Cutting and drilling of glass

Another important aspect of contact behavior of glass is the cutting and drilling of glass sheets. Cutting is divided into three operations: scribing, flexuring and breaking.

Scribing of the glass under a sharp tip (in fact a glass-cutter wheel) allows the generation of median cracks similar to those described under a sharp indenter (one observes that for scribing a lateral movement is to be added. The applied vertical force Fs while scribing has to be controlled carefully to introduce median cracks while avoiding the generation of lateral ones that yield chips and degrade the glazing edge quality. Subsequently, the introduced median cracks are driven through the glass thickness applying tensile stresses on the indented side by bending the glazing. Scribing of good quality is achieved using sharp rollers to mark the glass and cutting oil to protect the mark from the atmosphere and so from ageing.

Cutting oil is applied on the glass surface with a pad in front of the cutting head. The plate is then bent setting the crack tip under tensile stresses to force its operations are carried out automatically on float lines.

The success of this procedure depends on the control of crack generation and flexure stress, and also on the amplitude of residual stresses that may appear during the cooling of the glass. Because of the development of thin glass and fibres, laser cutting has been developed. Thermal shock induced by the laser allows for crack formation and further development. For very thin glass (also fibres) laser-induced melting permits the direct separation (cutting) of the two pieces. One advantage of laser cutting is that in principle operations like grinding can be avoided.

Glass cutting principle (scribing, flexuring).

Applications of glass

Glass is used for a very large range of applications.The most well-known applications are glazing, container sand glass wool which correspond to the largest industrial volumes. There are many more applications, some of them appearing just recently: glass containers for long-term conservation of industrial wastes,hard disk drives, displays, glass ceramics for cooking tops, amorphous semiconductors for photocopiers and metallic glasses for golf clubs and cutting tools, metallic glasses allow for large elastic energy storages. The applications are driven by one or several properties that make the use of glass attractive. The different glasses used in their relevant fields. Obviously many applications concern optical properties. Glass fibres are employed to fabricate and strengthen composites, while borosilicate glass chemical inertness has proved to be very attractive for medical purposes.

Glazing

Soda-lime-silica glazing is the most well-known application and is attributed to economical reasons. Glazing can be produced as flat glass with excellent surface quality, using the float process developed by Pilkington in the1960s. This good surface quality encompasses improved optical and mechanical performance. For safety reasons, in particular in buildings and transportation, glazing are either tempered (strengthened) or laminated, that is, formed by two substrates bonded by a polyvinyl butyral (PVB).Tempered glazing offer more mechanical resistance and eventually break into small harmless pieces. For very high strength levels required for airplanes and trains, chemically tempered glazings are preferred. Laminated glazing are preferred for roofs since, once breakage happens, roofs remain in one single piece thanks to the PVB bond. Optical transparency may be tailored by sanding, etching or rolling the glass. This is very popular in interior architecture. Insulated glasses allow for reducing heat transfer which occurs by radiation, conduction and convection. Through the use of insulated glass units with a low- conductivity gas fill between two or three panes of glass, conductive and convective heat loss can be significantly reduced. Radioactive heat loss requires tinted or coated glazing. In fact, while visual transparency is in most cases required, tailored properties have been developed in infrared (IR) and ultraviolet (UV) domains to reduce heat transfer. For vehicles, glazings with complex shapes are now being fabricated. In fact, glazing is an integral part of the design of new cars. Moreover, glass roofs have become more and more attractive. With increasing glazing surface area, anti solar coatings are required. Such coatings are detected on the wind- screen since they reflect blue (Renault vehicles) or pink (Pontiac vehicles) colours. As mentioned for building applications, IR insulation is now necessary to prevent extreme temperatures in vehicles during summer and at the same time to reduce gas consumption resulting from air-conditioning that equips most vehicles having large glazing areas.