Note: Descriptions are shown in the official language in which they were submitted.
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This invention relates to a method of coating a
continuou~ glaqs fiber filament with plastic.
In the field of gla~q fiber optics it is desirable
to coat a continuous gla~s fiber filament with a plastic in
order to ~trengthen the-strand. At presont such a coating
is applied by feeding the filament from a give-up reel con-
tinuously through a re-heating oven and then through liquid
plastic. The re-heating step improvQs the adherence of the
plastic to the filament. The problem with re-heating the
10" glass filament i8 that the heat cannot be applied uniformly
and d~fferential stresses occur. These stresses, added to the
tensions created when the filament is drawn from the give-up
reel, create strain~ in the filament which affect its oper-
ability as an optical device.
Another problem encountered in the known method of
coat~ng continuou~ glass fiber filaments is that moisture and
dust contaminate the surface of the filament between the time
it is drawn and the tim~ it i8 coated. Also, when the glas~
filament is drawn it contains micro-cracks in its surface and
if the filame~t i8 left to stand the micro-crack~ enlarge.
The surface conta~ination and the micro-cracks both adver~ely
affect the adherence of the plastic coating to the filament
which reduces its strength.
It is an object of the pre~ent invention to provide
an improved method of coating a continuous gla-qq fiber filament.
In its broadest aspect the invention consi~ts of a
method of coating a continuous glaqs fiber fila~nt by the
extru~ion of plastic, comprising the ~tep~ of: charging with
plastic coating material a closed vertical cylinder having a
die aperture in the lower end thereof and an axial core tube
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ter~inating in the die aperture to form an annular orifico,
the plastic being liquified to pas~ through the die orifice;
drawing the filament from a source of molten glass coaxial
with the core tube, continuouQly through the core tube; and
introducing pres~urized ga~ into the upper portion of the
cylinder to force the liquified plastic through the annular
orifice and onto the filament.
An example embodiment of the invention i8 shown in
the accompanying drawing in which:
10, -Pigure 1 i8 a cross-sectional view in elevation of
an extruder for use in carrying out the method of the invention.
The e~bodiment shown in the drawing con~ists of an
extruder 10 comprising a cylinder 12 which is vertically aligned
axially with the outlet orifice of a furnace 13 for producing
filaments from molten glas~.
Cylinder 12 ha~ an upper end closure 14 and a lower
die clo~ure 16 to form a chamber 18 within the cylinder. Uppex
end closure 14 is removably fixed to cylinder 12 by bolts 20
and ha~ a central bore 22 which i8 threaded to accept a boss
24 of a core tube 26 extending axially through the cylinder~
Bore 22 i8 sealed by a threaded nut 28 which bears a~ain~t an
0-ring 30 lying against an annular shoulder 32 in the bore.
Lower die closure 16 is removably fixed to cylinder
12 by bolts 34 and has a contoured aperture 36 into which the
free end or nozzle 38 of core tube 26 projects to form an
annular orifice 37. Nozzle 38 is removably attached to core
tube 26 by set screws 39 and is shaped to act as a valve to
~eat in aperture 36 o~ closure 16 when the core tube i~ moved
down~rdly.
A circular pressure plate 40 extends across cha~k~r 18
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normal to th~ axi~ of cylinder 12 and ha~ a central aperture
42 through which core tube 26 pxojects. Pressure plate 40 is
free to move dpwardly and downwardly in cha~ber 18. A support
plate 44 parallel to pressure plate 40 is located adjacent
lower die closure 16 with a plurality of large aperture~ 46.
Cylinder 12 carries two ports opening into chamber
18: a port 48 located adjacent upper end closure 14 for the
introduction of gas, and a port 50 located adjacent lower die
closure 16 for the introduction of liquid plastic. An annular
10~ ` band heater 52 circumscribes cylinder 12 between ports 48
and 50.
A level indicator rod projects through a seal 56
in upper end closure 14 and extends into chamber 18 to bear
against pressure plate 40. A thermocouple 58 projects through
cylinder 12 to chamber 18.
