Note: Descriptions are shown in the official language in which they were submitted.
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E~ACKGROUND OF THE INVENTION
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This invention relates to an ori~ice plate construction
for a glass fiber forming bushin~.
In the manufac-ture of glass fibers, a bushiny with a
bottom plate having 400 to 2000 minute orifices therein is
charged with molten glass, which then flows down -through the
orifices to draw the glass into fibers or filarnents. The
orifice plate typically becomes downwardly deformed in a convex
manner during prolonged usage, however, owing to the weight
of the molten glass in the bushing, the tension of the
withdrawn fibers, and the high operating temperatures of from
1100C to 1300C. Such deformation causes a difference in heat
radiation between the cones formed in the peripheral areas of ~he
orifice plate and those formed in the central area thereof, and in
addition the peripheral cones tend to drift or migrate
toward the central area of the deformed orifice plate. This
reduces the stability of the cones and increases filament
breakage, and when adjacent cones merge or become joined
together it is difficult to restore their separation.
To overcome this difficulty the distance between the
orifices or the thickness of the orifice plate may be increased,
or the plate may be strengthened by reinforcing ribs. These
conventional solutions are disadvantageous, however, in that
when the size of the orifice plate is increased the size of
the overall bushing is correspondingly increased. As a result
it is difficult to uniformly heat the molten glass and its
orifice flow properties therefore become uneven, which leads
to filament breakage and an attendant reduction in productivity.
Even when the filaments~do not break their diameters var~, which
~ reduces the strength~of the fibers In addition, as the bushings
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1 are mad~ of pla-tin~, any increa~e in their size involves
corresponding increases in equipment investment and in the
cost of the filaments produced.
An alternative technique has been to use a relatively
smaller bushing having ~000 -to 6000 orifices in the flat
bo-ttom plate at intervals oE less than 3 ~n, and to direct
an air flow at the bottom of the orifice plate to cool the cones
of molten glass and increase their viscosity, thus preventing
the cones from joining together. This technique increases
13 productivlty, but suffers from the disadvantage that as the
orifice density is increased the orifice plate becomes
necessarily weakened, which enhances i.ts convex deformation
tendency. Such deformation or curvature results .in a non-
uniform cooling effect from the air flow thereby causing severe
production problems such as, for example, fibers breaking off,
a variance in the diameter of the fibers and similar problems.
Thus, in spite of the many and varied prior art .
approaches, orifice plate deformation remains a significant
problem in the industry.
~20 SU~MARY OF T~IE INVENTION
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Accordingly, an object of this invention is to overcoma
the above-described difficulties accompanying conventional
; glass fiber manufacturing techniyues by providing a bushing
in which orifice plate deformation is prevented and in which
the orifice density can be incrèased without weakening the
bushing or increasing its dimensions.
Another object of the invention is to provide such a
bushing in which the cones of molten glass flowing out of the
orifice plate are more uniform and stable, thereby enabling
increased production efficiency.
g~ ~a~
These and o-ther objects are achieved by curving the
orifice pla-te inwardly in a uniform concave manner, thereby
greatly increasing its s-trength, rigiclity, and resistance -to
ou-tward deformation, even under extreme and prolonged operating
conditions. Such inward curving or bending also increases
the effective surface area of the orif.ice plate, which enables
the number and density of the oriEices to be increased, or
alternatively the size of the plate and bushing may be reduced
without reducing the total number of orifices.
1~ When the concave orifice plate of the invention is
applied to an air blowing process, the cooling air is found
to flow more smoothly and uniformly over the inwardly curved
surrace of the plate, whereby the cooliny effect on the cones
is much more even and the filament diameters are thus more uniform.
The orifice plate may be either rectangular or circular, whereby
its inward curvature is either arch-shaped or dome-shaped,
respectively.
BRIEF DESCRIPTION OF THE DRAWINGS
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In the Drawings:
Fig. 1 shows a sectional elevation of a glass fiber
~orming apparatus employing a forming bushing having a concave
or arch-shaped orifice plate according to this invention,
Fig. 2 shows a geometric diagram for explaining a
method of determining the radius of curvature of the orifice
: plate according to the invention, and
Fig. 3 shows an enlarged sectional elevation of a
modified bushing according to the invention, in which the
orifice plate is reinforcedO
~ErAILED DESCRIrlION OF THE PREFERRED EMBODIMENTS
ReEerring now to the drawings, Fig. 1 shows glass fibers
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1 being withdrawn at high density through a bushiny 1 having an
inwardly arched or concave rectangular orifice plate 2 extending
across the bottom of the bushing perpendicular to the longi-
tudinal bushing axis or ~ilament wi~hdrawal direction. Molten
glass in the bushing flows out through a plurality of circular
orifices 3 in the pla-te 2, forming cones 4 which are drawn
downwardly into glass fibers or filamen-ts 5. The relatively
small bushing in Fig. 1 has been recently developed, and the
plate 2 has so many orifices 3 that under ordinary conditions
the cones 4 formed on the lower surface of the plate would
merge and join together to preclude separate -Eiber formation.
