Shrinkage Porosity

Shrinkage porosity inside castings has a fundamentally different cause to gas porosity.

The density of a die casting alloy in the molten state is less than its density in the solid state. Therefore, when an alloy changes phase from the molten state to the solid state, it always shrinks in size.

This shrinkage takes place when the casting is solidifying inside a die casting die. At the centre of thick sections of a casting, this shrinkage can end up as many small voids known as ‘shrinkage porosity’.

Inside these voids there is no air or gas, but a vacuum.

If the shrinkage porosity is small in diameter and confined to the very centre of thick sections it will usually cause no problems.

However, if it is larger in size, or joined together, it can severely weaken a casting. It is also a particular problem for castings which need to be gas tight or water tight.

The general technique for eliminating shrinkage porosity is to ensure that liquid metal under pressure continues to flow into the voids as they form.

This can be achieved by careful attention to the gating system, the cooling system, the die layout and the heat flow paths in the die.

Some castings have both shrinkage porosity and gas porosity inside them, making it very difficult to see one for the other.

HotFlo! has a number of solutions for both shrinkage porosity and gas porosity and our engineering team is available to assist.