Last update: 20.08.2019
Safety & security
1.

Fully tempered glass

This glass type can be manufactured from float glass or practically every known flat structured ornamental and cast glass.

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In this process, the basic glass is thermally treated (tempered), providing it with three outstanding characteristics. The tensile strength is four to five times greater than annealed glass of the same thickness, allowing it to handle much higher suction or blunt impact forces. Tempering also makes glass more resistant to severe, short-term fluctuations in hot and cold temperatures, as well as enhancing its capability to handle large temperature differences within the pane of glass itself. However, should failure occur due to overloading, the glass will fracture into a blunt-edged mass of loosely connected pieces that pose less of an injury risk than the sharp-edged shards produced by shattered annealed glass.

Tempered glass

 

Production

The only glass panes that reach the tempering unit are those cut from basic glass. These glasses are precisely measured, the edges have already been worked, and drilled holes and boundary cuts have already been made. These panes are heated to approx. 600 °C using controlled and uniform heating, then quenched using cold air to rapidly cool the surfaces, followed by further cooling with cold air back to room temperature.

Manufacturing process of tempered glass

 

This “quenching” or, in professional terms, blow off causes the glass surface to cool down faster than the centre of the glass, creating a durable tensile strength in the glass. The tensile stress of glass surfaces subject to compressive force increases relative to the core of the glass cross-section.

Thermal strengthening glass

 

This tension structure gives the glass its outstanding features and also explains why all machin- ing should be carried out on the glass in advance. If drilling, for example, is carried out on the glass after it has been tempered, the entire glass will shatter as drilling breaks up, or interrupts, the tension structure, causing the glass to break apart. The tension zones are visible under polarised light and can be viewed at certain angles as coloured, optical effects.

 

Tension dynamic - visible

 

Tension strength distribution

 

Building physical characteristics

Thermal conductivity, light and energy transmittance, thermal expansion, compressive strength and elastic modulus remain  identical to the base glass, as do the weight, sound insulation characteristics and the chemical properties. Other parameters, however, will vary considerably.

Resistance to impact and shock

Fully tempered glass is resistant to shock from soft, deformable objects such as the human body and conforms to EN 12600 (pendulum impact test for glass in buildings). The respective field of application determines the required glass thickness.

Tensile bending strength

Fully tempered glass can be made from various basic types of glass and is also frequently coated with ceramic colours. The tensile bending strength should therefore be classified according to the design:
•    Tempered glass made from float glass
σ = 120 MPa
•    Tempered glass made from patterned glass
σ = 90 Mpa
•    Tempered glass made from enamelled glass, whereby the enamelled side is under tensile stress
σ = 75 Mpa

 

 

Resistance to ball impact

At 6 mm thick, fully tempered glass is particularly suitable for use in large surface glass applications in gyms and sports halls, as is typical in countries such as Germany (in acc. with DIN 18 032 “Test of safety against throwing balls”).

 

Heat influence and thermo-shock resistance

Fully tempered glass is capable of resisting temperatures exceeding 300 °C for short periods of time, and temperatures exceeding 250 °C for extended periods of time. The resistance versus temperature differences within a glass pane, for example between the centre and the edge of the pane, is very high, at 200  Kelvin (K), compared to 40 K for float glass.

Anisotropies (strain pattern)

These are typical irisation formations on thermally tempered glass due to the internal stress distribution in each pane. Air quench nozzles discharge air in a fixed or reciprocating motion. Areas of air quench (stress differences) are visible typically under polarised light conditions as arrays of iridescent spots or lines. Under some lighting conditions, these patterns can be seen in ordinary light. Anisotropies are considered as visible effects, not as defects, according to EN 12150-1.


However, state-of-the-art tempering lines are capable of limiting the effect of anisotropies significantly by making adjustments in the oven and quench zone. Furthermore, measurement technologies are available today for monitoring surface strength and tendency of anisotropies.

Optical quality

Typically, tempered glass possesses the basic optical qualities of annealed glass.

Minor surface deviations can occur in tempered glass, as it oscillates on rollers during the heating phase. These minor surface deviations are referred to as “roller waves” and are, to some extent, unavoidable. However, modern tempering lines have the capability of minimising the effect. This oscillation on the rollers can, in exceptional cases, also cause dots known as „roller pickup“ which form on the glass surface and are visible under adverse lighting conditions.

The European standard EN 12150-1 for tempered glass describes specific requirements for optical distortions:

 

Overall bow: 

Overall bow

The induced stress conditions can produce a slight bow. 

Maximum allowable tolerances
≤ 3,0 mm/m (0,3 % of measured distance)

 

Roller waves

 


Roller waves: 

Glass that is passed horizontally through an oven may contain a very slight surface wave caused by contact with the rollers. The reason for this is the softening of the glass at the end of the heating time.

Maximum alloable tolerances

≤ 0,3 mm over a distance of 300 mm 

Roller waves glass
Moisture film on tempered glass

The wettability of the surface can differ due to pressure from rollers, suction cups, trowelling compounds or lubricants. During subsequent formation of a moisture film on the glass surface, this varying wettability within a glass surface is visible but does not indicate any deficiency.

Identification

Each piece of tempered glass must be clearly and permanently marked in accordance with EN 12150.

Identification of tempered glass