Scab is a sand casting defect. Sand casting is a popular and widely used casting process in which molten metal is poured into a mold cavity formed in a sand mold. During the solidification of the metal, various defects can occur, and one of these defects is a scab.

Scab is a defect that occurs on the surface of a casting and is characterized by a raised or rough area. It is caused by the detachment of a thin layer of sand from the mold surface during the pouring of the molten metal. The detached sand then becomes embedded in the casting, resulting in a scab.

There are several reasons why scabs may occur in sand casting:

1. Mold Material: The quality and properties of the mold material can influence the occurrence of scabs. If the mold material is not properly prepared or if it contains impurities, it may not bond properly with the molten metal, leading to scab formation.

2. Mold Design: The design of the mold can also contribute to scab formation. If there are sharp corners or edges in the mold cavity, the molten metal may not flow smoothly, causing turbulence and sand detachment.

3. Pouring Technique: The way the molten metal is poured into the mold can affect the occurrence of scabs. If the pouring is done too fast or too forcefully, it can create turbulence and dislodge sand particles, leading to scab formation.

4. Metal Temperature: The temperature of the molten metal can also play a role in scab formation. If the metal temperature is too high, it can cause rapid sand expansion and detachment from the mold surface.

To prevent scab defects in sand casting, several measures can be taken:

- Use high-quality mold materials that are properly prepared and free from impurities.
- Design the mold with smooth and rounded corners to promote smooth metal flow.
- Control the pouring technique to ensure a smooth and controlled flow of molten metal.
- Monitor and control the metal temperature to prevent excessive sand expansion.

By implementing these measures, the occurrence of scab defects in sand casting can be minimized, resulting in higher quality castings.