The glass floats like an endless ribbon on the tin.At the entrance where the glass first makes contact with the tin surface, the temperature of the liquid metal is about 600oC.Tin is the only metal that remains in a liquid state at 600oC.Immediately after the exit from the float chamber, special rollers take up the glass and feed it into the annealing lehr from which it exits at about 200oC. After cooling to room temperature on an open roller track, it is cut, packed, and stored either for shipment or for further processing into products such as safety glass, reflective glass, self-cleaning glass, mirrors or double glazed or multi-glazed units. Float glass can be made in thickness between 1.5 to 20mm.
There are two techniques to accomplish this. To produce thin float glass, rollers control the width and speed of the glass ribbon. For thick float glass, the glass floats against graphite barriers, so that the ribbon flows out thicker. Thus, the desired widths and thicknesses can be achieved.
While each glass plant is different from the other, the float glass production process can be divided into five universal steps:
1. Batching of raw materials: The main components, namely, soda lime glass, silica sand (73%), calcium oxide (9%),soda (13%) and magnesium (4%), are weighed and mixed into batches to which recycled glass (cullet) is added. The use of ‘cullet’ reduces the consumption of natural.
gas. The materials are tested and stored for later mixing under computerized control.
2. Melting of raw materials in the furnace: The batched raw materials pass from a mixing silo to a five-chambered furnace where they become molten at a temperature of approximately 1500°C.
3. Drawing the molten glass onto the tin bath: The molten glass
is “floated” onto a bath of molten tin at a temperature of about 1000°C.
It forms a ribbon with a working width of 3210mm which is normally between 3 and 25mm thick. The glass which is highly viscous and the tin which is very fluid do not mix and the contact surface between these two materials is perfectly flat.
4. Cooling of the molten glass in the annealing lehr: On leaving the bath of molten tin, the glass
now at a temperature of 600°C
Has cooled down sufficiently to pass to an annealing chamber called a lehr. The glass is now hard enough to pass over rollers and is annealed, which modifies the internal stresses enabling it to be cut and worked in a predictable way and ensuring flatness of the glass. As both surfaces are fire finished, they need no grinding or polishing.
5. Quality checks, automatic cutting, and storage: After cooling, the glass undergoes rigorous quality checks and is washed. It is then cut into sheets of sizes of up to 6000mm x 3210mm which are in turn stacked, stored and ready for transport.
1. Float glass is used for glazing wherever full transparency is required in buildings.
2. It is used as a base material for safety glass, reflective glass, and self-cleaning glass, among others. It can be used in precision mechanics, especially where extreme surface flatness is required. E.g., for visual displays.