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Rule 4. Avoid Bubble Damage

No air bubbles should be entrained by the running system. If they are entrained, then they should not be allowed to pass through the liquid metal in the mold cavity. If this happens, then a mixture of oxide bubble trails, together with residual misshapen bubbles in the casting, will result. This is by far the most common defect in castings, and is commonly mistaken for shrinkage porosity. This severe defect may be avoided by:

Fast backfill of filling system by virtue of properly designed sprue and pouring basin, and use of a stopper in small castings, ceramic foam filters placed close to sprue/runner junction, and bubble traps. Any solutions must be demonstrated as effective using some suitable technique such as real-time X-ray radiography.
No interruptions to pouring.

This drawing illustrates the most common defect in castings - bubble damage as a mixture of oxide cracks and residual entrapped bubbles.

Illustration of bubble damage

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