Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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GLASS DRAWING PROCESS A~iD FURNACE
TECHNICAL FIELD
This invention is directed to the working of
softenable dielectric materials, particularly the drawing of
clad and unclad fibers and iber bundles from primary and
later-stage preforms of glass. The invention particularly
relates to processes for drawing glass fibers, bundles, and
composite products from the fused or softened end of a
preform introduced into a furnace.
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BACKGROUND ART
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; The art of glass drawin~ is presently the most
effective mode o producing either continuous, flexible
fibers or of producing relatively short segmer;ts for later
combining and processing into composite products such as
fiberoptic screens, faceplates, and image modifiers of
various types. ~esides being used for drawing of fibers and
multi-fiber bundles, drawing techniques of the type to which
the invention relates are applied to late-stage processing
o the composite products. Such processing includes
cross-sectional reduction, either uniform or graduated, the
latter technique used to form image expanders and reducers.
Such processing also includes various degrees of twistincJ
and other manipulations to form image re-orienting de~ices
such as partial rotators, inverters, etc.
An lmportant goal in this technical field is
uniformity of heating and a high degree of temperature
control in the critical softened area of the preform or
workpiece. Failure of uniformi~y in heating tl~e work ~one
is a major cause of product defect and rejection, resulting
in waste of expensive materials and production time. This
consideration is particularly critical in the case of a
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product formed from a preform of highly complex
cross-sectional character in which large, sometimes sharp,
gradients of optical, physical, and thermodynamic properties
are likely to be present. The requirement of uniform
heating reaches ultimate criticality when the conventional
upper limits of heating and drawing speed and of preform and
product cross-sectional dimension are reached and exceeded.
It has been the unrealized goal of skilled workers in this
art to produce uniform heating in the drawing furnace at the
moderate temperatures needed for drawing relatively delicate
composite products. Among the main reasons for failure to
acnieve this goal has been the difficulty of achieving
uniform radiation of heat from the radiant heating elements
lS at temperatures of around 1100~ to 1400F. t600 to 750C)
Separate radiant elements produce inherently non-uniform
heating. ~ttempts to produce a radiant source continuously
surrounding the fusing area or to embed discrete elements in
a dif~using matrix have not produced the desired uniformity.
Moreover, most composite products do not absorb radiant
energy uniformly even if it is introduced uniformly. This
compounds the pro~lem of non-uniform radiant sources, and
limits the level of uniformity even for an ideal uniform
radiant source.
This problem of absorbtion differential exists in
any application where there are difterent glasses in the
same product, the glasses having different infra-red
absorbing characteristics (for instance the core relative to
the cladding) or any product involving an extremely thick
preform or drawn diameter. In the latter case, the rate of
heating by absorption at the surface of the working area
must be carefully regulated according to the rate of
conduction of the heat toward the center of the piece. At
locations toward the axis the rad7ant energy per se fails to
penetrate at levels comparable to that at the surface.
A prior method of approximating uniformly
radiating elements has been developed which involves turning
the preform and the product on their common axi~, at a rate
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sufficient to smooth out the variations in the radiational
heating sources. This technique requires complex mechanisms
to coordinate the turning of the preform and product, as
well as the turning and lateral translation of the take-up
reel if the product requires. At best, the technique
produces horizontal (stratifie~) uniformity withcut
producing vertical uniformity and results in hot "rings"
instead of hot "spots". Moreover, the technique does not
address the problem of non-uniform absorption by a composite
product.
q'he problem of non-uniform absorption is
especially acute when the product contains light-absorbing
elements such as E~ cladding or fibers which tend to absorb
infra-red radiation in disproportion to the remaining
materials. Such elements, in a radiant furnace, produce
internal anomalies of temperature and viscosity which limit
and complicate the choice of drawing speed.
As a secondary consequence to the inability to
20 achieve uniform heating in the drawing process, both the
preform size and the reduction ratio in the drawing process
are severely limited. The result is that a co~posite
product having very small diameter fiber-optic components
must be produced by a many step process. Typically, the
steps include drawing a single fiber, drawing down a
25 multi-fiber bundle, drawing a multi-multi fiber bundle, and
fusing a bundle of these latter products into a block. Such
many-stage processes consume production time, and each step
has its own percentage rejection rate (on the order o~ ~0%).
