Active decompression to prevent melt drool from a mold (12, 14) or runner system (20) is achieved through the selective coupling and de-coupling of an injection piston (34) to a plunger (38). Following successive injection (FIG. 5a) and hold phases (FIG. 5b) of an injection molding process (FIG. 5), the runner and channel system is partially de-compressed (FIG. 5c) by drawing back together, over a short distance, the plunger (38) and the injection pressure (34) as one unit. The injection piston is then mechanically de-coupled from the plunger (FIG. 5e), with the injection piston (34) withdrawn to essentially its final shot position, but minus a customary packing distance (.delta.). Plastic melt (100), extruded into a shooting pot (28) positioned in front of the plunger (38), is permitted to push the plunger backwards, but now with minimal work expenditure. When the plunger again contacts (FIG. 5f) the injection piston (34), melt pressure now causes both units to be moved back to reach a final shot size volume. Finally, the injection piston (34) drives the plunger (38) forward to eject melt accumulated within the shooting pot (28).