Discussion on the formation control and application of white fog phenomenon in physically tempered glass.

1. Preface

Physically tempered glass transitions from the initial clamp-type vertical tempering to horizontal tempering, which effectively solves the problem of vertical tempering clamps. At the same time, the ceramic roller with high specific heat capacity is used to carry, transport, and heat the tempered glass, providing physical tempering for mass production. Glass provides effective protection. However, ceramic rollers can transport tempered glass, which must be due to the relative friction between the ceramic roller and the tempered glass surface, especially when different glass surfaces are heated, bending, local force is obvious, and the ceramic roller is rough, etc., the glass surface will appear. White fog called toughened glass or tempered glass white fog phenomenon in the industry, will affect the visual effect and aesthetics of glass.

2. The principle of tempered glass white fog phenomenon

During the tempering glass process, the glass sheet is driven horizontally on the conveyor roller table, as shown in Figure 1. The force of the transmission comes from the friction between the glass and the transmission roller table. The dust on the surface of the ceramic roller, the tiny particles on the lower surface of the glass, or the friction between the rough surface of the ceramic roller and the glass surface will cause white mist. The bending of the glass when it is heated unevenly will aggravate this phenomenon. Especially in the tempering furnace, the glass is deformed due to the heating of the upper and lower surfaces. The white fog is evident in other locations.

^{Figure 1 Schematic diagram of the transmission on the transmission system when the glass is tempered}

3. Influencing factors of tempered glass white fog phenomenon

3.1 Surface Quality of Ceramic Roller

The quartz ceramic roller table is a solid cylindrical roller, with steel shaft heads and rolling bearings at both ends and a drive wheel for transmission at one end. Since the glass is still in contact with the quartz ceramic roller table after being heated and softened at high temperatures, to ensure that the flatness and surface quality of the glass is not affected, the geometric size, shape, position tolerance, diameter deviation, straightness, radial runout and The coaxially of the center of rotation of the bearing has put forward very high requirements. Especially the surface roughness of the roller is an important technical index of the quartz ceramic roller table. It can be tested by a roughness tester, and the products at home and abroad control this index at Ra<2.0 um.

The rough and dusty ceramic rollers of the tempering furnace or the small particles on the lower surface of the glass are the root causes of white fog. Through observing the surface of ceramic rollers used in the LIJIANG Glass tempering glass furnace for more than 10 years and collecting industry data, it is found that the glaze layer on the outer surface of quartz ceramic rollers will fall off after 5 to 10 years of use.

The main factors affecting the quality of ceramic rollers are:

(1) The raw material of the roller is impure, contains impurities, and the surface is rough, which will affect the performance of the roller. In particular, the thermal performance of the roller is unstable, and many uneven spots are produced on the surface of the roller under high heat, which affects the surface quality of the tempered glass.

(2) Ceramic rollers with poor surface quality have low surface hardness, high apparent porosity, and a slightly poor finish. Observing with a magnifying glass, it is obvious that there are many small pits on the surface of the roller, which in turn affects the surface quality of tempered glass.

(3) The ceramic rollers used in float and deep processing will lubricate the roller table by injecting an appropriate amount of sulfur dioxide to reduce friction and improve the surface quality of the glass.

3.2 Flatness of glass in the heating furnace

When the glass is in the heating furnace, the upper and lower surfaces of the glass will show different bending states after absorbing heat. The uneven local heating of the glass will also make the glass show different bending states, and it is also affected by the glass specification and shape. Combining the above factors, heating will cause deformation and local stress on the glass in the heating furnace.

(1) The influence of the temperature difference between the upper and lower surfaces of the glass plate on the flatness of the glass in the heating furnace. Especially when there is an obvious temperature difference between the upper and lower surfaces of the glass: when the temperature of the lower surface of the glass in the heating furnace is high, the expansion of the lower surface of the glass will extend, forcing the four corners of the glass to bend upwards. Due to the effect of gravity, only The middle position of the glass is in contact with the ceramic roller, and the friction force at the contact position between the glass and the ceramic roller increases and white fog appears. When the temperature of the upper surface of the glass in the heating furnace is high, the expansion of the upper surface of the glass will extend, forcing the four corners of the glass to bend downward. The ceramic rollers are in contact with each other, the contact position is partially stressed and white fog appears, as shown in Figure 2.

