Encyclopedia of Glass Science, Technology, History, and Culture. Группа авторов. Читать онлайн. Newlib. NEWLIB.NET

Автор: Группа авторов
Издательство: John Wiley & Sons Limited
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Жанр произведения: Техническая литература
Год издания: 0
isbn: 9781118799499
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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.

      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.

Schematic illustration 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. Schematic illustration 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.

      5.1 Principle

Schematic illustration of the single-pass wire roll out process. The wire is inserted into the molten glass, which is pressed and cooled by rotating water-cooled rolls.

      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).

Schematic illustration of the overview of a float-glass plant.