Glass is a visco-elastic material whose mechanical properties change very rapidly over a small temperature span. Between 500o-600oC its viscosity falls by a factor of 10,000 as it transforms from a brittle solid to a plastic substance. The science of glass-bending is to use this plastic phase to produce shapes which are complex, yet free from wrinkles and other optical aberrations. It has attracted considerable research effort since the 1950s.

Sag bending is the most widely used process for windshields. The glass, supported peripherally and heated to the plastic phase, is allowed to sag under its own weight to the desired shape (gravity sag bending). Control is through the pattern of temperature distribution across the sheet.

Designers of many new models are seeking an enhanced process which, through closer temperature control and physical intervention, offers greater curvatures and more precision without sacrificing optical performance. A die can be used to press home the final five per cent of the shape - a technology known as die-assisted sag bending. Pilkington is tackling the most challenging shaping problem with its Advanced Press Bending technology.

Computer modelling of bending can simulate the behaviour of any of the Group's shaping processes, using finite element analysis to show the temperature distribution needed to achieve any given shape, and where the limits lie.

The main purpose of the toughening processes is to introduce compressive stresses into the surface and thereby raise the loads that float glass, shaped or unshaped, can be permitted to bear.

Thermal toughening is the most common way of toughening glass for use in products such as car side and rear windows. Glass is heated to about 650oC, then quenched with air jets so that the surfaces are cooled quickly and the core more slowly. At ambient temperature the core continues to cool and compression stresses develop in the surfaces, balanced by tension in the core. This produces a parabolic stress distribution through the glass thickness. A crack which propagates through the compression zone into the tensile zone will cause rapid release of the strain energy built up in the glass through the development of multiple cracks. This results in the formation of many small glass particles which are less likely to injure than shards.

Chemical toughening, used particularly to strengthen thin glass, involves an ion-exchange reaction which replaces sodium ions at the surface with bigger potassium ions, putting the surfaces in compression. It can be done in a bath of molten potassium nitrate at about 450oC. The stress distribution through the glass is much sharper than for thermal toughening, with a relatively shallow compression zone and a lower, flatter tensile stress in the core. When this glass fails, it shatters into large pieces.

Lamination by sandwiching a plastic layer between sheets of glass can hold splinters together and help absorb the kinetic energy of projectiles. Multi-layer combinations of glass and plastic can provide laminates capable of resisting almost any projectile, from a loose chipping to a high-velocity bullet. Laminated glass is used in windscreens for cars, aircraft and locomotives.

Residual stresses in toughened glass are inspected with a polariscope, enabling scientists to predict how glass will behave when it fractures.