Blank core vents (or dummy core vents) can be used to close holes in the mold.
Blank Core Vents
Core Vents with Holes
Core vents with holes let the air exhaust and the steam enter into the mold. They are easier to fit into the mold than slotted core vents
Slotted Core Vents
Slotted core vents let the air exhaust and the steam enter into the mold. They enable maximum performance but they demand higher skill to fit them into the mold.