GB962435A - Improvements in or relating to tubular film manufacture - Google Patents

Abstract

962,435. Making tubular films. IMPERIAL CHEMICAL INDUSTRIES Ltd. Feb. 26, 1962 [March 27, 1961], No. 11051/61. Heading B5B. In the production of tubular film by feeding a tube of thermoplastic material to a zone where it is expanded by gaseous pressure and is hauled off longitudinally to stretch it biaxially, the effect of thick spots in the tube wall is reduced by heating the tube to or maintaining at the stretching temperature differentially around its circumference to heat thicker parts of the tube wall more than the thinner parts of the tube wall so that it stretches to a greater extent in the thicker parts. The stretching temperature may be at or above the melting point or softening point of the material, i.e. at a temperature too high for molecular orientation to take place, or at a lower temperature at which the film is biaxially oriented. The tube of thermoplastic material is usually produced by melt extrusion and may be solidified by cooling in a sizing die before feeding it to the stretching zone. The differential heating of the tube may be effected by means of a heated gas such as air or nitrogen or by an infra-red heater divided into sectors surrounding the tube, or by a number of separate infra-red heaters surrounding the tube, one or more of the sectors or heaters being maintained at a higher temperature than the others. The infra-red heaters may be controlled from a thickness sensing means which senses the film output and which is operably connected to increase the heat supply to particular heaters to remedy any wall thickness variations sensed. As shown in the drawing, a tube of solidified thermoplastic material 1 is fed through the nip of rollers 2 through which passes a probe 3 for admission of an inflating gas to the interior of the tube in the stretching zone. The tube is thereafter heated to the stretching temperature by infra-red heaters 4 and is longitudinally stretched and hauled off by nip rollers 5. To reduce a thickness situated at one side of the tube, a fish-tail jet of air 6 heated to a temperature above the stretching temperature of the tube is played on to it. Additionally or alternatively, differential heating of the tube may be effected by the heater 4 which is divided into sectors around the tube. The collapsed tubular film is slit at 7 into two single thickness films, which are each wound up on a drum, one of which is shown at 8. Before reaching the drum each single thickness film is passed between the sensing members of a #-ray thickness gauge 9 of the type in which a radioactive source emitting #-radiation and a counting head traverse the film to produce an indication of transverse thickness variations in the film. The output of the gauge 9 is fed to electronic equipment 10 comprising a rate meter and a voltage discriminator responsive to it which passes a signal wherever the thickness of the film is greater than a predetermined amount. The output from the discriminator controls the heater 4 by switching in additional power or more sectors above the basic load of the heater in accordance with the position of the gauge 9 on its traverse across the width of the film. An alternative is to use a capacitance thickness gauge instead of the gauge 9. Specification 787,497 is referred to.