Novel fiber-forming bushing and tip plate

This invention relates to a novel bushing apparatus and nozzles or projecting orifices known as tips for the production of glass fibers. Particularly, it relates to a bushing having tips with cross sections in the form of finite-sided polygons. It has been shown that the use of noncircular tips to produce round fibers can result in higher tip packing densities than are now present in the art and that tips with a square cross section represent the most preferred embodiment. Still, more preferred is the embodiment which is the subject of the instant invention wherein rows of tips are interconnected by an integrally formed rib that increases the bending stiffness of the tip plate and its resistance to high temperature thermal creep.

The present invention relates to a novel bushing and tip plate for the 
production of glass fibers. More particularly, the invention relates to a 
bushing apparatus for the production of round glass fibers from a tip 
plate with rib stiffeners integrally formed and interconnected with the 
individual tips. Still more particularly, the invention relates to a 
bushing having square tips interconnected by such a reinforcing rib. The 
use of noncircular tips, such as square ones to produce round glass fibers 
is more fully described in my copending application U.S. Ser. No. 
07/357,157 filed May 26, 1989 describing a Novel Fiber-Forming Bushing and 
Tips. 
In the forming of glass fibers utilizing modern technology, 
electrically-heated containers known as bushings, typically constructed of 
precious metals such as platinum or palladium and alloys thereof are used. 
Molten glass is fed into the bushing and flows out through a multiplicity 
of nozzles or projecting orifices (hereinafter referred to as "tips") 
carried on what is commonly referred to as a "tip plate" which typically 
forms the bottom of the bushing. The flow of glass through the tips is 
usually driven by the hydrostatic pressure exerted by the molten glass 
above the tip plate. In some cases, it may be desirable to pre-pressurize 
this hydrostatic head by applying a pressurized gas, such as air, above 
the glass. 
Considerable quantities of heat are generated at the surface of a tip plate 
in a conventional fiber glass bushing. As glass exits the tips, the same 
mechanisms which cool the glass, i.e., natural convection and enhanced 
radiative heat transfer due to the presence of fin coolers, will also 
partially remove some of the heat associated with the tip plate. 
Nevertheless, the tip plate must still be capable of withstanding 
temperatures well in excess of 2,000.degree. F. while maintaining its 
structural integrity. The hydrostatic head mentioned above, which is 
relied upon to maintain the driving force for the flow of glass through 
the tips, also exerts a continuous load on the tip plate. At the high 
temperatures used in forming, this load will eventually lead to thermal 
creep and can result in a severe sag in the surface of the tip plate. This 
ultimately limits the useful life of the bushing. 
In recent years, the size of production bushings has increased to the point 
where tip plates carrying as many as 1,200, 1,800 and even 4,000 or more 
tips are commonplace. Unfortunately, the deformation associated with 
thermal creep has also become more acute. Grain stabilized platinum alloys 
have been developed to help resist creep and there are indications that 
the addition of small amounts of iridium or ruthenium to conventional 
precious metal alloys may also improve the strength of the tip plate. 
Furthermore, since a considerable investment in costly precious metals is 
required to construct a bushing, it would be advantageous to fabricate as 
many tips per square inch that can be feasibly accommodated on the tip 
plate to reduce the quantity of precious metals used while, at the same 
time, minimizing the problem associated with sag due to creep. The number 
of tips or orifices per square inch will hereinafter be referred to as the 
"packing density" of the tip plate. 
The prior art teaches one method for lowering the quantity of precious 
metals used in bushing construction by eliminating the tips entirely and 
replacing them with a flat perforated plate having a large number of holes 
or orifices to accommodate the flow of glass. Unfortunately, as the 
packing density of the orifices increases, both the effective elastic and 
plastic constants of the plate are reduced so that the same hydrostatic 
head produces even greater deformation than would be observed in a 
conventional tip plate of the same size. 
