a screw for use in an plasticating injection apparatus includes a plurality of interrupted primary flights . an outer surface of a primary valley is formed by the primary flights and features a plurality of plateaus . the plateaus span adjacent sections of the primary flights and define shallow levels between deeper levels of the primary valleys . this depth differential contributes to a turbulent mixing action which , along with the primary and secondary valleys , contributes to improve mixing action within a metering section of the screw . the primary flights are interrupted by secondary valleys which are of opposite hand to the primary valleys .