an extruder having a multichannel wave screw is disclosed wherein at least one portion of the screw is provided with a helical passage that is divided into a pair of parallel flow channels . the helical passage is defined by a helical conveying land which extends radially outwardly to a first diameter from the screw root , which defines the bottom surface of the helical passage . to divide the helical passage into the pair of flow channels , a helical barrier land extends radially outwardly to a second diameter which is less than the first diameter . each flow channel is further characterized by the existence of an undulating screw root which defines a plurality of waves , each with a crest and a valley . the crests of one flow channel are helically displaced relative to the crests of the adjacent flow channel . in addition , the difference between the second diameter and the first diameter is selected such that plasticated synthetic resinous material may flow from one flow channel into the other flow channel while unplasticated particles are subjected to intensive shearing forces by the crests of the respective flow channels . the wave cycle of each flow channel , defined as the distance between successive crests , is relatively short , eg . less than about 360 ° along each channel . preferably , the wave crests are positioned 180 ° apart along the channel .