Thus, there is a need to reduce the amount of precious metals used in the 
construction of bushings, especially large bushings having several 
thousand tips, while minimizing the problems associated with thermal 
creep. The instant invention addresses this need. 
SUMMARY OF THE INVENTION 
In accordance with the instant invention, a fiber glass bushing having a 
plurality of tips interconnected by, and integrally formed with, a 
plurality of reinforcing ribs is disclosed. The ribs run parallel to the 
width of the tip plate and structurally incorporate the individual tips 
thereof, thereby increasing the bending stiffness and resistance to high 
temperature thermal creep. The tips themselves may be conventional ones 
having a round cross section or they may be in the form of finite-sided 
polygons as described in my aforementioned patent application. 
Up until now, noncircular tips have been employed only when it was desired 
to produce fibers having noncircular cross sections. For example, U.S. 
Pat. No. 4,636,234 discloses a tip plate containing trilobal orifices for 
the production of similarly shaped fibers. U.S. Pat. Nos. 4,622,054 and 
4,759,784 both disclose other shapes and methods of production. These 
references also teach that in order to accomplish the production of the 
noncircular fibers, extremely high bushing pre-pressures must be used in 
order to force the glass through the tips. Also, a rapid quenching of the 
glass must occur before its surface tension tends to coalesce it into a 
fiber having a round cross section. Furthermore, while this physical 
phenomenon has long been known in the art and methods such as those 
discussed above have been developed to avoid it, it has never been 
apparent to exploit this behavior to produce round fibers from a 
noncircular tip. This topic is more fully addressed in my aforementioned 
copending application. Regardless of the tip geometry used, the instant 
invention can provide a tip plate of increased strength which resists 
thermal creep deformation longer than a tip plate having a conventional 
design. 
Therefore, it is an object of this invention to strengthen the tip Plate of 
a fiber glass bushing in order to resist thermal creep deformation at 
elevated temperatures. 
It is a further object of this invention to increase the packing density of 
tips present on the tip plate of a fiber glass bushing assembly while 
maintaining or increasing the structural integrity of the plate. 
These and other objects of the invention will become more apparent as the 
invention is described in detail with reference to the accompanying 
drawings.

DETAILED DESCRIPTION OF THE INVENTION 
With reference to the drawings, FIGS. 1 and 2 illustrate a conventional 
continuous direct draw process for the production of glass fibers wherein 
molten glass is fed into the top of a bushing assembly (1) and exits from 
a plurality of tips (2) to form individual glass cones which are then 
cooled and attenuated by means of a winder (3) into individual glass 
fibers (4). The individual fibers (4) are brought in contact with an 
applicator (5) and coated with a chemical size or binder. The fibers (4) 
are then gathered into a single strand (6) by a gathering shoe (7), 
normally in the form of a wheel having a grooved rim. The strand (6) is 
then wound over a rotating spiral (8) and onto a cardboard forming tube 
(9) which is rotated by an appropriately powered winder (3). The winder 
may cause either the forming tube (9), spiral (8) or both to reciprocate 
back and forth along their axis of rotation so that the strand (6) passing 
over the spiral (8) is laid down along the length of the forming tube (9). 
Cooling fins (10) are inserted between adjacent rows of tips (2) with one 
end of each fin being attached to a manifold (11) through which a cooling 
fluid, such as water, is pumped. The fins (10) are positioned so as to 
absorb radiative heat from the individual glass cones and conduct it to 
the manifold (11) where it is removed by the cooling fluid. The fins also 
remove some heat radiated by the tip plate (13). 
FIGS. 3 and 4 present an elevational and top plan view respectively of a 
typical bushing (1). The top of the bushing (1) is brought in contact with 
a glass supply source. The supply source may be the forehearth of a direct 
melt furnace in which glass flows directly along the length of the 
forehearth and into the bushing. Examples are disclosed in the book 
entitled, "The Manufacturing Technology of Continuous Glass Fibers", by K. 
