Method and apparatus for adjusting the thickness of a plastic film produced by cast extrusion process

A method for producing reels of plastic film by cast extrusion process, comprising the steps of:

• a) causing the film to flow out from a slit of an extrusion head by adjusting a thickness of the film by means of an adjustment device arranged in the extrusion head,
• b) drawing the film exiting the extrusion head by means of an internally cooled roll,
• c) winding the film coming from said roll by means of a winding unit placed downstream of the roll so as to form said reels,
• d) measuring the thickness of the film by means of a thickness measuring device placed between the roll and the winding unit,
• e) controlling said adjustment device to adjust the thickness of the film based on a thickness measurement provided by the thickness measuring device,
• f) measuring a reference quantity of the reels thus produced, said reference quantity being a quantity that depends on the thickness of the film,
• g) comparing a measured value of said reference quantity with a design value of said reference quantity corresponding to a design value of the thickness of the film, and
• h) if the measured value differs from the design value of said reference quantity, acting on said adjustment device and/or on the thickness measuring device so that the thickness of the film corresponds to a desired value.