Venting
Venting is an important process feature that must be considered in the design of the exterior of the mold. As the parison is trapped by the pinch-off, a certain amount of air is also trapped between the outer wall of the parison and the mold cavity. As air is injected internally to expand the parison, this trapped external air becomes more and more compressed until an interior and exterior pressure equilibrium is reached. When this occurs, the part will form without completely touching the mold wall. The results are visible as surface abnormalities, loss of texture or engraving detail, the appearance of creases and draglines, and longer cycles from poor mold contact.

Venting can be easily located at the edge of any insert in the cavity. Slotted vent inserts or porous metals can be purchased and fit into nearly any location. Most venting methods will produce visible markings on the finished part. Texture, stepping of inserts and other techniques can be employed to mask the markings made at the vent.