As shown in Fig. 1, however, a flow of cooling air is directed
at the lower surface of the orifice plate from a blower 6
to increase the viscosity of the cones and thereby prevent
their unwanted joining or merging.
The orifices in the plate 2 have an ordered or symmetri-
cal geometric arrangement, in the manner of mesh intersections,
for example. If the orifice plate is flat as in a conventional
bushing, it is thus liable to bend or deform downwardly in a
~ convex manner due to the weight of the molten glass and the
tension of the glass fibers being withdrawn. According to
this invention, however, the orifice plate is curved inwardly or
concave, and therefore has a considerably increased resistance
against any such weight deformation. Furthermore, with such
curvature the surface area of the orifice plate is increased
as compared with that of a flat plate mounted in the same
bushing, and the number of orifices can thus be correspondingly
increased.
The preferred radius of curvature range of the orifice
plate is, considering material dynamics and the actual fiber
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1 forming conditions, determlned from ~he dis-tance L between
the vertex of the orifice plate and the collection point of the
filaments 5, the span or width ~of the orifice pla-te, and the
angle a between the axial direction of an outermost orifice
and the lie of a filament 5 drawn therethrough. ~ccording to
experimental results it has been found that the angle a is
preferably less than 15 deyrees, whereby the radius of
curvature R of the orifice plate is within the range of from
35 mm to 200 mm. These dimensional rela-tionships are
illustrated in Fig. 2. When the radius of curvature is less than
35 mm the workability or uniform bendability of the orifice
plate is reduced to a difficult level, and in addition the
cones of molten glass flowing out of the orifices are prone
to sag or migrate toward the outer edges of the plate and join
together. When the radius of curvature is more than 200 mm,
on the other hand, the downward deformation resistance of the
plate becomesboo low and it is sub~ect to the same weight
sagging drawback as a conventional flat-plate.
The plate 2 is produced by drilling orifices of pre-
2~ determined diameter and spacing in a flat platinum plate and
then bending it to a desired, uniform curvature in a cold press
or the like, whereby the axes of all of the orifices are directed
at the center of curvature. As a result, angular fluctuations
between the directions of the orifice axes and attendant
filament tension differences are less than in a conventional
flat orifice plate, and each cone is more uniform in shape
which increases the fiber forming stability. Furthermore~
the air applied to the lower surface of the orifice plate flows
more smoothly between the cones owing to the plate curvature,
which enhances the cooling effect.
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1 In a comparative test a conventional bushing wi-th
a flat bottom plate having 2000 orifices drilled therein at a
density of 70 orifices/cm2 and a bushing with a curved orifice
plate having the same number and density oE orifices were
operated together to wi-thdraw glass fibers at a rate of 800 g/min.
The service life of the flat orifice plate was 52 days, while
that of the curved oriflce plate according to the invention was
more than 200 days.
Reference numeral 8 designa-tes a heat insulating
packing, such as asbestos or the like, disposed around the bushing
1 within a frame cup 9 having a bottom opening to accommodate
the orifice plate. A coating roller 10 is disposed at the
filament collection point for applying a sizing agent 11 to the
filaments to reduce frlction and prevent their adherence to each
other. The collection point is shown as being slightly offset
from the bushing axis in order to further enhance the uniformity
of the cooling air flow from the blower 6.
Fig. 3 shows a modified construction wherein one or
more reinforcing bars 7 are disposed in parallel between the
; 20 opposite interior walls of the bushing. Both ends of each
bar are fixedly secured to the walls and to the top or crown
portion of the ori~ice plate 2. The bars may be thin flat
plates or rods, and increase the deformation resistance of the
orifice plate to the point where a service life of
approximately one year may be expected. Some orifices in the
top portion of the plate are closed or sealed off when the
reinforcing bars are welded thereto, but their number is very
small when compared with the total number of orifices and the
.
effects thereof axe thus negligible.
Although the invention has heen described with reference
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1 to an arched, rectangular orifice pla-te, it :is equally applicable
to a circular orifice plate bent upwardly in the form of a dome,
in which case a suitable number of reinforcing bars may be
mounted between the top portion oE the dome and the side walls
of the bushing.
Because of their strengthened, curved construction,
the orifice plates according to the inven~ion can be made
smaller in size and their service life increased from four to
seven times that of conventional plates, without any reduction
in the total number of orifices owing to the increased acceptable
density thereof. Accordingly, the amount of platinum used in
manufacturing the bushings can be reduced, which constitutes a
further economic advantage.
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