Explaining casting porosity

The voids in a cast metal component known as porosity take three basic forms:

Fully enclosed porosity

This is not a problem unless uncovered by subsequent machining.

Blind porosity

Porosity open on only one surface of a component, leaving a relatively large internal surface area of metal open to attack by chemicals or water. Often the cause of “spotting out” of paintwork and internal corrosion of parts and can be exposed through machining to create through porosity.

Through porosity

Microscopic holes stretching right through a casting wall. Through porosity provides an open leak path and can lead to fluid or pressure loss and complete failure of components.

 

There are three main causes of casting porosity:

Flow porosity

Flow porosity refers to surface or internal defects caused when the molten metal does not flow well around the mould and when pressure conditions are poor.

 

Gas porosity

Usually internal, gas porosity occurs when air, steam or residual die-cast release agent vapours are trapped within the molten metal.

 

Shrink porosity

The most common and most difficult to control, shrink porosity is caused by the change in volume as metal changes state from liquid to solid within the mold.

As porosity is an inevitable part of the casting process, casting impregnation as a post-casting treatment is often the best solution. During casting impregnation, components are subject to a vacuum which removes air from the porosity, then filled with a thermally and chemically resistant polymer.

Ultraseal International is the global leader in providing vacuum impregnation equipment, sealants and services, and can provide a solution to sealing porosity for all requirements.