SAND CASTING DEFECTS AND CAUSES ANALYSIS--SHRINKAGE
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Shrinkage occurs because metals are less dense as a liquid than a solid.
A shrinkage cavity is a depression in a casting that occurs during the solidification process.
Shrinkage porosity appears with angular edges, compared to the round surfaces of gas porosity.
Cavities might also be paired with dendritic fractures or cracks.
Large shrinkage cavities can undermine the integrity of the casting and may cause it to eventually break under stress.
Shrinkage can result in two types of casting defects.
1. Open shrinkage defects
These are open to the atmosphere. Air compensated as the shrinkage cavity forms.
Pipes are open shrinkage defects that form at the surface and burrow into the casting.
Caved surfaces are shallow, open shrinkage defects that form across the surface of the casting.
2. Closed shrinkage defects
Also known as shrinkage porosity, closed shrinkage defects form within the casting.
Macro shrinkage can be viewed with the naked eye, but micro shrinkage cannot.
Closed shrinkage defects usually appear at the top of hot spots, or isolated pools of hot liquid.
Prevent shrinkage cavities by improving casting structure
Alloys always shrink when changing from molten to solid.
This is because the density of a casting alloy in the molten state is lower than that in the solid state.
You should expect some shrinkage during solidification. Factor a shrinkage allowance into the pattern design before casting.
You can prevent shrinkage casting defects by improving the overall casting structure:
- Design a running (gate) system with risers that ensure a continuous flow of molten metal
- Increase local heat dissipation by inserting internal chills, cooling ribs or cooling coils
- Reduce casting temperature to limit the total volume deficit