Method of using bushing environmental control in glass fiber forming

A method is disclosed for maintaining a uniform airflow and thus uniform thermal conditions below a bushing in a continuous glass fiber forming operation. The method comprises passing the glass filaments through an enclosure and blowing a gaseous fluid through the enclosure co-current with the filaments at a substantially constant volume and velocity to draw air downwardly with the filaments from all directions below the bushing at a substantially constant volume and velocity which thus provides a more uniform airflow and temperature below the bushing. The downward airflow velocity is insufficient to attenuate the filaments. Suitable apparatus for accomplishing this result is also disclosed. This apparatus comprises an enclosure for the filaments having a pair of chambers, the glass filaments being passed through the inner chamber and gaseous fluid being flowed from the outer chamber to the inner chamber at a substantially constant volume and velocity to flow co-current with the glass filaments.

BACKGROUND OF THE INVENTION 
In the formation of continuous glass fiber strands, glass filaments are 
typically attenuated through bushing tips or orifices located at the 
bottom of a heated bushing having molten glass therein. The filaments are 
attenuated through the bushing tips at speeds from about 1,000 to 15,000 
feet per minute (304.8 to 4,572.0 meters per minute) or more. The 
filaments are drawn across the application surface of an applicator where 
they are coated with a binder and/or size to prevent abrasion and to add 
desired properties to the filaments. The filaments are then gathered into 
a unified strand in a gathering shoe, which is typically a grooved 
cylinder or wheel formed of material such as graphite, and are wound on a 
rotating collet as a forming package, with the rotation of the collet 
providing the attenuative forces necessary to form the filaments. 
Glass filaments may range from about 0.0070 inch (0.0178 centimeter) and 
larger to about 0.00018 inch (0.0004572 centimeter) and smaller. These 
very small filaments can sustain only a very small tensile force before 
breaking and the breaking of a single filament among the hundreds or even 
thousands of filaments being drawn from a bushing requires an interruption 
of the forming process which reduces productivity and increases 
manufacturing costs. 
Forces which break filaments in the forming process can originate from 
nonuniform thermal conditions in the space immediately below the bushing 
where the molten glass streams are attenuated and cooled. An increase in 
the rate of heat removal from an attenuating molten glass stream will 
increase its viscosity faster than desired, adding tension to the 
filaments being formed. If this added tension becomes sufficient to cause 
an increase in stress to the ultimate stress of the glass, the filament 
breaks. Changes in the air velocity or air temperature near the molten 
streams can change the viscosity and tension sufficiently to break the 
filaments. 
The hundreds or even thousands of filaments being drawn downwardly from the 
bushing at speeds up to 15,000 feet per minute (4,572 meters per minute) 
or more drag surrounding air downwardly with them. The air being dragged 
downward by the speeding filaments is replaced by air from the immediate 
vicinity of the bushing, and it is not uncommon for the air to be sucked 
away from the forming cone directly below the bushing tips to satisfy the 
need below. When this occurs, the forming cone space draws air from its 
surroundings to replace the air stolen by the filaments. If this 
replacement air in the forming cone zone is erratic in either velocity or 
temperature, the stage is set for an interruption in production due to 
filament breakout based upon a change in air velocity and/or air 
temperature. 
Just as a temporary excess of airflow below the bushing can cause filament 
breakage from high tension resulting from too rapid cooling of the molten 
glass forming cone, a temporary deficiency of airflow below the bushing 
can result in fiber disruption due to insufficient cooling of the forming 
cone and consequent separation of the glass stream as a result of the 
pinching forces of surface tension. 
It is desirable, therefore, to reduce or eliminate erratic airflow and air 
temperature immediately below the bushing tips and thus to provide a more 
uniform airflow and uniform temperature in the region below the bushing. 
It is known to attenuate discontinuous glass fibers by means of high 
velocity downward gas or steam jets. Typical of this attenuation are the 
methods shown in U.S. Pat. Nos. 2,224,466; 2,234,986; 3,021,558; 
3,532,479; 3,547,610; 3,836,346 and 3,881,903. The velocities of the gas 
jets employed to attenuate the discontinuous fibers typically ranges from 
about 150 to 1700 feet per second (45.7 to 518.2 meters per second). 
While these high gas velocities may be employed in the production of 
discontinuous glass fibers, such high gas velocities cannot be tolerated 
in the production of continuous glass fibers. These high velocities 
disrupt the operation of the bushing, due to erratic turbulent flow and 
thus erratic airflow and temperatures below the bushing, resulting again 
in discontinuous filaments. Thus, it is a further objective of the present 
invention to control the environment below a continuous glass fiber 
forming bushing with gas of a volume and velocity sufficient to produce 
uniform airflow and temperatures below the bushing but insufficient to 
attenuate the filaments or produce turbulent airflow below the bushing. 
