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Why the porous structure provided by the silica is necessary in casting process?

Why the porous structure provided by the silica is necessary in casting process? Correct Answer for the escape of both gases and vapors

Permeability is the important factor for sand casting. If we do not allow gases and vapor to escape from the mould it can damage the casting pattern and can also weaken the strength of the material. Silica allows the escape of gases and vapor and that’s why it is necessary to provide this porous structure.

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