Abstract:
A method and apparatus for eliminating short term yardage variation in the formation of glass filaments is disclosed. This method involves additionally cooling the bushing tips from which molten glass is drawn to form filaments utilizing a separate cooling means in addition to the fin coolers normally employed. The additional cooling means removes heat at a rate of from about 1,000 Btu&#39;s per hour to about 3,000 Btu&#39;s per hour (293 to 879 watts) based upon a drawing rate of about 35 to about 70 pounds of glass per hour (77 to 154 kilograms per hour). The method further involves either forming a controlled stream of air across the bushing tips from above the additional cooling means or preventing the direct impingement of air upon bushing tips by blocking the air with the additional cooling means, to provide a more stable thermal environment under the bushing.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to glass filament forming and more particularly to a method for providing a uniform thermal environment at each of a plurality of orificed filament forming tips on a heated glass fiber forming bushing. 
     It is well known in the art to produce filaments from various thermoplastic heat softenable materials, and especially glass, by flowing streams of molten material from a plurality of orificed tips provided on the bottom of a heated bushing. In the case of glass, the streams are attenuated, usually by mechanical means, into filaments which are then gathered into strands and processed into a variety of commercial products. 
     One typical use for such strand is in the formation of glass fabrics. In order that a satisfactory woven fabric be produced, it is imperative that the diameters of each glass strand be consistent in a linear plane. Variations in the diameters of glass strands along the length thereof results in a fabric which will not lie flat but rather becomes &#34;puckered&#34;. Such a fabric is unacceptable. 
     Attempts have been made in the past to provide a uniform thermal environment at the bushing tips in order to produce a uniform glass strand. In U.S. Pat. No. 3,068,699 a heat radiating strip was molded to the bushing bottom and extended around the tips. However, there is no external control over the environment that is created. 
     In U.S. Pat. No. 3,672,857 it is suggested to control the environment around the bushing tips by adding an additional heat fence adjacent to or directly connected to the front bushing tips,thus providing additional heat to the tips. 
     Another suggested solution (U.S. Pat. No. 3,607,164) has been to plug up the boundary tips such that all active tips are surrounded by similar patterns of heat radiating tips. This would, however, decrease the number of filaments a given bushing could produce, and, as the need for larger bushings have increasing numbers of tips occurs, such a solution is far less desirable. 
     SUMMARY OF THE INVENTION 
     According to the present invention, it has been discovered that, contrary to the above-mentioned references, a more stable environment around bushing tips may be obtained by providing cooling to the tips by means of an additional cooling element adjacent to the front bushing tips. Such an element not only provides cooling to the bushing tips, but also allows control of an air stream flowing above the cooling element and across the bushing tips. It is known that due to the high speed of attenuating glass fibers, air is pulled downwardly with the fibers. By controlling the vertical distance between the cooling element and the bottom of the bushing, it is possible to control the air stream drawn in by the formed glass fibers over the cooling element. By placing the cooling element flush with the bottom of the bushing, the direct impingement of air upon front bushing tips is prevented. This combination of cooling and controlling the air flow around the bushing tips aids in obtaining a more stable environment around bushing tips. 
     Accordingly, it is the primary object of the present invention to provide a method of creating a more stable environment around the bushing tips in the formation of glass filaments. 
     Another object of the instant invention is to provide apparatus for carrying out the aforesaid method. 
     Other objects and advantages of the invention will become apparent in the following detailed description, with reference being made to the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a side sectional view across a bushing illustrating the apparatus employed in the instant method. 
     FIG. 2 is a view from underneath a bushing including a first embodiment of the invention. 
     FIGS. 3, 4 and 5 are sectional views of alternative shapes for the cooling bar employed in the instant invention. 
     FIG. 6 is a front view, partially in section, of another alternative embodiment for the cooling bar. 
     FIG. 7 is a sectional view through lines 7--7 of FIG. 6 illustrating the location of hollow tubes in the alternative embodiment of FIG. 6. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now to FIG. 1 there is shown a bushing 10 through which molten glass 11 is drawn through bushing tips 13. As the molten glass is attenuated through the bushing tips, cones of glass 15 are formed. Upon further attenuation these cones are formed into filaments 17 which are later gathered into composite strands. 
