tempered glass coverThe uneven flatness and thickness of laminated glass can cause both reflective optical deformation and optical distortion during use. When used for laminated glass, uneven thickness of laminated glass after lamination can result in optical deformation, further affecting the visual effect of the product. Partial thinning of laminated glass can also affect the bonding performance of the product; If the curvature of tempered glass is not good, when applied to insulating glass, uneven isobutylene glue will be formed during extrusion, which will affect the appearance quality and sealing quality, and may also result in some excessive thickness, further affecting installation. Therefore, controlling the flatness of tempered glass is very important.

Analysis of the reasons for the curvature of tempered glass caused by 1. Tempered glass cover manufacturers

The deformation of tempered glass mainly comes from the aspect of beauty; The first issue is the accuracy of the roller conveyor, followed by the problem of uneven stress.

Poor flatness of tempered glass caused by problems with the roller conveyor

1.1.1 Heating roller deformation

Generally, the roller table of the horizontal roller table tempering furnace is made of fused silica or ceramic material, which has good thermal shock resistance and thermal stability. However, sometimes the uneven internal structure may lead to thermal deformation during heating, especially at high temperatures. The thermal deformation of the roller conveyor will inevitably cause the roller conveyor to bend and also cause the glass moving on its surface to deform.

1.1.2 The roller conveyor is worn out

Long term use and repeated finishing of the roller conveyor will inevitably lead to wear and tear, especially when dealing with firmly adhered impurities on the roller conveyor. Grinding is usually used for finishing. Long term use and multiple finishing can cause uneven wear of the roller conveyor. On the one hand, uneven thickness or discomfort may occur on the same roller conveyor. On the other hand, roller conveyors may sometimes be used together without replacement. The use of new or old roller conveyors with uneven thickness or different degrees of wear together can also lead to uneven thickness of the roller conveyor. The occurrence of the above results will cause unevenness in the surface of the roller conveyor, and the glass will eventually be heated to the softening temperature and transmitted on the uneven roller conveyor. The softened glass will definitely deform, and this deformation of the glass will be retained until it is reinforced, which will cause deformation of the tempered glass in the future.

1.1.3 Deformation of the wind grille roller conveyor

After being heated in the furnace, the glass is quickly transferred to the wind grid roller conveyor. At this moment, the glass is still in a softened state. If the wind grid roller conveyor deforms, it will definitely affect the flatness of the glass. The simplest reason for the deformation of the wind grid roller conveyor is twofold: firstly, the transmission roller conveyor is tortuous, and secondly, the insulation material on the surface of the roller conveyor is incomplete.

1.2 Thermal deformation caused by uneven heating temperature

1.2.1 There is a temperature difference between the upper and lower surfaces of glass during heating. When the glass is transferred to the heating roller, the lower surface of the glass touches the roller, and the heating roller directly exchanges heat with the glass through conduction. The upper surface of the glass is heated by thermal radiation, and the heat transfer rate of the lower surface is higher than that of the upper surface. When thermal balance assisted heating is not adopted, the temperature of the upper surface of the glass is lower than that of the lower surface. At the beginning of heating, the glass is a typical elastic body, and its coefficient of thermal expansion is relatively high (the coefficient of linear expansion of glass is 9 × 10/C), Because the temperature of the lower surface is higher than that of the upper surface, the expansion speed of the lower surface is higher than that of the upper surface, causing the glass to bend upwards, that is, the glass periphery lifts off the roller conveyor, forming a heated center of the glass. As long as the center is supported by the roller conveyor, when the glass is continuously heated, the direction of contact with the roller conveyor first reaches the softening temperature and bears the full weight of the glass. The central part of the glass will undergo "flow" deformation, and the middle part will become thinner, resulting in roller conveyor marks or even optical deformation. On the other hand, when the glass is heated to the softening temperature, the glass will flatten, but the temperature difference will not disappear. When this type of glass with temperature difference is uniformly cooled to room temperature, the shortening of the hot surface of the glass is greater than that of the ice surface.

1.2.2 Glass deformation caused by temperature difference between the center and edges

When glass is heated in a heating furnace, if the temperature in the middle of the glass is higher than the temperature at the edges, the hot center of the glass will shorten more than the cold edges during the cooling process. When the glass is cooled to room temperature and the temperature difference disappears, the edge scale of the glass will be larger than the center scale, forming a larger compressive stress at the edge of the glass. To balance this uneven stress, the glass appears in a saddle shape. Similarly, when a piece of glass is heated in a heating furnace, if the temperature distribution along the final plane is higher at the edges than in the middle, during the cooling process, the edge of the hotter glass will shorten more than the center of the cooler glass. When the glass is cooled to room temperature and the temperature difference disappears, the central scale of the glass will be larger than the edge scale, forming a larger tensile stress condition at the edge of the glass. To balance this uneven stress, the glass will eventually form a pot shape and this condition is bidirectional, that is, the protrusion at the center of the glass will change in two directions.

