Tempered glass is a type of glass that is processed with thermal or chemical treatments, which increase its strength. The process of tempering leaves the outer surface of glass in a state of compression, and the interior in a state of tension. Due to these mechanical stresses, tempered glass breaks into granular pieces instead of shards with sharp edges, reducing the chance of injury. Tempered glass is also known as toughened glass or safety glass.
Due to its safety and strength, tempered glass is used in many industries and applications, which include construction projects. Some uses for tempered glass include vehicle windows, shower doors, refrigerator trays, architectural components, cell phone screens and cookware.
Tempered Glass Properties
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 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.
The main properties of tempered glass are summarized below:
- Versatility: Tempered glass can be shaped into different forms and styles to meet various applications.
- Impact resistance: The impact resistance of tempered glass is very high compared with float glass. This helps withstand extreme conditions and temperatures.
- Strength: Tempered glass can be 3 to 7 times stronger than annealed glass, making it suitable for more demanding applications with high environmental loads.
- Optical distortion: Tempered glass is not as clear as float glass, since the tempering process causes some optical distortions.
- Fabrication: Any cutting or grinding must be done before tempering, since these processes will cause fracture once the glass is tempered.
1. Selection of Raw Glass
The process begins with selecting high-quality float glass of the desired thickness and specifications. Our float glass manufactured according to the European standard EN 1096 : 2012, BS EN 572-2 : 2012. The glass must be free of defects such as bubbles, scratches, or impurities to ensure the best final product.
2. Cutting and Shaping
The selected glass sheets are cut into the required sizes and shapes using precision cutting tools. After cutting, edges are polished or beveled to remove sharpness and prevent breakage. Holes or notches, if needed, are drilled at this stage.
3. Washing and Cleaning
The cut glass is thoroughly washed with deionized water and special detergents to remove dust, grease, and other contaminants. Any remaining impurities can affect the tempering process.
4. Heating Process
The cleaned glass is placed inside a tempering furnace, where it is heated uniformly to a temperature of approximately 600–700°C (1112–1292°F). This temperature is close to the softening point of the glass, allowing internal stresses to be adjusted.
5. Rapid Cooling (Quenching)
Immediately after reaching the desired temperature, the glass is rapidly cooled using high-pressure air jets in a process called quenching. This sudden cooling creates compression on the outer surfaces and tension in the inner layers, significantly increasing the strength of the glass.
6. Quality Inspection
After tempering, the glass undergoes rigorous quality inspections, including:
Visual Inspection: Checking for defects such as optical distortions, cracks, or uneven surfaces.
Strength Testing: Ensuring the glass meets industry standards for impact and stress resistance.
Fragmentation Test: If broken, tempered glass should shatter into small, rounded pieces rather than sharp, dangerous shards.
7. Heat Soak Test:
The Heat Soak Test minimizes the risk of spontaneous breakage in tempered glass caused by nickel sulfide (NiS) inclusions. Tempered glass heated to 280°C ± 10°C (536°F ± 18°F) for 2–4 hours to trigger NiS expansion, causing defective glass to break.
8. Packing and Delivery
Once the tempered glass passes quality control, it is carefully packed to prevent damage during transportation. Protective films, wooden crates, or custom packaging materials are used to ensure safe delivery.