a plasticating apparatus having a barrel and a screw rotatably disposed therein . the screw has a mixing section including a helical conveying flight to form a helical passage for transporting resinous material along the screw . the mixing section further includes two helical divider flights , each spaced from the conveying flight to divide the helical passage into three channels of varying cyclic depth in the helical direction of the channels . the portions of minimum depth of the channels define wave crests which are helically displaced from each other .