Each glass bottle must be annealed. The function of annealing process is to eliminate or reduce the heat stress of the glass bottle to the allowable value. After annealing, the glass bottle is not fragile under extremely cold and hot conditions.
Glass bottle annealing is to eliminate and reduce the stress. How is the stress produced? In the process of production, glass bottle undergoes intense and uneven temperature change, which causes thermal stress of glass products. This thermal stress will reduce the strength and thermal stability of glass bottle, and even cause the self rupture of glass bottle.
1、 Stress of glass bottle
The stress in glass is generally divided into three categories, namely thermal stress, mechanical stress and structural stress.
1. Thermal stress: in glass products, the stress caused by temperature difference is called thermal stress, which can be divided into temporary stress and permanent stress according to its special existence.
(1) Temporary stress: the thermal stress produced when glass undergoes uneven temperature change below the strain point. It exists with the existence of temperature gradient. The stress disappeared with the disappearance of temperature gradient is called temporary stress.
(2) Permanent stress: the thermal stress generated when the glass undergoes uneven temperature change below the strain point, which exists with the existence of temperature gradient. When the stable gradient of the glass disappears, the residual thermal stress is called permanent stress
2. Mechanical stress: refers to the stress caused by the external force in the glass bottle. When the external force is removed, the mechanical stress disappears immediately.
3. Structural stress: the stress of glass caused by the uneven chemical composition is called structural stress, which belongs to permanent stress. The stress caused by the inherent structure of glass cannot be eliminated. The stone, stripe and poor homogenization in glass will lead to structural stress.
2、 Annealing of glass bottle
In order to eliminate the permanent stress in the glass, it is necessary to heat the glass to a certain temperature lower than the transition temperature of the glass for heat preservation and heat sharing, so as to relax the stress to eliminate the structural gradient caused by the temperature gradient of each part. This heat preservation and heat sharing temperature is called annealing stability. The annealing of glass products is generally divided into four stages.
(1) It is called heating stage to heat the glass bottle from furnace to annealing.
(2) It is called the heat preservation stage that the glass bottle is kept for a certain period of time after reaching the annealing stability, so as to make all parts of the glass bottle stable and uniform and to eliminate the stress as much as possible
(3) From annealing stable slow cooling to minimum annealing stable, it is called slow heating stage
(4) From the lowest annealing stable cooling stage to the fast cooling stage at room temperature.
The glass bottle can pass the standard packaging container only after the annealing process, so annealing is the essential process to produce the glass bottle.