Which could affect the flatness of tempered glass and what is the solutions?

If the flatness of tempered glass is not good, it will cause reflection optical deformation of the glass, and on the other hand, it will also produce optical distortion. When used in laminated glass, it can also cause optical deformation of the laminated glass, further affecting the visual effect of the product. The local thinning of laminated glass can also affect its bonding performance; If the flatness of tempered glass is not good, when applied to insulating glass, uneven butyl adhesive will be caused during extrusion and lamination, affecting the appearance quality of the glass, sealing quality, and may cause local over thickness, further affecting installation.

Therefore, guarantee the good flatness of tempered glass is very important.

1. Analysis of the reasons of tempered glass bending:

The deformation of tempered glass mainly comes from two aspects, first is the accuracy of the conveyor roller, and second is the uneven stress.

1.1. Poor flatness of tempered glass caused by problems with the conveyor rollers.

1.1.1. Deformation of ceramic rollers

Generally, the transportation roller inside heating oven of glass tempering furnace is the fused silica ceramic roller, which has good thermal shock resistance and thermal stability. However, due to the uneven internal structure, it may cause thermal deformation during heating, especially at high temperature condition. The thermal deformation of ceramic rollers inevitably causes the bending of the conveyor rollers and also inevitably causes deformation of the glass which moving on the ceramic rollers.

1.1.2. Wear of ceramic rollers

After prolonged use and repeated cleaning, ceramic rollers will inevitably experience a certain degree of wear and tear. Especially when cleaning the firmly bonded impurities on the ceramic rollers, it is usually necessary to use polishing methods for cleaning. Long time polishing and cleaning can lead to uneven wear of the ceramic rollers. This will result in uneven thickness or eccentricity on the same roller conveyor. On the other hand, if new and old ceramic rollers are used simultaneously, it will also cause uneven thickness on the roller conveyor, all of which will lead to uneven level of ceramic rollers. When glass swings on ceramic rollers with varying heights, softened glass will definitely deform, resulting in significant deformation after tempering.

1.1.3. Deformation of conveyor rollers of the cooling section

After the glass is transferred from the heating furnace to the cooling section, it is still in a softened state. If the level of the conveyor rollers in cooling section is not consistent at this time, it will inevitably affect the flatness of the glass. There are two main reasons for the deformation of the cooling section roller conveyor: one is the bending of the transmission roller, and the other is the wear of the aramid kevlar rope on the conveyor rollers.

1.2. Thermal deformation caused by uneven heating temperature

1.2.1. There is a temperature difference between the upper and lower surfaces of the glass during heating. When the glass is transported to the heating section and swing on ceramic rollers, the lower surface of the glass contacts the ceramic roller, and the ceramic roller directly conducts heat exchange with the glass through conduction, but at this time, the upper surface of the glass is heated by radiation. At this point, the heat transfer rate on the lower surface of the glass is much higher than that on the upper surface. If we do not adopt thermal balance for auxiliary heating, the temperature on the upper surface of the glass will be lower than that on the lower surface. At the beginning of heating, glass is a typical elastic body, with a relatively high coefficient of thermal expansion (coefficient of expansion of 9x10/° C). Due to the lower surface temperature being higher than the upper surface temperature, the expansion speed of the lower surface is higher than that of the upper surface, causing the glass to bend upwards. This causes the glass to warp away from the ceramic rollers, resulting in excessive heating in the middle and only the middle being supported by the ceramic rollers. When the glass is further heated, The position in contact with the ceramic roller first reaches the softening temperature and bears the full weight of the glass. The middle part of the glass will undergo "flow" deformation, and the thinning of the middle part will lead to roller imprints and even optical deformation.

On the other hand, when all the glass is heated to the softening temperature, the glass flattens, but the temperature difference does not disappear. When this type of glass with temperature difference is uniformly cooled to room temperature, the contraction of the hot side of the glass is greater than that of the cold side, forming a bending towards the hot side.

1.2.2. Temperature difference between the middle and edges causing glass deformation

When glass is heated in a heating furnace, if the temperature in the middle of the glass is higher than the temperature throughout, the shrinkage rate of the middle part of the glass will be greater than that of the edges during the cooling process. When the glass is cooled to room temperature and the temperature difference disappears, the size of the edges of the glass will be greater than that of the middle part. A large compressive stress is formed at the edges of the glass, and in order to balance the uneven stress of the philosophy, the glass will take on a saddle shape.

Similarly, when a piece of glass is heated, if the temperature at the edges is higher than the middle temperature, the shrinkage of the hotter glass edges is greater than that of the colder middle parts during the cooling process. When the glass is cooled to room temperature and the temperature difference disappears, the middle size of the glass will be larger than the edge size, forming a large tensile stress state at the edges of the glass. In order to balance this uneven stress, the glass will take on a pot bottom shape, And this state is bidirectional, that is, the protrusion in the middle of the glass will change in two directions.

