1,271,694. Making tubular films. IMPERIAL CHEMICAL INDUSTRIES Ltd. 5 June, 1970 [25 June, 1969], No. 32068/69. Heading B5B. Tubular films are produced by melt extruding thermoplastic materials, suitably polyethylene terephthalate, through an annular extrusion orifice and withdrawing the tube on to an internally cooled, matt-surfaced, coaxial sizing mandrel of smaller diameter than the extrusion orifice at a greater rate than the rate of extrusion, the mandrel being tapered over at least the length in which it is in contact with the contracting tube and passing the tube over a seal positioned within the tube at a point where the tube is cool enough not to stick to the seal, heating the tube to its stretching temperature and introducing gas under pressure to within the tube at a position beyond the seal and finally collapsing the expanded tubular film. In apparatus suitable for the process of the invention tubular film 4 is extruded from a cross-head extrusion die 3 and cooled by contact with a mandrel 5 which passes through the centre of the extrusion die. The mandrel is cooled internally by circulating water through pipe 8 and out of pipe 9 and the exterior of the tube is cooled by passage through a water bath 11 surrounding the tube 4 and continuously supplied with water entering through pipe 12 and exiting through pipe 13. Excess water is removed from the surface of the tube by vacuum chamber 14, evacuated via pipe 15. The tube is drawn over the tapered mandrel by nip rolls 16 which are driven at a faster peripheral speed than the rate of extrusion of the tube and which are narrower than the width of the collapsed tube so that air introduced under pressure to the inside of the tube through pipe 6, which has an exit at the end of the mandrel, can pass the rolls into the zone in which the tube is blown. The drawn tube is then heated to its optimum stretching temperature by passage through circular infra-red heater 17, oven 18 and a second ring heater 19 and stretched in a transverse direction by being blown by the air under pressure supplied through pipe 6 and in the longitudinal direction by nip rolls 25 which are rotating at a greater speed than rolls 16 and which also serve to collapse the expanded tubular film 20 after it has been cooled by air in tube 23 and guided over rolls 24 and prior to winding on reel 26. The diameter to which the heated film is blown is controlled by the air pressure supplied at 6 and measured by means of pipe 7 and sensing devices 22, which enable adjustments to be made, and also partially by cooling ring 21 which directs cold air on to the tube. The surface of the mandrel may be roughened mechanically or by coating with a particulate material, suitably a ceramic or polytetrafluoroethylene. In Fig. 3 the extruded tube 4 contracts on to mandrel 5, which is preferably tapered such that its diameter decreases at least 0.0008 inch for each inch of its length, and remains in contact therewith until it is fully cooled when it diverges from the mandrel and passes over a sealing ring 36 which prevents air under pressure which exits from pipe 6 from passing between the surface of the mandrel and the extruded tube 4. Any pressure of gas which may be generated in this section is vented to atmosphere through a hole 35. A gap 27 is provided in the extrusion head for removal of any gas pressure which may be generated in the space between the head 3 and the point of contact of the film on the cooled mandrel. Air pressure may also be supplied down this gap, if necessary, to prevent premature collapse of the freshly extruded film. The tube is suitably drawn over the mandrel at a rate of between 1.05 and 10 times, preferably between 1.05 and 5 times, faster than the rate of extrusion and when polyethylene terephthalate is used it is suitably stretched at between 85‹ and 105‹ C. to between 3 and 4¢ times biaxially by the stretching and blowing step of the process.
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