Abstract: A plasticating screw having a main channel defined by a helical flight disposed within and cooperating with an inner-wall of a heated barrel. The screw has a longitudinal portion with a plurality of staggered rows of noncontinuous advancing grooves arranged in the main channel thereof. The axis of each row of advancing grooves being substantially parallel to the helical axis of the adjacent helical flight of the longitudinal portion. A noncontinuous helical channel is formed therein traversing in a reverse direction, compared with the direction of the helical flight, the channel having a plurality of retracting grooves. Each retracting groove cuts through at least one pass of the main channel of the longitudinal portion, including the advancing grooves therein, and intersects and passes through the helical flight at least once, so that molten resin can back flow and re-circulate therethrough.