THE PRESENT INVENTION 
By means of the present invention, air velocity and temperature variations 
below the bushing tips of a continuous glass fiber forming bushing can be 
substantially reduced or eliminated. The present invention comprises 
passing the glass filaments as they are attenuated from the bushing 
downwardly through an enclosure approximately vertically spaced at a 
distance below the bushing. This enclosure has an inner and an outer 
chamber. The glass filaments pass through the inner chamber. The outer 
chamber is connected to a source of gaseous fluid such as air, nitrogen, 
oxygen, carbon dioxide, and the like. This gaseous fluid flows through the 
outer chamber at a substantially constant volume and velocity. The inner 
and outer chambers are connected by means of an opening designed to allow 
all of the gaseous fluid in the outer chamber to pass through the inner 
chamber co-current with the filament flow through the inner chamber. At 
the point where the inner and outer chambers are connected, the dimensions 
of the inner chamber are smaller than at the points where the filaments 
enter and where the gaseous fluid and filaments exit the inner chamber. 
This produces a venturi effect which draws air from above the unit and 
below the bushing from all directions through the inner chamber at a 
substantially constant volume and velocity along with the glass filaments. 
The air is drawn to the inner chamber at this substantially constant 
volume and velocity from all directions to result in a more uniform and 
constant airflow directly below the bushing tips and into the apparatus. 
This uniform, laminar airflow substantially reduces or eliminates 
variations in velocity and temperature below the bushing tips and thus 
removes heat from the bushing tips at a substantially constant rate thus 
substantially reducing the chances of breakouts of the filaments caused by 
erratic airflow and temperatures below the bushing tips. The downward 
airflow through the enclosure is, however, insufficient in velocity to 
attenuate the filaments. In addition, the present invention isolates the 
bushing from the harmful effects caused by adjacent bushings and their 
operation.

DETAILED DESCRIPTION OF THE DRAWINGS 
Turning now to FIG. 1, molten glass 11 is contained within a heated glass 
fiber forming bushing 10. The bushing 10 contains a plurality of bushing 
tips 12 at its bottom through which continuous glass filaments 14 are 
attenuated. The filaments 14 are formed from small cones at the bottom of 
the bushing tips 12 as these cones are drawn into the filaments 14. The 
filaments 14 are then passed downwardly and through the airflow control 
apparatus 40 which will be more fully described below. This apparatus 40 
is vertically spaced from the bushing tips 12. Typically, this spacing is 
from about 2 to 6 inches (5.08 to 15.24 centimeters), more or less, below 
the bushing tips. This spacing will vary according to the size of the 
bushing and the type of filaments being produced. As the filaments 14 exit 
this apparatus 40, they are passed across the application surface 31 of an 
applicator 32 where they are coated with a binder and/or size. The coated 
filaments 14 are then gathered into a unified strand 18 by passing them 
across the grooved surface of a gathering shoe 16. This gathering shoe 16 
typically is a graphite wheel or cylinder having a groove in its surface 
across which filaments 14 pass and in which they are gathered into a 
unified strand 18. The strand 18 is then traversed across the face of a 
rotating spiral 28 and is gathered in a generally crisscross pattern as a 
forming package 21 on the face of a rotating collet 22. Optionally, the 
strand 18 could be attenuated by a belt or wheel attenuator and collected 
in a container or on a moving surface as a mat. 
FIGS. 2 and 3 illustrate the airflow control apparatus employed in the 
present invention. The apparatus 40 comprises an intake duct 41 connected 
in fluid transfer relation to an outer chamber 42. The intake duct 41 is 
in turn connected to a source of gaseous fluid, such as air, nitrogen, 
oxygen, carbon dioxide, and the like which enters the duct 41 at a 
substantially constant rate. Typical of the gas flow rates employed are 
from about 100 to 300 cubic feet per minute (2.83 to 8.49 cubic meters per 
minute) and preferably from about 150 to 200 cubic feet per minute (4.25 
to 5.66 cubic meters per minute). These flow rates will vary according to 
the specific needs of the different bushings employed. The gaseous fluid 
flowing within the outer chamber 42 can exit only through the two openings 
49 which connect the outer chamber 42 with an inner chamber 43. The inner 
chamber 43 is surrounded by the outer chamber 42 on all sides. 
