Glass manufacturing – Processes of manufacturing fibers – filaments – or preforms – Process of manufacturing optical fibers – waveguides – or...
Reexamination Certificate
1999-06-09
2001-11-27
Colaianni, Michael (Department: 1731)
Glass manufacturing
Processes of manufacturing fibers, filaments, or preforms
Process of manufacturing optical fibers, waveguides, or...
C065S355000, C065S356000, C065S384000, C065S385000, C065S434000, C065S510000, C065S511000, C065S512000, C065S513000, C065S514000
Reexamination Certificate
active
06321573
ABSTRACT:
The present invention concerns a process for manufacturing an elongated, porous, SiO
2
preform by deposition of SiO
2
particles on the mantle surface of a cylindrical carrier which rotates about its longitudinal axis. The SiO
2
particles are formed by a plurality of deposition burners arranged at a distance from one another in at least one burner row which extends parallel to the carrier's longitudinal axis. The burners are moved back and forth along the forming preform in a repeating motion cycle and between turnaround points where the direction of burner motion is reversed. Measures are taken to prevent or reduce an overheating of the forming preform in the turnaround point regions.
In addition, the invention concerns an apparatus for manufacturing a porous SiO
2
preform, with a cylindrical carrier rotatable about its longitudinal axis, with a plurality of deposition burners arranged in at least one burner row extending parallel to the longitudinal axis of the carrier and at a distance from one another and connected with one another, by means of which burners SiO
2
particles are deposited on the carrier, forming the preform, and with a jog device which moves the deposition burners back and forth along the carrier between turnaround points where the direction of the motion is reversed.
A device of the kind described above is known from EP A1 476 218. Described therein is the forming of an elongated porous preform from SiO
2
particles whereby flame hydrolysis burners deposit SiO
2
particles in layers on a horizontally arranged substrate rod rotating about its longitudinal axis. The burners are mounted equidistantly 10 cm apart on a burner block which extends parallel to the longitudinal axis of the substrate rod. The burner block is moved back and forth by means of an adjustable jogging device between a left and a right turnaround point, along the porous cylindrical preform being formed. The amplitude of this translational motion of the burner block is smaller than the length of the preform. The slowing of the translational motion of the burner block during the reversal of motion at the turnaround points can result in an overheating of the preform surface and hence in local axial density fluctuation in the preform. This would produce regions of differing reactivity in the preform, becoming apparent especially during subsequent chemical reactions in further treatment of the preform and could result, after, for example, sintering, in non-homogeneity of the quartz glass body. In order to avoid this it is proposed in EP A1 476 218 that both the left and the right turnaround point be relocated by a few millimeters during each burner pass. Even though this equally distributes the local density fluctuations created in the preform at the turnaround points, it does not prevent them.
To solve this problem, a process of this kind is proposed in DE A1 196 28 958, where an overheating of the preform at the turnaround points is prevented or reduced by increasing the circumferential velocity of the forming preform, or by lowering the flame temperature of the deposition burners, or by increasing the distance between the deposition burners and the preform surface. Any one or a combination of these measures makes it possible to completely or partially compensate for the temperature increase of the preform surface at the turnaround points, so that in terms of time and space the preform is subjected to as even a temperature impact as possible over its entire surface. Axial density gradients in the preform are thus largely avoided.
Spatial arrangement of turnaround points according to EP A1 476 218 requires high equipment and control expenditures. Measures proposed in DE A1 196 28 958 for avoiding local overheating in the turnaround point area require a time-related modification of gas flows, distances or relative velocities in the turnaround point regions, and these measures can cause an undesired variation of the deposition rate.
The object of the present invention is therefore to provide a simple process for manufacturing a preform with small density fluctuations while avoiding the disadvantages of the measures indicated above, as well as providing a simple apparatus suitable for implementing the process.
As far as the process is concerned, this object is achieved on the basis of the generic process in that the measures comprise local removal of heat from the turnaround point regions, or local heat shielding of the turnaround point regions, and in that the measures can be held time-constant within a motion cycle.
The deposition burners produce a temperature profile along the surface of the preform being formed, said profile being formed by the flame temperature in the area around the surface, and by the oscillating motion of the burner row. This surface temperature profile has peaks in the turnaround point regions. Such peaks are avoided by the process according to the invention by measures for local cooling (cooling measures), in that heat is removed in the turnaround point regions or in that the preform being formed is shielded from heat in the turnaround point regions. The first-mentioned cooling measures by means of local heat removal will be subsequently called “active cooling” and the last-mentioned cooling measures by means of heat shielding will be called “passive cooling.”
The action of the measures for the active and passive cooling of the preform is above all limited locally to the respective turnaround point regions. Their action is based either on shielding the preform surface from heat in the turnaround point regions or conducting heat in these regions away from the preform surface. In either case, the cooling measures contribute to a homogenization of the preform surface temperature. In the ideal case the extent of heat shielding or of heat removal is precisely such that the temperature increase resulting from the burner motion is exactly compensated in the turnaround point regions. Density gradients in the preform are avoided or reduced in this way.
The cooling measures are placed at the turnaround points of the burner motion. In cases where the turnaround points are spatially constant the effect of the cooling measures is spatially constant; otherwise, in conjunction with a shifting of the turnaround points, it is spatially variable.
A special feature of the process according to the invention is that the cooling measures are kept—at least within one motion cycle of the burner row —constant in relation to time. What is meant by this is that a controlled or regulated changing of the parameters of the cooling measures, for example depending on the motion cycle of the burner row, is not required; the corresponding expenditures for control and regulation are thus eliminated.
The time-related constancy of the cooling measures does not relate to their effectiveness. The latter can be variable in relation to time because the effect of active or passive cooling increases in proportion to an increase of the heat quantity to be shielded or removed in the area of a turnaround point. The respective cooling measures are therefore most effective just when heating of the preform by the deposition burners is especially great, that is, when they sweep over the regions around the turnaround points.
In a kinetic reversal, it is the carrier that can be moved back and forth between the turnaround points.
With active cooling heat is removed from the preform surface in the turnaround point region. Heat can be removed by, for example, cooling elements such as, for example, fluid- or gas-cooled components which act in the turnaround point regions, or by streams of cooling gas directed at the preform surface in the turnaround points regions. As an alternative, heat can be removed from the turnaround point regions by heat conductors, by convection, or by heat flow. Flow devices which act in the turnaround point regions need to be provided to generate effective heat convection or flow.
One technique has been shown particularly effective in producing suitable heat conduction, convection or heat
Fritsche Hans-Georg
Neubauer Frank
Peper Udo
Röper Jürgen
Schaper Hartwig
Colaianni Michael
Heraeus Quarzglas GmbH & Co. KG
Tiajoloff Andrew L.
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