A screw extrusion press for processing thermoplastic and not reticulating polymers comprises a conveying screw (1) in a cylinder (2). The screw comprises a conveying zone (7) and a mixing zone with 10 mixing rings (3). These mixing rings comprise a passage hole (5) that extends from the bottom (10) of the screw to the vicinity of the cylinder wall (20) where it opens out into a shear gap (9).