Tempered glass : a basic idea

 

You may have heard about tempered glass before, seen it as an advertised feature in gadget specifications or in the form of screen protectors for mobile devices. But what is tempered glass exactly? What makes it different from standard glass or plastic screen protectors? Most importantly, what will tempered glass offer you as a consumer and it is worth the money?

The main difference between standard glass and tempered glass – is in the way they are made. Tempered glass is created by heating and then quickly cooling the material – and thus comes with a different structure.

One of the main characteristics of tempered glass is that it breaks differently from standard glass. Whereas normally, glass would break into big uneven pieces, tempered glass shatters into small even pieces. This gets rid of the danger of sharp edges and makes it a much safer option should something happen – for this reason, tempered glass is often referred to as “safety glass”.

How a tempered glass is better than conventional glass?

Mechanical Strength of Tempered Glass = 4 x Mechanical strength of Annealed Glass

Tempered glass is both physically and thermally stronger than conventional glass. As previously mentioned, tempered glass is treated to create exterior and interior stress in the material. The compressive surface stress of tempered glass is balanced by the tensile stress in the body.

For glass to be classified as tempered, its surface must have a minimum compressive stress of 69 megapascals (10,000 psi). Then, for a tempered glass to be considered a safety glass, its surface compressive stress must exceed 100 megapascals (15,000psi). If tempered glass breaks, the increased surface stress will result in small circular pieces instead of sharp shards. This property makes tempered glass suitable for high pressure and explosion proof applications.

Tempered glass gets its strength from the compressive stress on its surface. On the other hand, annealed or conventional glass has almost no internal stress, making it very fragile - it forms microscopic surface cracks that break under tension. Tempered glass does not have these surface cracks, and this prevents propagation or expansion when breaking.

How is tempered glass made?

To prepare glass for the tempering process, it must first be cut to the desired size. (Strength reductions or product failure can occur if any fabrication operations, such as etching or edging, take place after heat treatment.) The glass is then examined for imperfections that could cause breakage at any step during tempering. An abrasive such as sand paper takes sharp edges off the glass, which is subsequently washed.

Next, the glass begins a heat treatment process in which it travels through a tempering oven, either in a batch or continuous feed. The oven heats the glass to a temperature of more than 600 degrees Celsius. (The industry standard is 620 degrees Celsius.) The glass then undergoes a high-pressure cooling procedure called "quenching." During this process, which lasts just seconds, high-pressure air blasts the surface of the glass from an array of nozzles in varying positions. Quenching cools the outer surfaces of the glass much more quickly than the center. As the center of the glass cools, it tries to pull back from the outer surfaces. As a result, the center remains in tension, and the outer surfaces go into compression, which gives tempered glass its strength.