Glass strengthening through heat treatment
One of the most common ways glass is processed for safety and structural enhancement is through heat treatment. This involves reheating fully tempered glass to just below its softening point then rapidly cooling it using air or water. The rapid cooling process creates compression stresses on the surface and balancing tension stresses within.
Chemical strengthening
Another glass strengthening method is chemical strengthening. This involves submerging cut Heat Treated Glass in a molten salt bath of potassium or sodium ions at high temperatures. The larger ions diffuse into the outer surface regions of the glass. When removed from the bath, the ions cause the glass outer surface to be compressed and the interior stretched. This results in a glass 4-5 times stronger than standard glass against impact.
Thermal tempering
Thermal tempering is the heat treatment method used for the majority of strengthened glass. It uses precisely controlled heating and cooling to produce surface compressive stresses and internal tensile stresses. Roller-heated glass ribbon or individual glass sheets/covers are heated to temperatures above 800°C (1470°F). The glass is then rapidly cooled using either air or water jets to around 320°C to cause the desired stress pattern.
The rapid cooling rate of 55°C/second or faster creates the balanced compression-tension temper. This allows the glass to be 4-5 times stronger than untreated glass due to its ability to withstand forces by the compressed outer surfaces counteracting any applied stresses. Thermally tempered glass breaks into small rounded pieces instead of large jagged shards for enhanced safety.
Contact heat strengthening
Contact strengthening is a less common variation of thermal tempering that uses direct contact heating. Roller-heated glass passes between steel plates that transmit heat rapidly and uniformly. The glass is then air cooled on the opposite side to achieve toughening with slightly increased tensile stresses over regular tempering. This process is useful for heat-sensitive glass or glass with coatings that may be degraded by high air temperatures.
Surface compression measurement
The level of surface compression achieved through heat treatment is critical to the safety qualities of tempered glass. Optical stress measurement tools evaluate ring patterns under cross-polarized light to quantify compression levels. A minimum surface compression of 70-100 MPa typically meets safety standards for various applications like automobile glazing, furniture glass, shower doors and more.
Optical distortion reduction
While heat strengthening enhances glass durability, it often introduces some degree of optical distortion visible as wave-like patterns or rainbow effects when viewing an object through the glass. Glass producers continually refine tempering techniques and glass chemistries to minimize these visual aberrations. Low-iron glass compositions for example demonstrate reduced optical distortion compared to standard soda-lime silicate glass after thermal processing.
Additional processing capabilities
Glass fabricators can offer additional enhanced capabilities beyond basic tempering. Low-E coatings for instance may be applied prior to heat treatment and retain their performance qualities to provide solar control, improved insulation values and aesthetics. Laminating glass with safety interlayers like PVB films is also possible, combining the strength and breakage characteristics of tempered glass with protection against shards in the event of failure.
Patterned and specialty glass
Glass manufacturers have expanded heat strengthening applications beyond flat glass. Innovations allow curved, bent and customized patterned glass shapes to undergo thermal processing uniformly. Complex glass geometries for shower enclosures, automotive backlites, privacy hardware and decorative building façades can now be safely heat strengthened. Emerging technologies continue working to expand possibilities for glass protective treatments.
heat treatment strengthens glass structurally by carefully controlled heating and cooling processes that induce surface compression stresses balancing internal tension stresses. This reinforcement makes toughened glass 4-5 times stronger than standard glass. Various heat treatment approaches are used depending on glass type and desired result. Controlled optical distortion reduction and integration of additional manufacturing capabilities have broadened the viable uses of heat treated strengthened glass.
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