Glassrod elongation heating furnance having double bellows

摘要

In a heating furnace which holds at least one end of a glass rod with a holding portion and elongates the glass rod by softening the glass rod successively from the other end portion thereof with heating while applying a tensile force thereto, the heating furnace comprises a tubular portion through which the glass rod to be elongated is inserted such as to be longitudinally movable; heater, positioned within the tubular portion such as to circumferentially surround the glass rod, for heating the glass rod; a moving portion through which one end of the glass rod is inserted; bellows the ends of which are respectively secured to the moving portion and the tubular portion and which is longitudinally expandable and contractible and composed of at least a double cylinder surrounding the part of the glass rod such as to block an outside air from flowing into the heating furnace; and gas supply line for supplying, for purging, an inert gas into a space within the tubular portion and the inner bellows as well as the space between inner and outer bellows.