u2032 fig1 shows the pressure function of an apparatus according to the invention with a solid line and the pressure function of a conventional extruder with a broken line in a conventional extruder the pressure p is increased at the end of the length l of the extruder and therefore it is naturally the highest in point a which represents the place of the nozzle of the apparatus in the apparatus according to the invention the pressure is increased inside the apparatus already at an early stage in the shear zone at the end the pressure may even fall towards the nozzle fig1 is a partial cross sectional side view of an extruder according to the invention the reference numerals of fig1 correspond to those of the preceding figures both the rotor 1 and the stator 2 are cylindrical the rotor grooves 7 extend with substantially the same cross section along the entire length of the rotor 1 the cross section of the countergrooves 8 provided in the stator 2 decreases all the time the countergrooves 8 extend approximately to the middle of the apparatus preferably at a point that is u2153 of the length of the apparatus fig1 is a cross sectional side view of a part of an apparatus according to the invention the reference numerals of fig1 correspond to those of the preceding figures the rotor 1 and the stators 2 and 3 are conical the rotor 1 is formed such that in the final section the clearance between the rotor 1 and the stators 2 and 3 grows substantially larger than what it is in the initial section for example in the initial section.