Laminated glass is probably the most significant safety development in the auto industry since the seatbelt. It prevents more injuries than seatbelts each year. Laminated glass is used in a variety of industries around the world, but the most well-known is the auto industry. Laminated glass is used in windshields primarily because it is nearly impossible to break into shards or splinters, thus keeping the driver and passengers safe from flying shards of glass.
Structurally, there are three layers required to make laminated glass. There are two sheets of everyday glass, just like window glass, and the third layer, which is sandwiched between the two layers of glass, is a substrate which adheres to the glass. Both pieces of glass must be cut to the proper size, then be perfectly clean, free of any dust or debris. Dust or debris of any kind will compromise the integrity of the laminated glass, causing it to break, or even impairing the view of the driver when it is used in a windshield. There are often many more layers of glass and substrate used in applications other than the automobile windshield.
Only after the two pieces of glass are perfectly clean is the substrate heat-bonded to them. A substrate is a type of plastic, known as Polyvinyl Butyral, or PVB. There are two less well-known types of substrate used in the manufacture of laminated glass, Polyurethane and Ethylene Vinyl. The first substrates used to create safety glass were cellulose based, and darkened in the sun, thus impairing the driver's vision. There have been many attempts to use liquid substrates, but the cost of using chemicals to harden them or exposing them to UV light is very expensive, and is currently not a solution for use in the auto industry due to its cost-prohibitive expense.
Substrates must be applied to the glass in a climate-controlled environment. Temperature and humidity play a large role in whether the substrate adheres to the glass. Amazingly, the substrate is applied by hand, one of the few jobs in the auto industry that are less mechanized. The substrate is glued to the bottom sheet of glass, and then the top layer of glass is laid on the substrate and the bottom layer of glass, making a sort of sandwich. Any excess substrate is then trimmed off the glass around the edges. In order to make the clearest and safest windshield possible, the auto industry requires only three layers to a windshield, the two layers of glass and one layer of substrate. Other industries require more layers of glass and substrate.
Air bubbles are a problem which must be addressed when creating laminated glass. Air bubbles form no matter how perfectly the lamination process is done. The method of getting the bubbles out is to heat the glass, and then press the bubbles out with a huge press roller. This pushes the bubbles out to the edge of the glass, where they are released. This process is usually repeated several times before the glass is free of air bubbles.
The final step in the creation of laminated glass is to place the object, in this case, the windshield, into an autoclave. The heat of the autoclave further bonds the layers together with heat. The windshield must stay in the autoclave for the entire indicated time to ensure proper bonding of the glass and substrate. The heat-rolling and autoclave processes will force some of the substrate out around the edges of the glass sandwich. This excess is then trimmed from around the edges of the glass.
After the windshield comes out of the autoclave machine and the excess substrate is trimmed off the edges of the glass, it is inspected. Inspectors search for any imperfection in the glass, such as a dark spot, an air bubble, or a crack in the glass. If the windshield passes inspection, it is then ready to be shipped to auto manufacturers all over the world.