The glass filaments 14 pass downwardly through the inner chamber 43. The 
inner chamber 43 has an upper opening 50 through which the glass filaments 
14 enter and a bottom opening 52 through which the glass filaments 14 
exit. The inner chamber 43 and outer chamber 42 are preferably generally 
rectangular. Walls 44 and 46 which separate the inner chamber 43 from the 
outer chamber 42 are designed such that a narrower opening 47 is formed in 
the inner chamber 43 at the point where the air from the outer chamber 42 
enters the inner chamber 43 through the openings 49 than the openings 50 
and 52 through which the glass filaments enter and exit. Walls 44, 49 and 
46 define a venturi within enclosure 40, with the air entering the inner 
chamber 43 of the enclosure 40 at or about the throat of the venturi. 
Preferably, at this narrow opening 47, walls 44 and 46 do not meet. 
Rather, the walls are separated by the openings 49 through which the 
gaseous fluid from the outer chamber 42 passes to the inner chamber 43. 
Optionally, walls 44 and 46 could meet, or be a single wall, with a 
plurality of openings in the walls at the point where the gaseous fluid 
enters the inner chamber 43. Side walls 48 help guide the flow of the 
gaseous fluid in the downward direction so that the gaseous fluid flows 
co-current with the glass filaments 14 to the exit opening 52 in the inner 
chamber 43. This downward flow of the higher velocity gaseous fluid draws 
air from all directions from below the bushing into the opening 50 along 
with glass filaments 14 as they enter the apparatus 40. The walls 46 
separating the inner chamber 43 and the outer chamber 42 are preferably 
angled, as shown in FIG. 1, to provide a large volume for the outer 
chamber to minimize resistance to flow and to give substantially uniform 
airflow within the chamber 42, as well as to provide a larger cross 
section for flow at the exit end of the inner chamber 43 to prevent flow 
back or upward flow. Since the gaseous fluid flow to the duct 41 is at a 
substantially constant rate, the airflow into the apparatus 40 is also at 
a substantially constant rate from all directions. The purpose of the 
narrow opening 47, as opposed to an opening equal in size to either of the 
openings 50 and 52, is to eliminate flow back of the gaseous fluid and/or 
air which enters the apparatus 40 and, by venturi effect, to direct all of 
the gases in a downward direction to exit opening 52. Typically, the gas 
velocity, at the openings 47 and 52 may range from about 2 to 15 feet per 
second (0.61 to 4.57 meters per second). The dimensions of the apparatus 
40 will vary with the size of the bushing employed. 
The substantially constant airflow into the opening 50 results in a more 
uniform airflow above the apparatus 40, i.e., more uniform airflow below 
the bushing tips 12. This substantially uniform, streamlined, laminar 
airflow will result in more uniform air velocities and more uniform 
temperatures below the bushing tips 12 and substantially reduces breakouts 
of the fine filaments 14 due to erratic airflow and/or temperatures below 
the bushing tips 12. This airflow is, however, insufficient to attenuate 
the filaments, being typically in the range of from about 1 to 10 feet per 
second (0.31 to 3.1 meters per second) in velocity. 
The apparatus 40 is formed of a material which can withstand the hot and 
damp environment in a glass fiber forming level. A particularly suitable 
material is stainless steel. 
EXAMPLE 
Employing the apparatus as illustrated in the figures, 1600 C-75 continuous 
glass filaments having an average diameter of 0.00018 inch (0.0004572 
centimeters) were attenuated from a bushing at a speed of 11,600 feet per 
minute (3,535.8 meters per minute). The opening 50 at the top of the air 
control apparatus was 82.5 square inches (532.3 square centimeters). The 
opening 47 at the inlet between the inner and outer chambers was 54.25 
square inches (350.0 square centimeters). The opening 52 at the exit of 
the apparatus was 62.5 square inches (403.2 square centimeters). While 
entering airflows were not measured, at the exit airflows of between 110 
and 185 cubic feet per minute (3.1 and 5.2 cubic meters per minute) and 
velocities of between 4.22 and 7.10 feet per minute (1.29 and 2.16 meters 
per minute) were measured. Substantially fewer filament breakouts under 
the bushing were noticed than is normally obtained without the air control 
apparatus. 
From the foregoing, it is obvious that the present invention provides a 
method and apparatus for substantially improving the environment below the 
bushing tips in a glass fiber forming operation. 
While the invention has been described with reference to specific 
embodiments thereof, it is not intended to be so limited thereby except as 
set forth in the accompanying claims.