PURPOSE: To correct thickness errors by prestressing an elastically deformable ring gap nozzle lip by radially affected adjusting elements to correct the thickness errors through the elastic deformation of the ring gap nozzle lip. CONSTITUTION: Adjusting elements 11 which are constructed as piezoelectric adjusting elements define the starting position of a prestressed ring gap nozzle lip 4 by a predetermined electrical starting voltage. This pressure prestress is effective in the radial direction and thickness-error correction is effected by changing the electrical starting voltage in individual adjusting element 11 or individual adjusting element group to be either positive or negative. This change of the voltage causes the change of the voltage applied in the ring gap nozzle lip 4 and the resulting change of the voltage is superimposed upon the compressive prestress so as to cause the deformation every region, whereby the deformations provide the thickness-error corrections.
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