~rhus, the main object of the present invention is
to provide a process for acting on the working area of a
pre~orm to produce highly controlled, uniform heating.
~ nother object of the invention is to provide a
process in which the limits on the size of the preform, the
product, and the drawing reduction ratio are greatly
extended.
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A further object of the invention i5 to provide a
process for drawing glass which allows the elimination of at
least one of the successive reduction drawings in certain
fiber-optic processes, without loss of quality.
Another object oF the invention is to provide a
process in which the size of the working zone at which
reduction takes place may be chosen and control]ed. In
another of its aspects, the invention provides and apparatus
particularly adapted to assist in carrying out the uniform
heating and control of the working zone of a drawable
preform.
Further objects of the invention will become
apparent as specific embodiments are described.
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DISCLOSURE: OF THE INVENTION
The present invention uses a controlled,
high-velocity flow of temperature-regulated air or other
fluid, preferably produced in a separate heating chamber and
introduced a the drawing chamber. The process ta~es
advantage of the temperature distributing qualities of a mix
of forced-and free-convection to uniformly heat the working
zone of a preform. The process involves removing
heat-depleted fluid, preferably by cycling the cooled fluid
back to the separate heating chamber for reheating. The
process involves using data from temperature sensors at
critical points in the flow cycle to con~rol fluid heating
to maintain a desired srnooth temperature/time profile.
In another aspect, the process involves
temperature regulation of a small part of the preform just
outside of the drawing chamber of the furnace in order to
insure a consistently temperature-prepared preform entering
the working area; this reduces the requirement to regulate
ambient temperature and to compensate for different thermal
conductivity of different preforms and preform clamping
means. If effect, this step Eorms a thermal insulator which
keeps the heat from the furnace from undesirable transfer up
the preform.
A further aspect of the process of the invention
involves controlled movement of an extendable insulation
means in order to regulate the effective distance from the
inlet to the outlet of the drawing chamber of the furnace,
whereby the size of the working zone is regulated at will.
A still further aspe~t of the invention is a
drawing furnace specifically adapted to carry out the
process. The furnace comprises a drawing chamber with a
~ preform inlet and outlet and a preferably separate fluid
; heating chamber having controllable heating means. The
separate fluid heating chamber communicates with the drawing
chamber by input passageways or channels, the communication
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mediated by forced convection means. In accordance with
other aspects of the invention, the furnace is provided
with a return channel communicating between the drawing
chamber and the heating chamber, a pre-entry temperature
regulating means at the preform inlet, and a movable
insulated sleeve associated with the outlet of the drawing
chamber.
In a variation of both the process and the
apparatus, the preform inlet is at the bottom of the
furnace, and the product is drawn upwardly from an outlet
on the top of the furnace drawing chamber. The steps and
elements of the variations may be independently employed.
In a further aspect the invention provides a
process Eor drawing a fiber or fiber-bundle product from a
preform of heat-softenable, drawable material having a
drawing temperature and a working zone, comprising the
steps of: ~a) feeding the preform into an inlet in a
drawing chamber, (b) heating a gaseous fluid to a
temperature at or above the drawing temperature of the
preform using heating means which are substantially
completely radiatively shielded from the preform working
zone; (c) causing the heated fluid to flow past the
preform working zone until the preform is at a drawing
temperature in the working zone~ and (d) drawing the
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product from the preform through an outlet in the drawing
chamber.
The disclosed process and apparatus solve the
technical problem of uniformly heating large-diameter
drawable workpieces, possibly having complex shape in all
dimensions and which, in addition, may be a composite of
materials having different radiant absorbing
characteristics. The high-velocity, forced convection
supply of fluid at the work zone allows transfer of heat
energy to the work without the extreme differences in
temperature encountered in radiant heating. Thus, the
avoidance of surface hot spots or internal hot spots does
not depend exclusively on the conduction rate into the
workpiece or on small amounts of free convection of dead
air, but can be controlled by regulation of the velocity
and temperature of the forced convection currents. At a
given rate of heat energy application, moreover, the
technical peoblem of controlling the size of the work zone
(which is related to the shape of the draw and the shape
of the diameter reduction profile,) may be readily
resolved by extension of an insulating sleeve into the
forced convection flow pattern.
