Patent GB-1089863-A (GB)
Title: Process and apparatus for the continuous production of biaxially stretched tubular films of linear polyesters
IPC: B29C48/03
Filed: 1965-02-19
Abstract: 1,089,863. Making polyester tubular films. KALLE A.G. Feb. 19, 1965 [Feb. 21, 1964], No. 7340/65. Heading B5B. Biaxially stretched tubular films of thermoplastic linear polyesters having melt viscosities lower than 2000 poises at 518 F. are made by extruding a polyester melt through the annular orifice of an extrusion die at a linear speed V o , stretching the extruded melt longitudinally in a gas tight manner over an internally cooled sizing mandrel having a diameter smaller than that of the die orifice, the tube formed being simultaneously cooled from the exterior, conveying the tube from the mandrel at a linear speed V 1  > V o  to a stretching zone, a gas having been introduced under pressure with the tube through the mandrel, for the purpose of inflating the tube, heating the tube in the stretching zone to the stretching temperature, stretching the tube in this zone longitudinally by means of an increase. in the linear speed of the tube to V 2  > V 1  and transversely by the maintenance of a volume of gas under pressure in the interior of the tube, cooling the thus formed tubular film during and/or after this stretching step, and collapsing the tubular film produced to a flattened condition whereby the gas is confined under pressure within the portion of the tube passing through the stretching zone. If desired the tubular film after its passage through the nip of a pair of draw rolls constituting the flattening means, collecting the flattened tubular film on a wind-up roll. The extruded tube, upon contacting the mandrel may be urged into intimate contact with the mandrel by means of an air jet directed concentrically on to the tube, the direction of the air jet and the section of the tube between the extrusion die and the line at which the tube first contacts the mandrel forming an angle of 91 to 110 degrees with the surface of the section of the tube between the extrusion die and the said line of initial contact. The cooling of the tube during its passage over the mandrel may be effected by contacting the exterior of the tube with a cooling liquid. The tube may be heated to the stretching temperature by means of hot air or infra-red radiation. In the latter arrangement the heating chamber may be a hollow cylinder rotatable about the axis of the tubular film and provided with infra-red radiators disposed at equal intervals around its inner surface. In the illustrated example a polyester melt is extruded at a linear speed V 0  from the die 1 (Fig. 1) and drawn by means of the pinch rolls 4 over the internally cooled mandrel 2 the diameter of which is somewhat smaller than the inner diameter of the die orifice 1a The line of initial contact L between the extruded tube S and the mandrel is advantageously established by pressure from an air blast discharged from a slot die 3 surrounding and concentric with the mandrel 2. The cooled unoriented tube S is drawn over the mandrel 2 by the pinch rolls 4 at a linear velocity V 1  > V o . The amorphous tube S is then drawn through a heating chamber 5 by a pair of pinch rolls 6 which impart a speed V 2  > V 1  to the stretched tubular film. The tube is heated to the stretching temperature in the chamber 5 by hot air fed into the chamber at 5a and leaving the chamber at 5b. The stretched tubular film is collapsed by pairs of idler rolls 7 before reaching the pinch rolls 7. The tubular film is inflated for transverse stretching by gas introduced under pressure through the pipe 8. The nip rolls 4 are rubber covered and shorter than the width of the collapsed tube S to allow the gas from the pipe 8 to flow into the portion of the tube in the stretching zone.