In injection molding the plastic melt shrinks during cooling down.
Therefore after a volumetric filling of the cavity additional material is pushed into the cavity to compensate the shrinkage.
The phase where the screw still apply pressure to the melt is called packing phase.
Packing pressure (and packing time) is hereby essential.
Too much packing pressure leads to overmolding and flash formation.
Not enough packing pressure results in sink marks, voids and higher shrinkage.
Often a profile is used to apply pressure during the packing phase.