Читать книгу Encyclopedia of Glass Science, Technology, History, and Culture - Группа авторов - Страница 112

As already pointed out, in container‐glass manufacturing, the gob is first formed into the parison, and then the parison into the final container. The (de‐)formation of both the gob and parison depends on the actual viscosity of the glass. A low forming‐ or interface‐temperature leads to a high viscosity at the glass surface. Hence the glass surface starts to get “brittle.” If such a glass is then subjected to high deformation rates, as it happens not only upon pressing and blowing but also earlier in the process upon gob‐cutting, it can experience too high tensile or shear stresses. The critical tensile stress σ_c (in MPa) that a hot soda‐lime‐silica glass can sustain at a given temperature T may be estimated from an empirically derived correlation [8]:

(3)

Hot fracture occurs if the tensile stresses exceed this critical value. The maximum velocity v_max at which a glass container with a thickness d can be formed at viscosity η without experiencing hot fracture can be approximated by:

(4)

One thus concludes that at temperatures of about 1000°C, deformation velocities of ca. 500 m/s are, for instance, possible without hot fracture for a 2 cm‐thick soda‐lime‐silica glass layer. At 900°C, the maximum allowed velocity is already down to 100 m/s and is lower than 10 m/s at 800°C. Below 700°C the risk of defects caused by hot fracture becomes significant. Because usually such defects cannot be inverted (“healed”) in later forming steps, care must be taken to prevent them from appearing.