Note: Descriptions are shown in the official language in which they were submitted.
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METHOD A~D APPARATUS FOR INSULATING ELECTRICAL CONDUCTOR
This invention relates to methods and apparatus for
insulating electri Cdl conductor wires.
The e1ectrical properties of an insulated electrical
conductor wire are determined partly by the properties of the
insulating material itself and its thickness surrounding the wire and
also partly by the diameter of the conductor wire. In the manufacture
of insulated conductor wires, it is essential for certain predetermined
electrical characteristics to be obtained, e.g. where the conductor
wire is to be used in the manufacture of a telecommunications cable.
These essential electrical properties may only be obtained if the
diameter of the conductor wire is controlled within extremely close
tolerances during the insulating process. It has been found that
while an electrical conductor is being provided with a covering layer
of polymeric insulating material by extrusion, there is a tendency for
the conductor to become reduced in diameter as it passes through the
extrusion die.
The degree of diameter reductiDn is a function of the
differential in velocities of the conductor wire and the molten
polymeric material moving through the die orifice. For instance, in a
case where the velocity of the molten polymeric material is equal to
or greater than that of the conductor wire through the die orifice,
then no stretch occurs in the wire dntl the didmeter 1s mdlnldirl~d a~
that desired. On the other hand, in d case where the speed of the
molten polymeric material is slower than that of the conductor wire at
the die orifice, it has been found that a diameter reducing drawing
action takes place upt)n the wire. Such diameter reductions are in
fact excessive dS they cduse d departure from the desired electrical
characteristics of the insulated wire. For instance, when insulating
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a 22 AWG wire, i.e. of 0.0201 inches diameter, it is not uncommon for
d drdwiny action to result in d final diame-ter of .OI90 inches. While
such a reduction is of the order of 0.0011 inches and may appear to be
minimal, nevertheless i-t lies outside the tolerance reduction of
0.0003 inches (i.e. to a diameter of 0.0198 inches) for achieving the
desired electrical characteristics.
The attainment of substan-tially equal speeds for the molten
po1ymeric material and the conductor wire through the die orifice is
desirable but cannot be achieved on a continuous basis.
While no diameter reduction results in conductor wire if the
extrusion speed of the molten polymeric material exceeds that of the
conductor wire through the die orifice, there is the disadvantage that
this results in die swell in the polymeric material after extrusion.
Die swell produces an outside diameter to the insulation which is
greater than that required and causes variation from the desired
electrical characteristics of the insulated wire. In addition to
this, when die swell takes place, more of the dielectric material is
used than would be necessary i~ the insu1ated conductor wire were made
to its desired diameter.
The invention provides a method and apparatus which produces
an insulated conductor in which the diameter of the conductor wire and
also the outside diameter of the insulation mdy be held within
extremely close tolerances.
Accordingly, the present invention provides an appdrdtus for
insulating an electrical conductor wire cDmprising an extruder
cross-head having an extrusion die deFlning a die orifice, internal
wall means defining a flow passage for directing molten polymeric
dielectric material to the die orifice, a core tube within the
cross-head and upstream from the die orifice, the core tube defining a
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guide passage for directing the conductor wire through the die orifice
as it moves along a passline, and mounting means for the core tube to
render it adjustable into any desired position along the passline so
as to controllably adjust the distance between the core tube and the
5 wall means and thereby adjust the cross-sectional area of the flow
passage.
As may be seen, the apparatus of the invention includes a
core tube which is movable in the direction of the passline thereby
providing for adiustment in the cross-sectional area of the flow
passage and a control in the relative velocities of the molten
material and of the conductor wire as they pass through the die
orifice. As a result, the speed of the molten material may be
controlled to be substantially equal to that of the conductor wire or
insignificantly in excess of the velocity of the conductor wire so
lS that no reduction or an insignificant reduction in the thickness of
the conductor wire results.
