Throttling means for extruders of the transfermix type

An extruder for visco-elastic compounds comprising a barrel in which a screw is rotatably mounted and driven to coact with said barrel, said screw having a helical groove, and said barrel having an operative contact surface for material to be extruded said extruder comprising successively an inlet for material to be extruded, a compression section, a plasticising section and an exit section, said plasticising section embodying a Transfermix geometry in which aid helical groove in said screw varies in cross-section from full area to zero area and in a substantially corresponding length of said barrel the cross section of an oppositely handed helical groove varies from zero area to full area, whereby in operation the material is transferred from said screw into said barrel while being mixed and plasticized, and in which following said plasticizing section is a return section of which an internal diameter of said operative contact surface for the material in said barrel is reduced from that corresponding to the maximum depth of said barrel groove to one having substantially a running clearance to an outside diameter of said screw and said helical groove in said screw varies in cross-sectional area from zero to full depth;

• in which said operative contact surface of the said return section in said barrel is formed by a sleeve of at least partially cylindrical shape and having a radial thickness substantially equal to said maximum depth of said barrel groove, said sleeve being mounted in said barrel so as to form a substantially cylindrical gap between an innermost edge of the operative contact surface of said sleeve and a diametral edge of said screw at which the depth of said helical groove thereon is substantially zero, said sleeve being mounted in the said barrel by means adapted to prevent rotation thereof and by means for axially displacing said sleeve as to close or open said gap to produce a continuously adjustable throttling effect between substantially fully open and fully closed.