Thermal stresses
During the production and processing of glass, hazardous thermal stresses may be introduced. During the cooling of molten glass, in the range between the upper and lower annealing point, the transition from the plastic state to the brittle state takes place. At this stage, existing thermal stress must be eliminated through a carefully controlled annealing process. Once the lower annealing point is reached, the glass may be cooled more rapidly, without introducing any major new stress. Glass responds in a similar way when heated, e.g., through direct exposure to a Bunsen flame, to a temperature higher than the lower annealing point. Uncontrolled cooling may result in the "freezing in" of thermal stress which would considerably reduce resistance to breakage and mechanical stability. To eliminate inherent stress, glass must be heated up to a temperature between the upper and lower annealing point, be kept at this temperature for approx. 30 minutes and be cooled by observing the prescribed cooling rates.
Resistance to temperature changes
When glass is heated to a temperature below the lower annealing point, thermal expansion and the poor thermal conductivity result in tensile and compressive stress. If, due to improper heating or cooling rates, the permissible mechanical loads are exceeded, breakage occurs. Apart from the coefficient of expansion α, which varies with each kind of glass, the wall thickness, the geometry of the glass body, and any existing scratches must be taken into account. Therefore, it is difficult to state specific nu- merical values for thermal shock resistance. However, a comparison of the α values shows that DURAN® is much more resistant to thermal changes than, e.g., AR-Glas®. |
