Investigations on sticking temperature and wear of mold materials and wear of coatings

For characterization and assessment of service behavior of mold materials and mold coatings for glass forming opterations, like e.g. glass container manufacturing, a testing procedure and corresponding laboratory apparatus was developed. The testing procedure resembles industrial working conditions, the mold material specimens are subjected to cyclic loading by pressing small gobs (soda-lime glass) from a melting crucible under defined non-isothermal conditions. A function of pressing parameters, especially mold temperature, molding pressure and service time of the mold specimens, the sticking behavior, surface structure of the molded glass objects and corrosion and wear of the mold surfaces are analyzed. The sturdiness of sticking may be described by a Gaussian distribution, and is quantified by recording the time interval necessary for separating the glass blank from the mold surface using a special separation procedure. The duration of sticking increases steeply with mold temperature within a narrow temperature interval of 20-50 K. The sticking characteristics of each tested bulk material and coating can be described by two material specific values: the lower and the upper sticking temperatures. The lower sticking temperatures exhibit a linear dependence on the thermal diffusivity of the mold materials, whereas the upper sticking temperature additionally depends on chemical and structural properties of the mold surfaces. Corrosion and wear of the mold materials in contact with hot glass is enhanced in this testing procedure due to conducting those pressing experiments well above the lower sticking temperature, which yields significant differences in corrosion behaviour in short time laboratory experiments.