The cavity contains air, which is pushed out by the arriving melt.
Therefore, venting is necessary to allow the air to escape the cavity.
Otherwise a compression of the air occurs, which leads to a rise of the temperature (burn mark).
The venting consists of small gaps, which are situated for example at the parting line, at the sliders and at the ejector pins.
Depending on the used polymer, the height of the gap needs to be adjusted.
If the gap is too small, it prevents the air from escaping.
If the gap is too big, flash formation occurs at the part.

Venting in Plastic Industry