1299955 Extruder screw FRIED KRUPP GmbH 20 Feb 1970 [22 Feb 1969] 8352/70 Heading B5A [Also in Division B1] Improved homogenization of elastomers or thermoplastic material is obtained in a twostart extruder screw 1, by the cross-section of one screw channel 9 decreasing from the cylinder inlet 6 in the direction of conveying, and that of the other 10 increasing. In the Fig. 1 embodiment, the screw channels are of constant width but the root diameter of one 9 increases, and of the other 10 decreases. In the Fig. 2 embodiment (not shown), the root diameter is constant, and the cross-sections are varied by the channel widths. In the Fig. 3 embodiment (not shown) the root diameter of one thread increases and of the other decreases, with the width of both channels decreasing in the direction of conveying. Preferably, the screw threads 8 are of trapezoidal nature in cross-section as in Fig. 4. A plurality of sequential screws may be provided a single barrel as a continuous mixer.