There have been many articles and entire textbook chapters written about venting, but very few explain the complexity of what’s actually going on inside the mold and the effect it can have, not only on the molded parts, but on the mold itself. Without this basic understanding, the decision on where to put vents, how many you need, how wide they should be, and how deep they should go is probably an educated guess, or worse—a trial-and-error endeavor.
Let’s start off with a couple of basic facts: 1) The mold is not empty before the plastic enters it. It’s full of air. That air must escape in order to produce a good part. 2) Vents in an injection mold should let the air out, but not the liquid plastic. If you turn on the spigot to a garden hose, you will hear a hissing sound at the nozzle end for the first few seconds. That’s the air escaping out of the nozzle until the hose is full of pressurized water. Mold vents should function the same way.