GB732894A - Improvements in or relating to the manufacture of films of organic thermoplastic materials - Google Patents

Abstract

Film is produced by continuously extruding a molten, normally solid, organic thermoplastic material through an annular die, continuously inflating the tubing so extruded, before it has cooled to the solid state, by means of a gaseous medium, and drawing the inflated tubular film downwards from the die into a cooling liquid, the inflating medium within the tubing extending from the face of the die to a point where it is trapped below the surface of the cooling liquid by collapsing means such as a pair of nip rolls or by a liquid seal formed by a column of liquid introduced into the tube. The film is preferably partially cooled externally by directing a current or currents of air substantially uniformly round the circumference of the film during its passage from the die to the cooling liquid. The cooling liquid, normally water, is preferably maintained at least 20 DEG C. below the softening temperature of the thermoplastic material and may also be used for treating the film, e.g. with a dissolved or dispersed anti-static or sizing agent. The organic thermoplastic material may be polythene, an ethylenecarbon monoxide copolymer, vinyl phthalimide, vinyl carbazole, a hydrolysed ethylene/vinyl acetate interpolymer, plasticized polyvinyl chloride, a superpolyester, particularly polyethylene terephthalate, or a superpolyamide. The inflating medium is most conveniently supplied through a passage in the extrusion die and when the film collapsing means is a pair of nip rolls, the volume of this medium within the tubular film may be controlled by providing direct communication between this passage and a reservoir, e.g. of air, which communicates by means of a pipe provided with a tap with a source of compressed air and into which water may be allowed to flow by gravity from a reservoir through a tube provided with a tap. The pipe connecting the reservoir with the passage in the die is provided with a branch outlet tube carrying a tap. The initial volume of air admitted into the inflated portion of the tubular film is adjusted by means of the taps on the compressed air supply to the reservoir and on the branch outlet tube on the pipe connecting the air reservoir with the passage in the die. Thereafter any adjustment of the air volume which may subsequently be required, to allow for the escape of air from the inflated portion of the tubular film through the nip rolls, may be made by admitting water at the required slow rate from the water reservoir. For the production of flat film the extruded tube inflated by a bubble of air sealed within it by the cooling liquid may be drawn over a conical forming shoe which is immersed in the cooling bath and is provided with a razor edge which slits the film as it is drawn over the shoe. The slit film is then drawn over a second forming shoe (also immersed in the liquid) so shaped that at its end adjacent to the base of the conical shoe it follows the curve thereof, but the curvature is gradually reduced until, at the opposite end, the shoe becomes flat. The flat film then passes under a guide roller immersed in the cooling bath, and is finally withdrawn from the bath by a pair of nip rolls situated outside the bath. In an example, polythene is extruded at 145 DEG C. through a die to form a tubular film and is drawn by means of nip rolls, through a bath of water at 15 DEG C., the surface of which is 18 inches below the die, the rolls being one inch below the surface. During its passage from the die to the bath the film tube is cooled by air supplied through a ring cooler having an internal diameter of 6 inches and located 5 inches below the die. The flat width of the film is one inch. In other examples tubular film is similarly produced from polyvinyl chloride plasticized with 50 per cent of its weight of dibutyl phthalate extruded at 200 DEG C. and from highly polymeric polyethylene terephthalate melting at 258 DEG C. and extruded at 290 DEG C. Specification 656,735, [Group IV (b)], is referred to.