Improvements in method and apparatus for casting sheeting

摘要

In the production of sheeting, a thin layer of fluid dope is spread from a hopper upon a moving casting surface over which air is circulated adjacent to the area where the dope is being applied, at least a portion of the air approaching the area in the direction of travel of the casting surface being diverted into two or more thin parallel or slightly diverging streams which are substantially as wide as the casting surface and flow away from it, in order to prevent substantial amounts of air from moving towards the area. As shown in Fig. 1, a liquid dope 7 of the evaporative type in the hopper 4 flows down between the hopper blades 5, 6 on to the surface 3 of a casting wheel 1 mounted on trunnions 2 and turned in a clockwise direction, after which it is carried around the wheel and stripped off, after setting and becoming self-supporting, by the rollers 8-12. Air is drawn into the casing 16 through the inlet 17 in the top of the casing and through an opening 18 through which the set-sheet 20 passes out. To prevent solvent vapours from collecting in casing 16, air is circulated to draw them away from the casting surface 3 and the sheet laid thereon. To this end air is drawn by means of the air pump 24 through the air ducts 23, 22 from an opening in the partition 21. The solvent vapours are reclaimed by passing through the pipe 25 to a recovery apparatus and the purified air may be recirculated to the duct 28. There is a partition 26 separating the air duct 23 from the air duct 27 which encircles the periphery of the wheel 1 and leads to the air duct 28 from which air passes into air duct 27 and then round most of the wheel. There is also an opening 29 into the chamber 16 through which the sheeting passes before it is stripped from the wheel over the roller 8. Part of the air entering the casing 16 through the inlet 17 is withdrawn from the casing through the exit 13 connected by duct 14 to the main exit duct 25. In order to prevent these various air currents, and others such as that due to the movement of the casting surface, from reaching the stream of dope 31 flowing from the hopper blades on to the casting surface, there is provided an air baffle 32 (Figs. 1 and 2) located between and preferably supported by side walls 33, 34 of the casing 16. The baffle comprises a box-like frame consisting of end members 35, 36 mounted on the walls 33, 34 with a number of partition plates 37 extending between the end members and front and back frame members 38, 39. The plates or slats 37, 38, 39 are parallel with one another and spaced apart to provide flue-like openings 40 between the slat-like members by means of the distance pieces 42 supported by a series of rods 41. From three to six or more baffle-plates may be used depending upon the rate of air circulation in the chamber 16 (usually 300-1500 cubic feet per minute). The baffleplates may be orientated radially to the wheel. The spacing of their lower edges from the wheel is not critical and may, for example be 0.055 to 0.065 inch. For convenience the baffle-plates unit is generally made in rectangular form so that the bottom of the unit does not exactly follow the curvature of the casting wheel. Another baffle-plate 43 may be provided at the end of the baffle unit nearer the hopper, and this may have a strip of plush or soft cloth 44 mounted along the bottom edge (see also part 66 in Fig. 3). The lower edge of the baffle-plate 43 (or of the plush strip 44) is spaced from the casting surface, e.g. by a distance of 0.02 inch. As shown in Fig. 3, the hopper 50 is mounted on cross-members 51, 52 and the hopper blades 53, 54 lay the fluid dope 55 on the casting surface 56 of a wheel 57. Air is extracted from the suction chamber 61 through the suction pipe 62 by means of an exhaust fan (not shown) so that the pressure in the chamber 61 is somewhat lower than that in the casing surrounding the casting wheel. Air is thus drawn through the space between the casting surface 56 and the casing plates 58, 59. The lower edge of the baffle 65 may be provided with a strip of plush or soft cloth 66 and is attached to an air baffle assembly consisting of a series of spaced plates 67 with vertical flues 68 extending between them. The entry of air into the suction chamber 63 from the direction of approach of the casting surface 56 is thus reduced and the difference in pressure between the two sides of the bead 69 of plastic dope is maintained steady, the air in the chamber 63 being kept undisturbed by eddy currents. A similar arrangement (but with the suction chamber omitted) is also shown in which the plastic dope is laid from a hopper on to a belt of flexible polished metal mounted on a series of rollers.