Encyclopedia of Glass Science, Technology, History, and Culture. Группа авторов
[10].
Figure 10 Sketch of the float process: (a) for sheets thinner than the equilibrium thickness, where the glass is stretched, from its edges by top rolls and from its downstream part by conveyor rolls; (b) for sheets thicker than the equilibrium thickness, where water‐cooled carbon fenders serve as slipping guides to glass flowing [7].
5.4 A Complex Industrial Problem
In spite of the simplicity of its principles, the float process is not readily implemented because flow in both tin and glass and heat transfer among tin, glass, and the radiative field are really complex processes. Glass forming is mainly determined by parameters such as the glass flow rate, conveyor speed, rotating speed and angle of top rolls, and by the viscosity distribution within the glass ribbon. It goes without saying that the viscosity of the glass strongly depends on temperature, but the temperature distribution in the bath is itself influenced by radiative heat transfer, the flow of molten tin, and the glass forming conditions. Radiative heat transfer is predominant in the bath at temperatures higher than 600 °C, but the flowing molten tin also contributes markedly to heat transfer as a result of its high heat capacity, high thermal conductivity (about 50 times higher than that of the glass), and low kinematic viscosity (about 8 times lower than that of water), whereas the glass flow carries a large amount of convective heat. On the other hand, molten tin flows by traction from the glass ribbon (i.e. velocity distribution) and buoyancy convection (i.e. temperature distribution in the bath). In order to understand forming conditions, one thus needs to understand the whole set of processes taking place in the bath since glass forming, heat transfer in the bath, and flow of molten tin affect one another in a very complex manner.
As summarized by C.K. Edge [4], the float bath thus is “a remarkable entity which, although first envisioned as a finisher of glass surfaces, also functions as a container, a conveyor, a forming unit, a chemical reactor and a heat exchanger.” In view of this complexity, valuable information has been drawn from mathematical simulations not only of the glass forming mechanisms, but also of the temperature field and the mutually related dynamics of the molten tin and glass ribbon. As examples, calculations with finite‐element methods of the thickness contour over the glass ribbon and of the lateral thickness distribution of the ribbon at the bath exit are shown in Figures 11 and 12 [10]. The rather good agreement of such model values with the temperatures, thicknesses, or ribbon shapes that can be measured on line illustrates how simulations can be used to optimize the operating conditions, to design new facilities, and to check new ideas for process improvement and development.
From a chemical standpoint, potentially annoying impurities are oxygen from leaks or the N2–H2 gas mix, and sulfur originating from the molten glass. Both deteriorate productivity and glass quality if they induce alteration on the bottom surface of the glass caused by reactions with tin, and contamination adhesion on top and bottom surfaces (Chapter 5.6). In the so‐called oxygen and sulfur cycles (Figure 13), SnO and SnS vapors form, condense, and precipitate, the former as SnO2 and the latter as SnS (which is ultimately reduced to Sn particles). Thanks to extensive research, however, these impurities are now carefully managed through monitoring, sealing, cleaning, atmosphere controlling on flow and pressure, etc.
5.5 Trends in Float Production
The float process is advantageous because it yields excellent flatness, high flexibility with regard to thickness and width, and high productivity owing to completely continuous operation during the whole lifetime of the melting furnace, which can now reach up to two decades. Ever since its conception, plenty of technological improvements have been conducted for achieving higher throughput, larger width, and higher still quality, the thickness currently extending down to less than 0.4 mm and up to around 25 mm.
Figure 11 Shape and thickness distribution of a 2 mm thick float‐glass ribbon calculated with an integrated glass‐forming model [10]. Forming (i.e. shape, thickness, and velocity distributions of the glass ribbon) and the flow of molten tin are first calculated for a given temperature distribution of the glass ribbon. Heat transfer (i.e. the temperature distribution) in the float bath then is simulated, and the whole calculation is iteratively repeated until convergence is reached for the three interrelated mechanisms.
Figure 12 Simulated thickness distribution at the exit of the bath for 2 mm thick glass ribbon. The lateral distance is normalized [10].
Originally the float process was designed to produce glass sheets for architectural window and mirrors. Since the 1970s, the technology has evolved to meet other demands, especially that emerging from automotive market for higher optical quality and thinner sheets along with higher throughput to keep production costs reasonable. In addition, the float process has contributed to the growing solar generation market with products such as mirrors for solar power systems and cover glasses for photovoltaics.
Figure 13 The complex interactions of impurities with the atmosphere, tin bath, and glass ribbon in the float process: (a) oxygen cycle; (b) sulfur cycle.
Source: After Pilkington [9].
In the early 1980s, the float process achieved production of ultrathin glass of less than 1.1 mm for twisted nematic (TN)/super TN (STN) liquid crystal display (LCD) substrates, touch panels, and other electronics products with a remarkably high quality for flatness, thickness constancy, defect level, etc. Beginning in the 1990s, the float process has been producing flat glass of various kinds of compositions other than the traditional soda‐lime silicate. Examples are alkali‐free glass for thin film transistor (TFT) LCD substrate, high strain‐point glass for plasma display panel (PDP) and solar panel substrates, specialty glasses for heat‐resistant products, hard disk drive (HDD) substrates, and other products such as touch panels and display covers that are then chemically strengthened.
6 Downdraw Processes
6.1 Slot Downdraw
It was for forming thin glass sheets that the slot downdraw process was developed in the 1940s. As driven by pairs of rolls, the molten