The gas-assist process differs from standard injection molding in some very distinct ways. The gas-assist process does not require a cushion, whereas standard injection molding needs a cushion in order to properly pack the part. The polymer melt that is introduced into the mold controls the gas-assist process. The gas takes the path of least resistance and displaces molten material and compensates for volumetric shrinkage. The gas pressure takes the place of the machine's pack and hold pressures, and packs the part throughout the cooling stage. In standard injection molding, the part is only packed until the gate freezes. Warp is reduced in the gas-assist part due to a more even distribution of molding pressure through the thicker gas channels. The material and gas channel geometry primarily determines the wall thickness of the gas channel. It is almost impossible to influence the wall thickness with the processing parameters. Once injection of the gas has commenced, it is no longer possible to influence the filling process from outside the mold. Basic techniques, shrinkage and warpage, residual wall thickness, weld line prediction, and troubleshooting are covered in this section. At the end of the cooling phase the gas is vented to atmosphere and a vent opening remains in the part.