Method and device for checking an extrusion apparatus

A method for checking a setting of an extrusion device which produces a tubular strand conveyed along a longitudinal direction, wherein exit widths for extruded material on a top side and on a bottom side of an extrusion die of the extrusion device are set differently, comprising:

• measuring a refractive index over a cross-section of at least one wall of the tubular strand at a first measuring position downstream of a first cooling section for the tubular strand when the tubular strand has not yet completely solidified;
• measuring geometry values of the tubular strand at the first measuring position at at least one measuring location on the top side and at at least one measuring location on the bottom side of the tubular strand, wherein the geometry values comprise at least one of:
• (i) a wall thickness;
• (ii) an internal diameter;
• and (iii) an external diameter of the tubular strand;
• determining a calibration correlation between the refractive index at the first measuring position and a ratio of the exit widths for extruded material on the top side and on the bottom side of the extrusion die;
• and
• comparing a ratio of the exit widths set on the extrusion device for extruded material on the top side and on the bottom side of the extrusion die for the measured refractive index for a ratio of the geometry values measured on the top side and on the bottom side of the tubular strand with the calibration correlation,
• wherein the measuring is done by a terahertz measuring apparatus.