Vent marks appear as small nipples, dimples, rings, or witness points where the hot sheet is drawn into or across a mold vent. They are most visible on glossy surfaces and in regions that become especially soft or thin.
Oversized holes are an obvious cause, but size alone is not the whole mechanism. A sharp or burred vent edge, excessive local sheet temperature, prolonged pressure differential, poor vent placement, and a very low-viscosity material can make an otherwise acceptable vent visible.
Correction
The affected vent should be inspected under magnification and compared with nearby vents that do not mark. Burrs must be removed without rounding the surrounding mold surface. An oversized opening may need to be plugged and re-machined rather than covered with release agent.
Local heat and forming sequence should also be checked. Reducing sheet softness in one zone may remove the mark while preserving overall detail. Vent design should be validated on the actual material and finish requirement; a hole suitable for a textured thick-gauge part may be unacceptable on clear thin sheet.