In a preferred arrangement, the care tube is mounted within
the cross-head by a mounting medns which camprises à screw thread
means. In this screw thread means, a screw thread provided on the
core tube is received within a complementary screw thread on the
cross-head for mounting the core tube within the cross-head. Rotation
of the core tube to cause relative rotation of the screw thrl~ads
causes the core tube to move dlong the pdssl~ne. Thus d ldr(Je dn~Uldr
movement of the core tube w~ll ef~ect a short movement along the
passline and whence its movement into desired positions may be
lnfinitely controlled.
In use of the apparatus, the thickness or diameter of the
conductor wire after the insulation has been extruded around it may be
monitored manually, e.g. by ~aking sdmples of the wire at intervals
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and measuring them. The core tube may then be adjusted in position at
intervals as necessary so as to change the velocity of the molten
material passing throuyh the die orifice and thereby adjust the
diameter of the wire towards that required. However it is preferred
to be able to provide d continuous check upon the diameter or
thickness of the conductor wire. In a preferred apparatus, a
monitoring means downstream from the cross-head is provided for
monitoring a parameter indicative of the thickness of the wire when
covered with the insulation 1ayer. In this case analyzing and control
I0 means is also required to analyze the monitored values and for
controlling the position of the core tube along the passline. The
analyzing and control means i5 operable upon an analyzation of a
monitored value differing from that required for a desired thickness
of conductor wire to actuate a driving medns and move the core tube in
the appropriate direction to change the conductor wire thickness
towards that desired.
The invention also includes a method of insulating an
electrical conductor wire comprising;- passing the wire dlong a
passline through a core tube in a cross-head and then through a die
orifice of an extrusion die of the cross-head while extruding a molten
polymeric dielectric material around the wire; monitoring a parameter
indicative of the thickness of the wire at a position downstream From
the cross-head to deternl~ne whether ~h~ wire th~ckness ~Fers from
that deslred; and upon any monltored value differing from that
required for a desired conductor wire diameter, controllably moving
the core tube in an appropriate direction along the passline and into
a desired position so as to controllably adjust the distance between
the core tube and an internal wall means of the cross-head which
defines a flow passage for directing molten polymeric dielectric
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mdterial to the core tube and thereby ~djust the cross-sectional area
of the passdge to cause the thickness of the wire to approach that
desired.
Embodiments of the invention will now be described by way of
example, with reference to the accompanying drdwings, in which:-
Figure 1 is a diametric view, partly in cross-section, of
apparatus for insulating an electrical conductor wire according to a
first embodiment;
Figure 2 is a cross-sectional view of part of an extruder
cross-head of the apparatus shown in Figure 1; and
Figure 3 is a view similar to Figure 1 of d second
embodiment.
As shown in Figure 1, in a first embodiment, an apparatus 10
for insulating an electrical conductor wire 12 comprises an extruder
cross-head 14 secured to an outlet end of an extruder (not shown) for
molten polymeric dielectric material 16. Flow pdssages within the
cross-hedd convey the molten material 16 to a die orifice 1~ provided
by an extrusion d~e 20 held withi~ a die holder 22. The die hclder is
held in position within a main housing 24 of the cross-hedd by a
retaining nut 26 screwed into the upstream side of the housing 24.
A core tube 28 is disposed within the die holder 22. The
core tube is basically of conventional construction and comprises a
tapering downstream end 30 and has a coaxial passdyt? 32 for directin~J
the conductor wire 12 through the die orlfice IIS thL` wire moves along
its pdssline. The core tube is provided with d mounting means to
render it adiustable into any desired position atong the passline.
