What is the formation of casting shrinkage cavity?

Silica sol investment casting ridge prism body manufacturers analyze the whole process of shrinkage cavity formation. It is assumed that the poured cylindrical casting solidifies step by step from shallow to deep. After the liquid metal fills the mold, the liquid shrinks due to the drop in temperature, and this shrinkage is filled by the liquid metal in the pouring system. Therefore, there is always metal liquid in the die during this period.

When the surface temperature of the stainless steel casting drops below the temperature of the solid line, the live line, and the zero line, a hard shell solidifies on the surface of the casting to enclose the liquid metal inside, and the inlet of the pouring system is frozen.

With further cooling, the liquid metal in the hard shell will shrink due to the temperature drop, and it will make up for the hard shell's solidified shrinkage. At the same time, the solidified hard shell will also make the surface area of the casting smaller due to the temperature drop. If the volume reduction of the liquid metal in the hard shell due to liquid shrinkage and solidification shrinkage is equivalent to the volume reduction on the surface of the casting, the solidified shell is still in close contact with the liquid metal in the internal structure, which is not easy to cause shrinkage cavities. However, the liquid shrinkage plus solidification shrinkage of the aluminum alloy far exceeds the solid shrinkage of the hard shell, so the liquid state of high-quality aluminum alloy die castings will separate from the top of the hard shell and drop. As it continues to develop, the hard shell will continue to thicken, and the liquid level will continue to decrease. After the metal is completely solidified, an inverted conical shrinkage cavity is formed on the upper end of the stainless steel casting. As the temperature continues to drop to room temperature, the size of the entire casting continues to decrease, and the absolute volume of the shrinkage cavity also decreases relatively, but the relative volume of the silica sol investment casting remains unchanged. If a gate is set at the top of the casting, the shrinkage cavity will move into the riser.

In the case where the amount of gas contained in the liquid metal is not large, when the liquid metal separates from the top of the hard shell, a vacuum should be formed on the liquid level. Under the action of atmospheric pressure, the upper shell may be concave in the direction of shrinkage. As shown by the slash in the figure below. Therefore, the shrinkage cavity should include two parts: the outer shrinkage cavity and the inner shrinkage cavity. If the shell strength at the top of the casting is relatively strong, it is also possible that sinking will not occur.

In general, the basic reason for high-quality zinc alloy die castings to cause concentrated shrinkage is that the liquid shrinkage plus solidification shrinkage of aluminum alloys is far greater than the solid shrinkage; the premise of causing concentrated shrinkage is that the casting is from shallow In the deep step-by-step solidification, the shrinkage cavity is concentrated at the final solidification position.