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~3RlEE` LJ~SCRIPTION OF T~h DRAWINGS
Both the process and the apparatus of the present
invention may be bes-t understood by reference to specific
embodiments as shown in the drawings, which are illustrative
and not limiting.
Figure 1 shows a flow diagram of an embodiment of
the process of the present invention,
~igure 2 is a sectional view of a furnace
embodying the principles of the present invention,
E~igure 3 illustrates a simple embodiment of part
of a pre-cooling collar,
Figure 4 is a top elevational view of a preferred
form of the drawing chamber inlet with adjustable diameter
means,
E'igure 5 is a sectional view on the line V-V of
E'igure 2 showing an embodiment of the heated fluid
distribution means,
~igure 6 illustrates the action of the adjustable
insulated sleeve, and
~igure~ 7 shows a modified apparatus designed to be
capable of carrying out the drawing operation upward instead
of downward.
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D~TAII,~D D~SCRIPTION
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There are several modes of carrying out the
process of the present invention in which some of the
details depend upon the product being made and the raw
materials being worked. ~he following is a cletailed
description of successful applications, including .he best
mode contemplated at present. A detailed description of an
apparatus specifically designed for carrying out the process
is also laid out here.
I'he process diagram shown in Figure 1 outlines the
basic steps in the general process. Since the process
allows very close control of the preform heating, based on
calculated and experiential air temperature and flow-rate
data, optimal use o~ the process first requires a certain
degree of pre-entry temperature control of the preform.
This pre-entry temperature conventionally depends not only
on ambient temperature (control of which is inefficient) but
also on the conduction rate from prior furnaces through the
preform and its feeding mechanisms. The effect of these
~actors is reduced in the process of the present invention
by the step of bringing the temperature of the preform close
to a "normalized" temperature just before it enters the
furnace, This will most often be a cooling step, although
at some stages in the process and for some ambient
conditions there may be mild warming. The simplest
25 embodiment of this temperature normaliæing step 10 involves
bringing more or less constant temperature air Erom a source
and blowing it onto the periphery of the preform at the
entry point to the furnace. At the same time, the preform
lS. fed into the furnace drawi~g chamber in a feeding step 11
30 using available feeding mechanisms. These include motor
driven drive screws. The mechanism may include several of
these drive screws if the core and a cladding or a plurality
of claddings must be driven at differe~t rates. It is not
necessary for these mechanisms to include rotating means.
Such means were a complicating expedient to achieve furnace
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uniformity as mentioned above and have had only qualified
success.
1'he crux of the present invention involves the
step of heating air or otAer heat exchange fluid in a
chamber, preferable separate from the chamber in which the
fiber drawing will take place. If the chamber is not
separate, at least the working zone should be shielded from
any radiant elements used therein. The heated air is then
introduced or delivered 13 into the drawing chamber using
controllable forced convection means: fans, air pumps, etc.
The air heating step 12 is preferably controlled (for
instance by variable resistance or by introducing a
controlled cool gas stream) under the guidance of a
temperature measurement step 14. This measurement is
preferably carried out as the air is introduced into the
drawing chamber. This measurement, whi~h is carried out for
example by inserting a thermcouple into the air flow
pattern, controls the air heating means by a preselected
algorithm, mediated by electrical or electronic processing
means. These may involve feedback or feedforward processing
with calculated or tabulated parameters, leading to a
discret or contlnuous heat control setting step 15. This
step may also encompass control of variation of the flow
rate of the heated air. The goal is the development of a
temperature/time profile appropriate for the uniform heating
of a given preform in a given working zone. To this end,
the heated air is directed to flow past the working zone of
the preform, step 16. The preferred mode of flow ls rather
turbulent, but with a directional trend through the working
, 30 zone in a distinct direction. It is also preferable that
the inflow be distributed aro,und the periphery of the
preform inlet end of the dra~ing chamber.
