At the end of filling phase, in injection molding there is still melt in front of the screw of the plastification unit.
This amount of melt is needed to to compensate the shrinkage within the cavity during the solidification of the melt.
At the end of packing there is still melt left in front of the nozzle. This amount of melt is the so called melt cushion.
It is used to validate the stability of the injection molding process. There shall be only a small variation in the melt cushion from one shot to another.
A difference might indicate an abrasion of the non-return valve.
This glossary of plastic industry is provided by PLEXPERT Canada Inc.