Thermal Stress in Glass Products

The stress generated by the temperature difference during the forming process of glass products is called thermal stress. According to the characteristics of thermal stress, thermal stress can be divided into temporary stress and permanent stress.

Temporary stress The so-called temporary stress is to heat the glass product below the strain point temperature. For soda-lime-silicate glass, this temperature is generally about 500 ° C, and then let the glass product cool rapidly, although the cooling of the glass product exists at this time. With a certain temperature difference, it is subjected to uneven temperature changes, but the thermal stress generated at this time exists with the existence of the temperature gradient and disappears with the disappearance of the temperature gradient. This thermal stress is generally called temporary. stress.

Although the temporary stress disappears with the disappearance of the temperature gradient of the glass product, its value must also be controlled. If the temporary stress exceeds the tensile strength limit of the glass product itself during the cooling process of the glass, the glass will also break. This principle can be used to cut tubes and hollow glass products by quenching. At present, in the cooling process of glass annealing in foreign countries, the trend is to let the glass products be cooled quickly at a lower temperature as much as possible. The purpose is to prevent the glass bottle products from bursting due to excessive temporary stress.

Because when the glass product is below the strain point temperature, the glass is basically in the elastic deformation temperature range and belongs to the brittle state. It has the characteristics of a solid object with elastic and brittle characteristics. The structure of the glass is basically fixed and the movement of ions is hindered. The structure and performance of the glass have little influence, so below this temperature, the glass product is cooled rapidly, although there is a temperature difference, it will not produce permanent stress, and the thermal stress produced is temporary stress. This is the reason why glass products can be allowed to cool quickly below the annealing lower limit temperature.