This mounting means comprises a screw thread medns 34 consisting of a
mdle screw thread on the core tube upstream from its tapered section
and a complementary female screw thread Formed within the upstream end
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of the die holder 22. As can be seen from Figure 1, the die is thus
locatable in position for operation by its screw threaded reception
within the die holder. Thus when in operating position, the rotation
of the core tube in either direction will cause a corresponding
movemenl of the core tube in the appropriate direction along the
passline of the conductor wire 12. As a result of this movement, the
downstream end 36 (Figure 2) of the core tube mdy be caused to move
towards or away from internal wall means, i.e. frusto-conical wall 38
of the die holder, which defines a flow passage for the molten
lo material as it moves towards the die orifice. Any movement in this
fashion by the core tube results in an adjustment in the
cross-sectional area of the flow passage at the position where the
core tube and the wall 38 lie closest together, i.e. between the wall
and the peripheral edge 40 of the end 46 as shown in Figure 2. As
will be appreciated, such displacement of the core tube will create a
change in the pressure upon the polymeric material within the extruder
and d resultant change in velocity of the molten material as is passes
between the wall 38 and the care tube towar~s the die orifice. This
chdnge in velocity of the molten material is accompanied by a change
in the relative velocities of the molten material and the conductor
wire.
With the core tube disposed in one particular position along
the passline, the velocities of the molten material and the conductor
wire are substanti~lly e~qudl and the~ con(luctor wlre Is not drawrl dowrl
25 dS it pdsses through the die ori~ice. With this set of conditions, it
is also found that there is substantially no die swell in the molten
material so that the finished outside didmeter of the insulation is as
required. However should the core tube be closer to the wall 3~ than
its optimum position to provide the desired set of requirements, then
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there is d pressure bullt up upon the molten ma-terial wi-thin the
extruder and the material is forced through the gap between the core
tube and the wall 38 at a greater velocity thereby beins extruded
through the die orifice also dt d gredter velocity. In these sets of
conditions, the diameter of the wire still remains at that required
because there is no drdw down upon the wire. However more molten
material is used than is required thereby resulting in die swell upon
exit from the die. This is a corresponding incredse in the outside
diameter beyond that needed for obtaining optimal electricdl
characteristics in the finished insulated conductor and certain of the
electrical characteristics may depart from their desired values.
On the other hand if the core tube is moved upstream away
from the optimal position, then the distance between the wall 38 and
the core tube increases and the pressure upon the molten material is
reduced thereby resulting in the velocity of the material being less
than the conductor wire as it passes through the die orifice. Under
this set of conditions, while the diameter of the insulated conductor
may be substantially as required, a resultant draw down upon the wire
decreases its diameter and also causes a departure of the electrical
characteristics in the finished product from those required.
Hence it is possible to be able to adjust the position of
the core tube within the cross-head easily by causing its rDtation.
As can be seen, a small rotation ~F the r;ore tuhe will prodlJce d
corresponding dnd srnaller movement of the core tube axidlly along the
~5 passllne so that an infinite adjustment of the core tube is rendered
possible. Such an adjustment may be perforlned mdnudlly dS a result of
measurements tdken dt intervals upon samples of insulated wire.
However in d preferred arrangement of the invention as shown in this
embodiment provision is made to monitor a parameter indicative of the
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conductor wire thickness. As will be described, this will allow for
the provision of a continuous control upon the position of the core
tube so as to ensure that there is no significant draw down of the
conductor wire and that the outside diameter of the insulation is
substantially that required in the finished product.
A monitoring means for the above purpose comprises a speed
monitoring device 42 which is of conventional construction for
monitoring the speed of wire. Electrical signals are sent from the
device 42 to an analyzing and control means in the form of a
1~ microprocessor 44 which analyzes the ~alue of the signals received
from the sensor 42 together with datum signals received from another
speed sensor 46 which is disposed at the upstream end of the
cross-head. The signals received from sensor 46 correspond to the
speed of the uninsuldted wire. As can be seen, if the signal received
from sensor 42 corresponds to d speed which is greater than thdt at
sensor 46 then this must be as d result of a reduction in diameter or
thickness of the conductor wire. When the signals differ in this way
then d control signal is sent from the microprocessor 44 to a driving
means in the form of a stepper motor 48 which is drivably connected to
an endless drive member or belt 50 of the driving means. Belt 50
drivably connects d pulley wheel 52 of the motor 48 with d pulley
wheel 54 secured coaxially at an upstream end of an extension 56 of
the core tube.
