The core enclosed by the molten metal develops gasses (see Core gasses).

In the best case, with sufficient permeability to gas of the molding material they can escape from the core to the ambience through the core prints. However, in many cases an additional core vent is required.

The fie molding material does not comply with the requirements for venting, the following options may be helpful:

• Venting subsequent to shooting and prior to hardening
• Insertion of venting rods into the provided holes prior to shooting
• Insertion of venting hoses made of web material (not suitable in cases when the core is shot)
• Coke filling for large-scale cores

Additional references:
Gas blister