Glass is made up of several oxides that fuse and react with one another upon heating to create a glass. These include silica, sodium oxide, and calcium oxide. Raw substances from which these materials are formed are sand, soda ash, and limestone. Soda ash acts as a flux; put simply, it reduces the melting point of the batch composition. Lime is added to increase the hardness of the glass. Glass designed for windscreens also contain several other oxides: magnesium oxide, potassium oxide and aluminium oxide.
The raw materials are weighed in the correct amounts and combined with a small amount of water. Once made, it will be fed to a large tank for melting, implementing the float glass procedure. Initially, the batch is heated to a molten state, and then it will be fed into a tank known as the float chamber, which contains a bath of molten tin. At its entrance, the temperature of the tin is about 1,835 degrees Fahrenheit, while at the exit the tin’s temperature is slightly cooler 1,115 degrees Fahrenheit. In the float chamber, the glass doesn’t submerge into the tin but floats on top of it, moving through the tank as though on a conveyor belt. The perfectly flat surface of the tin causes the molten glass also to become flat, while the high temperatures clean the glass of impurities. The decreased temperature at the exit of the chamber allows the glass to harden enough to move into the next chamber. After the glass exits from the float chamber, rollers pick it up and feed it into a furnace called a Lehr. In this furnace, the glass is cooled gradually to about 395 degrees Fahrenheit, after the glass exits the Lehr, it cools to room temperature. It is now very hard and strong and ready to be cut.
Cutting and Tempering
The glass is cut using a diamond scribe. Diamond is used because it is harder than glass. Next, the cut piece must be moulded into shape. The sheet of glass is placed into a metal mould. The glass filled mould is then heated in a furnace to the point where the glass shapes to the mould.
After this shaping step, the glass must be hardened in a heating step called tempering. First, the glass is heated to about 1,565 degrees Fahrenheit, and then blasted with jets of cold air, this process toughens the glass. This allows the auto glass, when damaged, to break into many small pieces of glass without sharp edges.
In this process, two sheets of glass are bonded together with a layer of plastic. The lamination takes place in an a special oven. The plastic layer is often tinted to act as an ultraviolet filter. When laminated glass is broken, the broken pieces of glass remain bound to the tear resistant plastic layer, and the broken sheet remains transparent. Unlike traditional safety glass, laminated glass can be further processed
After laminating, the windscreen is ready to be assembled with plastic moulds so it can be fitted. Known as encapsulation, this assembly process is usually done at the glass manufacturer. First, the windscreen is set in a mould. Next, molten plastic is injected into the mould, when it cools it forms a plastic frame around the glass. The windscreen is then shipped to the car manufacturer, where it is fitted to a vehicle. The installation is done by a process that uses a polyurethane adhesive to bond the windscreen to the vehicle’s body.
This step includes testing of raw materials and checking melting temperature. As the glass is made, special devices are used to look for defects in the glass. Other automatic devices have been developed to measure dimensions and radius of curvature after the windscreen has been shaped.
Safety glass used in windscreens must meet certain specifications regarding properties such as impact resistance, and strength. Safety standards have been developed to ensure the quality of the glass.