Toughened or tempered glass is a type of safety glass processed by controlled thermal or chemical treatments to increase its strength compared with normal glass. Tempering creates balanced internal stresses when broken which cause the glass, to crumble into small granular chunks instead of splintering into jagged shards. The granular chunks are less likely to cause injury.
Glass is a naturally fragile material. To boost its functional properties and enhance its operational safety, it undergoes the process of tempering.
Tempering or toughening is a process where the glass is heated at high temperatures to make it stronger and more resistant to breakage. This process creates a balance in the product’s internal stresses, so that when the glass is broken, it would crumble into tiny granular chunks instead of breaking into sharp, jagged pieces.
Because of its increased strength and safety, builders and architects utilize them in a multitude of demanding applications. This includes showers, vehicle windows, refrigerator trays, glass tables, diving masks, glassware, cookware, fireplace grates, bulletproof windows, architectural glass doors and virtually anywhere else that needs safe and strong glass.
Tempering is a term used in metallurgy, and defines the degree of hardness and strength imparted to a metal, as by quenching, heat treatment, or cold working. Glass is also 'hardened' the same way, i.e., heated to very high temperatures, then cooled quickly. (The melting point of glass is between 1400°C and 1600°C depending on its composition). This changes the structure of the glass. It has lower stress points and forms cube-like crystalline structures. Because of this, tempered glass shatters into cubes rather than into long, sharp shards so it is a lot safer to use.
After toughening, bit becomes highly resistant to heat and shock. Tempered glass is four to five times more resistant to breakage than annealed glass.
THE PROCESS OF TEMPERING GLASS
Tempered glass goes through a process similar to that of a tempered steel.
Stage 1: All toughened glass begins life as a float glass. Before it undergoes tempering, the glass is examined for imperfections. Bubbles, inclusions, and cracks may cause the float glass to break during toughening. So if any signs of such flaws are found, the glass can’t be tempered.
Stage 2: Prior to toughening, it must first be cut to the desired shape as it won’t be possible to cut or etch the finished product in its toughened state. Once cut, the edges are smoothed and any burrs produced during etching or cutting are removed.
Stage 3: To completely remove the grains of glass that were deposited during sanding, the float glass is thoroughly washed. This also ensures that dirt and any other tiny debris won’t interfere with the tempering.
Stage 4: In the tempering process, the surface of the float glass is heated at over 600 degrees Celsius as it travels through a furnace. Some manufacturers heat the glass above its annealing point of approximately 720 degrees Celsius.
Stage 5: The scorching glass is then rapidly cooled through quenching by a high-pressure blast of air for a period of three to 10 seconds at various angles. As it cools and begins to shrink, tensile stresses temporarily build-up in the interior zone of the glass while its surface consequently develops surface stresses. These compressive stresses eventually enhance the strength of the glass, making it tougher to break.
A properly tempered glass should be able to withstand pressures of a minimum of 10,000 pounds per square inch (psi) and can be expected to break at about 24,000 psi.
Beyond the added tensile strength and safety, tempered glass has a greater resistance to thermal shock and thermal stresses. Essentially, it can withstand constant exposure to temperatures as high as 243 degrees Celsius.
Toughened glass has hundreds of applications. Used in automobiles, toughened glass windshields and windows reduces the occurrence of deadly cuts and bleeding in case of accidents because the glass breaks into small, cube-like pieces. Because of its structural strength, tempered glass is widely used in architectural applications.
Toughened glass acquires a degree of strength for excess of the strength of normal glass sheet or plate glass, which if broken shatters into small and comparatively harmless pieces. It is claimed that the resistance to mechanical stock of toughened plate glass is 4 to 5 times more than that of ordinary plate glass. A toughened glass has better resistance to the vibration, mechanical shock and abrasion.
Process of Manufacture:
The glass plate is heated to a temperature above its softening point and then subjected to rapid cooling. The glass is suddenly chilled and in this process contracts towards the core. It stretches until it has solidified and is no longer able to contract further at this stage the core is still soft. It contracts against restrained exercise by the solidified upper layer of the glass. This compression is responsible for the strength of the glass sheet, which is limited to about 20,000 lb/sq.inch. Thus it is highly stressed and the resultant force is able to nullify the external impact.
The intensity of the stresses depends on the rate of cooling, co-efficient of expansion, thermal conductivity of the glass, its specific heat, elasticity, and certain other physical properties.
Toughening Process:
The raw plate glass sheet which is free from waviness, distortion etc., is cut to required size and shape and then all the edges are ground and polished as per end use of the product. This is called edge 4 grinding and polishing and is very important for toughening because it will lead to breakages during process. No glass sheet can be toughened without edge grinding and polishing.
Washing and Drying:
After the edge grinding and polishing the glass sheets are washed manually or by machine and then dried. The glass sheets are fed into the furnace (Electrically operated). The sheets are kept in the furnace above its softening point, which varies according to the composition of glass. After attaining required temperature the glass sheets are removed out of the furnace and placed in the air blowing quenching boxes for 20 to 25 seconds. After quenching glass sheet is toughened. For bend glass toughening, the glass sheets passes through a set of dies (as per shape) after furnace and then to the quenching boxes.
Testing:
After toughening all the sheet glasses are passed through the polariscope inspection.
Tempered glass is also termed as toughened glass as it is believed to be four times stronger than normal glass. Tempered glass, when broken results into harmless granular pieces. In toughened or tempered glass, activities such as drilling, cutting, sandblasting, and machining are not possible.
Increasing government initiatives on infrastructure facilities accompanied with widening application outlook in construction sector will favor tempered glass market demand. Improving standard of living along with rising consumer spending on interior furniture designing owing to disposable income and rapid urbanization in India, China and Brazil will stimulate industry growth.
Rise in tempered glass market demand owing to high strength, safety, anti-breakage, and heat resistance properties coupled with its wide usage in public buildings including phone booths, bus terminals, canopies gymnasiums and sports arenas should stimulate market demand. Rising government regulatory support to favor infrastructure development towards residential and commercial buildings should stimulate the market growth.
Global Tempered Glass Market was valued at $46 billion in 2016, and is projected to reach $65 billion by 2023, registering a CAGR of 5.0% from 2017 to 2023. Tempered glass also known as toughened glass, is known to be four times stronger than simple annealed glass. Tempered glass is produced by heating the silica mix up to 600°C and then rapidly cooling the molten silica. Tempered glass is widely used in automotive window panes, building windows, furniture, and interior activities of buildings.
Growth in use of tempered glass in automotive and construction industry drives the market. However, stringent government regulations in the automotive and construction industry restricts the market growth. Increase in construction activities in emerging economies such as India, Indonesia, Brazil, Argentina, and the Middle East offers growth opportunity for the tempered glass market.
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