the invention is directed to a bi - directional check ring for a two - stage injection unit . more specifically , a close fitting ring is inserted behind the plunger head of the melt accumulator so that normal running clearances between the plunger and barrel bore can be used . the outer diameter of this check ring has a very close fit with the inner diameter the injection barrel , but is not rigidly connected to the plunger . accordingly , the plunger is free to “ floats ” while injection and plastication / filling take place . the open volume between the ring and plunger is small , but sufficient to permit the ring to “ float ” very close to the barrel inner diameter , without being influenced by the alignment of the plunger and injection drive motor .