Annealing glassware is the process where the glass is heated and kept for a defined period to relieve internal stresses. Careful cooling under controlled conditions is essential to ensure that no pressures are reintroduced by chilling/cooling.
The glass must be heated to a temperature near the low dry glass transition temperature for insulation and ambulate to clean the temperature gradient of each part of the glass and relax the stress. This selected thermal insulation temperature is called the annealing temperature of the glass.
The annealing temperature of most glass is 550±20℃. For example, the plate glass is 550~570℃, and the bottle glass is 550~600℃. The annealing temperature of the glass is related to its chemical composition, such as alkali metal oxide Na20, K20, and other Sio2, Al203, and CaO can increase the viscosity of glass, so with the increase of their content of the annealing temperature is increased.
The glass annealing system and the type of products, shape, size, allowable stress value, annealing furnace temperature distribution and other conditions related to the current annealing system have a variety of forms according to the annealing principle. The annealing process can divide into four stages: heating stage, heating stage, slow cooling stage, and fast cooling stage. According to the above four steps can be made a temperature-time curve, and this curve is called the annealing curve as follows:
1. Heating phase
Different varieties of glass have other annealing processes. Some glass forms directly into the annealing furnace called one-step annealing. However, some products develop cooling after heating annealing, called second-step annealing. Hence, the heating stage is not necessary for some products. In the heating process, the glass surface produces compressive stress. Therefore, the heating rate can be correspondingly higher. For example, the flat glass at 20℃ can enter the annealing furnace at 700℃. The heating rate can be as high as 300°C/rain. The heating rate of 20/ A2 ~30/ A2 (C/min) is generally used in producing optical glass products.
2. Average temperature heating phase
The glass products are heated to the annealing temperature for insulation and are carried out to clean up the stress in this stage. The first to determine the annealing temperature, followed by the holding time, is generally 20~30℃ lower than the annealing temperature as the annealing temperature.
3. Slow cooling phase
To make glass products that do not produce stress after cooling, or reduce to the required stress range of the product, slow cooling after heating is necessary to prevent too significant temperature difference.
4. The phase of the fast cooling
As mentioned above, only temporary stress is generated when the glass is cooled below the strain point. As long as it does not exceed the glass’s limited strength, the cooling rate can be accelerated to shorten the entire annealing process, reduce fuel consumption, and improve productivity.
Generally, a lower cooling rate is used when we produce the glass. It’s caused by the dry products more or less there are some shortcomings, so as not to make tensile stress on the interface between the defects and the central glass. 15%~20% of this value is used for the general technology glass. In addition, we will adjust the numeric value based on the specific situation in the production process.
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