METHOD OF CONTROLLING THICKNESS OF PARISON

PURPOSE:To perform continuous extrusion molding of parison in desired thickness, by a method wherein a target value of the thickness of the parison corresponding to a calculated length of the parison is obtained every fixed period of time and a gap between a die and core is controlled so that the target value of the thickness is obtained. CONSTITUTION:A target value (a) of a thickness corresponding to a given calculated length l of parison is calculated through interpolation, through which a stroke Cs of a core 12 is calculated. The same is applied to a servoamplifier 18 through a CPU 22. The servoamplifier 18 controls a hydraulic cylinder 14 through a hydraulic servovalve 16 so that the same turns given core stroke Cs. Feedback of a movement of a core 12 to the servoamplifier 18 is performed by a potentiometer 20 and the core 12 is moved so that the movement of the core 12 turns given core stroke Cs. The calculated length l of the parison is operated in succession. With this construction, a preset actual thickness of the parison 15 to be extruded through a gap 13 between a die 10 and the core 12 comes true.