An extruder has a housing formed with an axial passage and a relatively large-diameter central screw in the passage. A plurality of small-diameter outer screws angularly spaced within the passage about the central screw mesh therewith. The outer screws form outer flow passages of substantially smaller volume and the screw thread of the central screw forms central flow passages of larger volume than where these screws had geometrically complementary threads. The central screw and the outer screws are rotated in opposite senses at the same peripheral speed. In this manner the synthetic-resin material plasticized in the extruder exits from the extruder at the same temperature both in the region of the outer screws and the central screw. The axial length of the screw threads can be different, the gap between their flanks can be different, or the radial gap between their crowns and roots can be different so as to effect this difference in volume.