| 摘要 |
PURPOSE:To reduce air bubbles to such size that they can be ignored and to improve the durability by adding operations, where a glass is heated at a temperature higher than the melting point and is flowed and the atmospheric gas pressure is raised and a core is cooled in this state, to a glass melt-fixing process. CONSTITUTION:In the figure, the temperature in the melt-fixing process is indicated by a solid line, and the pressure in a furnace is indicated by a dotted line. The pressure in the furnace is raised when the period in which a temperature TW of glass melt-fixing is held is terminated, and this pressure is held until the temperature falls to Tb. It is sufficient if Tb is lower than a strain point (a temperature corresponding to 10<14.5> poise viscocity), and about several-10 atmospheres are enough for this pressure, and the result is not changed even if a pressure higher than them is applied. Thus, not only air bubbles having 10-50mum diameter but also air bubbles 7 having 1-2mum diameter shown in the figure disappear and only about <=0.5 air bubbles having about 5mum maximum diameter appear because the size of generated air bubbles is reduced by application of the pressure and gas in air bubbles is dissolved in the glass. |
