Glass manufacturing – Fiber making apparatus
Reexamination Certificate
1999-09-21
2002-05-07
Hoffman, John (Department: 1731)
Glass manufacturing
Fiber making apparatus
C065S530000, C065S540000
Reexamination Certificate
active
06381990
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to a method and assembly for sealing a draw furnace, and more particularly, to a sealing assembly and method for sealing the top of an optical waveguide draw furnace.
2. Technical Background
Relatively high temperature heat sources are required for drawing high strength, low loss optical waveguide fibers from high silica-content optical waveguide fiber preforms or blanks. Two of the predominant heat sources utilized for drawing such fibers are zirconia furnaces and graphite furnaces. Fiber draw furnaces generally operate at temperatures greater than about 1900° C., typically as high as about 2050° C.
A disadvantage associated with zirconia induction furnaces is that extended use and thermomechanical stresses due to poor thermal shock resistance cause cracks in the muffle and susceptor. This cracking causes zirconia particles to migrate from the inner surface of the furnace onto the preform and/or fiber being drawn from the preform resulting in substantially weakened fiber and unacceptable product losses. Moreover, zirconia induction furnaces are sensitive to rapid changes in temperature. Accordingly, significant amounts of time are necessary for increasing and decreasing the temperatures within the furnace. Rapid heating and cooling of the furnace results in fracturing of the zirconia muffle, which necessitates replacement of the muffle and results in significant furnace down time.
Such shortcomings lead to the development of graphite induction furnaces. Graphite induction furnaces typically include a graphite muffle which is insensitive to rapid changes in temperature and thus is less susceptible to cracking. It has been found, however, that graphite furnaces are susceptible to oxidation at temperatures approaching about 450° C. and that oxidation is particularly problematic when the graphite is exposed to the high temperatures used to draw optical waveguide fibers. For this reason, among others, it is preferable to maintain an inert atmosphere within the draw furnace. Oxidation occurs when gasses from ambient atmosphere react with the solid carbon muffle at high temperatures according to the following reactions:
C+O
2
→CO
2
; and (1)
C+CO
2
→2CO. (2)
A typical onset temperature for reaction (1) for a graphite grade used in a draw furnace is about 700° C. Reaction (2) becomes significant above 900° C. These reactions of the furnace muffle with oxygen and carbon dioxide cause the furnace muffle to be consumed, especially at elevated fiber drawing temperatures, and are referred to generally as CO events.
The graphite muffle material is a composite of graphite grains bonded together by a carbon binder matrix. It is believed that the binder material is more susceptible to oxidation than the graphite grains. Therefore, when the composite of the two materials is exposed to air, and thus oxygen at temperatures above the oxidation onset temperatures, the matrix binder material preferentially oxidizes. The graphite grains, having no binder left to hold them in place, are then free to fall away from the composite structure. It is believed that this mechanism causes graphite particulate to migrate from the muffle wall to the fiber preform and/or fiber during drawing.
Graphite particulate that becomes incorporated into the fiber during drawing causes unacceptable product losses due to point defects. Point defects manifest themselves as sharp attenuation increases in the signal transmitted through the fiber. Point defect product losses due to graphite particulate from draw furnaces can be greater than about 5%, which is an unacceptably high loss. Graphite particulate that has adhered to the fiber during the draw process also contributes to fiber breaks.
In order to reduce graphite particulation produced by oxidation of the graphite muffle material, and thus the number of resulting point defects, an inert gas is typically supplied to the interior of the furnace to prevent ambient air and other gases from entering the furnace. Unfortunately, there are a number of operations that occur during optical waveguide draw operations which present ambient air and other gases with the opportunity to enter the furnace despite delivery of high inert gas flow rates into the furnace. When a mistake is made during these draw operations, either by human error, by mechanical failure, or otherwise, ambient air and/or other gases are often permitted to enter the furnace and a CO event occurs.
Many of these CO events occur during movement or operation of the top hat assemblies presently used to close the opening in the top of a draw furnace during blank load, unload, and idle periods. Typical top hat assemblies include a viton/rulon seal that is positioned adjacent the top of the optical waveguide draw furnace when the top hat assembly is positioned over the downfeed handle and lowered onto the draw furnace. In addition, these top hat assemblies incorporate a plurality of hooks used in manually lowering and raising the top hat assembly. The hooks are specifically employed to raise the top hat assembly while the downfeed handle is being removed from the draw furnace. Currently, an operator must engage the hooks prior to removal of the downfeed handle from the draw furnace to ensure that the top hat assembly is being withdrawn from the top of the draw furnace as the downfeed handle and the remainder of the waveguide blank are removed from the hot zone of the draw furnace. Otherwise, that portion of the downfeed handle exiting the hot zone of the draw furnace will approach the top hat assembly, and the extreme heat retained by the downfeed handle will melt the viton/rulon or other sealing member of the top hat assembly. Often times, however, an operator fails to engage the hooks, which ultimately results in the melting and thus the premature replacement of the top hat sealing member.
Moreover, if an operator fails to disengage the hooks after the top hat assembly has been positioned on the top of the draw furnace, certain aspects of the fiber drawing operation itself can create CO events. Generally, once a seal is formed between the top hat and the top of the draw furnace, the bottom door assembly of the draw furnace is opened and fiber drawing commences. During the start of optical waveguide fiber draw, the optical fiber blank is typically partially raised back out of the hot zone of the draw furnace to stretch and thin the fiber in order to obtain a desired fiber diameter. If the hooks are still engaged with the top hat assembly during this time, the top hat raises off of the top of the draw furnace and the seal is broken. Since the bottom door assembly of the draw furnace is open during fiber draw, the broken top hat seal creates what is known in the art as a chimney effect. The chimney effect is the rapid intake of air through the bottom opening in the draw furnace caused by, inter alia, the opening of the top of the draw furnace. Ambient air is thus forced into the muffle of the draw furnace and the oxygen contained within the air reacts with the carbon muffle causing a CO event. The graphite particulate is then free to migrate to the fiber which results in fiber being formed with an unacceptable number of point defects. In addition, the entrance of air into the draw furnace during fiber draw also results in diameter upsets which also creates unusable fiber. Moreover, each CO event degrades the draw furnace muffle which in turn reduces draw furnace efficiency and necessitates muffle replacement.
In view of these and other shortcomings, an improved assembly and method of sealing an optical waveguide draw furnace is needed to prevent the entrance of ambient air and other unwanted gases into the draw furnace.
SUMMARY OF THE INVENTION
One aspect of the present invention relates to an apparatus for cooperating with a downfeed handle to seal the top of an optical waveguide draw furnace. The apparatus includes an assembly that is constructed and arranged to removably cover the t
Ball Steven Craig
Barnard, Sr. John Morris
Snipes James Alan
Corning Incorporated
Hoffman John
Krogh Timothy R.
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