GB1087720A - Improvements in or relating to the manufacture of thin film - Google Patents

Abstract

1,087,720. Making films of uniform thickness. KING PACKAGING Ltd., and P. J. M. PARDOE-MATTHEWS. June 15, 1964 [June 14, 1963], No. 23899/63. Heading B5B. In the manufacture of continuous lengths of flat film sheet by the " trapped bubble " process, in which hot plastic organic polymeric material is extruded through an annular extrusion orifice to form a thin-walled tube which is flattened by being drawn through the nip of a pair of rolls and which is inflated and thereby converted into a tubular film by introducing a gas under pressure into the portion of the tube between the extrusion die and the flattening rolls, the flattened tube being then slit longitudinally, one or more streams of a gas at superatmospheric pressure are directed on to a portion of the exterior surface of the tubular film, for example, as shown in Fig. 2, from a ring of inwardly directed nozzles 25 encircling the tubular film 22 in the zone in which the film is inflated to its maximum diameter. The nozzles 25 communicate with a conduit 26 which in turn communicates with a supply of air maintained at a constant pressure. A branch line 27 leads from the conduit 26 to the controller 28 of an air pressure controlled variable aperture valve 29 in a conduit 24 one end of which communicates with a second supply of air at constant pressure and the other end of which discharges the air into the interior of the tubular film and inflates the tubular film. If the diameter of the tubular film increases so that the wall of the film becomes thinner, the resulting reduction in the distance between the nozzles 25 and the film 22 and consequent increase in the resistance to the discharge of air from the nozzles causes the air pressure at any given point in the conduit to rise and partially close the valve 29. The air pressure in the portion of the conduit 24 between the valve 29 and the inlet to the tubular film will therefore decrease and the wall thickness of the tubular film 22 will increase. Conversely, if the diameter of the film 22 decreases and its wall thickness increases, the air pressure in the conduit 26 will fall, causing the valve 29 to open. The pressure of the air flowing through the conduit 24 into the film 22 will increase and the wall thickness of the film consequently decrease. The variations in the air pressure in the conduit 27 resulting from variations in the wall thickness of the tubular film 22 may be caused to correct the wall thickness of the film by varying the speed of rotation of the rolls which flatten the film and draw it from the extrusion die.