GB779645A - Improvements in or relating to continuous manufacture of films - Google Patents

Abstract

<PICT:0779645/IV(a)/1> <PICT:0779645/IV(a)/2> <PICT:0779645/IV(a)/3> <PICT:0779645/IV(a)/4> Apparatus for extruding films with beaded edges comprises an extrusion die with a slit orifice one edge of the slit having two bead-forming slots, a rotatable casting drum and a roller with bead-shaping grooves which presses against the film on the drum. The apertures of the bead-forming slots are preferably adjustable. The drum may also have bead-forming slots whereby film with beads on both faces is produced. The casting drum and the bead-forming roller are preferably provided with means for regulating their temperature, such as circulation through their interior of a heat-exchange liquid. A preferred form of die is shown separated into its parts in Fig. 2 and in cross-section (assembled) in Fig. 4. The die consists of two blocks 14, 15 which are bolted together with the interposition of a spacer plate 16, whose thickness determines the thickness of the extruded film. Die block 14 has a conduit 12 through which the material passes into a recess 18 with sloping bottom wall 19. Uniform distribution of the material is assisted by bar 17, which is removably located by lugs 22 in spacer plate 16 (see also Fig. 5) and which is tapered upwardly and downwardly on its side which faces the block 14 and is also tapered from its centre towards its ends. Spacer plate 16 has an aperture 20 corresponding in shape to the recess 18 in die block 14, and has sloping portions 23 adjacent the opening to provide a larger flow of material at these positions to assist formation of the beads. The width d of the aperture determines the width of the film as extruded. Die block 15 has two slots 24, which also permit an increased flow of material to form the beads. The aperture of the slots is adjustable by a pair of plates 29 carried by a transverse bar 28 slidably arranged in slots 26 in the die block 15. The die is preferably made of corrosion-resistant steel capable of being annealed, hardened and highly polished. Suitable metals are stainless steel, alloy steels such as vanadium steel, brass, bronze, titanium or titanium alloy. The spacer plate 16 can be made of steel, titanium, copper or aluminium. The assembled die is disposed with its aperture slightly above the casting drum and the peripheral speed of the drum is faster than the rate of extrusion. A bead-forming roller is located near to the extrusion die and presses against the film on the drum. Two kinds of bead-forming roller are described, one of which contacts the film at its edges only, to form the beads, and the other contacts the entire width of the film to give a calendering effect. Fig. 7 shows the first kind of bead-forming roller, and also shows a casting drum 10 having grooves 11 which co-operate with grooves 37 in the roller to produce a film F having beads on both faces. The roller has hollow stub shafts 43 through which heatexchange fluid is passed, as shown by the arrows, to cool the surface of the roller. In an example, polyethylene terephthalate having a melting point above 200 DEG C. is extruded at 265-275 DEG C. under a pressure of50-100 lb. per sq. in. through an orifice measuring 4.5 ins. long and 0.09 in. wide spaced 0.35 in. above the casting drum. The casting drum has a diameter of 24 in., a peripheral speed of 50-300 in. per min., and a surface temperature maintained at 40-70 DEG C. The bead-forming roller has a diameter of 4.375 in. In addition to polyethylene terephthalate, other materials which may be used are synthetic resins and superpolymers, e.g. nylon, polyethylene, polystyrene, polyvinyl halides, polyvinyl acetate, rubber hydrochloride, cellulose derivatives, polyesters from glycols and aliphatic dibasic acids, and polyesteramides. Specifications 578,079, [Group IV], 617,129, 746,386 and 779,646 are referred to.