Blowholes or Gas
Small or large number of spherical cavities, superficial or in depth
Indicates that either air was entrapped at casting or that gas separated from the metal at solidification
Causes of this defect are evident in the metal and other portions of the process; to cure, R&R recommends taking the following actions:
Turbulent metal flow or low shell permeability.
Reduce pouring height, avoid turbulence, cast through bottom, use ceramic casting filter and improve ceramic shell permeability.
Excess picking up of N and H gas when metal is kept at superheat temperatures for too long, enhanced by low shell permeability.
Cast at lowest possible metal temperature, avoid long holding times and improve shell permeability.
Improper burn-out of shell.
Remove any organics from the shell in the burn-out/preheat process.