High quality glass production starts with the float process, developed by Pilkington and now the industry standard. Automotive glass production includes pre-processing, toughening, laminating, encapsulation, extrusion and assembly, adding functionality.
Float Glass
The float glass process, developed by Pilkington in 1952, is now the world standard for high quality glass production. Float glass is often processed further before being fitted into buildings and vehicles. The process, originally able to make only 6mm thick glass, now makes it as thin as 0.4mm and as thick as 25mm. A 'batch' of precisely mixed raw materials is melted in the furnace. Molten glass, at approximately 1000ºC, is poured continuously from a furnace onto a shallow bath of molten tin in a chemically controlled atmosphere. It floats on the tin, spreads out and forms a level surface. Thickness is controlled by the speed at which the solidifying glass ribbon is drawn off from the bath. After annealing (controlled cooling) the glass emerges as a 'fire' polished product with virtually parallel surfaces. A float plant, which operates non-stop for a 'campaign' of between 11 and 15 years, makes around 6,000 kilometres of glass a year in thicknesses of between 0.4mm to 25mm and in widths of over three metres. The NSG Group operates or has interests in 51 float lines worldwide.
Pilkington Technology
Over the past 50 years almost every major advance in glass has come from Pilkington, from the invention of the float process to self-cleaning glass. Pilkington invests around £29 million a year in research and development focused on product development and manufacturing efficiency improvement.
Pilkington Automotive has pioneered techniques that have led to major industry firsts such as;
- the production of the wraparound windshield
- the S-bend backlight
- solar reflective automotive glazing
- full vehicle encapsulated glazing systems
- electrically heated filament windshields
Pilkington Automotive conducts R&D at its European Technology Centre in the UK and also in Sagamihara, Japan, Toledo, USA, Witten, Germany and Turin, Italy.
Toughening (tempering)
Toughened glass is most frequently used in the rear and side windows of vehicles. It is designed to be much stronger than standard glass. In the case of a breakage, it shatters into very small blunt pieces thereby significantly reducing risk of injury. The toughening process introduces internal stresses into the glass through a combination of controlled heating to very high temperatures, (>640°C) and differential cooling. The heating cycle is also used to shape or curve the glass, either by allowing the heated glass to 'sag' to a pre-defined mould shape under gravity, or for more complex shapes, by being pressed to shape by male and female moulds.
Laminating
Lamination is a form of safety glazing where normally two thin glass plies create a sandwich around a polyvinylbutyral (PVB) interlayer. Normally used for a vehicle's windshield, in the case of breakage, the glass is held in place by the interlayer, retaining emergency visibility for the driver. Laminated glazing is now also increasingly being specified for car side windows. Usually the glass plies are shaped (curved) as matched pairs through heating to a temperature of around 620°C. As with tempering, the shape can be achieved through gravity 'sagging' or through pressbending for the more complex shapes. Differential heating to control temperature across the surface of the glass, and hence the resultant degree of bending, is also used for more complex shapes. The shaped glass pairs are then gradually cooled to room temperature before the PVB laminate is sandwiched between them. At this point the PVB is opaque and only becomes transparent at completion of the lamination process. This involves the removal of any air trapped in the glass sandwich through a mechanical or vacuum squeezing process, followed by the heating of the windshield to 140°C within an autoclave, under a pressure of 10 to 15 kg/cm2, to complete the bonding of the two glass plies.
Increasingly, glazing systems rather than a simple piece of glass are being supplied to the vehicle manufacturers. Much of this value-added activity is undertaken once the glass has been laminated or tempered. Glazing systems help to simplify the vehicle assembly process. Modular systems such as encapsulation and extrusion are designed to facilitate adhesive bonding of the glazing to the vehicle.
Encapsulation
Encapsulation involves the injection moulding of a polymer trim, shaped precisely to fit the vehicle body, to the periphery of the glazing. It also provides the opportunity to incorporate within the moulding additional styling features, fixing mechanisms and even hinges for opening windows in minivans and estate cars.
Extrusion
An alternative glazing system is provided by the robotic extrusion of a polymer profile to the periphery of the glazing, with the extruded profile or seal being precisely shaped to the vehicle.
Assembly
Fixing of certain glazings within a vehicle can also be achieved through the application of locator clips to the glass. The attachment of such clips, together with other hardware, is most commonly referred to as assembly.