In the operation of the example embodiment, cha~ker
18 is opened by removing bolts 20 and lifting off upper end
closure 14 together with core tube 26 and indicator rod 54.
Pressure plate 40 iæ also removed from chamber 18 and the
cha~ber i8 charged with a machined billet of ~olid plastic
coating material, such a~ nylon or polypropylene, which rest~
against support plate 44. The billet ha~ a central passage
to receive core tube 26. Pre~sure plate 40 is then replaced in
ch~mber 18 to rest on the billet below ga3 port 48 and upper
end clo~ure 16 is replaced on cylinder 12 with core tube 26
extending through pressuro plate 40 and through the billet,
and with indicator rod 54 resting on the pressure plate. Gas
port 48 i8 connected with a ~ource of inert ga3, ~uch as
nitrogon, at constant pressure and liguid plastic port 50 is
2~
closod by suitable p~ug mean~. Boss ~ is screwed downwardly
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in bore 22 to have free end 38 of core tube 26 seat in die
aperture 36. Band beater 52, connected to a source of elect-
ricity, is switched on to melt the billet of pla~tic within
cha~ber 18. The temperature of the pla_tic within chamber 18
is sensed by thermocouple 58.
Extruder 10 iq now ready to coat a cont~nuou_ gla~3
fiber filament 60 which is pulled rom glas~ furnace 13 and
pas~ed through core tube 26. Boss 24 of core tube 26 i~
scxewed upwardly t~ un_eat nozzle 38 from die aperture and
inert ga~ at constant pressure i8 introduced into chamber 18
through port 48 above pressure plat- 40. The pressurized gas
acts against plate 40 to force the liquified plastic out
through die aperture 36 around nozzle 38 as strand 60 is pa~sed
through core tube 26, producing a plastic coating 6~ on the
filament. me thickness of coating 62 is governed by the
po_ition of nozzle 38 in die aperture 36, by the gaQ pre~sure
and by tho speed of filament 60 which attenuates the coating.
The device of the invention applies a constant
pres~ure to the liquid plastic material which produce~ a uni-
form extrusion of the plastic onto filamsnt 600 When using
a machined billet of plastic, pres~ure plate 40 i~ not nec-
e~sary but the plate minimize~ ga~ tracking through the die.
Support plate 44 is optional to seat the billet above lower
end clo~ure 16.
If desired, liquid pla~tic may be introduced dir-
ectly into chamber 18 instead of using a machined billet of
pla~tic. The liquid plastic i_ introduced by connecting port
50 with a suitable ~crew or ram extruder. The use of the
device of the prqsent invention in conjunction with a convent-
ional screw extruder would s~ooth out the pressure fluctuations
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of such an extruder and provide a smooth coating of plastic
on a continuous filament. The leval o~ liquid pla~tic in
cham~er 18 using either a,mRchined billet or a conventional
extruder i8 sensed by indicator rod 54 which could be employed
to provide feedback control for the conventional extruder.
If it is necessary to debydrate the plastic in
cha~ber 18 kafore extruding it, a vacuum can be applied to
port 48 in the absence o~ pres~ure plate 40.
Because liquid plastic i~ a non-Newtonian fluid,
1~ shear stresses complicate its flow characteris~ic~. The
device o the present invention reduces the shear stresses
applied to the plastic by conventional extruders and cause~
the material to behave more like a ~ewtonian fluid which i8
more predictable and hence!~more controllable~
By drawing glass fiber filament 60 directly from
a molten source through extr~der 10 the ~ filament is
coated with plastic kefore the ~urface of the filament can
become contaminated or the micro-cracks can become enlarged.
Also, the residual heat of fila~ent 60 can be employed to
obtain the required bonding of plastic coating 62 to the
filam~nt, Of course extruder 10 mu~t be spaced below glass
furnace 13 a distance to allow filament 60 to cool to a
temperature which will not degrade plastic coating 62. It
will also be appreciated that no lateral stresse~ are imposed
on ila~ent 60 before it is coated.
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