Method for controlling fusion pipe sag

#This invention relates to fusion pipes used in the production of sheet glass and, in particular, to techniques for controlling the sag which such pipes exhibit during use.

#The fusion process is one of the basic techniques used in the glass making art to produce sheet glass. See, for example, Varshneya, Arun K., “Flat Glass,” Fundamentals of Inorganic Glasses, Academic Press, Inc., Boston, 1994, Chapter 20, Section 4.2., 534-540. Compared to other processes known in the art, e.g., the float and slot draw processes, the fusion process produces glass sheets whose surfaces have superior flatness and smoothness. As a result, the fusion process has become of particular importance in the production of the glass substrates used in the manufacture of liquid crystal displays (LCDs).

#The fusion process, specifically, the overflow downdraw fusion process, is the subject of commonly assigned U.S. Pat. Nos. 3,338,696 and 3,682,609, to Stuart M. Dockerty, the contents of which are incorporated herein by reference. A schematic drawing of the process of these patents is shown In FIG. 1. As illustrated therein, the system includes a supply pipe 9 which provides molten glass to a collection trough 11 formed in a refractory body 13 known as an overflow downdraw fusion pipe or, more simply, a “fusion pipe.”

#FIG. 1 is a schematic drawing illustrating a representative construction for a fusion pipe for use in an overflow downdraw fusion process for making flat glass sheets.

#FIG. 2 is a schematic drawing illustrating the off-center axial forces used to control sag in accordance with the invention.

#As discussed above, in the overflow process for making glass sheet, hot glass flows into a trough 11 formed in a fusion pipe 13 and then flows over the top of the trough (the top of the weirs) and down the sides of the pipe to the root 15 of the pipe where it is drawn off as sheet glass.

#Because of the high temperatures at which the process operates, the material of the pipe is susceptible to creep. Hence, the pipe sags steadily under gravity. Eventually the sag reaches a point where the quality and/or the dimensions of the finished glass are no longer within specifications and the pipe needs to be taken out of service and replaced. It is accordingly desirable to reduce the sag rate of the pipe, and thereby extend its useful life.

#The present invention achieves reduction in sag through the use of axial forces which apply favorable moments at the ends of the pipe which reduce the sag of the pipe due to gravity. FIG. 2 is a schematic drawing of the applied axial forces and the relevant pipe geometry. In this figure, pipe 13 is supported at its ends by supports 21 and has a neutral axis 19. The neutral axis is that axis which does not elongate or contract as pipe 13 undergoes bending based on its mass distribution, its temperature distribution, and its material properties as a function of temperature. Put another way, the neutral axis is that axis which would not elongate or contract if pipe 13 were to undergo bending in the absence of axial forces F of FIG. 2 but with all other conditions the same.