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The slot downdraw process has disadvantages in terms of imperfect flatness and other defects caused by slot deformation and foreign contamination on the inside of the slot. It was to overcome them that the fusion downdraw process was developed by Corning [3, 5, 7, 11]. As sketched in Figure 15, the well‐stirred molten glass is delivered through a conduit tube to one end of a rectangular trough that is the upper part of a fusion pipe. The molten glass flows over the weirs uniformly along the full length of the trough and then runs down on both sides of the fusion pipe. Two glass streams join and merge together at the “root,” which is a bottom apex of the fusion pipe. A pair of rolls grips the edges of the glass sheet just below the root to prevent the sheet from becoming narrower as it is stretched downward. The glass sheet is then cooled down while its edges are still held by pulling rolls as it proceeds through a vertical annealing lehr, and is finally conveyed to the cutoff station. The distinguishing advantage of the fusion downdraw process thus is that the glass is formed without touching anything except air so that one obtains a smooth and defect‐free fire‐polished surface. To be achieved, however, this result requires a highly homogeneous molten glass and a minute control of the distribution of glass temperature and flow.

Since the 1960s, the fusion downdraw process has produced photochromic glass, heat‐resistant glass, and glass for chemically strengthening. It provides ultrathin specialty glass of less than 1.1 mm thickness used for electrical capacitors, microscope slides, optical filters, touch panels, micro electronic mechanical systems (MEMS), substrates of TN LCD, and thin film solar cells. Besides, production of alkali‐free glass for TFT LCD substrate was started in 1984 by Corning. Although the specifications are in this case much more severe, the fusion glass can also be used for TFT LCD substrate without polishing, thanks to its excellent surface quality. To meet the market demand for larger size substrates, the width has been extended up to around 3 m. In addition, the downdraw process is applied to specialty glass for emerging chemically strengthened products such as touch panels and display covers.

Figure 14 Sketch of slot downdraw process in cross section. The molten glass is pulled downward through a narrow slot driven by rolls [7].

Figure 15 Sketch of fusion downdraw process in a bird's‐eye perspective. Molten glass flows over weirs and run down on both sides of fusion pipe. Two glass streams join and merge together at the root and are stretched downward [3].