GB1073605A - Improvements in and relating to perforating films of thermoplastic material - Google Patents

Abstract

1,073,605. Making perforated films. T. J. SMITH & NEPHEW Ltd. Dec. 19, 1963 [Dec. 21, 1962], No. 48339/62. Heading B5B. Thin films of a thermoplastic material having spaced recesses on one or both surfaces are perforated by being held in a slightly stretched condition over and with one surface, preferably the recessed surface when only one surface of the film is recessed, in contact with a continuous cylindrically curved convex metal surface and while so held and with or without moving the film in relation to the curved metal surface, subjecting the free or outer surface of the film, i.e. the surface, which is not in contact with the metal surface, to the heat of either a flame having a temperature at least sufficient to melt the material of the film, or preferably higher, or to a jet of a heated gas, which flame or jet is directed on to the free surface of the film for a short period, which may be a small fraction of a second, the flame or gas temperature, gas pressure and the period being so chosen that only those portions of the thin film which constitute the base or bottom wall of each recess are melted and the film thereby perforated, the portions of the film between the recesses not becoming molten. The film material may be polyethylene, polypropylene, polyvinyl chloride, a polyester, a polyamide or polystyrene. Recessed films which may be used as starting materials for the perforating process are depicted in section in Figs. 1, 2, 4, 5 and 10 (not shown). In Fig. 1 the film is of uniform thickness in which the recesses have been formed by pressing-out or casting. In Figs. 2 and 4 the recesses have been formed by casting in one surface of the film only, the other surface of the film being flat and continuous. In Fig. 5 and 10 recesses have been formed by casting in both surfaces of the film opposing recesses having a common thin bottom wall. As depicted in Fig. 7 (not shown), the recessed film 22 is drawn from a let-off roller 23 by a take-up roller 24 and is drawn under slight tension over and in contact with a rotatable hollow metal roller 21 having a smooth plain continuous peripheral surface. The peripheral speed of the metal roller 21 may be equal to or higher or lower than the linear speed of the film. A portion of the film at any moment in contact with the cylinder 21 passes in front of a row of flames 25 from a burner 26. The film is of the type shown in Fig. 4 and the surface of the film having recesses is in contact with the cylinder 21, so that the air in the recesses where the film contacts the cylinder is held imprisoned in the recesses. The hollow roller 21 is cooled by means of a circulating liquid coolant entering and leaving the interior of the roller through pipes 27 and 28. The portions of the film which are between the recesses and in full contact with the surface of the roller 21 do not melt as they are cooled by the cylinder, whereas the thinner portions of the film which constitute the bottom walls of the recesses cannot transmit the heat directly to the roller 21, but only to the air imprisoned in the recesses, which is a relatively poor conductor of heat. Consequently, these portions of the film melt and form perforations. The formation of perforations is assisted by the expansion of the air imprisoned in the recesses. If the thickness of the thin portions of the film at the base of each recess is only a small fraction of the total thickness of the film and only one surface of the film is recessed, the non- recessed surface of the film may be in contact with the cylinder 21.