active decompression to prevent melt drool from a mold or runner system is achieved through the selective coupling and de - coupling of an injection piston to a plunger . following successive injection and hold phases of an injection molding process , the runner and channel system is partially de - compressed by drawing back together , over a short distance , the plunger and the injection pressure as one unit . the injection piston is then mechanically de - coupled from the plunger , with the injection piston withdrawn to essentially its final shot position , but minus a customary packing distance . plastic melt , extruded into a shooting pot positioned in front of the plunger , is permitted to push the plunger backwards , but now with minimal work expenditure . when the plunger again contacts the injection piston , melt pressure now causes both units to be moved back to reach a final shot size volume . finally , the injection piston drives the plunger forward to eject melt accumulated within the shooting pot .