an extruder for plasticating and extruding an extrudate , and an extrusion screw used with the extruder form the subject of this invention . the extruder screw includes an elongated root body and a thread portion helically wound about the root body and extending along the length . the screw having a low compression ratio , flight with a reduced helical angle , and a conical , tapered front end section where at least one - half flight of the thread has a pitch associated therewith which is different from the pitch of the remainder of the thread and where the diameter is less than the diameter of the remainder of the root body .