Screw extruder with improved dispersive mixing

A method of extruding material from a screw extruder comprising the steps of:

• providing a screw extruder including a barrel having input and output ends, a bore defining an inner surface, and an extrusion die at said output end;
• at least one extruder screw positioned within said bore, each screw including a central shaft and at least one screw flight, each flight including a front pushing face and a rear face, each front pushing face having a profile which interacts with said inner surface of said barrel to form a progressively narrowing passage through which material is forced into multiple regions of high elongational and shear stress;
• introducing extrusion material into said input end of said barrel;
• rotating said screws to mix said material dispersively and distributively;
• conveying said material towards said extrusion die at said output end to be shaped;
• and,
• extruding said extrusion material from said extrusion die.

A screw extruder comprising:

• a barrel having a bore defining an inner surface;
• at least one extruder screw having a screw diameter D, said screw being positioned within said bore, each screw including a central shaft, a mixing section having a mixing section length L, and at least one screw flight, each flight including a front pushing face, a rear face and a flight tip having a flight width w.sub.f, the profile of each said front pushing face acting with said inner surface of said barrel to form a progressively narrowing passage through which material is forced into multiple regions of high elongational and shear stress;
• said screw flights being arranged in a helical pattern around said central shaft and having defined a helix angle .phi.;
• said screw extruder having a channel depth H defined as the distance between said central shaft and said bore inner surface, a radial flight clearance .delta. defined as the distance between said flight tip and said bore inner surface, and a flight clearance/channel depth ratio .delta./H;
• and
• for a range of .delta./H of 0.05-0.5, mixing section length L lies in the range 1-20 times the screw diameter D, the helix angle .phi. lies in the range from -90.degree. to -30.degree. and +30.degree. to +90.degree., and the flight tip width w.sub.f, lies in the range 0-0.5 times the channel width.