Annealing vs. Tempering – Glass Strengthening
Annealing – During Initial Manufacturing
When glass undergoes the annealing process, it is slowly cooling the hot glass to relieve internal stress once it has been formed. Annealing glass makes it more durable. Glass which has not undergone the annealing process is susceptible to cracking or shattering when subjected to relatively small temperature changes or mechanical shock. It may in fact retain many of the thermal stresses caused by quenching (i.e. heat treating) and significantly decrease the overall strength of the glass.
During the manufacturing process the glass is heated until the temperature reaches the annealing point which is the stress relief point glass reaches during the cool down phase. At this point, the glass is too firm to distort or bend but remains soft enough for any built up stresses to relax. Soaking or holding the piece of glass at this temperature helps to even out the temperature throughout the piece of glass. The time required for this soak or holding action can vary depending on the mass and type of glass. Once this point has reached and the hold time has been accomplished, the annealed glass is slowly cooled through the strain point. Following this process, the glass can carefully be cooled until it reaches room temperature.
Tempered Glass – Strengthening a Finished Piece of Glass
When glass is tempered, it goes through a thermally controlled toughening process to increase its strength compared with normal or conventional glass. The tempering process puts the outer surfaces of the glass into compression and the inner surfaces of the glass into tension. This stress causes the glass when broken to crumble into small granular chunks vs. jagged shards preventing possible injury. Annealed glass which has not been heat tempered, if broken will not break into the “safety” chunks or dices, but will in fact break into jagged shards.
Tempered glass is considered to be safety glass and is used for applications such as car windshields, shower doors, glass tables and doors and more.
Abrisa Technologies is a recognized global supplier of high quality, fabricated glass components, optical thin film coatings, and custom glass solutions for a wide variety of industries. From our US based Abrisa Industrial Glass fabrication facility in Santa Paula, CA and our ZC&R Coatings for Optics division in Torrance, CA we serve diverse industries such as microelectronics and displays, semiconductor, military, automotive, aerospace, medical, biomedical and scientific R&D. We provide custom specialty flat glass and coating products for applications such as: flat panel display, touch and gesture recognition; visible to IR imaging and surveillance; entertainment, indoor and outdoor lighting; advanced instrumentation; and photonics.