Dual screw extrusion apparatus having a mixing chamber and a conveying chamber downstream thereof with the mixing chamber having a wall clearance greater than that of the conveying chamber

1. An extrusion apparatus for forming an extrudate, the apparatus comprising:

a mixing chamber comprising two parallel intersecting housing bores;

two screw shafts rotatably cantilevered for self-journaled support for rotation about respective generally parallel axes of said housing bores, said screw sections positioned for co-wiping intermeshing rotation within their respective housing bores within said mixing chamber;

said housing bores within said mixing chamber having an outside diameter D OM ;

a conveying chamber having said screw shafts disposed therein with conveying portions arranged generally parallel to one another downstream of the mixing chamber said conveying chamber having an outside diameter (D MCO ) and wherein D OM is greater than D MCO ; and

wherein said screw shafts within said mixing chamber 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 within said mixing chamber; and

wherein said conveying chamber and the conveying portions of the screw shafts therein have a screw-to-chamber wall clearance in the conveying chamber of less than D OC /128, where D OC =an outside diameter of the conveying portions of the screw shafts, and

wherein said wall clearance within said mixing chamber is greater than said wall clearance within said conveying chamber.

2. An extrusion apparatus as defined in claim 1 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 where D OC is the outside diameter of the conveying chamber.

3. An extrusion apparatus as defined in claim 1 in which the screw sections of the screw shafts in the mixing chamber each have an outside diameter to inside diameter (OD-ID) ratio greater than 1.5.

4. An extrusion apparatus as defined in claim 1 in which screw sections of the screw shafts in the mixing chamber each have an outside diameter to inside diameter (OD-ID) ratio in the range of approximately 1.4-1.8.

5. An extrusion apparatus as defined in claim 4 in which the screw sections of the screw shafts each have an outside diameter to inside diameter ratio of approximately 1.78.

6. An extrusion apparatus as defined in claim 4 in which the screw sections of the screw shafts in the mixing chamber are configured to develop a torque density in excess of approximately 11 Nm/cm 3 .

7. An extrusion apparatus as defined in claim 4 in which the screw sections of the screw shafts in the mixing chamber are configured to develop a torque density in the range of approximately 8.7-13.6 Nm/cm 3 .

8. An extrusion apparatus as defined in claim 7 in which the screw shafts in the mixing chamber include engagement sections having sinusoidal spline configurations configured to engage complementary receptacles of a drive mechanism.

9. 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.

10. An extrusion apparatus as defined in claim 9 and further including a degassing port disposed downstream from the second inlet.