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Sheet Glass Manufacturing : The Float Glass Process

Glass, which has been made since the time of the Mesopotamians and Egyptians, is little more then a mixture of Silica sand, soda ash and lime. When heated to about 1500 degrees Celsius this becomes a molten mass that hardens when slowly cooled. The float process for making flat glass was invented by Alistair Pilkington. This process allows the manufacture of clear, tinted, and coated glass for buildings, and clear and tinted glass for vehicles. 

Float plants today make glass of near optical quality. Several processes – melting, refining, homogenizing – take place simultaneously of up to 2000 tons of molten in the glass furnace. They occur in separate zones in a complex glass flow driven by high temperatures. The principle of float glass is unchanged since the 1950s. However, the product has changed dramatically, from a single thickness of 6.8mm to a range from sub – millimetre to 25mm, from ribbon frequency marred by inclusions and bubbles to almost optical perfection. 

Float glass is sold by the square meter, and at the final stage computers translate customer requirements into patterns of cuts designed to minimize waste.

Float glass is sheet glass made by floating molten glass on a bed of molten tin. This method gives the glass uniform thickness and a very flat surface. Float glass is more commonly known as window glass. Because it is inexpensive and sometimes free, it is often used in the glass fusing process. The molten glass spreads onto the surface of the metal and produces a high quality, consistent level sheet of glass that is later heat polished. The glass has no wave or distortion and is now the standard method for glass production; over 90% of the world production of flat glass is float glass. Float glass is a specialized type of glass with an extremely smooth and uniform structure, with superb optical properties. For this reason, it is used in a wide variety of different applications including windows, solar panels, LCD displays, and automotive wind screens. Float gas is produced via the float glass process, which involves floating the molten glass on a bed of molten metal and then allowing the glass to set. The method means large panes can be produced easily, and thickness can also be controlled. The float glass process has significantly reduced the cost of producing glass that is flat and smooth. Modern glass is produced from several different materials including sand, dolomite, limestone, carbonate, sodium sulfate and also scrap glass. All of these materials are heated to extreme temperatures (2,800 °F) to form molten glass. This glassy liquid is the

n poured onto a ‘tin bath’ consisting of molten tin, which acts as

a level template for the glass to distribute over and then harden. As the glass hardens

he tin template ensures the glass has flat top and bottom surfaces. Once it has cooled and completely solidified the monolithic sheet of glass can be cut into smaller sections. The molten tin is prone to oxidizing

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This is prevented by keeping the tin bath in an atmosphere of 90% hydrogen and 10% nitrogen. Today, float glass is the most widely used form of glass in consumer products. Due to both its high quality with no additional polishing required and its structural flexibility during production, it can easily be shaped and bent into a variety of forms while in a heated, syrupy state. This makes it ideal for a variety of applications such as
 
Automobile glass (e.g. windshields, windows, mirrors)
Mirrors
Furniture (e.g. in tables and shelves)
Insulated glass. Windows and doors Most forms of specialized glass such as toughened glass, frosted glass, laminated safety glass and soundproof glass consist of standard float glass that has been further processed.

The phrase “to float” means “to be buoyant”.

The float glass manufacturing process is based. In the float glass process, molten glass is fed onto a float bath of molten tin. This tin bath is 4-8 meters wide and up to 60meters long. To prevent the tin surface from oxidizing with the atmospheric oxygen, thetin bath is placed under a protective gas atmosphere. This atmosphere must becarefully controlled since its composition is instrumental for the properties of the contactsurface between the glass and the tin which, in turn, influence the thickness of the glasssheet.The glass floats like an endless ribbon on the tin. At the entrance where the glass firstmakes 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 andfeed it into the annealing lehr from which it exits at about 200oC. After cooling to roomtemperature on an open roller track, it is cut, packed, and stored either for shipment orfor further processing into products such as safety glass, reflective glass, self-cleaningglass, mirrors or double glazed or multi-glazed units.Float glasscan be made in thickness between 1.5 to 20mm. There are two techniquesto accomplish this. To produce thin float glass, rollers control the width and speed of theglass ribbon. For thick float glass, the glass floats against graphite barriers, so that theribbon 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 canbe 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 whichre

cycled 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.

Basically the principle on which the float glass manufacturing process is based. In the float glass process, molten glass is fed onto a float bath of molten tin. This tin bath is 4-8 meters wide and up to 60meters long. To prevent the tin surface from oxidizing with the atmospheric oxygen, the tin bath is placed under a protective gas atmosphere. This atmosphere must be carefully controlled since its composition is instrumental for the properties of the contact surface between the glass and the tin which, in turn, influence the thickness of the glass sheet. 

The Glass floats like an endless ribbon on the tin.

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. 

Applications:

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.

Pyrolytic Deposition – Hard coat and on-line coating:

Pyrolytic coating deposits metallic oxide directly onto the glass surface whilst it is still hot. The low-e coating is effectively ‘baked-on’ to the surface and the resulting low-e coating is very hard and durable. The pyrolytic coatings are often referred to as ‘hard coats’. Pyrolytic coatings can be up to 20 times thicker than sputtered coatings (they are still 500 times thinner than a human hair) and the baking process makes them much harder and resistant to wear. Pyrolytic ‘hard coats’ have a low emissivity, but this is higher than those achieved for soft coats. Hard coat products have emissivity’s ranging from 0.15 to 0.2. The ability to apply ‘hard coats’ whilst the glass is still hot means that hard coated products are cheaper than soft coated products.

Steps:

1. The first stage is the loading of raw material

2.Melting: in melting furnace at a temperature of 1500°C or higher where it flows as molten glass.

3.Tin Bath: The molten glass at 1050°C flows gently over a refractory and is spread evenly over the surface of this bath, floating on the molten tin.

4.Molten glass leaves the float bath as a solid ribbon at 600°C.

5.Annealing:  annealing process in produced and the glass leaves with 25°C.6.The glass is ready to be cut and shipped.

The float glass process involves floating a glass ribbon on a bath of molten tin and creates a smooth surface naturally. 

Floating is possible because the density of a typical soda-lime-silica glass (~2.3g/cm3) is much less than that of tin (~6.5g/cm3) at the process temperature.

After cooling and annealing, glass sheets with uniform thicknesses in the ~1–25mm range and flat surfaces are produced. 

The float glass process is used to produce virtually all window glass as well as mirrors and other items that originate from flat glass.

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