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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aforementioned equilibrium thickness Te is given by

      (1)equation

      5.2 Float Bath

Symbol Denotation
T e Equilibrium thickness
ρ t Density of molten tin
ρ g Density of molten glass
S ga Surface tension at glass–atmosphere interface
S gt Surface tension at glass–molten tin interface
S ta Surface tension at tin–atmosphere interface
g Gravitational constant
Schematic illustration of the equilibrium thickness of floating glass on the molten tin when the gravitational forces and surface tensions are balanced.

      Once poured onto the tin bath with a thickness of about 50 mm, the glass spreads out and thins to its equilibrium thickness in the upstream area in the float bath. As formed to the required thickness and width in the forming area (see Section 5.3), the glass ribbon is taken out from the bath either to receive appropriate reflective, low‐emissivity, solar‐control, self‐cleaning, or other specific coatings (Chapters 6.7 and 6.8) or to enter directly the annealing lehr at the temperature at which the viscosity is about 1010 Pa·s (i.e. about 600 °C for soda‐lime silicate). At the end of the lehr, whose length can reach 120 m, the ribbon is finally cooled down to room temperature and brought into the cutting area. Whereas both edges are cut out (to be recycled as cullet) because of the imprint left by the top rolls, the ribbon itself is cut either according to customers' specifications or as standard sheets, for instance, 6.0 × 3.21 m in Europe where tools used in the flat‐glass transportation industry have been fitted to this size (which, by the way, is too large to allow flat glass to be shipped in containers).

      5.3 Thinner (Top‐Roll Process) and Thicker (Fender Process) Glass Ribbons

      For producing float glass thicker than the equilibrium thickness, a pair of water‐cooled carbon fenders serves as slipping guides to the flowing glass in the bath (Figure 10b). The glass thus proceeds with a restricted width and a large thickness. As it passes down the fender area, the effects of gravitational forces and surface tensions make both its upper and lower surfaces flat and thickness uniform. The glass is then cooled to an appropriate temperature in the downstream area of the fender where its viscosity is high enough not to allow width changes. In contrast to what is taking place in the top‐roll process, stretching is not significant at all and there is no drive to return to the equilibrium thickness because there is no glass–tin–atmosphere interface in the fender area.