When the preform is at working temperature, the
reduced diameter product is drawn in the usual manner, using
combinations of gravity and traction in a drawing step 17.
Since the rate of drawing and the degree of reduction in
diameter of preform product depend (among other factors) on
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both temperature profile and the length of the working zone,
an additional working zone shaping step 18 may be employed.
The working zone shape may be set before the drawing begins
or may be adjusted at various stages of the drawing process
according to the requirements of the product and/or the
character of the preform.
A successful method of shaping the working zone
involves varying the effective distance from the inlet of
the drawing chamber to the effective outlet. Specifically,
~his may be accomplished by positioning an insulating sleeve
which is capable of variable extension from the outlet of
the chamber toward the inlet, concentrically of the axis of
the draw.
Io facilitate temperature control, the heated air
which has been flowed past the working zone may be withdrawn
from the chamber in an air return step 19 to be reheated in
the heating chamber. If an additional air temperature
measurement step 20 i8 performed on withdrawal of the air,
refinement of the heat setting control may be made. Such an
adjustment may be made, for example, on the basis of
calculation of absorbed heat from the temperature
differential.
After drawing of the product, the usual product
processing is carried out, indicated generally A9 step 21.
For continuous fiber, a rotating and reciprocating take--up
reel may be provided. Additional coatings may be applied.
For more discrete product, means may be provided for
periodic cutoff of draw segments, either as quasi-finished
product or for bundling into multi- and multi/multi-element
composites for further processing.
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Because this process allows the drawing of ve-y
; large diameter product from very large diameter preforms due
to the extraodinarily uniform heating that the process
provides, it has been found to be possible and advantageous
to feed the preform up from below the furnace and draw the
product up from above. This process variation
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advantageously modifies the effect of gravity on the working
zone shape and on the take-up qualities of the product. The
variation is especially applicable in a working ~one which
is relatively long with a slow taper.
The general process of this invention has been
applied without any extraordinary control measures to a
composite preform over 4.5 inches (12 cm) in diameter which
was drawn down to a diameter of .030 inches (.080 cm) in one
stage. The preform was normalized to about 70F. (21C).
Air was heated so that it could be delivered to the drawing
chamber at about 1380F (750~C). The air was forced at a
high velocity past the working zone and withdrawn at about
1375~ (747C). The rate of production and the quality of
the product were at least comparable to conventional
radiative drawing which would have required multiple stages
for this reduction.
In another case the uniformity of heating in this
20 process allowed a one-step production of a one-inch product
from a three-inch preform with excellent quality and
production rate. The preform was introduced from below the
furnace and taken up from above.
L~o apparent limit has been found to the size of
preform to which the process may advantageously be applied.
25 ~hen applied to conventional drawing processes, the
production rate approaches 5 to 6 times the usual rate and
is presentl~ lirnited by the capacity of available or readily
modified take-up mechanisms.
~rhe ability of this system to deliver heat
30 effectively to large diameter workpieces in the normal ,
drawing operation without distortion in the workpiece allows
use of the system in a remarkable manner~ Ordinarily, large
; diameter products formed of large numbers of fine fibers are
; formed by a very inefficient procëss involving stacking of
the fibers and the fusing of the fibers under heat and very
high pressure to remove voids. It has been four.d that these
large diameter products can be formed far more efficiently
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in the following manner. F'irst, a preform is formed by
stacking fibers into a bundle with a diameter slightly
larger than the desired product. The bundle is then enclosed
in a gas-tight glass envelope which is then evacuated. The
resulting preform is then passed through the furnace in the
manner of this invention except that the preform is only
drawn down a small amount to the desired diameter. The
result is a large diameter product formed of uniformly
fused, voidless and undistorted fibers. This product is
capable of being sliced into gas-tight plates. In practical
operation, it may be necessary to continuously evacuate the
envelope during the draw. Because of the very low distortion
caused by this draw, it is possible to apply a twisting
15 motion to the workpiece in the draw zone. This results in a
product in which the fibers have a uniform spiral
orientation. The resulting proauct can be cut to form image
rotators or inverters.