It follows thdt ~n use~ with the conductor 12 pdssing
through the cross-head and being provided with a layer of lnsulation
to form an insulated conductor wire 58, then the two speed sensors 42
and 46 provide signals which are being continually analyzed within the
microprocessor 44, The microprocessor operates to send signals to the
stepper motor 48 to control the position of the core tube 28 to ensure
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that the speed of the insulated conductor 58 is not significan-tly
greater than the input speed of the conductor as measured by the
sensor 46. The stepper motor i5 tended to be driven by the
microprocessor to urge the core tube upstream in order to create a
slight draw down of the conductor. ~owever, this causes a
differential with signals received from the sensors 42 and 46. Thus,
control signals from the microprocessor control the stepper motor so
as to hold the upstream movement of the die to absolute minimum
consistent with holding the draw down to an insignificant amount.
The apparatus is also provided with a pressure limit sénsor
59 which is dttdched to the housing 24 and senses the pressure of the
molten material within the cross-head as it moves towards the die.
This sensor 59 is connected with the microprocessor and operates to
modify the signals for operation of the core tùbe to ensure that
pressure conditions within the extruder do not exceed an upper limit.
It has been found that with use of the invention according
to the first embodiment, a satisfactory product has resulted. In this
product, the degree of draw down o~ a 22 AWG conductor wire has been
minimal and has resulted in finished diameters for the wire within
dcceptable tolerances, i,e, the wire has been reduced to d diameter no
less than 0.0198 inches from a starting diameter of 0.0201 inches. In
addition to this the finished diameter of the insulation has differed
insignificant~y from that required in the finished ~3roduct tu l3r(3duce
the desired electrical chdrdcteristics ~or the insulated conductor,
The invention is dlso ap~licdble to insulating conductor
wire with more than one layer of insulation. For instance as shown in
Figure 3, in d second embod~ment, a conductor wire 12 is insuldted
with two layers of polymeric materidl. In this embodiment, some parts
o~ apparatus have the same design and operation as corresponding parts
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described in the first embodiment. In such cases the same reference
numerals are used in the second embodiment.
In the second embodiment, the core tube 28 is disposed
within a die holder 60 which is longer axially than the holder 22
described in the first embodiment. The holder 60 is desi~ned to
accommodate two dies 62 and 64. The upstream die 62 and its
positional relationship to the core tube 28 is substantially as
described for the die 20 in the first embodiment. Thus a flow passage
66 is provided between the core tube and an internal wall 6~ of the
die 62 and the affected cross-sectional ared between the core tube and
the wall may be varied by axial movement of the insert. The die 64 is
disposed slightly downstredm from the die 62 so dS to form the
cross-head into a dual cross-head construction. A second extruder
~ot shown) is provided for supplying molten mdteridl for the outer
layer dt die 64. With this arran~ement, the two layers of insul~tion
mdy both be solid, i.e. non-cellular or alternatively one of the
layers may be of cellular construction. In this particuldr
embodiment, the inner layer formed by the die 62 is of cellular or
foam construction and the outer layer Formed with the die 64 is of
solid construction.
Operation of the apparatus according to the second
embodiment is identical with that in the first embodiment with re~ard
to the application oF the flrst l~yer of insul~ion dnd thre eoni;rol oP
the thickness of the conductor wire. It hàs ~een found that drdw down
oF conductor wire occurs only during application of the inner
insulation layer. The control function therefore rnust be applied
dur;ng the appl;cation of the first layer. Hence in the second
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embodilnent dS shown in Figure 3, the movab1e core tube operates in the
position upstream from the first die and no adjustment requirements
are necessary for movement of -the conductor through the second die.
