TOOLING: Venting: Where And How Deep?

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 jus

Critical Design Considerations for Sprue Bushings

Last month I discussed how to reduce the massive area where the sprue, runner, and cold well intersect at the parting line. This month let’s take a look at some additional sprue-bushing design considerations to further improve the cycle time and to help avoid several processing problems. Cooling: The sprue doesn’t need to completely solidify before opening the mold. It only needs to be able to shrink enough to negate the taper lock and have sufficient physical integrity to remain adjoined to the part or runner. But that doesn’t mean cooling the sprue bushing isn’t critical, because it is. Do you have any molds that eject ice-cold parts, but the sprue is still as soft as butter? For maximum e

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