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5 5 Jebsen‐Marwedel, H. and Brückner, R. (1980). Glastechnische Fabrikationsfehler, “Pathologische” Ausnahmezustände des Werkstoffes Glas und ihre Behebung; Eine Brücke zwischen Wissenschaft, Technologie und Praxis, 229. Berlin: Springer.
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Note
1 Reviewers: E. Muijsenberg, Glass Service a.s, Vsetin, Czech RepublicC. Rüssel, Friedrich Schiller University, Jena, Germany
1.4 Primary Fabrication of Flat Glass
Toru Kamihori
Production Technology Center, Asahi Glass Co., Ltd., Yokohama‐shi, Kanagawa, Japan
1 Introduction
Flat glass is ubiquitous in the modern world, from the facades of high‐rise buildings to the large windows of automobiles, the solar power generation systems, and the various kinds of displays that are now integral components of daily life. Not only did these new applications cause a tremendous increase of the world production from less than 7 105 in the early 1960s to 59 106 metric tons in 2014 (then with an annual growth rate of 7%), but they have also yielded dramatic improvements in glass quality and functionalities. For a glass material that had been manufactured for 2000 years with very little change, the industrial evolution observed during the last 50 years has been incredibly rapid indeed!
As experienced by ancient Roman glassmakers, flat glass made by pouring the melt on a solid substrate has a surface that is not smooth enough to ensure good transparency. Until the beginning of the twentieth century, flat glass had for this reason to be produced out of hollow glass to keep the defect‐free surface conferred by fire polish. As made in this way with either the crown or the cylinder process (Chapter 10.8), production of flat glass was very labor‐intensive, restricted to relatively small sheets and subject to wastage when cut into pieces for use. Besides, it did not yield high‐quality products as is obvious to anyone looking at an old window where objects are often seen distorted through the glass in which defects and streaks are also generally present as a result of the detrimental effects of temperature or composition heterogeneities that could not be avoided during the melting and forming processes.
Mechanization was pioneered from 1894 to 1916 by J. H. Lubbers at the American Window Glass company. Glass cylinders of constant diameter and wall thickness began in 1904 to be blown successfully with a well‐controlled low‐pressure air flow issued for 15–18 minutes from a machine that was dubbed iron lung [1]. Making much bigger cylinders, which eventually reached 1 m in diameter and more than 13 m in length (Figure 1), did reduce considerably cost and labor (whence the very strong Union opposition met by the new process), but did not result in consistently good quality because of the wavy surface and optical distortion induced by the flattening stage. Thanks to updraw processes designed in Belgium and in the United States, it then became possible in the following decade to bypass the hollow‐glass step. But these processes remained discontinuous because devitrified material had to be removed periodically from the production line.
A true revolution in glass producing thus occurred when the float process was introduced in the 1950s to produce sufficiently good flat glass to make the grinding and polishing steps ensuring high‐quality sheets obsolete. Production became in addition completely continuous, which allowed the productivity to be considerably increased without affecting surface quality. Because of the very large amount of glass produced, however, the float process may be impractical when production volumes are small or the glass composition has to be changed frequently. For specialties such as crown glass for niche markets or new glass for electronics markets, older processes are thus still used or new ones have been designed to achieve in particular a high flexibility of throughput and broad ranges of thickness and width.
In this chapter, we will review the various processes that have been designed to achieve these goals, beginning with the first developed ones whose interest is now only historical. The early processes are denoted as updraw because the glass was drawn upward, in contrast to the downdraw processes, which have subsequently been developed for specialty glasses. No attention at all will be paid to the synthesis of the glass itself, which is described in Chapter 1.3. The emphasis will thus be put on the forming process and on the material parameters such as viscosity, density, heat capacity, and surface tension that control it. For more detailed descriptions, the reader will be referred to the available technical literature [1–9].
Figure 1 Very large glass cylinders blown mechanically with the Lubbers process for flat‐glass production [2].
2 Overview
The main features of past and current processes are summarized in Table 1. Although updraw processes are no longer used for commodity applications, it remains worthwhile to examine their mechanisms and forming principles from a technological viewpoint. For flat‐glass forming, the essential requirement is to achieve the constant desired thickness, which now ranges from around 25 mm to less than 50 μm, with the specified width at a commercially admissible cost. Additionally, a flatter and smoother surface is requested. The essential forming defects are mainly of two kinds depending on whether they are derived from locally uneven deformation or undesirable stress. The former is caused by viscosity heterogeneity of glass originating from chemical impurities or temperature irregularity, and the latter is caused by fluctuation of forming condition or stress imbalance.
Architectural glass must, for instance, satisfy appropriate transparency and reflection, but glass for automobiles and electronics products has to meet much more demanding quality specifications even for thinner glass whose production becomes increasingly difficult.
In all forming processes, two distinct steps are involved once the glass has been melted at about 1500 °C, refined, and homogenized in the melting tank. The first is its delivery under conditions at which the temperature, thickness, and flow rate must be as stable and uniform as possible throughout its whole width at a viscosity of about 102–103
