THICKNESS CONTROL DEVICE FOR INFLATION FILM

PURPOSE: To keep the thickness accuracy of an inflation film high without generating a hunting phenomenon by controlling the rotational speed of a taking- over roll so that an objective control non-sensitive zone is set to a control non-sensitive band to set a thickness average value to objective thickness. CONSTITUTION: In feedback control, a microcomputer 25 sets the max. taking- over speed and also sets an objective control non-sensitive zone on both sides of objective thickness and further sets a usual-control non-sensitive zone larger than the objective control nonsensitive zone in width and, when a thickness average value is out of a usual control non-sensitive zone, the objective control non-sensitive zone is set to a control non-sensitive zone by a preset predetermined number of times (min. thickness control number of times) and the rotational speed of a taking-over roll is controlled by a servo motor 17 so that the thickness average value becomes objective thickness. On and after, until the thickness average value becomes out of the usual control non-sensitive zone, thickness control not altering the rotational speed of the taking-over roll 7 by the servo motor 17 is performed.