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Tempered Glass

Table of Contents

What are the main characteristics of tempered glass?

The main characteristics of tempered glass are high mechanical strength, particularly high resistance to bending and impact, and good thermal stability. Its resistance to bending is three times that of ordinary glass, and its deflection is 3 to 4 times greater than that of regular glass. Its impact resistance is five times higher than that of ordinary glass. Tempered glass also has good heat resistance.

What is the process flow of the horizontal tempering method?

The horizontal tempering method is currently the most widely used glass tempering method in the world. Glass is conveyed horizontally through a radiation conveyor to the heating furnace and cooling device for heating and blowing tempering. The main process is as follows:

Microcomputer operation: Set the process parameters such as heating time, top temperature, bottom temperature, unloading temperature, and pressurization time according to the type and thickness of the tempered glass required.

Loading: Load the glass onto the loading table according to the specifications and paste or print the tempering mark according to the product requirements.

Heating: The glass enters the heating chamber from the loading table and is heated on a reciprocating ceramic roller until it reaches the suitable tempering temperature.

Quenching: After heating, the glass enters the air blower and undergoes blowing quenching and tempering on a reciprocating fiber roller.

Unloading: After tempering, the glass is cooled to the set unloading temperature (usually around 40°C) and can be unloaded from the unloading table.

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What are the characteristics of the horizontal tempering method for glass production?

Compared with other tempering methods, the horizontal tempering method has the following characteristics:

High production efficiency, good product quality, and high economic benefits.

Wide processing range, suitable for tempering various types of glass with different thicknesses and sizes, including clear glass, tinted glass, patterned glass, colored glass, and LOW-E glass.

Convenient operation and easy loading and unloading. The heating, quenching, and mechanical conveying are all automated, and the entire system can be controlled by one person and a central control computer.

The flatness of tempered glass produced by the horizontal tempering method is greatly improved compared with that produced by the traditional vertical hanging method. The deformation of the clamp is eliminated, and when the glass moves on the quartz radiation conveyor, it is like being placed on a mold, which limits the deformation of the softened glass.

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What is stress pattern in tempered glass?

After the tempered glass is produced, due to the uneven heating and cooling during the tempering process, there will be different stress distributions on the glass surface. By placing the tempered glass under polarized light, one can observe the color and brightness changes in different areas on the glass surface, which is called stress pattern in tempered glass. Polarized light in sunlight has a certain composition, and its intensity is affected by the weather and the angle of sunlight incidence. By observing tempered glass through polarized sunglasses or at a large angle perpendicular to the glass surface, the stress pattern in tempered glass will be more visible.

What are the reasons for the formation of stress spots in tempered glass?

There are several main reasons why stress spots occur after glass tempering.

(1) Poor overall uniformity of furnace temperature: The overall uniformity of furnace temperature directly affects stress spots. If the temperature is hotter at the top and cooler at the bottom, it can cause the glass to bend. If the temperature is higher in the front and lower in the back, it may cause wave patterns or glass to burst. Poor overall uniformity of furnace temperature will inevitably lead to stress spots in the glass.

(2) Local uniformity: The position, size, angle, height, depth, and chamfer of the air vents all affect stress spots. For example, blocking an air vent will cause an air vent pattern in that position of the glass. When drilling holes, different sizes can also cause stress spots. In addition, the smoothness of the inner wall of the hole is directly related to the blowing speed. The difference between smooth and rough is significant. If the hole wall is rough, the blown air will be significantly blocked, causing a large impact. Moreover, the angle and size of the chamfer also have an impact.

(3) Poor stability of furnace temperature: The instability of furnace temperature (sudden highs and lows) also affects stress spots. High temperature can cause wave patterns, while low temperature can cause glass to burst. Therefore, the stability of temperature is crucial.

(4) Poor overall uniformity of blowing: The overall uniformity of blowing affects flatness and stress spots. Uneven blowing is similar to uneven furnace temperature. If the blowing is not uniform, the glass will sometimes curl up and sometimes curl down, making it impossible to adjust. At the same time, the unevenness of blowing will inevitably result in stress spots. Therefore, the overall uniformity of blowing from top, bottom, left and right is the most basic and important factor, which has a significant impact on stress spots in the glass.

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How to reduce stress spots in tempered glass?

The following measures can effectively reduce stress spots during the tempering process of glass:

Maintain the overall uniformity of furnace temperature. The intelligent matrix heating method can be adopted to control the temperature well.
Maintain the local heat uniformity. A thick heat-resistant steel plate can be used as a radiation plate to solve local problems.
Maintain the stability of furnace temperature. The radiation plate and intelligent control method can be used. The purpose of placing the radiation plate is to stabilize the furnace temperature, and the control will be more stable.
Maintain the overall uniformity and symmetry of blowing air.

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What are the reasons for scratches on tempered glass and how to deal with them?

Overlapping handling of glass. Single-sheet handling should be adopted.
The conveying rollers or surfaces are not clean. The radiation channels should be cleaned.
The radiation channel surface is uneven or out of sync. Adjust the radiation channel to make it level or in sync.

What are the causes of scratches and pitting on tempered glass? How to deal with it?

Adhesion of foreign substances on rollers. When it is minor, use S02 gas. When it is severe, stop the furnace and clean the rollers.
Glass heating time is too long. The heating time should be shortened.
The temperature at the edge of the glass is too high, and defects are concentrated. The gap between the glass sheets should be reduced, and the sheets should be loaded staggered to make the loading rate of each furnace similar.
The pressure in the middle of the glass is too high, and there are more defects in the middle. Reduce the temperature difference between the top and bottom as much as possible to minimize the warping of the glass edge after entering the furnace.

