a distribution head having an inlet for accepting a stream of fluid plastic and an outlet for discharging the stream , two pairs of discrete channels in the distribution head extending from the input to the output for splitting the streams into corresponding discrete substreams , selected ones of the channels being of predetermined different lengths to provide lateral mixing of the stream as it passes through the head . the invention also includes a combination of the distribution head and a connected extrusion tool and the process of making homogeneous plastic articles , particularly pipes .