Annealing of glass

The interaction force on a unit cross section inside a substance is called internal stress. The internal stress of glass can be divided into three categories according to the different causes of its generation.
(1) Thermal stress in glass The stress generated in glass due to temperature difference is called thermal stress. According to its existence characteristics, it is divided into temporary stress and permanent stress.
1 Temporary stress The thermal stress generated when the glass in the elastic deformation temperature range with a temperature lower than the strain point undergoes uneven temperature changes during heating or cooling is called temporary stress. This stress exists with the existence of temperature gradient and disappears with the disappearance of temperature gradient.
2 Permanent stress The thermal stress that remains in the glass after the temperature gradient between the inner and outer layers of the glass disappears is called permanent stress. The generation of permanent stress in glass is the result of stress relaxation within the strain temperature range. In order to reduce the generation of permanent stress, the appropriate annealing temperature and cooling rate should be selected according to the chemical composition of the glass and the thickness of the product so that the residual stress value is within the allowable range.
(2) Structural stress in glass The stress generated in the glass due to structural unevenness caused by uneven chemical composition is called structural stress. Structural stress is a permanent stress. For example, during the melting process of glass, due to poor melting homogenization, defects such as streaks and stones are generated. The chemical composition of these defects is different from that of the main glass, and their expansion coefficients are also different. After the temperature reaches room temperature, the adjacent parts with different expansion coefficients shrink differently, causing stress in the glass. This stress caused by the inherent structure of the glass cannot be eliminated by annealing.
(3) Mechanical stress of glass Mechanical stress refers to the stress caused by external force acting on the glass. It is a temporary stress. It disappears when the external force disappears.

Annealing of glass is a heat treatment process to reduce or eliminate the permanent stress generated in the glass during the molding or heat processing process and improve the performance of the glass.
According to the cause of the formation of stress in the glass, annealing of glass is essentially composed of two processes: reducing and eliminating stress; preventing the generation of new stress. Glass does not have a fixed melting point. It cools from high temperature and transforms from liquid to brittle solid material. This temperature range is called the transition temperature range. The upper limit temperature is the softening temperature and the lower limit temperature is the transition temperature. In the transition temperature range, the particles in the glass can still move, that is, at a certain temperature near the transition temperature, heat preservation and equalization can eliminate the thermal stress in the glass. Since the glass is a viscoelastic body at this time, although the stress can be relaxed, it will not change the appearance of the product.

(1) Annealing temperature and annealing temperature range of glass In order to eliminate the permanent stress in the glass, the glass must be heated to a certain temperature below the glass transition temperature Tg for heat preservation and equalization to eliminate the temperature gradient of each part of the glass and relax the stress. This heat preservation and equalization temperature is called the annealing temperature. The highest annealing temperature of the glass refers to the temperature at which 95% of the stress can be eliminated after 3 minutes, which is equivalent to the annealing point (n-1012Pa·s), also called the upper annealing temperature; the lowest annealing temperature refers to the temperature at which only 5% of the stress can be eliminated after 3 minutes. It is also called the lower annealing temperature. The temperature range from the highest annealing temperature to the lowest annealing temperature is called the annealing temperature range. The annealing temperature range is generally 50~150℃. The highest annealing temperature of bottle glass is 550~600o℃. In actual production, the annealing temperature generally used is 20~30℃ lower than the highest annealing temperature. The lowest annealing temperature is 50~150℃ lower than the highest annealing temperature. The annealing temperature of glass is related to its chemical composition. Any composition that can reduce the viscosity of glass can also reduce the annealing temperature.

(2) Glass annealing process The annealing process of glass products includes four stages: heating, heat preservation, slow cooling and fast cooling. According to the heating and cooling speeds, heat preservation temperature and time of each stage, a curve of the relationship between temperature and time can be drawn. Figure 2-35 is the annealing curve.
The first stage is the heating stage. The main task is to heat the product to the annealing temperature. When the glass product is formed and sent to the annealing furnace, due to the temperature drop of the product itself during the forming operation and transportation process, the temperature of the product is generally lower than the annealing temperature of the glass when entering the annealing furnace, especially for some thin-walled products. Therefore, when the product enters the annealing furnace, the product must be heated to the annealing temperature determined in advance.

