding can not be obtained, and holes often appear in the solidification part of the casting. Different from general gravity casting, low pressure casting is filled from bottom to top, and the gate is at the lower part. In order to make the casting get enough feeding, it is necessary to form a sequential solidification from top to bottom, that is, solidification away from the sprue and solidification at the sprue, otherwise shrinkage cavity and shrinkage porosity defects will occur. So how should manufacturers solve this problem?
As low-pressure casting is counter gravity casting, gravity always hinders feeding, so whether for sand casting or metal casting, whether for castings solidified at the same time or sequentially, the quality of liquid level pressurization control system is the key link to determine the compactness of castings. Especially for thin-walled metal mold casting, the solidification time is not long. When the mold is filled to the top of the mold, the solid content in the liquid metal has accounted for a considerable proportion. At this time, the pressure should be increased rapidly immediately in order to overcome the negative effect of gravity and carry out feeding. At this time, the compactness of the casting is a very critical moment.
At present, some liquid level pressurization control systems still pressurize slowly according to the filling speed at the critical moment, while others are worse. They can pressurize normally when the pressure is low, but the higher the pressure is, the slower the pressure rise speed is, that is, the so-called parabolic filling with the opening downward.
When the solidification of liquid metal has basically ended, the control system will raise the pressure of pressurization and feeding. Obviously, it is too late, which will not play a good role in the density of castings. In production, sometimes the feeding pressure is already very high, but the castings still have shrinkage porosity defects, resulting in too high pressurization leakage rate. When the feeding channel is reasonable, this is mainly because the timing of pressurization of the control system is not well controlled, rather than the wrong statement that "the feeding pressure has little effect on the compactness of castings".
Editing: WMM