BACKGROUND OF THE INVENTION
Field of the Invention
The present invention relates to coating die which is used
for drawing an optical fiber, and to an optical fiber drawing
method therefor.
Description of Related Art
It is common for a plurality of resin coating to be applied
to an optical fiber from a viewpoint of maintaining mechanical
strength and transmission characteristics. An ultraviolet
curable resin having relatively lower Young' s modulus are applied
to an optical fiber as a first coating layer and an ultraviolet
curable resin having relatively higher Youngs modulus and a second
coating layer.
An optical fiber drawing apparatus for applying an
ultraviolet curable resin is explained with reference to FIG.
2 as follows.
In FIG. 2, a reference numeral 1 indicates an optical fiber.
The optical fiber lis formed by performing heat-melt ing processing
so as to extend a raw material 2 of an optical fiber in a drawing
furnace 3. A liquid first ultraviolet curable resin is applied
to an outside of the optical fiber 1 by passing the optical fiber
1 through a first resin coating apparatus 4. Furthermore, by
passing the optical fiber 1 through a first hardening apparatus
5 (UV lamp) for irradiation with ultraviolet light, the resin
is hardened; thus, a first coating resin is formed on the optical
fiber 1.
Consequently, a second coating resin layer such as an
ultraviolet curable resin is formed on the first coating resin
layer by passing the resin-coated optical fiber through a second
resin coating apparatus 6 and a second hardening apparatus 7.
A resin-coated optical fiber 8 is wound by a winding apparatus
9. The resin coating process is performed vertically.
Recently, it is required that the productivity of the optical
fiber be improved in accordance with an increase in demand therefor.
In particular, high speed ultraviolet curable resin coating
processing is required to be realized. However, if high speed
coating processing is realized, there is a problem in that
ultraviolet curable resins cannot be applied stably and in uniform
thickness.
For a solution to the above-mentioned problem, a coating
process which is described in Japanese Examined Patent Application,
Second Publication No. Hei 7-5336 is known. According to this
coating process, defects in the coating resin is prevented from
occurring in coating die by controlling the application
temperature of the liquid ultraviolet curable resin and by
controlling the viscosity of a region of the liquid ultraviolet
curable resin, in which shear rate is slower than a critical shear
rate, in the applying temperature with in a predetermined range.
. v
In this coating process, it is disclosed that the viscosity is
in a range of 500 cps to 3000 cps in an application temperature
of 60°C to 100°C in a region in which shear rate in the applying
temperature is slower than a critical shear rate.
However, as disclosed in the above-mentioned prior art
document, shear rate of an ordinary ultraviolet curable resin
is in a range of 104 to 105 sec"1. If the drawing process is performed
while the shear rate must be slower than the critical shear rate,
the line speed cannot be high. If a high speed drawing such as
a line speed of 10 m/sec is performed, interfacial shear rate
becomes quite high; thus, it easily becomes higher than its critical
shear rate.
On the other hand, designing aspect of coating die becomes
important along with an increasing drawing speed of the optical
fiber. Japanese Examined Patent Application, Second Publication
No. Hei 7-91092 is an example of such design for coating die.
A coating die which is disclosed in this prior art document is
explained with reference to FIGS. 3, 4, and 5 as follows.
In FIG. 3, reference numeral 1 is an optical fiber. A resin
12 is applied to the optical fiber 1 while the optical fiber 1
is passing through a guiding die 10 and a die 11. Reference numeral
13 indicates a holder for supporting the guiding die 10 an the
die 11. Reference numeral 14 indicates a guiding die hole.
Reference numeral 15 indicates a bottom face of the guiding die.
Reference numeral 16 indicates a tapered section of the die 11.
Reference numeral 17 indicates an exit hole of the die 11.
Reference numeral 18 indicates a top faceofthediell. Areference
numeral 19 indicates a meniscus..
In FIGS. 4A and 5A, the guiding die 10, a die 11, and shapes
thereof are shown. In the above-mentioned prior art document,
the relationship between B and H in FIG. 4, the relationship between
G and C, the relationship between G and D, and the range of angle
a are disclosed.
In the above-mentioned prior art document, a case in which
a tapered section of the die 11 has two stepsbymodifyinga structure
shown in FIG. 4B is mentioned. However, the detail of such
structure is not disclosed. Also, a case inwhicha tapered section
of the dies 11 has a curve by modifying a structure shown in FIG.
