A screw element which has an axially asymmetrical screw cross-sectional profile which has at least two construction points located within the screw cross-sectional profile is provided. The screw element is suitable for use in an extruder having two drive shafts which rotate in the same direction and at the same speed, in order to process or produce plastic masses as an extrudate. Also, an arrangement of two identical or different screw elements in an extruder having two drive shafts which rotate in the same direction and at the same speed is provided. An extruder which is equipped with two identical or different screw elements and has two drive shafts which rotate in the same direction and at the same speed is provided. Furthermore, the use of the screw element according to the invention for processing or producing plastic masses is provided.

An internal mixer includes a mixing chamber having a mixing room, a material input portion having an inner space, a hopper provided in the material input portion, an exhaust passage having an exhaust port provided in the material input portion, and a driving device that raises and lowers a weight. The driving device can stop the weight at a lower position where the weight having entered the upper opening of the mixing room can press the material to be kneaded from above and an exhaust adjustment position where the weight closes a part of the exhaust port. When the weight is at the lower position, a gap is formed between the weight and the peripheral edge of the mixing chamber defining the upper opening.