Partially filling the cavity with the correct volume of polymer melt is generally accomplished by trial and error and can be time consuming for non-transparent parts. Parts must be cut open to examine the gas channel penetration as processing adjustments are made. If the melt cushion or prefill is too large, then there will be material accumulations, which can lead to sink marks, longer cooling times, warping and other undesirable effects such as increased costs. If the melt cushion is too small, then there will not be enough material to flow outward to the cavity extremities and therefore the gas will break through the melt and escape through the mold parting line and be wasted along with the material. An optimum melt cushion will complete the form of the part with the smallest possible prefill and allow consistency from shot to shot.