1.2.3. Uneven random temperature distribution

The uneven temperature distribution of the above two types is mainly caused by improper operation. However, uneven random temperature distribution may be caused by poor equipment condition. The heating wire of the glass tempering furnace is locally damaged, the position of the temperature sensor (i.e. thermocouple) is changed or distorted, and the glass is placed improperly on the roller conveyor, all of which can cause uneven heating of the glass. This random temperature distribution is uneven, which can lead to uneven temperature distribution in the glass plane direction during heating. During cooling shrinkage, different areas of the glass experience irregular and irregular shrinkage, resulting in poor glass flatness.

1.3. Thermal deformation caused by uneven cooling

As is well known, after heating to the softening point, glass is rapidly cooled, generating sufficient compressive stress on the surface of the glass, and the process of producing tensile stress in equilibrium with compressive stress inside the glass is called the tempering process of glass. Tempered glass first releases compressive stress when subjected to external forces, and then generates tensile stress on the glass surface, making the strength of tempered glass higher than that of ordinary float glass.

Tempered glass is a type of glass that undergoes permanent stress and can also deform when the internal stress of the glass is unbalanced. When the compressive stress on the upper surface is greater than the tensile stress on the lower surface, in order to balance the stress, the glass will bend downwards; Similarly, when the compressive stress on the current surface is greater than that on the upper surface, in order to balance the stress, the glass will bend upwards. During the cooling process of glass, due to the different cooling rates on the upper and lower surfaces, surfaces with faster cooling rates generate greater stress than surfaces with slower cooling rates.

2. Solutions/Methods to ensure the flatness of tempered glass

2.1. The solution to the problem of ceramic roller deformation is:

Before glass tempering, allow sufficient time for the ceramic rollers to preheat, so that ceramic rollers could be uniformly heated, stress is eliminated, and the stress deformation of the ceramic rollers is reduced.

When cleaning ceramic rollers, pls polish the entire ceramic roller and avoid polishing only local areas as much as possible. This can make the ceramic roller wear evenly and maintain the consistency of the ceramic roller.

Regularly check and adjust the height of the ceramic rollers, and ensure that all roller tracks are in the same plane.

In cooling section, the conveyor rollers are usually wrapped with aramid kevlar rope outside the metal roller. If the glass breaks in the cooling section, the kevlar aramid rope is easily cut, so it is necessary to regularly check the aramid kevlar rope in the cooling section. If the aramid kevlar rope is worn or damaged, pls replace them in time.

2.2. The solution to thermal deformation caused by uneven heating temperature is:

2.2.1. If there is a temperature difference between the upper and lower surfaces, when the glass is bent upwards, it indicates that the temperature on the upper surface of the glass is high. At this time, the temperature on the upper surface should be reduced, and to maintain the overall heating temperature of the glass, the temperature on the lower surface should be appropriately increased;

When the glass is bent downwards, it indicates that the temperature on the lower surface of the glass is high. At this time, the temperature on the lower surface should be reduced, and to maintain the overall heating temperature of the glass, the temperature on the upper surface should be appropriately increased;

If the yield and quality of tempering are affected when adjusting the temperature, the heating time can be appropriately extended or reduced.

2.2.2. The solution for glass deformation caused by temperature differences between the middle and edges is:

If the glass shows a saddle shaped shape, it indicates poor insulation in the heating furnace or a low temperature setting at the edge. At this time, it is necessary to adjust the temperature setting at the edge; When the glass shows the shape of the bottom of the pot, if it is caused by historical reasons, the placement of the glass can be temporarily stopped, and the furnace can be run empty first to evenly distribute the temperature inside the heating furnace before placing the glass again. If it is a temperature setting issue, it is necessary to adjust the temperature setting parameters to make the temperature distribution inside the heating furnace uniform.

2.2.3. For random temperature non-uniformity, the solution is:

Timely maintenance of equipment to avoid poor condition. Regularly clean the glass slag near the thermocouple to ensure that the temperature measured by the thermocouple is the accurate furnace temperature. At the same time, don’t place many loads in the same way when loading.

2.3. For the problem of thermal deformation caused by uneven cooling, the solution is:

For the uneven stress generated during the cooling process, the cooling rate of the glass surface can be adjusted by adjusting the air pressure of the air grille. If the glass is bent downwards, the cooling rate of the lower surface of the glass can be improved by increasing the lower air pressure; If the glass is bent upwards, the cooling rate of the upper surface of the glass can be improved by increasing the upper air pressure.

In last, in order to improve the flatness quality, we suggest you:

1. Choose the glass tempering furnace with force convection system, for this glass tempering furnace, it is easy to adjust the heating speed for upper and lower surface of glass

2. Choose a little bigger big blower for the quenching.

With a bigger blower, it is more easy to adjust the air pressure of quenching. And now, for many glass, the thickness is minus. For example, if the glass thickness is 4mm, maybe its actual thickness is only 3.8mm. So with a little bigger blower, it could guarantee the same quality.