Cores are pieces that are placed into casting moulds to form internal cavities of the casting, or to form extra sections of the mould for castings that have external projections or negative draft, which, if included in the pattern, would prevent the pattern from being removed from the mould.  Multiple cores may be used in complex castings.

Cores can be made from metal (in shapes that are easily removed from the casting, and used in permanent mould processes) or chemically bonded sand (complex shapes, and used in all mould types).  Metal cores need to be configured such that they are parallel to the mould parting line, or can be removed before the casting is removed from the mould, and shaped so that is readily freed from the casting.

Metal cores are typically made from cast iron or steel.  Sand cores are made from materials similar to those used for chemically bonded sand moulds.  These cores are formed in core boxes - similar to pattern boxes used to make moulds.