US Patent 7270471 Extruder

In an extruder for continuous working and/or processing of flowable substances having at least two parallel, corotating shafts equipped with intermeshing screw elements whose face is limited by circular arcs corresponding to the outside screw diameter, the screw core diameter and at most the center distance of the screw elements, and guided on circular segments of the extruder housing that are parallel to the shafts, at least one screw element (1) has a plurality of concentric ring portions (8) spaced a distance apart and formed by grooves (12) turned into the screw element (1).