The amount of resin required to fill the sprue of a plastic mould

The amount of resin required to fill the sprue, runner and cavities of a plastic mould comprises a "shot". Trapped air in the mould can escape through air vents that are ground into the parting line of the plastic injection mould, or around ejector pins and slides that are slightly smaller than the holes retaining them. If the trapped air is not allowed to escape, it is compressed by the pressure of the incoming material and squeezed into the corners of the cavity, where it prevents filling and can also cause other defects. The air can even become so compressed that it ignites and burns the surrounding plastic material.

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