Glass Manufacturing - Automotive Processing
Because glazing is an integral part of any vehicle’s styling, each piece of automotive glass is unique to a specific opening within any individual vehicle. Hence three-dimensional shaping of the glass, together with imparting its increased strength and safety properties are at the heart of automotive glass processing. However, ahead of this, certain pre-process operations must be undertaken.
Pre-Processing
Pre-processing involves a number of preparatory activities, ahead of submitting the glass to heat treatment.
They include:
- cutting out the flat glass template from standard, rectangular ‘block sizes’ of automotive float
- edge-working the shaped, but still flat, piece of glass to provide a smoothed glass edge
- drilling any required holes within the glass
- washing the glass, before clean-room printing is undertaken
- printing of shade bands, logos etc. on the glass in a single pass process for simple features but a two or three pass process for demisting circuits, antennas and alarms.
On completion of these pre-process activities, the glass proceeds to be shaped and to receive its safety properties.
Automotive Glass Shaping and Strengthening
There are two basic forms of glass shaping and strengthening for automotive applications, though hybrid processes have more recently been developed.
Automotive Toughening
Toughened glass, or tempered glass as it is sometimes called, is most frequently used in the rear and side windows of vehicles. It is designed to be much stronger than non-safety glass. However, in the case of a breakage, it shatters into very small pieces rather than sharp shards of glass, thereby significantly reducing the risk of injury. This is made possible by the toughening process which 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.
Automotive 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 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 620ºC. As with tempering, the shape can be achieved through gravity ‘sagging’ or through press-bending 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 heating of the windshield to 140ºC 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 Processes
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.