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 creating the so called melt cushion.
During packing phase, this cushion is used to compensate the shrinkage within the cavity.
The quality of an injection molded part can be observed by controlling the melt cushion.
A difference might indicate an abrasion of the non-return valve.