Mixing process and screw extruders for carrying out the same

Claim 1

An extruder for homogenizing, kneading, dispersing, coloring and mixing a material, comprising a cylinder; and an extruding screw arranged for rotation within said cylinder, said extruding screw having one end threading extending in one direction, another end threading extending also in said one direction, and two mixing threadings located between said one and other end threadings and including a forward threading extending in said one direction, and a counterthreading extending in the opposite direction and intersecting said forward threading, said forward threading and said counterthreading having the same pitch to delimit two helical channels that cross each other at a plurality of free intersections, one of said channels imparting a forward movement to the material while the other channel imparting a return movement to a part of said material to create a backward flow of material which, on meeting the forward flow causes the material from the two crossing helical channels to mix whereby the mixing becomes increasingly thorough as the forward flow of material proceeds, said one threading, said two mixing threadings, and said other end threading having the same diameter.