GB1375231A - - Google Patents

Abstract

1375231 Drawing sheet glass GLAVERBEL 24 Jan 1972 [24 Feb 1971 (2)] 3278/72 Heading C1M In the manufacture of sheet glass by feeding molten glass into a kiln to establish a continuous forward flow of glass towards a drawing zone at which the molten glass is continuously upwardly drawn from the surface in the form of a ribbon, in at least one place, which in plan aspect of the kiln is spaced inwardly from a boundary of the surface of the molten glass, the molten glass is heated to maintain at that place a thermal barrier formed by an upward flow of molten glass which rises to the surface from a position which is at or above the bottom level of the forward flow and which is in the vicinity of a wall portion of or in the kiln so that molten glass behind the barrier is substantially prevented by such wall portion from flowing beneath the barrier. The thermal barrier may extend the full width of the kiln between the drawn glass and the rear wall, or over the width of the glass only, or on either side adjacent the edges of the drawn glass or a combination of such positions. As shown in Figs. 1 and 2, a kiln 1 for molten glass comprises a sole 2, a rear wall 3 and side walls 4, 5, each of which have L-shaped upper portions 6, 9; 7, 10 and 8, 11 respectively, and a continuous ribbon of glass 13 with side edges 14, 15 is drawn from the upper surface 12 of the glass. Over each wall part 6, 7 and 8, sets of three horizontal electrodes 16, 17 and 18 are provided spaced from the outer walls 9, 10 and 11, each set of electrodes being connected to a source of electricity such as 19. The current passes between the electrodes, which are at different potentials, to form a thermal barrier extending upwardly from the wall parts 6, 7 and 8. In a number of embodiments, a threshold extends upwardly from the base of the kiln and may be itself heated, as in Fig. 12, by gas flames 140 in a hollow threshold 131 or by electricity either by passing current through a part of the threshold or an electrode attached thereto. Or the threshold may form the base from which the thermal barrier extends by a series of electrodes thereover, similar to those of Figs. 1 and 2, or by electrodes extending therethrough. In the construction shown in Fig. 9, in a kiln 102 with soles 105 and 106 and thresholds 107 and 112, and in which glass ribbon 115 is drawn upwardly and turned over a roller 103, electrodes are formed by layers of molten tin extending between rear wall 104 and threshold 107 and between the two thresholds, both layers extending the full width of the kiln. Electrodes layers 110, 111 are connected to an electrical source which provides a path over the threshold 107 to give a thermal barrier thereover. Tin oxide blocks 114, are also provided in each side wall 113, and are each connected to the pole of the source connected to layer 110 so that current also flows between the layer 111 and both of the blocks 114 to heat the inner surfaces of the side walls adjacent the side edges of the ribbon 115. In a modification, the threshold 107 is provided with a plate of molybdenum with an upwardly extending curved plate, such plate influencing the electric field lines and current density. In a further modification using two molten layers as electrodes, the sole below the one layer, and the rear wall, are both extended to give thicker portions in which channels leading to recesses are formed, the molten tin extending through the channels to the recesses, where the tin is solidified and the power source is connected to these solidified sections of tin. As shown in Fig. 19, in a Pittsburgh machine, the kiln 159 has a sole 160 and rear wall 161 provided at its upper end with a threshold 162 and a top rear portion 163 slightly below the level 164 of the molten glass in the kiln to allow a continuous overflow of molten glass thereover. In the trough between, a tungsten plate 165 is positioned to form an electrode. In one embodiment (Fig. 18, not shown), the plate 165 is connected via a power source to a pool of molten lead extending in an upper recess in the normal draw bar. As in Fig. 19, the draw bar is formed in an extension of the rear wall 161 giving a ledge 180 in which a recess 181 extends the full width of the kiln and holds a pool 182 of molten tin, other molten metal or a molten metal salt such as a lithium salt. Molten glass for the surface can be removed, to assist flow either by removal, as in Fig. 19, over the end wall or by shim openings in the walls opposite the heating positions. Materials used for the electrodes may be in solid form, and Pt, Mo, Sn or tin oxide are referred to, or in molten form, molten metals such as tin or lead, or molten metal salts such as salts of lithium. Specifications 1,375,232, 1,129,416 are referred to.