^{Figure 2 The obvious tempered white fog phenomenon on the surface of tempered glass}

(2) The aspect ratio of the glass also has a great influence on the bending of the glass in the heating furnace. Through daily observations, when the upper and lower temperatures in the heating furnace are set at the same level, the square glass (with a size close to 1:1) whose plate surface reaches 60% of the heating area in the furnace will bend downwards, while the narrow strip glass (with an aspect ratio of 3: 1 or so) It is very easy to bend up. The small panel glass is subjected to strong heat from the ceramic roller at the bottom of the heating furnace, and it is easy to bend up in the first few furnaces of intermittent production. Mastering the glass size and the law of glass bending in the heating furnace, and recommending different parameters for different glasses, is one of the effective means to improve the deformation of glass in the heating furnace in advance and improve product quality.

(3) The uneven local heating of the glass leads to local deformation and stress, which leads to an increase in the contact friction between the glass and the ceramic roller, and obvious white fog appears at the contact position. Figure 3 is the temperature distribution of the same piece of glass under thermal infrared imaging after it comes out of the furnace.

^{Figure 3 The uniformity of the surface of the high-temperature infrared scanning glass after it leaves the heating furnace}

A relatively uniform temperature field will be formed in the heating furnace. However, there will still be local temperature unevenness, which makes it impossible to form an uniform temperature inside the glass. The main factors leading to uneven heating are:

(1) There are differences in the power of the heater. The temperature in the area with high heating power is high, and the temperature in the area with low heating power is low.

(2) The heater is faulty and cannot be heated normally.

(3) The temperature sensor is out of order or the calibration is inaccurate.

(4) The temperature setting is unreasonable. There is more heat dissipation at the edge of the furnace body, and the temperature setting should be slightly higher than that in the middle. The middle of the glass absorbs more heat than the edge, and the temperature in the middle is slightly higher than that of the edge. Failure to take these factors into account in the temperature setting will result in uneven heating.

(5) The stacking of glass is unreasonable. Since the roller table is a poor conductor of heat, if a certain area is used repeatedly, the temperature of this area will drop, while the temperature of the area where the glass is not placed is relatively high. When the next batch of glass is heated in this area The temperature is relatively high.

(6) The heating furnace is poorly sealed, and there is a lot of local heat dissipation. Strong convection will also cause local temperature unevenness in the heating furnace.

3.3 Drive Influencing Factors

Friction is divided into static friction, rolling friction, and sliding friction. Relatively speaking, the friction of rolling friction is much lower than that of sliding friction. The transmission of glass on ceramic rollers belongs to rolling friction: the contact between glass and a single ceramic roller belongs to rolling friction. However, in the actual design and use of tempering furnaces, to ensure the consistency of the transmission of the entire heating furnace, the same motor is often used through the belt, The active belt or steel belt drives all the ceramic rollers, and different transmission belts or steel belts will affect the synchronization of the transmission. Through experiments to compare the elasticity of the transmission belt, which can ease the impact and vibration loads, and compared under the same equipment conditions, the white fog effect and surface quality of the glass products produced by the belt transmission are better than those of the active belt and steel belt products.

3.4 Surface and thickness of glass

Relatively speaking, the larger the glass surface, the greater the thickness, and the greater the mass, the friction force will increase accordingly when the local force is applied, resulting in the white fog phenomenon.

4. Conclusion

Tempering white fog is caused by the friction between the glass and ceramic rollers during the tempering heating process. How to make the friction force uniform and minimize the impact on glass surface quality is the key point to improving the white fog state. Improving tempered white fog through equipment transmission improvement, equipment heating maintenance, and process parameter adjustment is the key for all equipment manufacturers and craftsmen. LIJIANG Glass will be committed to serving 1000+ tempered glass production and processing factories around the world, and contribute to the optimization and improvement of tempered glass processing equipment.

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