What can be the reason for shrink-spot porosity?

Shrink-spot or localized shrinkage porosity These are large irregular voids usually found near the sprue-casting junction. It occurs when the cooling sequence is incorrect and the sprue freezes before the rest of the casting. During a correct cooling sequence, the sprue should freeze last. This allows more molten metal to flow into the mold to compensate for the shrinkage of the casting as it solidifies. If the sprue solidifies before the rest of the casting no more molten metal can be supplied from the sprue. The subsequent shrinkage produces voids or pits known as shrink-spot porosity. Avoid by Using sprue of correct thickness. Attach sprue to thickest portion of wax pattern. Flaring the sprue at the point of attachment or placing a reservoir close to the wax pattern.