Typically a core coating will act as an impermeable barrier between the core substrate and the mould cavity, ensuring that gases formed through the thermal degradation of the core binder exit the core through predefined vents. Where adequate venting is not available or possible, there is a risk that gas pressure build-up within the core will rupture the coated core surface, resulting in scab defects, such as:

• A Non-metallic inclusion consisting of coating and core sand
• Scab defects where metal has replaced the ejected coating and core sand
• Metal penetration and burn-on in the area of the core no longer protected by a coating layer
• Gas blow-holes related to the binder decomposition gases which exit the core and become entrapped within the solidifying metal