Incidence of the defect
Scabs may appear on all casting surfaces when pouring into green sand moulds. They occur primarily at the top of the mould cavity, although their formation on the mould bottom in the form of “rat tails” is likewise possible. Highly compacted parts of the mould cavity are also at risk. Scabs can be created when the mould wall is heated by radiant heat, but may also be caused by the direct flow of heat from the molten metal into the mould wall.
Explanation
During pouring, parts of the sand mould near the surface are