Patent GB-843629-A (GB)
Title: Improvements in or relating to film or sheet-forming apparatus
IPC: B29C48/91
Filed: 1956-06-29
Abstract: <PICT:0843629/IV(a)/1> <PICT:0843629/IV(a)/2> <PICT:0843629/IV(a)/3> A film-forming apparatus comprises a continuously moving casting surface on to which a film-forming dope consisting of film material dissolved in a volatile solvent is deposited from a hopper having a discharge slot in its bottom surface disposed above the casting surface, a substantially airtight enclosure embracing the bottom of the hopper and enclosing two portions of the area of the casting surface which at any given moment are on opposite sides of, and adjacent to, the area of the casting surface upon which a line of dope is being deposited from the discharge slot, means for preparing a mixture of air and solvent vapour in which the solvent has a predetermined concentration, and means for pumping the mixture into the enclosure at a predetermined rate.  As shown in Fig. 1, a multiple casting apparatus comprises a conventional hopper 2 containing a dope 3 positioned above a casting wheel 4, from which hopper there flows on to the wheel a stream of dope to form a sheet 8 which sets as the wheel rotates, due to evaporation of the dope solvent, and is stripped from the wheel at 9.  The partially cured sheet 8 then travels in a sinuous path over rollers 12-14 through a drying chamber 11 and then over rollers 15, 16 to a second smaller casting wheel 17.  Dope is fed under pressure through a pipe 19 to a pressure hopper 18, Figs. 1 and 2, from which it is extruded in a thin stream on to the partially cured sheet 8 from a discharge slot formed between the spaced ends of blades 20 and 21.  The dope-containing part of the hopper is enclosed by a box 23 made up of side walls 24, end walls 24a (Fig. 3) and a bottom wall 24b. That portion of the bottom wall to the right of the discharge slot and designated 261 (Figs. 1 and 2) is inclined upwardly to allow entrance of a solvent vapour-air mixture from a pipe 78 to an enclosure surrounding the discharge slot.  A heating medium may be circulated within the box 23 in order to maintain the temperature of the dope in the hopper at a suitable level.  The composite sheet 25 passes over rollers 26-28 in the drying chamber 11 and then through discharge slot 29 to a take-up roll. Drying air enters the chamber 11 at an inlet 30 and is discharged at an outlet 31.  The space between the bottom of the hopper 18 and the upper half of the casting/wheel 17 is enclosed by vertical walls 35, 351 extending parallel with the axis of the wheel and from a point below the wheel up to the hopper and terminating in flanges 38, 40 which support a rim 47 encircling and fixed to the hopper 18.  The joint between rim 47 and the flanges 38, 40 is sealed by means of a rubber gasket 48.  The ends of the wheel and hopper are enclosed by offset walls 41 (Fig. 3) each terminating at the upper end in a horizontal flange 42 and at the bottom end in a labyrinth seal 43 having concentric grooves 45 and supported by the bearing 44 of the wheel 17.  The seals 43 are arcuate in shape and closely spaced from the end faces of the wheel or drum. The enclosure is completed by a pair of labyrinth seals 52, 53, the faces of which are concentric with the wheel 17 and are provided with a number of parallel grooves.  To permit access to the enclosure, walls 35, 351 are provided with openings 54, p  55 closed by doors 56, 58 hinged at 57, 59 and bounded by sealing gaskets 60, 601.  The doors are latched by means of permanent magnets engaging soft iron plates.  The labyrinth seals 52, 53 are adjustably mounted on supports 64, 65 extending from the doors 56, 58 so that, as the doors are opened, the seals move away from the wheel surface.  The seal 52 is mounted on a block 66 which slidably engages the block 64 and is fixed to the end of a rod 67 slidably mounted in a tubular sleeve 68 carried by the door 56.  The seal 53 is similarly mounted, and in both cases the rod 67 can be adjusted axially by means of a double-threaded connection between itself and a knob 69 adapted to be rotatable through a single revolution.  The solvent vapour-air mixture of adjustable concentration is fed into the enclosure by a device 75 comprising a pair of flow meters for air and solvent, a vaporizer for converting a liquid solvent such as methylene chloride into vapour, and a cooler for the air to adjust the temperature of the vapour-air mixture to a point just above the dew point of the mixture.  The proportions of air and vapour fed into the pipe 78 are controlled by valves 76, 77. The first casting wheel may also be provided with an enclosure and accessories as described above. The arrangement prevents the formation of incrustations of film material on the edges of the hopper blades and the dropping of condensed solvent on the casting surface and/or on the sheet of dope deposited on the casting surface. Specification 550,560 is referred to.