A single-screw extruder (100), and a method. The extruder (100) comprises—a cylindrical rotor member (1) having diameter (D) and length (L) and comprising a feeding zone (14), —the rotor member (1) arranged in a barrel (2), —the cylindrical surface of the rotor member (1) carrying cavity/cavities and/or projection(s) (5) arranged in helically extending rows, —the helically extending row(s) of the rotor member (1) having a pitch (P) and depth (d) in the feeding zone (14) of the rotor member, and the extruder (100) further compris-ing—a drive system (4) for the rotation of the rotor member (1) in the barrel (2). The relation of the depth (d) to the diame-ter (D) of the rotor member, i.e. d:D, is not more than 1:20, and the relation of the pitch (P) of the rotor member to the di-ameter (D) of the rotor member, i.e. P:D, is not more than 1:4.