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APPARAT~S
An apparatus specifically designed to carry out
the process of the present invention is disclosed in FIGS.
2-7 and is shown most comprehensively in a modified
schematic manner in Figure 2.
Although forced convection ovens have been
available in the past, the furnace of the present invention
is disclosed in unique com~ination with other elements and
in configuration adapted to utilize the special properties
of forced convection in a glass drawing operation. The
application of the force~ convection heating to the glass
drawing art have resulted in extraordinary, unexpected, and
surprising increase in the diameter capacity, drawing rate
and quality (in terms oE product rejection rates) of this
art.
The general combination as illustrated in Figure 2
and indicated generally by the numeral 30, includes a
complex of feed mechanisms 31, preferably having a capacity
for differential feeding of various components of a
composite preform. The feeding complex may include a vacuum
pull assembly 32 including the required gas tight seals, and
clamping means 33. The wor~piece of the apparatus is a heat
softenable, drawable preform 34. To carry out the
temperature normalization or "pre-cooling" of the preform
just before entry into the furnace (indicated generally by
40) this embodiment includes a hollow collar 35 or thermal
isolator which is supplied with relatively constant
temperature air from a source 36 via a conduit 37 with
controllable valve 38. The "pre-cooling" air is directed
inwardly toward the preform through a plurality of inwardly
facing, radially direc-ted apertures such as 39. It has been
successful and convenient to choo~e, as a standard, a
temperature near room temperature of 70~ (about 21C). To
achieve a finer degree of control over keeping the preform
temperature constant, temperature sensors above and/or below
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the collar could be provided to control pre-cooling air
temperature and/or volume.
The preform 34 is fed into the furnace 40 through
a drawing chamber inlet 41. This inlet is preferably
supplied with diameter adjusting means 42 such as a fairly
refractory "iris" assembly, to insure moderate resistance to
heat loss.
lrhe drawing chamber 43 is one of two chambers
which ~his embodiment of the furnace comprehends, the other
chamber being the separate air heating chamber 44. The
heating chamber is supplied with heating means which may be
combustive, inductive, arc-induced, diaelectric, etc., but
in the preferred embodiment comprised large area resistive
coils 45. These are supplied with power from a source 46
and a control mechanism 47 such as variable impedance or a
variable transformer.
A forced convection element 50 (a high temperature
fan, pump, etc.) draws air through the heating elements and
directs a flow through a channel 51 which communicates with
the drawing chamber 43. Tnis delivery is preferably
mediated by a clistribution means 52 such as an internally,
radially perforated plenum.
At a point along the path of flow, a temperature
sensing transducer means 54 is provided sending its signal
to a central control system 60.
The flow of air is actively or passably drawn past
the preform/workpiece to create the working zone 55
terminating at or near the effective drawing chamber outlet
56. The flow continues through a return channel 57 which
may be provided with a return forced convection element S8
and a return air temperature sènsing transducer means 59
also sending to the central control 60.
The effective drawing chamber outlet may be
provided with diameter adjusting means 59 similar to the
means 42 at the inlet.
The effective drawing chamber outlet 56 is
distinguished from the actual outlet 61 by an insularing
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sleeve 62 which movably engages the actual outlet and
extends into the drawing chamber toward the inlet 41. When
the product is drawn past the inner end of this sleeve, the
product is shielded from the heated flow. This movable
point thus defines the end of the workin~ zone 5S. The
position of the sleeve may be temporarily fixed as by set
screws or clamps, or may be under variable central control
effected by extension means 63 such as servo-activated rack
and gear or friction wheels.
The product is drawn in the usual manner by a
drawing mechanism 64 which employs gravity, traction means,
e~c. The product is then passed on to further processing
elements 65: take-up reels, cutters, bundlers, slicers, etc.
An effective pre-cooling collar 35 may be
constructed as detailed in Figure 3. Compressed air of
relati~ely constant temperatllre is brought via a conduit 37
from a constant temperature air source such as an air
compressor into the collar and is directed inward toward the
preform through radial apertures 39.
