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About the previous article, The Birth of Glass: An Integrated Introduction to Float Glass Manufacturing Processing①, introduced the first two processing procedures of float glass production.This article will continue to introduce the three production processes of float glass forming and coating, annealing, cutting and packaging during the production of float glass.

Floating Glass Forming and Coating

The process of forming the clarified liquid glass into a glass plate is a process of mechanical manipulation according to the natural tendency of the material. The natural thickness of this material is 6.88 mm thick. The molten glass gushes out from the furnace through the flow channel area, and its flow is controlled by an adjustable gate called a gate, which is about ±0.15 mm deep into the molten glass. It floats on top of molten tin-hence the name float glass. Glass and tin do not react with each other and can be separated; their molecular form of resistance to each other makes glass smoother.

Figure 5  Floating Glass Forming and Coating

Figure 5  Floating Glass Forming and Coating

The tin bath is a unit sealed in a controlled nitrogen and hydrogen atmosphere. It includes support steel, top cover and bottom shell, refractory materials, tin and heating elements, reducing atmosphere, temperature sensor, computer process control system, about 8 meters wide, 60 meters long, and the production line speed can reach 25 meters per minute. The tin bath contains nearly 200 tons of pure tin with an average temperature of 800°C. When the glass forms a thin layer at the end of the tin bath entrance, called a glass plate, there are a series of adjustable edge-drawing machines on both sides to operate. The operator uses the control program to set the speed of the annealing kiln and edger. The thickness of the glass plate can be between 0.55 and 25 mm. The upper zone heating element is used to control the glass temperature. As the glass plate continuously flows through the tin bath, the temperature of the glass plate will gradually drop, making the glass flat and parallel. At this point, AcuraCoat® pyrolysis CVD equipment can be used for Online Coating Glass of reflective film, Low-E Glass film, Solar Control Glass film, Photovoltaic Glass film, and self-cleaning film. The glass is now ready to cool.

Figure 6 Cross section of tin bath

Figure 6 Cross section of tin bath

Figure 7 spreads into a thin layer on the molten tin

Figure 7 spreads into a thin layer on the molten tin, keeps it separated from the tin, and is shaped into a plate. The hanging heating element provides heat, and the speed and angle of the edger control the width and thickness of the glass.

Floating Glass Annealing   

When the formed glass leaves the tin bath, the temperature of the glass is 600 degrees. If the glass plate is placed in the atmosphere to cool, the glass surface will cool faster than the inside of the glass, which will cause severe surface compression and cause harmful internal stress to the glass plate. 

Figure 8 Floating Glass Annealing 

Figure 8 Floating Glass Annealing 

The heating process of glass before and after forming is also the process of internal stress formation. Therefore, it is necessary to gradually reduce the temperature of the glass to the ambient temperature by controlling the heat, that is, annealing. In fact, annealing is performed in an annealing kiln (see Figure 7) with a preset temperature gradient of about 6 meters wide and 120 meters long. The annealing furnace includes electric control heating elements and fans to keep the horizontal temperature distribution of the glass sheet continuously stable.  

The result of the annealing process is to carefully cool the glass to room temperature without causing temporary stress or stress.

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