Patent GB-1037176-A (GB)
Title: Improvements in or relating to the manufacture of tubular films
Inventor: FUNNELL PETER ALAN
IPC: B29C48/03
Filed: 1962-05-24
Abstract: 1,037,176. Making tubular films. BRITISH CELLOPHANE Ltd. May 13, 1963 [May 24, 1962], No. 20068/62. Heading B5B. An apparatus for producing a stretched tubular film of an organic thermoplastic material comprises an extrusion die 2, Fig. 1 (not shown), having an annular orifice 1 through which the molten material is extruded to form a tube 3 which is drawn over a chilled former or mandrel 4 by a pair of short nip rollers 5, Figs. 1 and 2 (not shown), to cool the tube 3 rapidly to a solid state. The solidified tube is then drawn through a stretching zone 6 by a pair of nip rollers 7 which collapse the full width of the tubular film 8 to a " lay-flat " state for winding on a storage roll. The chilled former 4 is mounted on the face of the die 2 by stems, one of which is shown at 9 to minimize heat transference from the hot die 2 and cooling water is circulated through the interior of the former 4 by way of pipes 10 and 11. The former 4 is circular in cross-section and tapered in a direction away from the die 2 to facilitate the draw-off of the solidified tube 3. The face of the die 2 and most of the body of the former 4 are enclosed within a drum-shaped chamber 12 which has a vertically movable lid 13 between which and the wall of the chamber 12 is an annular sealing gasket 16. The circular opening 17 in the lid 13 is provided with an annular flexible seal 18 which presses lightly against the tube 3 as it is being drawn over the former 4. Air under pressure is admitted to the chamber 12 through a pipe 19 and to a space 20 between the face of the die 2 and the former 4 by way of a pipe 21 to balance the pressure in the chamber 12 and thus prevent the molten tube 3 from being pressed into the space 20. The interior of the tube 3 is maintained under pressure by air introduced through a pipe 22 and the pressure of air in the chamber 12 is adjusted to balance the pressure in the tube 3 so that the tube 3 is prevented from expanding while it is in a hot plastic state. The nip rollers 5 are considerably shorter than the width of the tube 3 in the " lay-flat " state and therefore only nip the central portion of the tube 3 so that the edges of the tube 23 bow outwardly (Fig. 2) providing passageways 24 for the air introduced into the tube 3 and avoiding the formation of creases at the edges 23. In the stretching zone 6 the tube 3 enters an enclosure 25 which is at atmospheric pressure and in which heat is directed uniformly upon the tube by circle of infra-red heaters 26. The amount of heat applied to the tube 3 is adjusted so that the temperature of the tube wall is raised to a level not exceeding a temperature which is 2 C. below the " transition point " of the tube material, the temperature being sufficiently high to allow the tube to expand under the influence of its internal gas pressure and thereby become stretched transversely and become converted into a tubular film 8. The tubular film is then withdrawn from the enclosure 25, cooled by passage through the air and collapsed across its full width between the nip rollers 5 which also serve to prevent the escape of air from the interior of the tube 3 and tubular film 8. The tube 3 may be stretched longitudinally during its passage through the stretching zone by rotating the nip rollers 7 at a greater peripheral speed than that of the nip rollers 5. The transition point of the film material is defined as the melting point in the case of materials having a definite crystalline melting point, i.e. the temperature at which the microcrystalline structure of the material is observed to disappear when the temperature of a film of the material is gradually raised and observed under a microscope. In the case of materials such as polyvinyl chloride which have no definite crystalline melting point, the transition point is defined as the temperature at which the tensile strength of the material, measured at normal temperature is no longer increased on stretching.