Differences in glass machinery

by:Enkong     2022-07-20

Due to the resistance to drastic temperature changes, the annealing of the basic rationale glass during the forming process. A temperature gradient is generated between the inner and outer layers, and irregular thermal stress is generated in the product due to differences in glass machinery such as the shape, thickness, and degree of cooling of the finished product. This thermal stress can reduce the mechanical strength and thermal stability of the product, and also affect the optical uniformity of the glass. If the stress exceeds the ultimate strength of the finished product, it will rupture by itself. Therefore, the existence of uneven thermal stress in glass products is a serious error and fallacy. Annealing is a heat treatment process that can eliminate or reduce the thermal stress in the glass to an allowable value as much as possible. In addition to fiberglass and thin-walled small hollow products, indeed all glass products must be annealed. Thermal stress in glass products can be divided into temporary stress and permanent stress according to its characteristics.

Due to its poor thermal conductivity, ① Temporary stress. When the glass is heated or cooled below the strain point temperature. Each part will form a temperature gradient, resulting in a certain thermal stress. This thermal stress, with the existence of the temperature difference, the greater the temperature difference, the greater the temporary stress, and disappears with the disappearance of the temperature difference. This thermal stress is called temporary stress.

But before the temperature is out of balance, it should be noted that the stress can be dispelled by itself. When the temporary stress value exceeds the ultimate strength of the glass, the glass will break by itself abnormally, so the heating or cooling speed of the glass in the brittle temperature range should not be too fast.

Thermal stress caused by temperature difference, ② permanent stress. When the glass begins to cool from above the strain point temperature. After the glass is cooled to room temperature and the temperature of the inner and outer layers is out of balance, it cannot be completely dissipated, and there is still a certain stress in the glass, which is called water stress. The size of the permanent stress depends on the glass machinery such as the cooling rate of the finished product above the strain point temperature, the viscosity of the glass, the thermal contraction coefficient and the thickness of the product.

The advantages and disadvantages of annealing thick glass that control the thickness of the edge are closely related to the thickness curve of the glass, and annealing improves the thickness curve. In particular, the thickness of the edge of the glass plate, the individual requires the thickness curve to ensure that the thickness of the edge at 25 mm is about 1 mm thinner than the average thickness. In the past production, I often failed to do this. The main reason is that the edge is too thin to reduce the load to reduce the tempering stress. Due to the increase in thickness, the heat penetration in the glass body will be delayed. If it is produced under high load Thick glass will increase the tempering stress in the glass body, which will bring difficulties to the subsequent cutting and the customer's re-cutting, and will also cause trouble for annealing, increase the explosion, and reduce the yield. Therefore, it is necessary to reduce the load to meet the appropriate tempering stress. Increase the gas spray gun in the A zone and increase the chilled water after the washing machine. As mentioned above, the advantages and disadvantages of edge stress directly affect the advantages and disadvantages of slitting, which requires glass in the annealing kiln. Areas A and B try to increase the edge temperature as much as possible, because a gap spray gun has been added at the entrance of the annealing kiln, so it is a good choice to add a pair of gas spray guns in the A area to prevent the tensile stress at the edge of the glass. In the same way, adding chilled water after the washing machine can also increase the weight and tensile stress of the edge, and achieve the purpose of improving the slitting.

It is to reheat the glass product with permanent stress to the temperature at which the particle inside the glass can move, and the annealing of the glass. The displacement of the controlled mass points dissipates the stress (called stress relaxation) to eliminate or weaken the permanent stress. The stress relaxation rate depends on the glass temperature, the higher the temperature, the faster the relaxation rate. Therefore, a suitable annealing temperature range is the key to obtaining excellent annealing quality of the glass. In the actual production process, it is impossible to completely eliminate the permanent stress. Strength and thermal stability characteristics of thick glass annealing The production methods of thick glass mainly include reverse method and baffle method. Here, it is important to discuss the annealing when baffle method produces thick glass. The main principle of thick glass produced by the baffle method is to 'thaw' the baffle area of ​​the glass liquid. The important characteristics of its annealing are as follows, 1. The thick glass produced by this method has a relatively cold edge and a thin edge, so the compressive stress on the edge is large. 2. The thicker the glass, the longer the heat stays inside the glass. 3. In order to meet the cutting requirements, the tempering stress must be reduced. 4. To prevent the edge from cooling too fast in the post-annealing zone, a high tentative tensile stress is generated at the edge, resulting in longitudinal crack loss.

The longer the heat is kept in the glass body, the thicker the glass is when the temperature of the A zone is properly lowered. Corresponding to its annealing time will be longer. Since the annealing length of the annealing kiln is fluid, it is possible to delay the annealing zone by properly reducing the temperature of the A zone, and finally achieve the purpose of reducing the glass tempering stress by adding a gas lance at the edge of the RET zone. In the production of thick glass, the largest loss is the longitudinal crack of the glass. If a longitudinal crack occurs, it will take as little as half an hour, and it may take a few hours for more time. Therefore, I have to take special precautions against the occurrence of similar situations in the production of thick glass. It may be possible to take measures such as improving the thickness curve to control the thickness of the edge; reducing the load; increasing the gas spray gun in the A area and increasing the chilled water after the washing machine; properly reducing the temperature of the A area; increasing the edge gas spray gun in the RET area to achieve the best yield and Best cut quality.

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