| 摘要 |
PURPOSE:To suppress occurrence of crystal and to make it possible to obtain long preform without cracking in double crucible preform drawing method, by specifying temperature control on and after the tip of a double structure nozzle and an atmospheric condition. CONSTITUTION:Glass melts 7 and 8 for core and clad are extruded from a double nozzle 20 by pressurizing the surface of the melts 7 and 8. Since the lower part of the nozzle 20 is cooled to approximately glass deformation temperature by a refrigerant 10 in a cylindrical column 9 for refrigerant, the melts 7 and 8 are cooled to approximately the glass deformation temperature and made high viscous in the extrusion. On and after the process, pressure in a furnace core pipe 1 is made >=1atm. and <pressure applied to the surface of the melts 7 and 8 by regulating a gas 11 for pressurizing the furnace core pipe and an outlet 12 of the furnace core pipe. Core clad glass made into high viscosity is continuously drawn by lowering a drawing bar 18 of a double crucible closed stopper. The glass is passed through a heater 19 kept approximately glass transition temperature of the glass by a high-frequency heating coil 14 for annealing. |
