The mold cavity is full of air when the cycle starts. During the injection cycle this air is pressurized by the incoming hot feedstock and forced out of the cavities through vents. Venting is needed in the last portion of the cavity to be filled (opposite the gate). Vents are very thin reliefs typically 0.015 mm (0.0006 inch) deep, and up to 12 mm (0.5 inch) wide for large MIM parts. Ideally, venting leaves no observable trace on the part surface.
Venting is also necessary to allow the part to be ejected from the tooling. For example, a shape with a closed end cannot be ejected without creating a vacuum; otherwise an additional ejector pin must be built into the forming tool which is pulled first to vent the mold.