What's the Difference in Gas and Shrinkage Porosity?

Posted by Andy Marin on Fri, Jan 12, 2018 @ 08:57 AM
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The definition of porosity is any void or hole in a casting. But this definition does not describe or give direction on the root cause of porosity. Casting porosity can be caused by gas formation or solidification shrinkage while the metal is a liquid.  If a casting needs to be pressure tight, then the porosity can allow gas and fluids to seep from the part.  In addition, the porosity can weaken the casting. In this blog, we will discuss the difference between gas and shrink porosity, and the best solution to seal porosity.


Gas-Related Porosity

What is it?
Gas-related porosity is caused by trapped mold or core gases in the liquid metal.  Air is present in the tool cavity, and is easily trapped as metal fills the cavity.  The air dispersed as small sphere of high pressure air when the metal fills the cavity.  


What does it look like?
Gas-related voids generally look like bubbles with a smooth interior. Gas porosity is always buoyant and will be near the top of the casting.    




Shrinkage-Related Porosity

What is it?
When a casting solidifies inside the tool, it always shrinks in size.  Shrinkage-related porosity is caused by sections of a casting that solidify later than the surrounding sections, and do not have enough metal flow into the section to completely fill.  


What does it look like?
Shrinkage porosity will have a jagged or linear appearance. Shrinkage porosity usually occurs in either the cope or drag portion of the casting and below the surface.  





Problems of Porosity
As the amount of porosity increases in an aluminum or iron casting, it can become inter-connected and cause a leak path. The porosity makes the casting unusable for holding pressure in applications like pumps, compressors, transmissions and plumbing fixtures. 

Porosity in powdered metal parts can cause severe plating problems where the plating chemicals are trapped in the pores. The entrapped plating chemicals expand with enough force to cause spotting on the plating. 


How to Stop Casting Porosity
The most economical and successful approach to stop casting porosity is through vacuum impregnation. Vacuum impregnation is a method that seals the casting resulting from porosity. The impregnating sealant is introduced into the voids within the wall thickness of the casting through vacuum and/or pressure methods. This method is a cost effective and permanent solution to casting porosity. There is no limit to the size of castings which can be impregnated.  Since the impregnation process occurs within the part, the vacuum impregnation process does not distort, discolor, or affect the casting. 


Vacuum impregnation achieves the results that casting engineers have sought for centuries: a leak-free and pressure tight casting that is economical and usable in all the applications required.



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