Abstract: A highly viscous mass to be homogenized is fed from a screw plasticizer into a mixing chamber, perpendicular to the plasticizer mouth, in which a mandrel rotates independently of the plasticizer screw to masticate the mass in a narrow annular clearance between itself and the chamber wall. The mass, force-fed through this clearance by the screw pressure, encounters several sets of free-floating balls or other rotary bodies loosely guided in annular grooves of the chamber wall, or of the mandrel, while revolving about the mandrel axis. The mandrel may have a screw-threaded rearward extension for admixing additional ingredients from one or more further chamber inlets with the mass to be homogenized, with exertion of a back-up pressure upon that mass. The chamber temperature may be controlled by the circulation of a cooling fluid through its wall and/or through the mandrel.