I. 2.3 Uneven Random Temperature Spread

The above two types of uneven temperature distribution are mainly caused by improper operation. Random uneven temperature distribution may be caused by poor equipment condition. Partial damage to the heating wire of the tempering furnace, changes or distortions in the orientation of the temperature sensor, and unreasonable stacking of glass on the roller conveyor can all cause uneven heating of the glass. This random uneven temperature distribution will lead to uneven temperature distribution in the plane direction of the glass during heating, and irregular and irregular shortening in different areas of the glass during cooling will result in poor flatness of the glass.

1.3 Thermal deformation caused by uneven cooling

As is well known, after heating to the softening point, glass is rapidly cooled, and a satisfying compressive stress occurs on the surface of the glass, while a tensile stress that balances the compressive stress occurs inside the glass. This process is called glass tempering. Tempered glass first releases compressive stress when subjected to external forces, and then tensile stress occurs on the surface of the glass, making the strength of tempered glass higher than that of ordinary glass. Tempered glass is a type of glass with permanent stress, and it can also undergo deformation when there is an imbalance of stress inside the glass. When the compressive stress on the upper surface is greater than that on the lower surface, it is called equilibrium stress, and the glass bends downwards. Similarly, when the compressive stress on the lower surface is greater than that on the upper surface, it is called equilibrium stress. The glass bends upwards, and during the cooling process, due to the uneven cooling rate of the upper and lower surfaces, the surface with faster cooling rate will experience greater stress than the surface with slower cooling rate.

2 Strategies for Controlling the Flatness of Tempered Glass

2.1 Countermeasures for deformation of heating roller conveyor: Preheat the roller conveyor at a sufficient time before glass tempering to ensure uniform heating, eliminate stress, and reduce stress deformation of the roller conveyor. In response to roller wear, it is necessary to polish the entire roller when organizing it, and try to avoid polishing parts of the roller as much as possible, so that the roller wear is uniform and the consistency of the roller is maintained. And often check and adjust the height of the roller conveyor to make the surface of the roller conveyor flat during operation, in order to maintain the flatness of the glass surface. The structure of the wind grille roller conveyor is to wrap nylon rope around the metal roller conveyor as insulation material. When the glass explodes inside the wind grille, the nylon rope is simply cut off, resulting in uneven surface of the roller conveyor. It is necessary to promptly inspect and replace the nylon rope.

2.2 Control measures for thermal deformation caused by uneven heating temperature

(1) Regarding the temperature difference between the upper and lower surfaces, when the glass twists upwards, it indicates that the temperature on the upper surface of the glass is high. The upper surface temperature should be lowered to maintain the heating temperature of the entire glass, and the lower surface temperature should be appropriately increased; When the glass bends downwards, it indicates that the lower surface temperature is higher than the upper surface temperature, and it is necessary to appropriately lower the lower surface temperature and increase the upper surface temperature together. If the yield and quality of tempered steel are affected during temperature adjustment, the heating time should be appropriately extended or reduced.

(2) In response to glass deformation caused by temperature differences between the center and edges, if the glass has a saddle shaped shape, it indicates that the furnace insulation is poor or the temperature setting value margin is low, and the temperature setting value at the margin needs to be adjusted; When the glass appears in the shape of a pot bottom, if it is due to thermal history

The reason for this is that glass should not be stacked temporarily. The furnace should be operated empty first to evenly distribute the temperature inside, and then the glass can be placed on top; If there is a temperature setting issue, the temperature setting parameters need to be adjusted to ensure even temperature distribution inside the furnace.

(3) In response to random temperature unevenness, it is necessary to promptly inspect the equipment to prevent poor condition. Glass shards near the temperature sensor should be regularly sorted out to ensure that the temperature sensed by the temperature sensor is accurate furnace temperature. Putting them together on the upper plate is to prevent multiple furnaces from being stacked in the same condition.

2.3 Tempered glass cover manufacturers can adjust the cooling rate of the glass surface by adjusting the wind pressure of the grille to control thermal deformation caused by uneven cooling. If the glass bends downwards, it can be adjusted by increasing the cooling rate of the outer surface of the glass through the wind pressure at the bottom of the reinforced grille; If the glass twists upwards, it needs to be adjusted by adding wind pressure from the upper part of the grille to increase the cooling rate of the surface of the glass.

Article source: Tempered glass coverwww.yinghuade.com