Abstract: An apparatus for comminuting grass clippings and other waste material comprising an elongate cylindrical barrel, an extrusion screw mounted for rotation within said barrel, an inlet opening at one end of the barrel through which material to be comminuted is presented to the extrustion screw by feeding means, an end cap closing the end of said barrel and having a shaped inner surface corresponding the shape of a generally conical end portion of said extrusion screw, said end cap having a plurality of openings through which the comminuted material is forced by said extrusion screw, said openings being directed towards the line of contact between the end of the flight of said extrusion screw and the end of said barrel.