The hot junction of the casting is full (there is a void inside). When shrinking, although the skin layer has a certain strength, it still shrinks internally, and the surface is concave when it is not broken.
During the filling process, subsidence occurs due to the Extrusion of gas between the metal flow and the cavity walls. This bumpy surface is smooth and clean, mainly in difficult-to-vent cavities, where castings are located at the end edges.
In the case of poor performance of the injection mechanism of the machine (such as the old vertical machine), when the working hydraulic pressure is unstable, the injection pressure is also unstable. The pressure that pushes the metal is discontinuous, causing the skin of the casting to form multiple times. Each time there is a different edge of the epidermis, resulting in streak depressions.
There is residue in the cavity. It is concave during production, and the residue of the cavity is not necessarily flaked but has various irregular shapes, so the depth of the casting is also shallow.
Methods to prevent dents in castings:
The wall thickness should be uniform.
The excessive thickness should be relieved.
Choose an alloy with low shrinkage.
Correctly select the alloy liquid introduction position and increase the cross-sectional area of the inner runner.
Increase the injection pressure and increase the filling time.
Appropriately reduce the pouring temperature and the temperature of the mold.
Some temperatures should drop.
Improve overflow conditions.