L. Lowenstein, published by the Elsivier Publishing Company, New York, 
1973, at pages 61-66, where a typical fiber glass direct melt forehearth 
system is shown with several configurations of forehearths and bushings 
attached thereto. Specifically at page 66, the author shows the attachment 
of a bushing to a typical forehearth. In the same book at pages 89-100, 
typical fiber glass bushings and their relation to the fiber drawing 
process are described. An alternative supply system, which may also be 
used, is one in which glass is supplied in the form of solid marbles to a 
special bushing. The marbles are then melted directly inside the bushing 
and the resulting glass fed through a plurality of tips located on its 
bottom. A bushing of this type is also shown in Lowenstein, supra, at 
pages 102-104. 
Two ears or terminals (12a and 12b) are provided for passing an electrical 
current through the bushing assembly (1) in order to heat it and the tip 
plate (13). The bushing has four sidewalls (14a, b, c and d) suspended 
from a flange (15) and attached to the tip plate (13) at their other 
extremity typically by welding. The bushing (1) also has an open top so 
that the sidewalls and tip plate form a cavity to accommodate molten glass 
flowing from the forehearth or the upper part of a marble melt bushing as 
described above. A screen (16) may also be provided in order to prevent 
any small particulate debris carried in the glass from reaching the tip 
plate (13) although bushings without screens may also be used. 
The tip plate (13) carries a plurality of tips (2) arranged in groups (17) 
of at least two rows parallel to each other and the minor axis of the tip 
plate. These groups (17) are usually separated by a space wide enough to 
accommodate the insertion of an individual fin (10) carried by the 
manifold (11). (Both the fins (10) and their associated manifold (11) are 
collectively referred to as "fin coolers" by those skilled in the art.) 
Bushings, as previously described, are constructed from precious metals 
such as platinum, rhodium, palladium, and alloys thereof. A typical alloy 
that has been widely used contains approximately 80 percent platinum and 
20 percent rhodium on a weight basis. Sometimes, grain stabilized platinum 
and grain stabilized platinum alloys have been employed where strength and 
creep resistance are a primary design criteria. Other alloys have included 
platinum and platinum-rhodium alloys containing small amounts of iridium 
or ruthenium to also increase strength. Gold has also been used 
occasionally to locally alter the wetting characteristics of the glass. 
In the manufacture of a conventional tip plate, a sheet of suitable 
precious metal alloy, along with a die, are first put through an 
appropriate rolling mill. As the sheet is compressed, the die produces a 
sequence of indentations at each location where a tip is to be formed. In 
the next step, a hydraulic punch press and a female die are used to push a 
series of pins through the tip plate material and into the female die. The 
metal alloy is plastically deformed and flows into the gap between the 
pins and the die whereby the walls of the tip (2) and its base shoulder or 
fillet (20) are formed. This cold drawing or coining process is more fully 
described by Lowenstein, supra, at pages 95-97. 
In the case of the instant invention, reinforcing ribs are formed during 
the initial rolling operation described above. The only difference is that 
the rolling die is modified so that the rib will be formed at the same 
time the indentations used to locate the tips are made. The remainder of 
the operation is the same with the exception that the female die used in 
the punch press operation is modified to accommodate the raised ribs of 
the tip plate. 
The rib (21) is preferably triangular in its cross section as illustrated 
in FIGS. 5 and 9 although other cross sections are possible. The base of 
the rib is generally slightly less than the widest dimension of the 
shoulder (20) of the tip but may also be slightly greater depending upon 
the actual dimensions of the dies used in the manufacturing process. The 
height of the rib extends from the surface of the tip plate anywhere up to 
a distance slightly below the tip exit. 
FIG. 5 better illustrates the relationship of the tips (2) and the rib 
structure (21) by providing a perspective view of them as they would be 
seen from below the tip plate, generally looking into line 5--5 of FIG. 3. 
