- Issue Time
Due to the tolerance of drastic temperature changes, the glass bottle produces a temperature gradient between the inner and outer layers during the molding process, and due to the differences in the shape, thickness, and degree of cooling of the finished product, irregular thermal stresses are generated in the product. Such 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 break on its own. Therefore, the uneven thermal stress in the glass bottle is a serious mistake, and annealing is a heat treatment process that can eliminate the thermal stress in the glass bottle as much as possible or reduce it to an allowable value. Except for glass fibers 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, when the glass is heated or cooled below the strain point temperature, a temperature gradient will be formed in each part, resulting in inevitable thermal stress. This kind of thermal stress, with the existence of the temperature difference, the greater the temperature difference, and the greater the stress temporarily, and disappear with the disappearance of the temperature difference. This thermal stress is called temporary stress.
But before the temperature is out of adjustment, what should be paid attention to, of course, the stress can be eliminated by itself. When the stress value exceeds the ultimate strength of the glass, the glass will rupture itself abnormally, so the heating or cooling speed of the glass in the brittle temperature range should not be too fast.
The thermal stress caused by the temperature difference, when the glass is cooled from above the strain point temperature, the glass is cooled to room temperature, and the temperature of the inner and outer layers is out of adjustment, but it cannot be completely dissipated. The magnitude of the permanent stress depends on the cooling rate of the finished product when it is above the strain point temperature, the viscosity of the glass, the thermal shortening factor and the thickness of the product and other glass machinery.
Annealing of glass is to reheat the glass product with permanent stress to the temperature at which the particles inside the glass can move, and the displacement of the particles is controlled to disperse the stress (called stress relaxation) to eliminate or weaken the permanent stress. The rate of stress relaxation depends on the temperature of the glass. The higher the temperature, the faster the relaxation rate. A suitable annealing temperature range is the key to obtaining precise annealing quality for glass. In the actual production process, it is impossible to completely eliminate permanent stress. Annealing can reduce or homogenize the residual stress to a minimum to achieve the enhancement of the glass to get purpose of mechanical strength and thermal stability.