Patent GB-1160330-A (GB)
Title: Manufacture of Glass
IPC: C03B18/10
Filed: 1967-12-29
Abstract: 1,160,330. Float glass. P. P. G. INDUSTRIES Inc. 29 Dec., 1967 [30 Dec., 1966; 8 Nov., 1967], No. 59018/67. Heading C1M. In a process for producing a flat glass ribbon having a thickness other than equilibrium thickness by the float technique the glass-metal bath system is subjected to electrically produced forces different at the margins than at the centre of the ribbon during travel thus establishing different levels of the tin at the edges as compared to the central section of the ribbon. The electrically produced forces may be electromagnetic and/or electrostatic. With the electromagnetic system bus-bars or core wound magnets are suspended above the marginal edge portions of the ribbon, with a pair of electrodes immersed in the molten tin at either end of core-wound magnets, and the field forces induced cause a trough to open along either side of the ribbon, the depth and location of which determines the thickness of the central portion of the ribbon. With the electrostatic system electrodes are suspended above the ribbon, along the marginal edge portions and/or the central portion and electrodes are immersed in the molten tin bath, the forces resulting causing the tin and the glass to be attracted upwardly beneath the suspended electrodes thus controlling the thickness of the central portion of the ribbon. In a prepared electromagnetic system a U-shaped bus-bar 40 is suspended above a floating glass ribbon 24 by refractory rods from which it is insulated, and is connected to an A,C. supply. The legs of the bus-bar extend up to one-third of the length of the float bath above the marginal edge of the ribbon. When current is applied to the bus-bars the molten tin of the float bath beneath the bus-bars, along the length thereof, forms a trough into which the marginal portions of the ribbon lie. The level differential thus gives a ribbon having a central area of less than equilibrium thickness whilst the extreme marginal portions retain the same thickness as they would under equilibrium conditions. In various other embodiments of this type D.C. fed core-wound magnets are used and by varying the polarity and or the distance and location of them above the bath and ribbon a trough can be formed whereby the central portion has a thickness less than or greater than equilibrium thickness. Core-wound magnets require to be cooled at the temperatures of operation. In these latter cases refractory ridges may be made on the bath floor to define channels which confine and concentrate the electrical forces and thus conserve the power used. In an electrostatic system electrodes 100 are suspended above the marginal edges of the ribbon for one-third the bath length and/or an electrode 112 (Fig. 14) is suspended above the central portion of the ribbon. Bus-bars 116 are located in the bath to energize the molten tin. When current is supplied to bus-bars 116 and either electrodes 100 or electrode 112 forces are set up which cause the tin of the bath and the glass of the ribbon in the area immediately beneath the energized suspended electrode to be attracted upwardly. Thus by which electrodes are energized and by controlling the degree of attraction so is the thickness of the central portion of the ribbon controlled. The electrodes are preferably constructed from molybdenum sheathed in a high nickelcontaining alloy. By using the process of the invention glass sheet of #" or #" can be obtained.