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Why is there porosity in my casting?

Updated: Oct 21, 2022

Porosity in casting is a common defect and can usually be attributed to one or two main causes. It is often easily rectified. Here's a quick question and answer to try and help you solve the issue.

#1 – What do the pores look like?

Are the pores round and spherical or are they more tree-like in shape?

Round, spherical pores. Source: Santa Fe Symposium

Round and spherical pores are indicative of gas porosity. Go to #2. Otherwise, go to #3

Non-spherical pores. Source: Santa Fe Symposium

#2 – Gas porosity – are they throughout the sample or just at the surface?

If they are located at the surface only (shown left), the porosity is likely caused by the evolution of gas due to a reaction at the mold wall. In general, clean burnout of the mold, use of cleaned scrap (especially recycled casting sprues) in the melt charge, and lower casting and/or flask temperatures will reduce the probability of gas porosity.

For more details, see: D. Ott, Handbook on Casting and Other Defects in Gold Jewellery Manufacture (London: World Gold Council, 1997).

If they are located throughout, the porosity is likely caused by dissolved gas in the alloy. Ensure high-quality start material (>99.9% high-purity metals or Pure master alloys) and that the material is all dry before melting.

#3 – Shrinkage porosity?

If caused by shrinkage, then this is common for investment cast products where there is essentially insufficient liquid able to fill the mold during solidification. It is important to carefully consider mold design, specifically the diameter and positioning of sprues.

Example of Shrinkage porosity in investment-cast 18K Gold. Source: Santa Fe Symposium


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