GB741295A - Improvements in or relating to apparatus for manufacturing film or sheet material - Google Patents

Abstract

Apparatus for manufacturing film or sheet material comprises a casting surface disposed beneath a hopper to receive a ribbon of film or sheet-forming composition, the hopper and casting-surface being adapted for moving relatively to one another, a substantially air-tight enclosure open at one end adjacent to the hopper so that the ribbon flowing between the hopper and casting surface flows across and substantially seals the open end of the enclosure which is located on that side of the ribbon which comes into contact with the casting surface, and pressure reducing means in communication with the enclosure to subject the side of the ribbon facing the enclosure to a reduced pressure. As shown in Fig. 1, a film-forming solution 7 flows from the hopper 4 through the narrow slit between the blades 5 and 6 on to the casting surface 1 of the casting wheel 2 in the form of a ribbon 80 which extends almost completely across the fourth side of an enclosure or reduced pressure housing 11 comprising a pair of side-plates 12 (Figs. 1, 3 and 4) closely fitting the smooth peripheries 13 of the wheel 2. The side-plates 12 are connected to a cross-bar 14, supported by the frame members 10, and there is a second cross-bar 16 which is not attached to the frame members 10 but which slides on the surface 1 of the wheel 2 and is normally held up against the cross-bar 14 by the rotation of the wheel 2. The bar 16 is completely covered by a layer of plush, or other soft material, so that it does not injure the surface 1 of the wheel 2 and so that it makes a reasonably air-tight contact with the wheel 2, the side-plates 12, and the fixed bar 14. The housing 11 has a top wall 17 which is tapered upwardly at 18 to a pipe 19 leading to the suction side 20 of an exhaust fan 21. A plate 28 extending between the two side plates 12 may be hinged at 29 to the hopper 4 (Fig. 1) or merely connected to the side plates 12, leaving the end 23 open for the free entrance of air. A small <PICT:0741295/IV(a)/1> <PICT:0741295/IV(a)/2> <PICT:0741295/IV(a)/3> <PICT:0741295/IV(a)/4> amount of solvent-laden air is drawn through the clearances 27 (Fig. 4) between the edges 25 of the ribbon of film-forming material as it first reaches the surface 1 by the reduced pressure in the chamber 11 and prevents the formation of small gelled beads or strings of film-forming material at the ends of the hopper blades 5 and 6. In the modification shown in Fig. 6 the film-forming composition flows from the hopper 56 as a ribbon 58 on to the casting surface 50 comprising an endless belt passing around in the direction shown by the arrow a pair of rollers (of which one is shown at 51) and supported by a series of idler rollers 53. A chamber 54 communicates through pipe 55 with a source of reduced pressure (not shown) and with a chamber 57 open to the atmosphere at one end 59. Apparatus according to the invention is particularly suitable for casting sheetings comprising partially hydrolised cellulose acetate dissolved in acetone, or cellulose triacetate dissolved in a mixture of 90 per cent by weight of methylene chloride and 10 per cent of methyl alcohol. The invention enables casting speeds to be increased. Specification 741,818 and U.S.A. Specification 2,078,261 are referred to.