In the practice of the instant invention, the use of a square tip is the 
most preferred embodiment although conventional round tips may also be 
used as well. The reasons for this preference are more fully described in 
my aforementioned copending application. 
FIG. 6 is a bottom plan view of a tip plate having square tips arranged in 
their most preferred orientation along with the rib (21) structure 
described above. The tips are oriented in such a fashion that their 
diagonals lie parallel to the major and minor axis of the tip plate (13) 
and tips in adjacent rows are staggered with respect to one another. This 
arrangement minimizes the pitch (18) between adjacent rows as well as the 
pitch (19) between adjacent tips in the same row. 
FIG. 7 presents a cross sectional view of two tips adjacent to one another 
in the same row as taken along line 7--7 of FIG. 6. As mentioned 
previously, a shoulder (20) near the base of each tip (2) usually results 
from the cold drawing process used to form the tip. 
FIG. 8 presents a cross sectional view of two tips taken along line 8--8 in 
FIG. 6. Here, the effect of the rib (21) is more clearly visible. Although 
the rib increases the bending moment of inertia and stiffness of the tip 
plate, it also allows the bending stresses to be more evenly distributed 
over a greater cross sectional area thereby resulting in a lower stress 
intensity and creep rate than would otherwise be present. 
With reference to FIG. 10, some individual noncircular tips (2) which can 
be formed on the tip plate (13) to allow the production of round glass 
fibers also utilizing the rib structure instant invention are shown in 
perspective. For example, an equilateral triangle (10a), a square (10b), a 
pentagon (10c), a hexagon (10d), and an octagon (10e) are all forms of 
regular polygons having sides of equal length. It is a property of regular 
polygons that they may be circumscribed by a circle that touches each 
vertex of the polygon. Although regular polygons are preferred, this does 
not mean that this invention neglects to contemplate the use of irregular 
polygonal shapes as well. For example, a tip having a cross section in the 
shape of a right triangle is envisioned as well as an equilateral one. A 
tip having four sides of equal or unequal length and formed in the shape 
of a parallelogram or rhombus, as well as a square tip, is similarly 
contemplated. 
Noncircular tips of the type described herein can be used to produce round 
fibers because near the tip exit, where the emerging glass has a very low 
viscosity, the surface tension will constrict the surface of the glass and 
cause it to assume a circular cross section even though the glass 
initially issues with substantially the same cross section as the tip. 
These surface tension forces may be so strong as to actually cause glass 
issued at a very high temperature to coalesce into beads or droplets 
rather than flow in a continuous stream. At lower temperatures, the 
behavior of the stream is largely dominated by the increased viscosity so 
that surface tension effects are insignificant by comparison. This is the 
reason it is necessary to rapidly quench glass issuing from a noncircular 
tip to "freeze" its shape by rapidly increasing the viscosity before the 
surface tension has time to coalesce it when the production of a 
noncircular fiber is desired. 
It is also believed that the use of noncircular tips helps to benefit the 
stability of the forming process and may reduce the frequency of breakouts 
as well. In the immediate vicinity of the tip, the irregular surface of 
the stream as it exits into the surrounding atmosphere will provide an 
enhanced but localized area for convective and radiative heat transfer 
that would not be present if the cross section of the stream were 
circular. This localized cooling results in stringers or ribbons of glass 
having a slightly higher viscosity than the rest of the stream. It is 
believed that this effect helps stabilize the cone and formation of the 
fiber while the stream is coalesced by the surface tension. 
The use of a noncircular tip in the form of substantially regular 
finite-sided polygon or irregular variations thereof also results in 
increasing the packing density of the tips and thus better utilizing the 
available surface area of the tip plate while an integrally formed rib 
structure as described herein increases the bending stiffness of the tip 
plate and thereby reduces the effect of thermal creep deformation. 
While this invention has been described with reference to certain details 
of construction and embodiments illustrated in the accompanying drawings 
and specification, it is not intended that it be limited except insofar as 
what appears in the accompanying claims.