5B is mentioned. However, the detail of such structure is not
disclosed. .
Also, the required shape of the die 11 is different between
a case in which a resin is applied to an optical fiber and a case
in which a resin is further applied to the resin-coated optical
fiber. Furthermore, in actual drawing operation, while the
drawing speed increases from a low speed such as a starting speed
(for example, 0.5 m/sec) to a high speed in which a product is
drawn on an operational basis (for example, 30 m/sec), temperature
of an optical fiber which is coated by a resin is not fixed.
Therefore, coating conditions becomes unstable to a certain degree.
In order to obtain a high quality optical fiber, coating operation
must be performed stably so as to avoid slipping phenomena of
resin in each drawing speed range.
SUMMARY OF THE INVENTION
The present invention was made in consideration of the
above-mentioned problems. An object of the present invention
is to provide a drawing method for an optical fiber which can
coat an optical fiber by resin stably even in a high speed drawing
operation and high productivity and cost reduction can be realized.
Another object of the present invention is to provide an optical
fiber drawing die which is used for the drawing method.
In order to solve the above-mentioned problems, a first
aspect of the present invention provides an optical fiber drawing
die which is used for a resin coating apparatus for forming a
resin coat by applying a resin to an optical fiber which is formed
by drawing process to a raw material for an optical fiber which
is characterized in that:
an interfacial shear rate of the optical fiber to the resin
coat is calculated in accordance with a pressure value of resin
inside a coating cup;
the interfacial shear rate is in a range of -1.5 X io5 to
0 sec"1.
A second aspect of the present invention provides an optical
fiber drawing die, which is used for a resin coating apparatus
for forming a resin coat by applying additional resin to a
resin-coated optical fiber, characterized in that:
the interfacial shear rate of the optical fiber to the resin
coat is calculated in accordance with a quantity of a coating
resin;
the interfacial shear rate is in a range of -3 X 105 to
2 X 105 sec'1.
By doing this, it is possible to perform stable resin coating
processing on the resin coated optical fiber even if the absolute
value of interfacial shear rate exceeds 105 sec"1.
A third aspect of the present invention is an optical fiber
drawing apparatus comprising:
a resin coating apparatus for applying a resin to an optical
y
fiber using the optical fiber drawing die according to the first
aspect of the present invention; and
a resin coating apparatus for applying a resin to the
resin-coated optical fiber by using the optical fiber drawing
die according to the second aspect of the present invention.
By doing this, it is possible to realize an optical fiber
drawing apparatus which can perform a resin coating operation
stably even in a high speed drawing operation.
A fourth aspect of the present invention is an optical fiber
drawing method for drawing an optical fiber by using an optical
fiber drawing apparatus according to the third aspect of the present
invention.
By doing this, it is possible to perform a stable resin
coating operation stably even if an absolute value of the
interfacial shear rate exceeds 105 sec-1. Therefore, it is
possible to perform a drawing operation at high speed, and it
is possible to realize a drawing method for an optical fiber which
can realize high productivity and cost reduction.
As explained above, according to the present invention,
interfacial shear rate of an optical fiber to a coating resin
is calculated in accordance with a pressure value of resin inside
a coating pocket, and an interfacial shear rate is set in a range
of -1. 5 X 105 to 0 sec"1. By doing this, it is possible to perform
stable resin coating operation on an optical fiber even if the
absolute value of interfacial shear rate exceeds 105 sec"1.
Also, the interfacial shear rate of the resin-coated optical
fiber to the coated resin is calculated in accordance with the
quantity of coating resin, and the interfacial shear rate is set
in a range of -3 X 10D to 2 X 10° sec"1. By doing this, it is
possible to perform stable resin coating operation in the
resin-coated optical fiber even if the absolute value of the
interfacial shear rate exceeds 105 sec"1.
Also, according to the present invention, interfacial shear
rate is set according to different shapes of dies between dies
for performing a resin-coating operation on an optical fiber and
a die for performing resin-coating operation to the resin-coated
optical fiber. Therefore, it is possible to realize an optical
fiber drawing apparatus which can perform stable resin coating
operation even in a high-speed drawing operation.
