| 摘要 |
PURPOSE:To prevent the remaining of bubbles without deteriorating productivity in vitrifying a large-sized glass preform by partially changing the length of a heater enclosing a furnace core tube in the axial direction and making its temp. distribution gentle. CONSTITUTION:A thin-walled cylindrical heater 9 is made of high-purity carbon, etc., connected to an electrode at a tongue 91 and energized. Grooves 92 are alternately provided in the axial direction to obtain a specified resistance value, and uniform heating is attained in the circumferential direction. A segment extended upward by L2 (about 50-100mm) from the cylindrical shape having the original length L1 (about 100-200mm) is formed at angles of 45 deg. to one another. In this case, the cross-sectional area of the heater at the extended part (L2) is half of that at the original part (L1). Although the glass preform to be vitrified is traversed from the upper part to the lower part at the normal velocity, the preform is heated at the same rate as in the case when the preform is traversed at the lower velocity, and the remaining of bubbles is prevented. |
