A rotational screw for mixing synthetic resins with various additives, comprising a plurality of helically arranged mixing recesses provided consecutively, at specified mixing positions, in the peripheral areas thereof and communicated transversely of the helical direction with one another so that the amount of delivered material is large and highly intimate mixing is effected. The improvement comprises a plurality of helically arranged recesses which are triangular in helical cross section, and are arranged to have their corresponding tips of adjacent recesses at 60 DEG angles from each other.
