Encyclopedia of Glass Science, Technology, History, and Culture. Группа авторов
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.
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].
7 Perspectives
Over the years the demand for flat glass has paralleled the growth of the global economy. In addition to architectural applications, the automotive, solar energy, and electronics especially flat panel display (FPD) application markets have all been at the same time growing and an important source of new, value‐added products (Chapter 6.10). Recently, glass sheets for chemically strengthened components such as cover glass for displays and ultrathin glass for touch panels have emerged as important products driven by the explosive diffusion of mobile phones and tablets with touch sensors [12]. These trends are supposed to continue and affect markets such as appliance, transportation, interior architecture, and many others. The important role of flat glass keeps increasing in these domains as well as in the field of information and communication, optics, healthcare, and so forth. Further improvements will thus be made to meet new specifications and respond to various market demands. From an industrial perspective, however, not only the cost and quality of the glass itself but also controllability, investment size, yield, delivery time, versatility, cost of post‐processing, and other factors of the manufacturing process have to be taken into consideration for each application. Therefore, an overall and comprehensive understanding of the forming process remains a key issue.
For new applications, work is in particular being conducted on ultrathin flexible glass (0.2 mm–30 μm) and on rolled glass for flexible display, OLED lighting, and organic thin‐film solar cell to take advantage of unique features of glass such as bendability, impermeability to gas, transparency, surface quality, chemical and thermal durability, and so on [13]. Such products are not yet on the mass market because the fundamental technologies are not mature, but flexible and rolled glasses are nonetheless expected to come out in the near future. The applications to the field of health care, electrical and optical packaging, MEMS, and so forth are anticipated to become more popular as well [14]. The relevant information can be found on the websites of glass manufacturers.
Two directions for development of the forming process can be followed. One is to improve further currently existing processes in terms of flatness, thickness, width, productivity, controllability, cost, versatility, facility lifetime, etc. The other direction is to add values through online introduction of other features such as coating and surface treatment (cf. Chapters 6.7 and 6.8). A closer match and harmonization between forming process and glass composition and properties might be also attractive.
As for forming commodity glass, invention of a novel process surpassing float with regard to energy consumption and investment costs would be desirable. For specialty glasses, innovative processes with higher quality and lower cost will of course also be sought after. Advances in basic science, simulation methods, sensing procedures, and information technology are presumed to become still more important either in operation and engineering or in development and innovation. Moreover, newly developed materials could make other innovative progress possible. In this respect, could unprecedented innovations based on novel mechanism make the processes described in this chapter obsolete in a near future? Their advantages should be considerable to write off the capital invested in current production plants all over the world. But would those innovations give rise to new applications and create new markets? A never ending challenge will change the world [15, 16].
References
1 1 Cable, M. (2004). The development of flat glass manufacturing processes. Trans. Newcomen Soc. 74: 19–43.
2 2 Yates, R.F. (May 1921). Revolutionizing the glass‐blowing industry. Popular Monthly: 30–32.
3 3 Hynd, W.C. (1984). Flat glass manufacturing processes. In: Glass: Science and Technology, Vol. 2, Processing I (eds. D.R. Uhlmann and N.J. Kreidl), 45–106. New York: Academic Press, Inc.
4 4 Yunker, R.W. (1984). Flat glass manufacturing processes, and C.K. Edge, Update. In: The Handbook of Glass Manufacture, 3rd ed., vol. 2 (ed. F.V. Tooley), 683–714 and p. 714/1–714/21. New York: Ashlee Publishing Co.
5 5 Cable, M. (1999). Mechanization of glass manufacture. J. Am. Ceram. Soc. 82: 1093–1012.
6 6 Mishima, Y. (1985). Flat glass forming, float process. In: Glass Encyclopedia [in Japanese] (ed. S. Sakka), 276–283. Tokyo: Asakura Shoten.
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