The injection cycle includes pressing the two mold halves together with tonnage and shooting plastic into the cavity during injection. However, thinking about that injection process, the plastic is displacing air that is in the cavity at the time of closure. Where does the air go?
Venting in the mold is there to allow the air and gas to escape. As it injects, the leading edge of plastic pushes the air and gas into the venting, allowing the plastic to fill out the cavity. The vents are so small, that the viscosity of the plastic prevents the plastic from entering or “flashing” as we call it. The press settings tell the injection press how much plastic to push into the cavity. If air and gas take up some of that volume, the injection press does not know that and will continue to push. If there was no venting, or as we see in production inadequate venting, the plastic would push air and gasses into the last areas of the cavity to fill. Then, with nowhere to go, the injection pressure super compresses the air and gas. Compression of gas leads to extreme heat being generated and the neighboring plastic will be scorched or even burned.
As different plastics have different viscosities when liquid, the mold designer calculates the vents based on the plastic that the part designer has selected for the part(s) the mold will make. This is one of many reasons why changing resins once a mold has been finished is not as easy as pouring a different resin in the dryer and ultimately shooting it.
Mold designers provide venting by placing them along the parting line edge of the mold or using the circumference of ejector or core pins. Naturally, the larger the part being molded, the larger the cavity, the more entrapped air and the more venting required. You cannot have too much venting. Once in place, vents have to be monitored and maintained. Sometimes during the build but certainly postproduction, cleaning and maintenance should be done prior to shelving the mold.
So yes, we plan for and constantly blow off “steam” on our production floor. It gets you the part you designed and need for your products!