What are the causes of rainbows on tempered glass? How to deal with it?

During the formation of float glass, SnO infiltrates into the tin-coated surface, and is oxidized into SnO2 during tempering, which causes the glass surface to produce fine wrinkles under pressure and interference colors due to light refraction, forming rainbows. It is recommended to choose high-quality original sheets, control the heating temperature at the lower limit, and use fine polishing powder for polishing.

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What are the reasons for the poor flatness of tempered glass?

The main reasons for the poor flatness of tempered glass are: deformation caused by heating rollers, severe wear of the conveyor belt, deformation of the air knife system, temperature difference between the upper and lower surfaces of the glass during heating, temperature difference between the center and edges of the glass causing deformation, and uneven random temperature distribution.

What is the impact of deformation of heating rollers on the flatness of tempered glass?

Usually, the conveyor belt of a horizontal roller hearth tempering furnace is made of fused silica or ceramic material, which has good heat resistance and thermal stability. However, the uneven internal structure of the conveyor belt may sometimes lead to thermal deformation, especially at high temperatures during heating. The thermal deformation of the conveyor belt will inevitably cause bending, resulting in deformation of the glass moving on its surface.

What is the impact of severe conveyor belt wear on the flatness of tempered glass?

After long-term use and repeated cleaning, the conveyor belt may become worn, especially when impurities adhere firmly to the surface of the belt, grinding is usually used for cleaning, resulting in uneven wear of the conveyor belt. On the one hand, uneven thickness or eccentricity may occur on the same conveyor belt. On the other hand, different conveyor belts may be replaced at different times, but used simultaneously, resulting in uneven thickness or wear, which will cause surface irregularities on the conveyor belt and reduce its accuracy. When glass is heated to its softening temperature and moved on such an uneven conveyor belt, deformation will inevitably occur and be retained after tempering.

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What is the impact of air knife conveyor belt deformation on the flatness of tempered glass?

After being heated in the furnace, the glass is quickly transferred to the air knife conveyor belt, and it is still in a softened state at this time. If the air knife conveyor belt deforms, it will inevitably affect the flatness of the glass. The most common causes of air knife conveyor belt deformation are: firstly, bending of the transmission belt; and secondly, damage to the insulation material on the surface of the belt.

What is the impact of temperature difference between the upper and lower surfaces during heating on the flatness of tempered glass?

When the glass is transferred to the heating roller, the lower surface of the glass contacts the roller directly and the heat exchange occurs through conduction, while the upper surface is heated through thermal radiation. The heat transfer rate of the lower surface is higher than that of the upper surface. Without auxiliary heating to achieve thermal balance, the upper surface temperature is lower than that of the lower surface. At the beginning of heating, the glass is typically an elastic body with a high coefficient of thermal expansion. As the lower surface temperature is higher than the upper surface temperature, the expansion rate of the lower surface is higher, causing the glass to bend upwards and the edges of the glass to lift off the roller, resulting in only the center of the glass being supported by the roller. As the glass continues to be heated, the part in contact with the roller reaches the softening temperature first and bears the entire weight of the glass, causing the center of the glass to undergo flow deformation, resulting in a thinner middle portion, which may lead to roller marks or even optical distortion. On the other hand, after the glass is fully heated to the softening temperature, it flattens out, but the temperature difference still exists. When the glass is cooled uniformly to room temperature, the shrinkage of the heated side is greater than that of the cold side, resulting in bending towards the heated side.

Similarly, when the upper surface temperature of the glass is higher than the lower surface temperature during heating, the center of the glass protrudes upwards, resulting in a curved tempered glass after quenching.

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What is the effect of temperature difference between the center and the edges on the flatness of tempered glass?

When glass is heated in the tempering furnace, if the temperature at the center of the glass is higher than at the edges, during the cooling process, the hot center of the glass will shrink more than the cooler edges. When the glass is finally cooled to room temperature and the temperature difference disappears, the dimensions of the edges of the glass will be larger than those of the center, creating large compressive stress at the edges of the glass. To balance this uneven stress, the glass will take on a saddle shape.

Similarly, if the glass is heated in the furnace and the temperature distribution along the surface is higher at the edges than at the center, during the cooling process, the hotter edges of the glass will shrink more than the cooler center. When the glass is finally cooled to room temperature and the temperature difference disappears, the dimensions of the center of the glass will be larger than those of the edges, creating large tensile stress at the edges of the glass. To balance this uneven stress, the glass will take on a pot shape, which is a two-way change where the protrusion at the center of the glass will change in both directions.

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What is the impact of uneven random temperature distribution on the flatness of tempered glass?

Uneven random temperature distribution may be caused by poor equipment condition. Local damage to the heating wire of the tempering furnace, changes or distortions in the position of temperature sensors, and unreasonable stacking of glass on the roller table can all result in uneven heating of the glass. This random temperature distribution will cause an uneven temperature distribution in the plane direction of the glass during heating. When the glass cools and contracts irregularly and randomly in different areas, it will result in poor flatness of the glass.

To achieve the desired flatness of tempered glass, effective control and adjustment must be carried out for the aforementioned factors to achieve optimal quality.

What are the indicators for measuring the quality of tempered glass?

The indicators for measuring the quality of tempered glass mainly include three aspects: size and appearance indicators, safety performance indicators, and general performance indicators.

Size and appearance indicators, including size and allowable deviation, thickness and allowable deviation, and appearance quality.

Safety performance indicators, including bending degree, fragment state, and hailstone bag impact performance.

General performance indicators, including surface stress and thermal shock resistance.

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