Tempered ( Toughened ) Glass

Toughened glass (or tempered glass) is a type of safety glass that has increased strength and a tendency to shatter in small, square pieces when broken. It is typically used in unframed assemblies such as frameless doors and in structurally loaded applications. Using toughened glass could pose a security risk in some situations due to the tendency the glass has to shatter utterly upon edge impact.

Toughened glass is typically four to six times the strength of annealed glass. However, this strength comes with a penalty. Due to the balanced stresses in the glass, any damage to the glass edges will result in the glass shattering into thumbnail sized pieces. Because of this, the glass must be cut to size before toughening and cannot be re-worked once toughened. Polishing the edges or drilling holes in the glass is carried out before the toughening process starts. Also, ironically, the toughened glass surface is not as hard as annealed glass and is more susceptible to scratching.

Toughened glass is made from annealed glass via a thermal tempering process. The glass is placed onto a roller table, taking it through a furnace which heats it to above its annealing point of 600 C. The glass is then rapidly cooled with forced draughts of air. This rapidly cools the glass surface below its annealing point, causing it to harden and contract, while the inner portion of the glass remains free to flow for a short time. The final contraction of the inner layer induces compressive stresses in the surface of the glass balanced by tensile stresses in the body of the glass. This compressive stress on the surface of the glass is typically as high as 50 MPa.

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