Extrusion method and apparatus

Abstract

An extrusion apparatus including a mixing chamber comprising two intersecting housing bores and an inlet positioned to receive material into the mixing chamber. Two screw shafts are supported for rotation about respective generally parallel axes and include respective screw sections positioned for co-wiping intermeshing rotation within the respective housing bores of the mixing chamber. The apparatus supports screw shaft rotational speeds greater than approximately 800 rpm and includes screw shaft conveying portions that are rotatably cantilevered for self-journaled support within respective separate conveying chambers arranged generally parallel to one another downstream of the mixing chamber.

1 . An extrusion apparatus for forming an extrudate, the apparatus comprising:
 a mixing chamber comprising two parallel intersecting housing bores;   two screw shafts supported for rotation about respective generally parallel axes and including:
 respective screw sections positioned for co-wiping intermeshing rotation within the respective housing bores of the mixing chamber; and 
 respective conveying portions rotatably cantilevered for self-journaled support within respective separate conveying chambers arranged generally parallel to one another downstream of the mixing chamber; 
   the apparatus being configured to support screw shaft rotational speeds greater than approximately 800 rpm.   
   
   
       2 . An extrusion apparatus as defined in  claim 1  in which:
 the apparatus is configured to support rotation of the screw shafts at rotational speeds in the range of approximately 100 to 1800 rpm; and   the housing bores and the screw sections of the screw shafts are sized to have a screw-to-chamber wall clearance in the range of approximately D os /64-D os /128, where D os =an outside diameter of the screw sections.   
   
   
       3 . An extrusion apparatus as defined in  claim 2  in which the conveying chambers and the conveying portions of the screw shafts are sized to have a screw-to-chamber wall clearance in the conveying chambers is less than D oc /128, where D oc =an outside diameter of the conveying portions of the screw shafts. 
   
   
       4 . An extrusion apparatus as defined in  claim 2  in which the screw-to-chamber wall clearance in each of the conveying chambers is in the range of approximately D oc /128 to D oc /150. 
   
   
       5 . An extrusion apparatus as defined in  claim 1  in which the screw sections of the screw shafts each have an outside diameter to inside diameter (OD-ID) ratio greater than 1.5. 
   
   
       6 . An extrusion apparatus as defined in  claim 5  in which screw sections of the screw shafts each have an outside diameter to inside diameter (OD-ID) ratio in the range of approximately 1.4-1.8. 
   
   
       7 . An extrusion apparatus as defined in  claim 6  in which the screw sections of the screw shafts each have an outside diameter to inside diameter ratio of approximately 1.78. 
   
   
       8 . An extrusion apparatus as defined in  claim 6  in which the screw sections of the screw shafts are configured to develop a torque density in excess of approximately 11 Nm/cm 3 . 
   
   
       9 . An extrusion apparatus as defined in  claim 8  in which the screw sections of the screw shafts are configured to develop a torque density in the range of approximately 8.7-13.6 Nm/cm 3 . 
   
   
       10 . An extrusion apparatus as defined in  claim 9  in which the screw shafts include engagement sections having sinusoidal spline configurations configured to engage complementary receptacles of a drive mechanism. 
   
   
       11 . An extrusion apparatus as defined in  claim 1  and further including:
 a first inlet configured and positioned to receive material into the mixing chamber; and   a second inlet positioned to receive material into the mixing chamber downstream from the first inlet.   
   
   
       12 . An extrusion apparatus as defined in  claim 11  and further including a degassing port disposed downstream from the second inlet. 
   
   
       13 . An extrusion apparatus as defined in  claim 1  and further including a common discharge cavity disposed downstream from the conveying chambers, the discharge cavity being in fluid communication with the conveying chambers. 
   
   
       14 . An extrusion apparatus as defined in  claim 1  and further comprising two screw tips carried by the respective screw shafts for coaxial rotation with the respective screw shafts, the screw tips being disposed within a common discharge cavity 
   
   
       15 . A method for rapid formation of an extrudate, the method including the steps of:
 providing a housing including a mixing chamber comprising two parallel intersecting housing bores and including separate conveying chambers arranged generally parallel to one another downstream of the mixing chamber;   supporting screw sections of two screw shafts for co-wiping intermeshing rotation within the respective housing bores of the mixing chamber and conveying portions of the screw shafts for rotation within the separate conveying chambers of the housing;   feeding material into the mixing chamber; and   mixing the material within the mixing chamber and conveying the mixture downstream through the mixing chamber and along the respective conveying chambers by rotating the screw shafts in the same sense at a rotational speed in the range of 800 to 1800 rpm.   
   
   
       16 . The method of  claim 15  in which:
 the step of providing a housing includes providing a discharge cavity downstream from and in fluid communication with the conveying chambers;   the step of supporting screw sections includes supporting screw tips on the respective screw shafts within the discharge cavity for coaxial rotation with the respective screw shafts; and   including the additional steps of:
 conveying and merging the mixture into the discharge cavities from the mixing chamber; and 
 providing additional mixing and propulsion to the mixture through rotation of screw tips within the discharge cavity. 
   
   
   
       17 . The method of  claim 15  in which the steps of providing a housing and supporting screw sections include forming the conveying chambers and conveying portions of the screw shafts to leave a screw-to-chamber wall clearance in the range of approximately D oc /128 to D oc /150 between respective inner walls of the conveying chambers and the conveying portions of the screw shafts where D oc =an outside diameter of the conveying portions of the screw shafts. 
   
   
       18 . The method of  claim 15  in which the steps of providing a housing and supporting screw sections include forming the housing bores and the screw sections of the screw shafts to leave a screw-to-chamber wall clearance in the range of approximately D os /64-D os /128 between an inner wall of the mixing chamber and screw sections of the screw shafts where D os =an outside diameter of each of the screw sections. 
   
   
       19 . The method of  claim 15  in which the step of supporting screw sections within the separate conveying chambers of the housing includes providing screw sections that each have an outside diameter to inside diameter (OD-ID) ratio in the range of approximately 1.4-1.8. 
   
   
       20 . The method of  claim 15  in which the step of feeding material into the mixing chamber includes:
 feeding polymeric material into the mixing chamber through an upstream inlet of the mixing apparatus; and   feeding organic material into the mixing chamber through a downstream inlet of the mixing apparatus disposed downstream from the upstream inlet;   and in which the step of mixing the material includes rotating the screw shafts at rotational speeds in the range of 600-1500 rpm.   
   
   
       21 . The method of  claim 20  including the additional step of heating the polymeric material to a molten state before mixing it with the organic material. 
   
   
       22 . The method of  claim 20  in which the step of feeding polymeric material includes feeding one or more of the thermoplastic resins selected from the group of thermoplastic resins consisting of high density polyethylene, low density polyethylene, linear low density polyethylene, polyvinyl chloride, and polypropylene. 
   
   
       23 . The method of  claim 20  in which the step of feeding organic material includes feeding one or more organic materials selected from the group consisting of wood flour, wood pellets, wood fibers, wastepaper, kenaf, flax, rice hulls, jute, sisal, coconut, and hemp. 
   
   
       24 . The method of  claim 15  in which the step of mixing the material and conveying the mixture downstream through the mixing chamber and along the respective conveying chambers includes developing a torque density in the range of approximately 8-11 NM/cm 3 .