Also, according to the present invention, it is possible
to perform stable resin coating operation even if an absolute
value of the interfacial shear rate exceeds 105 sec-1. Therefore,
it is possible to perform drawing operation at a high-drawing
speed and to realize an optical fiber drawing method which can
realize high productivity and cost reduction.
/ACCOMPANYING
BRIEF DESCRIPTION OF THE/DRAWINGS
FIGS . lAand IB are cross sections of an optical fiber drawing
die according to the present invention.
FIG. 2 is a general diagram of processes contained in the
optical fiber drawing apparatus according to the present
invention.
FIG. 3 is across section showing a conventional resin coating
apparatus.
FIGS. 4A and 4B are cross sections showing an example of
a guiding die and a die which are used in a conventional
resin-coating apparatus.
FIGS. 5A and 5B are cross sections showing another example
of a guiding die and a die which are used in a conventional
resin-coating apparatus.
DETAILED DESCRIPTION OF THE INVENTION
An embodiment of the present invention is explained with
reference to the drawings as follows.
In FIG. 1, an example of an optical fiber drawing die
according to the present invention is shown.
The drawing die is used in a first resin coating apparatus
4 which is contained in an optical fiber drawing apparatus shown
in FIG. 2 and a second resin coating apparatus 6.
The shape of the drawing die is determined in accordance
with dimensions A, B, D, and an angle C shown in FIG. 1A, and
dimensions A, B, D, and an angle C and E shown in FIG. IB. These
dimensions and angles are determined in accordance with an
interfacial shear rate of the optical fiber which is to be coated
with a coating resin.
The basic formula for calculating the interfacial shear
rate is as follows.
First, a solution of a Navier-Stokes equation Fl is
calculated by using a boundary condition according to formulae

. Here, v indicates a resin speed, r indicates a position
in a radial direction . p indicates a pressure value . z indicates
a position in an optical fiber under' conditions that a forward
direction is a positive direction. Rf indicates a radius of an
optical fiber. Vf indicates speed of an optical fiber. Rd
indicates a radius of a die (a function of Z).
A solution of the function of Z can be represented by a
formula 4.

Here, a value of the right-hand side of the formula F6 such
as dp/ dz is unknown. In order to obtain a value of <9p/<3z, it
is necessary to determine a pressure value Pini at an entrance
of an upper part of the die and a pressure value POUT: at an exit
of lower part of the die. Otherwise, it is necessary to introduce
a flow amount Q. That is, if formulae F7 and F8 substitute a
formula F6; thus, a formula F9 is obtained.

Here, integration range in the z axis direction is in a
¦ range in which a resin does not produce vortex flow.
When an optical fiber made from a glass is coated, interfacial
shear rate is calculated by substituting apressure value of coating
resin in a coating pot into a Pini at an entrance of top of a die.
When a resin-coated optical fiber is coated, an interfacial shear
rate is calculated in accordance with a flow amount of resin which
is calculated according to a coating diameter of resin which is
already coated.
As a result of an experiment which is executed for the present
embodiment, a range of stable interfacial shear rate is -1.5 X
105 to 0 sec-1 when an optical fiber which is made from a glass
is coated by resin. Also a range of stable interfacial shear
rate is -3 X iob to 2X105 sec-1 when a resin-coated optical fiber
is coated.
An optical fiber drawing apparatus according to the present
invention uses a die having -1.5 X 10s to 0 sec"'' of interfacial
shear rate in a first resin coating apparatus 4 for performing
a resin coating operation on an optical fiber. An optical fiber
drawing apparatus according to the present invention also uses
a die having -3 X 10° to 2X103 sec"1 of interfacial shear rate
in a second resin coating apparatus 6 for performing a resin coating
operation to a resin-coated optical fiber. That is, such a
structure is employed so as to improve drawing efficiency by having
a die for coating an optical fiber by resin and a die for coating
a resin-coated optical fiber respectively. The shapes of the
die are different each other.
Also, an optical fiber drawing method according to the
present invention uses the above-mentioned optical fiber drawing
apparatus to perform a drawing operation for an optical fiber.
By this method, it is possible to realize stable resin applying
operation and productive drawing operation in an resin coating
apparatus even in a high drawing speed.
Hereinafter, an example of a shape of an optical fiber drawing
die and calculation of interfacial shear rate is shown as follows.
