The glass-to metal interface during container forming processes
It is known that a newly formed glass container will only possess a very small fraction of its theoretical strength. This suggests that damage occurs on the surface of the glass melt during the forming process due to glass to mould contact and hot glass handling. It might be expected that any damage inflicted on the surface of a glass article during manufacture would heal at the elevated manufacturing temperatures used, however this does not appear to be true. Therefore, the actual mechanism by which glass strength is reduced during forming needs to be fully understood and the work presented in this thesis addresses this problem. Experiments, therefore, have been carried out here which simulate the formation of glass articles using an experimental pressing rig by systematically altering processing parameters such as the mould material, surface fmish of the mould, pressing temperature and atmosphere. Processing parameters that are used industrially for the formation of glass containers were generally reproduced wherever possible in order to investigate the glass-to-mould interaction. The interaction of both a cast iron mould material and carbon-carbon composite materials with a soda-lime-silica glass were examined using the techniques of scanning electron microscopy, x-ray photoelectron spectroscopy and atomic force microscopy in order to determine the type and extent of surface damage formed. The surfaces of the pressed glass samples made were found to contain defects of embedded particles and indented dimples. The embedded particles found were usually due to bulk material transfer from the plunger material used. The texture found on the surface of the pressed glass samples was found to be directly affected by the surface fmish of the plunger. Pressing glass samples using a cast iron plunger at an initial plunger temperature below 450°C resulted in a randomly rippled 'chilled' surface. As the initial temperature of the plunger was increased, the surface texture of the pressed glass became a closer replica of the plunger surface. The use of vacuum assistance to form the glass samples also resulted in the surface of the pressed glass becoming a closer replica of the original plunger surface, even at lower pressing temperatures. The surfaces of the cast iron and carbon-carbon composite plungers appeared to have been affected by the initial plunger temperatures used. As the initial pressing temperature was increased, the amount of oxidation for both material types increased. In the case of the carbon-carbon composite materials investigated, both the matrix and fibres were found to have broken down at the pressing temperatures used. X-ray photoelectron spectroscopy, of the pressed glass surfaces and the plunger materials indicated that sodium ions had migrated from the glass melt to the plunger surface during forming.