At the end of the injection molding process, the screw stops rotating and just pushes forward. At this time, the injection molding enters the pressure-holding stage. During the pressure holding process, the nozzle of the injection molding machine continuously replenishes the cavity to fill the volume vacated by the shrinkage of the part. If the cavity is filled and the pressure is not maintained, the part will shrink by about 25%, especially the rib will form shrinkage marks due to excessive shrinkage. The holding pressure is generally about 85% of the maximum filling pressure, of course, it should be determined according to the actual situation.