Example 1
In an example for performing a resin coating operation on
an optical fiber made from glass, an example of calculation of
interfacial shear rate and drawing operation by using dies (types
1 and 2) shown in FIGS. 1A and IB are performed under conditions
that coating pressure is 0.3 Mpa, resin viscosity is 0.9, 1.5,
and 2.0 Pa*sec, a glass diameter is 12 5 fim, and a finished-coated
diameter is 190 Aim. A result for the type 1 die is shown in TABLE
1. A result for the type 2 die is shown in a TABLE 2.
A, B, C, and D in the TABLE 1 indicate the same sections
and angles shown in FIG. 1A. A, B, C, D, and E in the TABLE 2
indicate the same sections and angles shown in FIG. IB. Symbols
"O" in TABLES 1A and IB mean that coating error did not occur
regardless of resin viscosity and speed of resin coating operation
such as accelerated speed phase or operational speed. Symbols
"A"mean that at least one coating error occurred during the
accelerated speed phase at at least one resin viscosity. Symbols
"X" mean that an optical fiber broke because of abnormal coating
during accelerated speed phase or an abnormal coating occurred
during an operational speed.
Example 2
In an example for performing a resin coating operation on
a resin-coated optical fiber, interfacial shear rate is calculated
by substituting an actual measured value so as to perform a drawing
experiment by using dies (types 1 and 2) shown in FIGS. 1A and
IB under condition such that resin viscosities are 0.9, 1.5, and
2.0 Pa*sec, diameters of already-coated optical fibers were 190
|ii. Results for the type 1 die are shown in TABLE 3. Results
for the type 2 die are shown in TABLE 4.
Symbols such as "O", "A", and "X" in TABLES 3 and 4 have
the same meanings the cases in TABLES 1 and 2. When a drawing
operation was impossible because the optical fiber broke, a value
which was inferred by data obtained from a case of low line speed
was used instead.
According to the above results, a stable range for
. interfacial shear rate is preferably -1.5 X 10'5 to 0 sec-1 in a
case in which an optical fiber made from a glass is coated by
, resin. It is preferable that a coated optical fiber be coated
at a stable interfacial shear rate within a range of -3 X io5
to 2 X 105 sec"1. According to this example of an optical
fiber drawing die, interfacial shear rate of an optical fiber
to the coated resin is calculated in accordance with a pressure
value of resin inside a coating pot, and the interfacial shear
rate is set in a range of -1.5 X 105 to 0 sec'1. Therefore, it
is possible to perform a stable resin coating operation on an
optical fiber even when an absolute value of the interfacial shear
rate exceeds 105 sec"1.
Also, according to this example of an optical fiber drawing
die, interfacial shear rate of the resin-coated optical fiber
of the coated resin is calculated in accordance with a coated-resin
diameter, and interfacial shear rate is set in a range of -3 X
105 to2 X 10° sec"1. Therefore, it is possible to perform a stable
resin coating operation on the resin-coated optical fiber even
when an absolute value of the interfacial shear rate exceeds lcf"1
sec"1.
Also, in this example of an optical fiber drawing apparatus,
interfacial shear rate is set according to different shapes of
dies between a die for performing a resin-coating operation to
an optical fiber and a die for performing resin-coating operation
on a resin-coated optical fiber. Therefore, it is possible to
perform a stable resin coating operation even in a high-speed
drawing operation.
Also, in this example of an optical fiber drawing method,
it is possible to perform stable resin coating operation by using
the above-mentioned optical fiber drawing apparatus even if the
absolute value of interfacial shear rate exceeds 105 sec"1.
Therefore, it is possible to perform drawing operation at higher
drawing speed and to realize an optical fiber drawing method which
can realize high productivity and cost reduction.

An optical fiber coating die is made such that an interfacial
shear rate of the optical fiber to the resin coat is calculated
in accordance with a pressure value of resin inside a coating
cup, and the interfacial shear rate is in a range of -1.5 X 105
to 0 sec-1. Also, an optical fiber drawing die is made such that
the interfacial shear rate of the optical fiber to the resin coat
is calculated in accordance with a diameter of a coating resin,
and the interfacial shear rate is in a range of range of -3 X
105 to 2 X 105 sec-1. By doing this, an optical fiber drawing
die which can be used in an optical fiber drawing method so as
to realize stable resin coating operation even in high-speed
drawing operation and high productivity can be realized.