Encyclopedia of Glass Science, Technology, History, and Culture. Группа авторов
1.1–0.7 mm can be produced, especially for electronics applications, with a width of 1.5–2 m, thanks to the forming stability derived from the Asahi blocks.
4 Roll Out Process
The continuous double‐roll process was developed in the United States in an effort led by the Ford Motor Company to meet a growing demand from the automotive industry. As delivered from the forehearth, the molten glass was pressed to a given thickness, cooled rapidly by a water‐cooled pair of rotating rolls, and then conveyed into a horizontal annealing lehr. The thickness was determined mainly by the gap between the rolls, whereas the output was fixed by the rotating speed of the rolls.
As made by Pilkington Brothers in the 1920s, this process was then improved to manufacture plate glass through online grinding after annealing, followed by polishing of the cut plates. The process was further developed by Saint‐Gobain in the 1950s to grind and polish on line the glass ribbon (Chapter 10.9). Along with a waste of about 20% of the glass, very high investment and operating costs were major disadvantages of these mechanical methods, however, which in fact prompted Pilkington to develop the float process as described in Section 5.
Figure 4 Sketch of the Pittsburg Pennvernon process in cross section. The molten glass is drawn upward from the free surface right above the drawbar immersed below the glass surface [3].
Figure 5 Sketch of the Asahi process in cross section. The rotatable Asahi blocks are immersed into molten glass instead of the débiteuse, and enable the parting line to be renewed where devitrification takes place [6].
Because the float process was not designed at all for patterned and wire‐reinforced glass, the continuous roll out process, with which these products have been produced since the 1920s, has escaped oblivion (Chapter 10.9). Thanks to its versatility and facility for customization, it has even found new special applications, for instance, to make cover glasses for solar cells with excellent light diffusion through patterned textures on the surface. Usually the pattern is impressed on the lower surface by the lower roll, which is engraved. Generally the thickness range is 2–7 mm for the patterned and 8–25 mm for the polished glass.
As to wire‐reinforced glass, it is produced in two ways depending on whether the wire is simply inserted into a molten glass (single‐pass process, Figure 6) or sandwiched between two glass layers (double‐pass process). Although more complex, the latter process has advantages over the former in terms of larger output, wider width, and higher quality, and better suitability for subsequent conversion into polished wired glass because the wire mesh is always precisely located at the center in the thickness direction [1, 3–8].
5 Float Process
5.1 Principle
When it was invented in the 1950s, the float process turned out to be an epoch‐making method to produce flat glass with a smooth surface without any additional polishing. Its production cost was low enough to make possible an extensive use of the material in buildings and automobiles, which is one of the hallmarks of current civilization. The basic process of making flat glass on a molten metal was in fact patented in various ways as early as in 1848 in England by H. Bessemer, of steel‐converter fame, and then several times in the United States by W. Heal and J. H. Forrest (1902 and then in 1925), and by Halbert K. Hitchcock (1905 and 1925), but a great many technical problems had to be solved before the process could be made practical. Through a seven‐year expensive research program of Pilkington Brothers in the United Kingdom, which in its last stage included 13 months of production that had to be discarded, all these problems had eventually been overcome in July 1958 by a team led by L.A.B. Pilkington (no relationship with the Company's owners) and K. Bickerstaff. The following year it even became possible to produce with the same process the distortion‐free glasses needed for mirrors, thus abolishing the long‐standing distinction between window and polished‐plate glass (Chapter 10.9).
Figure 6 Sketch of single‐pass wire roll out process (upper part insertion process). The wire is inserted into the molten glass, which is pressed and cooled by rotating water‐cooled rolls [8].
Although the float process became continuously profitable only in 1963, the previous year it began to be licensed all over the world by Pilkington Brothers in rapidly expanding markets. Within a decade, the good optical quality of float glass resulted in a vanishing share for other sheet‐ and plate‐glass processes. As of 2015, more than 400 float plants are operated worldwide, units being up to about 500 m long (Figure 7). With a typical size of about 25 × 60 m, melting tanks are bigger than Olympic swimming pools to produce 600 metric tons (and even up to 1300 tons in the biggest plants) of flat glass per day with an investment cost ranging from 70 to 200 million dollars or euros, depending on actual size, location, and product complexity. As for the inflation‐adjusted production cost (cf. Chapter 9.6), it has been almost continuously decreasing by a factor of 4 from 1965 to reach today a range of 200–300 dollars or euros per ton (Chapter 9.6), raw materials and energy accounting both for about 20% of it.
When a molten glass is poured onto a clean molten metal bath, it floats, thanks to its much lower density, spreads out, and thins to the point where the gravitational forces and the surface tensions among the glass, molten metal, and atmosphere are in equilibrium to reach the so‐called equilibrium thickness. The lower and especially the upper surfaces of the molten glass are fire‐polished, perfectly flat, and parallel except at the edges. The float process is based on this principle. Among metals or alloys that are liquid between 600 and 1050 °C, the relevant temperature range for glass forming, pure tin was the obvious choice because of its low melting temperature of 232 °C, high density of about 6.5 g/cm3 at 1000 °C, low vapor pressure of about 10−7 atm at 1000 °C, high boiling point of 2602 °C, low reactivity with silicates in the metallic state, and not too high cost (about 20 dollars/kg as of 2015).
Figure 7 Overview of a float‐glass plant (scale not right: size of the right‐hand side, for instance, much exaggerated) http://www.glassforeurope.com/en/industry/float‐process.php