     The bushing tips 13 are cooled by means of a base cooling bar 19 having a heat transfer fluid such as water, flowing in a pipe 25 therethrough. Heat transfer elements 21 are connected to the base bar 19 so that heat may be removed from the area surrounding bushing tips 13 through the heat transfer elements 21 to the base bar 19 and ultimately removed by the flowing heat transfer liquid in pipe 25. The heat transfer elements 21 may be hollow pipes which would also carry heat transfer fluid or typically are in the form of solid fin members. However, the exact means employed for such cooling is not important to the operation of the instant invention and are so well known in the art that further explanation is unnecessary. 
     A second cooling bar 23 is placed adjacent to, but spaced from, the front bushing tips 13 and opposite from the cooling bar 19. This cooling bar 23 has no additional heat transfer elements nor is it connected to the heat transfer elements 21 associated with the base cooling bar 19. This bar 23 has a pipe 27 located within it which has flowing therethrough a heat transfer fluid such as water. This pipe or channel 27 is separate from line 25 for the base cooling bar 19 and provides control of the rate of heat removel from the bar 23. The location of this cooling bar 23 is adjustable with relation to its distance from the bushing tips both in the horizontal and vertical direction. This enables control of the amount of heat removed by the cooling bar 23 and the air stream flowing over the bar and across the bushing tips due to the high speed attenuation of the filaments. 
     By means of this invention, it is possible to allow a controlled amount of air to flow above the cooling bar 23 and across the bushing tips and downwardly with the formed filaments or, by placing the cooling bar 23 in direct contact with the bottom of the bushing 10, to prevent direct impingement of air upon the front bushing tips 13. It has been found that suitable results may be obtained with the cooling bar directly touching the bottom of the bushing frame or from about 0.125 inch to 0.25 inch (3.17 to 6.33 millimeters) below the bushing. It is preferred to place the bar about 0.1875 inch (4.7625  millimeters) below the bushing. It has also been found that the bar is capable of removing heat from the environment surrounding the bushing tips at rates, for example, of from about 1000 to about 3000 Btu&#39;s per hour (293 to 879 watts) based upon a drawing rate of from about 35 to 70 pounds per hour of glass (77 to 154 kilograms per hour). This combination of effects leads to a more stable environment for drawing of the glass filaments, thus leading to a more consistent diameter of filaments. While the element 23 is illustrated as square in cross-section in FIG. 1, FIGS. 3 through 7 illustrate numerous other operative shapes that may be employed in the fabrication of such a bar. 
     FIG. 2 is a bottom plan view of the bushing of FIG. 1. This Figure illustrates rows of bushing tips 13 having heat transfer elements 21 therebetween. This figure also illustrates the connection of the heat transfer elements to the base cooling bar 19 and the direction of travel of heat transfer fluid through pipe 25 within the base cooling bar 19. This figure further illustrates the horizontal location of the cooling bar 23 with respect to the bushing tips 13 and the heat transfer elements 21. Of course, this horizontal location can be adjusted to give an optimum effect. Finally, a heat transfer fluid line 27 is illustrated within the cooling bar 23. 
     FIGS. 3, 4 and 5 are cross-sectional views through alternative shapes for the cooling bar 23 not showing the pipe carrying the heat transfer liquid. Such shapes include oval, circular, or rectangular configurations, however any other shape can be used to give a desired direction to the air stream flow or heat transfer pattern. 
     FIG. 6 is a side view, partially in section, of an alternative embodiment of the cooling bar 23. In this embodiment, a hollow support element 29 has a series of tubes 31 carried within. These tubes 31 are interconnected by reservoirs 33 and 35 at the entry and exit, respectively, of the tubes 31. Heat transfer fluid passes into reservoir 33, through tubes 31, into reservoir 35 and out of the cooling bar 29. Flow is restricted to the tubes by means of a face plate 37 located at the entry to the tubes 31 and a face plate 39 which is located at the exit of the tubes 31. These plates 37 and 39 block the heat transfer fluid from passing between the tubes 31 as well as helping to keep the tubes 31 in place. 
     FIG. 7 is a cross-section of FIG. 6 through line 7--7 illustrating a typical location of the tubes 31 within this alternative embodiment. 
     EXAMPLES 1-5 
     Five 2/G75 bushings were fitted with the cooling bar of the present invention. A 2/G75 bushing contains 800 orifices. The 2 designates a two-way split of the fibers into 400 filaments. The bars were 0.1875 inch (4.7625 millimeters) below the bushing bottoms. This allowed controlled air streams to flow below the bushings and onto the cones of molten glass formed below the bushing tips. Cooling water was provided to each cooling bar at a pressure of 50 psig (440,136 pascals) at a temperature of 86° F. (30° C.) The glass fibers were drawn at a linear rate of speed of 12,420 feet per minute (3,785.6 meters per minute). This resulted in 68 pounds per hour (149.6 kilograms per hour) of glass being drawn through each bushing. The results of these examples are shown in Table I. 
     
