GB781879A - Improvements in and relating to the manufacture of films - Google Patents

Abstract

<PICT:0781879/IV (a)/1> <PICT:0781879/IV (a)/2> <PICT:0781879/IV (a)/3> Heat-set flat films are produced from cold-drawn, tubular film of organic thermoplastic material by continuously withdrawing the cold-drawn, tubular film from a pair of pinch rolls which serve to hold it in the flattened form, inflating the film by continuously admitting air or other gas at a position downstream from the pinch rolls through an inlet means around which the tubing is at least partially flattened, continuously passing the inflated film, between the pinch rolls and the at least partially flattened portion, through a heating zone to heat it to a temperature between that at which the film has been drawn and the melting-point of the material, and then through a cooling zone, and thereafter slitting it along two lines substantially diametrically opposite to one another. As shown in Fig. 1, a continuous length of tubular film 1 passes through the nip of a pair of driven pinch rolls 2, which are the second pair of two sets of pinch rolls of a cold drawing unit provided to stretch the film simultaneously in transverse and longitudinal directions by enclosed fluid pressure in the film between the two sets of pinch rolls and by driving the second set of pinch rolls at a greater peripheral speed than the first set. From the pinch rolls 2 the film travels in contact with sets of cooling rollers 3, 31, over vertical baffles 7, 71, and thence past cooling rollers 4, 41, which also serve to collapse the film gradually to an almost flattened condition, to slitting knives (not shown) which slit the film into two lengths of equal width, and finally to a pair of wind-up rolls 8, 81. During its passage through the apparatus hot air or other gas under positive pressure is introduced into the interior of the tubular film through the pipe 5 (Figs. 1 and 2) which has an elliptical cross-section of a high degree of eccentricity and is provided with holes 6, 61, 611. The pipe 5 and baffles 7, 71 are covered with velvet 25 to facilitate the slipping of the film over their surfaces. The baffles are attached to two points on the air inlet pipe 5 by hinges which enable the baffles to be folded flat to lie in a horizontal plane, so that they can be inserted into the film by pushing the pipe 5, with the folded baffles attached, into the open end of the film and, when it has penetrated to the desired extent, releasing the baffles so that they become vertical. Two pairs of creasing rolls shown at 9, 91 and 29, 291 in Fig. 2 (which is a cross-sectional elevation of the apparatus viewed from the line A, A1 of Fig. 1), located between the cooling rollers 4, 41 and the slitting knives 10, 101, are adapted to form creases at opposite sides of the advancing, almost flattened film, the film being slit along these creases by the knives. As shown in Fig. 3, a continuous length of tubular film passes from a pair of driven pinch rolls 12 past two sets of cooling guide-plates 13, 131 over vertical baffles 17, 171 which are operated by the method previously described, and thence past cooling guide-plates 14, 141, which also serve to collapse the film gradually to an almost flattened condition, and between rolls 19, 191 to the slitting knives (not shown) and a pair of wind-up rolls 18, 181. A pipe 15 is provided having an elliptical cross-section of a high degree of eccentricity and one end open to the atmosphere, the pipe communicating with the interior of the inflated portion of the tubular film 11 via holes 16, 161, 1611. An air-tight casing 21, connected to a vacuum pump (not shown) via duct 23, surrounds the inflated film, and a needle valve 24 controls the degree of vacuum produced. A cylindrical radiant heater 22 is disposed around this casing over the area where the film is to be heated. Velvet 26 is used to cover the baffles, pipe and rolls, where shown, and serves both to facilitate the passage of the film over the pipe and baffles and to minimize leakage of air into the casing 21. The vertical baffles 7, 71 and 17, 171 shown in Figs. 1 and 3 respectively serve to insulate the air enclosed by the film in the film-inflating and film-cooling regions, respectively, from the adjacent mass of hot air within the heated portion of the film and may be made of or coated with glass, asbestos or other thermal insulating material. When the air introduced into the pipe 5 is preheated, the portion of the pipe within the cooling zone of the film should also be covered with heat-insulating material. The invention is particularly applicable to the production of films from polyethylene terephthalate,