METHOD OF CONTROLLING THE FILM THICKNESS AT A BLOWN FILM EXTRUDER INSTALLATION

摘要

A nozzle ring of an extruder for blowing tubular plastics film is divided into cooling sectors provided with adjusting elements. The thicknesses of the blown film are measured over the circumference thereof and a number of film sectors of equal circumferential length corresponding to the number of cooling sectors is established. Assuming that the film sector of maximum or minimum thickness has been extruded in the correct position for the purpose of determining the cooling sector associated with the film sector, the subsequent film sectors are in turn allocated to successive cooling sectors. The mean thickness distributions are determined from the measurements over the individual film sectors. Those cooling sectors to which film sectors were allocated having thin or thick portions exceeding a respective thin or thick portion threshold value which is a fraction of the value of the film sector with the absolute minimum or maximum thickness, respectively, are cooled or heated until the film thicknesses are within the permissible tolerance range.