In this column we will get into more specifics and details on venting. Let’s start with the most common type, parting-line vents. The old standard for parting-line vent design was to take a cutter, typically 0.250 to 0.500 in., and cut the relief of the vent to within 0.250 in. of the cavity. Typically a vent would be cut every 1.5 to 3 in. apart around the perimeter of the cavity (see detail). Relief channel depth can vary, depending on opinion. I have found that 0.010 to 0.015 in. is adequate in most cases. (At times this can be deeper for certain materials and for overflows, as discussed below.)
Shortly after moving from the tool shop to the molding side, I realized that the standards just cited were not the best and caused issues with inadequate venting. Two things were really the culprit, the vent land length and too much removal of parting-line bearing surface, with the number of vent relief channels contributing to coining of the parting lines and closing off the vents. I will get into better vent relief designs shortly.