Mould cavities need to be vented to provide a way out for the air that is displaced by the material flowing into the tool. Poor venting will lead to low weld line strength, incomplete mouldings, burn marks and moulded in stresses.

Vents can be located anywhere along the parting line of the tool. They should be positioned where the tool fills up last. Vents should also be placed on bosses and blind ribs. Here, a flat spot on an ejector pin usually does the trick; otherwise inserts have to be used.

For Polyamides, the geometry of the vent is shown in the figure below. These vents will not give flashing, but are big enough to allow the air to escape.