The detailed view in Figure 4 looking down on the
drawing chamber inlet illustrates an embodiment of an
adjustable diameter means 42 for causing the effective inlet
; diameter to approximate that of the preform 3~ (shown as a
typical single cladding fiber-optic preforrn.) A similar
mechanism can be used for the diameter adjusting mechanism
59 of the effective outlet 56.
Figure 5 shows a detailed horizontal section on
the line V-V of Figure 2 of a flow distribution means 52
embodied in a toroidal plenum with internal radial
perforations 53 shaped to facilitate flow control~ In the
preferred embodiment, the perforations would be elongated
along the axis of the workpiece.
A detail of a dynamic émbodiment of the adjustable
sleeve 62 is shown in E`igure 6 with an adjustment for a
shortening of the working zone in broken lines. Adjustment
is made by extending means 63 under central control 6~ The
extending means are preferably annular.
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The process variation which involves feeding the
preform from below the furnace and drawing the product from
above may be carried out satisfactorily on the appratus of
E'igure 2. Full advantage of this invention may best be
taken under the circumstances, however, by the apparatus
variation shown in Figure 7. In this variation, the furnace
is so mounted, as by the use of gimbles 70, 71, that it may
be rotated at will in a vertical plane. In this case, the
feed mechanisms 31 and the drawing mechanism 64 are
relocated to their respective appropriate locations as shown
in the figure.
As an alternative to the rotatable configuration
shown in Figure 7, the apparatus may be built with a
symmetry about a central horizontal plane. Thus, the input
channel from the heating chamber to the drawing chamber, as
well as the flow distribution means, may be located
; centrally. The "inlet" and "outlet" areas of the drawing
chamber may then be identically supplied with closable
return channels, extendable insulated sleeves, pre-cooling
collars, and aperture diameter adjusting means. In this way
; the preform may be fed and the product drawn upward or
downward with equal ease. The choice will depend upon the
character of preform, product, and production rate.
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IND~I'E~IAL APPLICA~ILIl'Y
Some of the modes of industrial applicability of
the uniform heating provided by the drawing process of the
present invention are readily apparent from the above
description of the equipment and its characteristics, while
other uses and advantages are totally unexpected.
A typical product rejection rate of about 20% at
each step of a multi-step manufacturing process obtains in
present radiation furnaces. The process and apparatus of
the present invention have significantly reduced this
rejection rate.
The drawing rate for moderate ~iameter products
has been greatly limited by the need for a slow enough rate
to allow complete and reasonably uniform heating of the
15 preform. The present process heats even large diameter
preforms quickly and uniformly to the point that some
product can be taken up at rates 5-6 times to that of
conventionally configured drawing furnaces, with more than
satisfactory ~uality.
The present process does not generally require
special treatment o~ products which incorporate radiation
absorbing claddings and fibers as compared to with the
treatment required of SUch products in radiative furnaces.
~uch products include most composite image manipulating
products: faceplates, image expanders, inverters, etc.
Most significantly, in prior art processes, the
normal composite product is produced in several stages.
This is due to limitations in the size of the reduction
ratio of the product to preform which can be achieved
without distortion. There has~also been an absolute size
30 limit of the preform that can be heated with threshhold
uniformity. The present process can uniformly heat the
working 20ne of a preforrn, including a composite preform, of
at least up to 4.5 inches (12 cm~ and possibly much ~arger.
lhis means:
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1. At fairly ordinary product-to-preform ratios, a final
product of large diameter may be drawn, possibly
eliminating the final pressing, annealing, devoiding,
ana shaping steps,
2. Advantage can be taken of the vastly larger r2duction
ratios possible to eliminate one or more stages of the
production of multi-multi-type composite products. The
elimination of such steps saves time and labor, as well
as the cost of materials lost through inevitable waste
and rejection~
Clearly, minor changes may be made in the form and
construction of this invention and in the embodiments of the
process without departing from the material spirit of
either. Therefore, it is not desired to confine the
invention to the exact forms shown herein and described but
it is desired to include all subject matter that properly
comes within the scope claimed.
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