shapes that can be measured on line illustrates how simulations can be used to optimize the operating conditions, to design new facilities, and to check new ideas for process improvement and development.
From a chemical standpoint, potentially annoying impurities are oxygen from leaks or the N2–H2 gas mix, and sulfur originating from the molten glass. Both deteriorate productivity and glass quality if they induce alteration on the bottom surface of the glass caused by reactions with tin, and contamination adhesion on top and bottom surfaces (Chapter 5.6). In the so‐called oxygen and sulfur cycles (Figure 13), SnO and SnS vapors form, condense, and precipitate, the former as SnO2 and the latter as SnS (which is ultimately reduced to Sn particles). Thanks to extensive research, however, these impurities are now carefully managed through monitoring, sealing, cleaning, atmosphere controlling on flow and pressure, etc.
5.5 Trends in Float Production
The float process is advantageous because it yields excellent flatness, high flexibility with regard to thickness and width, and high productivity owing to completely continuous operation during the whole lifetime of the melting furnace, which can now reach up to two decades. Ever since its conception, plenty of technological improvements have been conducted for achieving higher throughput, larger width, and higher still quality, the thickness currently extending down to less than 0.4 mm and up to around 25 mm.
Figure 11 Shape and thickness distribution of a 2 mm thick float‐glass ribbon calculated with an integrated glass‐forming model [10]. Forming (i.e. shape, thickness, and velocity distributions of the glass ribbon) and the flow of molten tin are first calculated for a given temperature distribution of the glass ribbon. Heat transfer (i.e. the temperature distribution) in the float bath then is simulated, and the whole calculation is iteratively repeated until convergence is reached for the three interrelated mechanisms.
Figure 12 Simulated thickness distribution at the exit of the bath for 2 mm thick glass ribbon. The lateral distance is normalized [10].
Originally the float process was designed to produce glass sheets for architectural window and mirrors. Since the 1970s, the technology has evolved to meet other demands, especially that emerging from automotive market for higher optical quality and thinner sheets along with higher throughput to keep production costs reasonable. In addition, the float process has contributed to the growing solar generation market with products such as mirrors for solar power systems and cover glasses for photovoltaics.
Figure 13 The complex interactions of impurities with the atmosphere, tin bath, and glass ribbon in the float process: (a) oxygen cycle; (b) sulfur cycle.
Source: After Pilkington [9].
In the early 1980s, the float process achieved production of ultrathin glass of less than 1.1 mm for twisted nematic (TN)/super TN (STN) liquid crystal display (LCD) substrates, touch panels, and other electronics products with a remarkably high quality for flatness, thickness constancy, defect level, etc. Beginning in the 1990s, the float process has been producing flat glass of various kinds of compositions other than the traditional soda‐lime silicate. Examples are alkali‐free glass for thin film transistor (TFT) LCD substrate, high strain‐point glass for plasma display panel (PDP) and solar panel substrates, specialty glasses for heat‐resistant products, hard disk drive (HDD) substrates, and other products such as touch panels and display covers that are then chemically strengthened.
6 Downdraw Processes
6.1 Slot Downdraw
It was for forming thin glass sheets that the slot downdraw process was developed in the 1940s. As driven by pairs of rolls, the molten glass is pulled downward through an accurately dimensioned narrow slot, made of platinum, which is fixed at the bottom of the forehearth. The glass sheet pulled from the slot is then gripped on its edges to prevent narrowing (Figure 14). The suitability of this process for making thin glass stems from the fact that a viscous molten glass can be pulled downward with a higher speed than if it were just subjected to free fall [3, 7, 11].
6.2 Fusion Downdraw
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.
