A screw extruder has a body (1) forming a chamber (2) of two barrels (3,4) housing two counter-rotating axis-parallel rotors (5,6) a supply port (7) for the material to be mixed in the chamber (2) at one end of the body (1), a discharge port (8) for discharging the mixed material at the other end of the body, a conveying section (34) with screws for feeding the material from the supply port (7) downstream to a mixing section (44) which comprises at least two mixing zones (45,46), each mixing zone having at least one forward-conveying wing (14,15;21,22) and at least one backward-conveying wing (17,18;24,25) downstream of the forward-conveying wing (14,15;21,22) on each rotor (5,6).