When glass is heated, its surface layer is subjected to compressive stress and its inner layer is subjected to tensile stress. Since the compressive strength of glass is about 10 times its tensile strength, the heating speed can be correspondingly faster. However, the sum of the temporary stress generated by the temperature gradient and the inherent permanent stress during the heating process cannot be greater than its tensile strength limit, otherwise it will break. In actual production, factors such as the uniformity of the thickness of glass products, the size and shape of the products, and the uniformity of the temperature distribution in the annealing furnace will affect the heating and heating speed.
Annealing immediately after the product is formed is called primary annealing, and annealing after the product is cooled is called secondary annealing. The production of bottle and can glass products always adopts the primary annealing method of entering the annealing furnace immediately after forming. For some products with complex shapes, uneven wall thickness or bottle bottom thickness exceeding 8mm, it is strictly forbidden to use a one-time annealing kiln for secondary annealing. If re-annealing is required, a secondary annealing kiln must be selected for annealing, otherwise the glass products will burst. For example, surface decal products belong to secondary annealing, and the baking kiln is used to anneal the product for secondary annealing. For some products that need to be processed by drying, if the stress is too large after drying, secondary annealing is also required to eliminate the stress.
The second stage is the insulation stage, the main purpose of which is to eliminate the temperature gradient generated by rapid heating and eliminate the inherent internal stress in the product. Make the temperature difference between the surface temperature and the inner layer of the product disappear. In this stage, the annealing temperature must be determined first, followed by the insulation time. Generally, the annealing temperature is 20~30℃ lower than the upper limit of the annealing temperature. In addition to direct measurement, the temperature at which the viscosity is 1012Pa·s can also be calculated based on the glass composition. When the annealing temperature is determined, the insulation time can be calculated according to 70a2~120a2, or according to the allowable stress value.
Generally, for products with thick walls, the temperature time should be longer, so that the stress in the product can be fully relaxed, otherwise, a large internal stress will remain in the product. For thin-walled products, the insulation time can be appropriately shorter.
The third stage is the slow cooling stage of the product in the annealing furnace. After a certain period of heat preservation at the annealing temperature, the original stress of the product has been eliminated. In order to prevent the permanent stress from being generated after cooling, or to reduce it to the stress range required by the product, slow cooling after equalization is necessary to prevent the formation of permanent stress.
The fourth stage is the rapid cooling stage of glass. The starting temperature of rapid cooling must be lower than the strain point of the glass, because the structure of the glass is completely fixed below the strain point. Although the temperature gradient is generated at this time, no permanent stress will be generated. In the rapid cooling stage, only temporary stress can be generated. Under the premise of ensuring that the glass products are not broken due to temporary stress, they can be cooled as quickly as possible.
In actual production, a lower cooling rate is used. For general glass, 15%~20% of this value is taken, and for optical glass, less than 5%.
The total annealing time of glass products is the sum of heating, heat preservation, slow cooling and rapid cooling time. The annealing rate of each stage must be limited to the allowable stress value that the product can withstand. First, determine the most suitable annealing curve by calculation, and usually adjust it in production practice. For bottle glass! Annealing system is shown in Table 2-34. 

(3) Issues that should be noted when formulating the annealing system The annealing temperature of bottle glass should be set according to the product size, weight, glass composition, product kiln temperature and the structural characteristics of each annealing kiln. At the same time, the following points should also be considered.
① The influence of temperature difference in the annealing furnace Despite many technical measures, the distribution of temperature in the cross section of the annealing furnace is still uneven, which makes the temperature of the product uneven. Therefore, when formulating the annealing system, the insulation time should be appropriately extended, and the slow cooling rate should be lower than the cooling rate corresponding to the actual allowable permanent stress value, generally half of the allowable stress value is taken for calculation. The determination of heating rate and fast cooling rate should also consider the influence of annealing furnace temperature difference.
When the product does not need cold spraying, the spacing of the bottles in the annealing kiln chain should be as close as possible without affecting the heat cycle and wind heat cycle in the kiln. Generally, 15~20mm is appropriate. In addition, the height and shape of the bottle should also be considered. If the bottle is taller, the upper limit of the distance can be taken, and if the bottle is shorter, the lower limit can be taken. When the product needs cold spraying, the distance of the bottle should be based on the cold end spraying being able to spray evenly on the bottle body.
③ Annealing problems of products with thick walls and complex shapes The temperature difference between the inner and outer layers of thick-walled products is large. Therefore, within the annealing temperature range, the insulation time of thick-walled products should be extended accordingly so that the temperature of the inner and outer layers of the products can be consistent, but the cooling rate must also be slowed down accordingly, and the total annealing time should be extended. It should be noted that the extension of the insulation time of thick-walled products is not proportional to the thickness of the products. This is because the load is larger after the thickness increases. If the products are kept at a higher temperature for a long time, they are easy to deform. Products with complex shapes are prone to stress concentration. Therefore, they should use a relatively low insulation temperature like thick-walled products, and the insulation time should be appropriately extended. Both the heating and cooling rates should be slow.
④ Annealing problems of different types of products in the same furnace When products with the same chemical composition and different thicknesses are annealed in the same annealing furnace, the annealing temperature should be determined according to the product with the smallest wall thickness to avoid deformation of thin products. However, the insulation time should be appropriately extended, and the heating and cooling rates should be determined according to the product with the largest wall thickness to ensure that thick-walled products will not break due to thermal stress.
When products with different chemical compositions are annealed in the same annealing furnace, the glass product with the lowest annealing temperature should be selected as the insulation temperature. At the same time, the insulation time should be extended so that products with different annealing temperatures can achieve good annealing.
⑤ The influence of inherent stress of products When heating rapidly, in addition to calculating temporary stress according to temperature difference, the influence of inherent stress should also be estimated.