                                           TABLE I__________________________________________________________________________Outlet Water Temp.      Flow Rate   Bush.Oper.Temp.                           Heat RemovalEx.   ° F.  (° C.)      Gal/Min.            (l/Min)                  ° F.                      (° C.)                           Btu/Hr.                                 (Watt)__________________________________________________________________________1  89  31.7      1.36  5.15  2150                      1176.6                           2032  595.42  90  32.2      1.05  3.97  2128                      1164.4                           2092  613.03  89  31.7      1.20  4.54  2140                      1171.1                           1793  525.74  90  32.2      1.50  5.68  2051                      1121.7                           2988  875.55  88  31.1      1.33  5.03  2127                      1161.7                           1325  388.2__________________________________________________________________________ 
    
     The resulting glass fibers of the above examples had highly uniform diameters which would be expected to produce a good quality glass fiber fabric. 
     In all of the above examples, heat was removed by the cooling bar in an amount of from 1,000 to 3,000 Btu&#39;s per hour and the air stream flowing above the cooling bar was controlled in such a manner to provide glass fibers having highly uniform diameters which would be expected to produce a good quality glass fiber fabric. 
     EXAMPLES 6-19 
     2/G75 bushings as in the previous examples were equipped with the cooling bar of the present invention. In these examples the cooling bar was connected directly against the bottom of the bushing frame. This prevented air from directly impinging on the front bushing tips. Cooling water was provided at an average inlet temperature of 86° F. (30° C.). The glass fiber was drawn at an average speed of 12,420 feet per minute (3,785.6 meters per minute), resulting in 68 pounds per hour (149.6 kilograms per hour) of glass fiber production. 
     In examples 6-19 the cooling bar was installed at the front of the bushing with standard fin coolers located at the rear. 
     The collected fibers were twisted with yarns on a standard twist frame and the yarns were tested for unevenness. Fabrics were then woven from the yarns and graded. The results of these tests appear in Table II. 
     
                       TABLE II______________________________________      % UnevennessExample    (Uster)        Fabric Grade______________________________________ 6         1.4            D 7         1.7            C 8         1.3            C 9         1.55           C10         1.25           D11         1.4            C12         1.65           D13         1.65           C14         1.05           B15         1.45           C16         1.75           C17         1.55           C18         1.55           D19         1.65           D______________________________________ 
    
     As can be seen from these results, the percentage of unevenness, as measured by the Uster test was quite low, however, the fabric grade formed from the strands produced in this manner were inconsistent. 
     EXAMPLES 20-57 
     Examples 6-19 were repeated, with the cooling bar lowered to 0.1875 inch (4.7625 millimeters) below the bushing. 
     The collected fiber were twisted and tested as in Examples 6-19 and fabric woven from the yarns tested as in Examples 6-19. The results of these tests appear in Table III. 
     
                       TABLE III______________________________________      % UnevennessExample    (Uster)        Fabric Grade______________________________________20         1.45           A21         1.4            A22         1.15           A23         1.2            A24         1.2            A25         1.1            A26         1.15           A27         1.25           B28         1.3            A29         1.15           A30         1.3            B31         1.55           B32         1.1            A33         1.25           A34         1.05           A35         1.0            A36         1.2            A37         1.4            A38         1.4            A39         1.4            B40         1.3            A41         1.55           A42         1.3            A43         1.35           A44         1.2            A45         1.8            B46         1.25           A47         1.8            C48         1.7            A49         1.95           A50         1.2            A51         1.55           B52         1.05           A53         1.25           A54         1.1            A55         1.2            A56         1.2            A57         1.1            A______________________________________ 
    
     As can be seen from these results, the percentage of unevenness, as measured by the Uster test, was quite low and, the fabric grade formed from the strands produced in this manner were highly consistent. 
     While the foregoing description of the invention has been made with reference to specific embodiments, it is not intended that the invention be limited except insofar as in the appended claims.