Abstract:
A mold for shaping end portions of glass tubes has first and second mating members which define a contoured bore into which the heated tube fits. Each member has a longitudinal bore from which extend radial bores opening into the interior cavity of the mold containing the tube. Each member also has radial vent bores for venting gases and vapor to the outside. Each longitudinal bore is connected to a lubricant, such as water, supply, which is applied to the surface of the tube through the radial bores.

Description:
FIELD OF THE INVENTION 
     This invention relates to shaping glass tubes for use in the production of optical fibers and, more particularly to the shaping of the end of a glass tube by means of lubricated molds. 
     BACKGROUND OF THE INVENTION 
     In the manufacture of optical fibers, a process known as MCVD (modified chemical vapor deposition), which is shown and described in U.S. Pat. No. 4,217,227 of J. B. MacChesney et al is widely used. An initial step of the process involves injecting into a glass tube, commonly referred to as a starter tube, a gaseous mixture containing certain reactants while the tube is being heated by a moving torch. The particle components of the gaseous mixture, referred to as “soot” are deposited on the interior wall of the tube in the region thereof of greatest heat. After sufficient soot is deposited, the starter tube is heated and collapsed into a glass rod from which optical fiber is subsequently drawn. During the deposition process, the remanant gases and reactants after deposition exhaust out of the end of the tube remote from the introduction end. It is not uncommon for a buildup of soot to occur at the exhaust end of the tube to the point where free flow of the mixture out of the tube is inhibited, which can affect the deposition process. As a consequence, it is the usual practice to butt fuse an exhaust tube to the exhaust end of the starter tube, the exhaust tube having a larger internal diameter to permit free flow of the exhaust gas mixture from the starter tube into and through the exhaust tube. However, if the fusion joint inside the tubes is not smooth or otherwise represents a discontinuity, there can still occur a building up of the soot at the joint which can cause clogging. It is equally as important as a smooth junction that the two tubes be precisely aligned so that they can be rotated as a unit without wobble. 
     It is common practice to shape the butting end of the exhaust tube by tapering its outside diameter to the diameter of the starter tube. This is usually accomplished through the use of manually manipulated graphite forming tools, such as paddles. The process requires a high level of skill on the part of the operator to the point that it deserves to be characterized as an “art”. Manual manipulation seldom results in any thing approximating exact duplication, hence each formed exhaust tube is, in essence, custom made. Tubes formed manually in this manner have little uniformity and dimensional commonality and, as a result, a high scrap rate of exhaust tubes is usually the case. A further common problem is that precise symmetry of the formed diameter relative to the centerline of the starter tube is quite difficult to achieve. As pointed out hereinbefore, precise alignment of the starter tube and the exhaust tube is a desideratum which is negated by any eccentricity of the exhaust tube resulting from the manual shaping process. 
     Some prior art methods are directed toward eliminating the use of manually manipulated paddles, by substituting therefor a mold, of graphite or other suitable material, which shapes the end of the exhaust tube, heated to ductility, while air or other fluid is applied to the interior wall surface to force the tube against the mold and to maintain the tube shape. The process requires a substantial air pressure, which tends to cool the tube to the point where the ductility thereof is insufficient for forming or shaping. In U.S. patent application Ser. No. 09/623,908; filed Jul. 31, 2000 of Jason Kay et al., the disclosure of which is incorporated herein by reference, there is shown an apparatus and method of accurately shaping the inner surface of the end portion of the exhaust tube which does not require pressurized air. The apparatus of that application includes an interior mold that has a variable configuration to allow insertion within and withdrawal from the exhaust tube. 
     There remain, despite the apparatus of the aforementioned Kay application, problems arising from the physical contact between the outer mold and the exhaust tube. Because the exhaust tube is rotating relative to the mold during the forming process, there is a great deal of wear of the mold which requires frequent replacement to maintain the necessary precision in the finished product. Further, the physical contact produces a distorted glass surface and, over time, undesirable irregularities therein. 
     SUMMARY OF THE INVENTION 
     The present invention is an apparatus which substantially reduces the wear of the mold and the resulting surface aberrations of the exhaust tube. 
     In greater detail, in a first preferred embodiment of the invention, the exterior mold for shaping the end of the rotating exhaust tube comprises first and second substantially identical members having contoured inner surfaces, and a plug for insertion into the end of the exhaust tube to maintain its internal diameter at the desired dimension and axially precise. In accordance with the present invention, each mold member has an elongated bore, preferably parallel to the axis of the assembled mold, extending from one end face thereof along substantially the entire length, but which is closed at the end of the member remote from the end face. Extending from the elongated bore, and in communication therewith, are at least two radial or transverse bores or channels which open at the interior surface of the member. Also extending from the interior surface and open thereto are one or more exhaust or vent bores offset from the radial bores and the elongated bore which connect the inner surface of the member to the exterior surface thereof. The vent bores do not intersect the elongated bore or the channels. Each of the mold members is mounted to a positioning arm for moving the member toward the central axis to form the mold and moving it away from the central axis to access the exhaust tube. 
     Further in accordance with the invention, the end face of each member has a suitable connection thereon for connecting the elongated bore to a fluid conduit which is, in turn, connected to a fluid (preferably water) supply for applying fluid to the elongated bore and channels. A delivery control means such as a valve member in each conduit can be used to regulate the flow rate of the water supplied to the elongated bore. In operation, the exhaust tube is mounted in, for example, a lathe chuck, and is rotated thereby. Suitable means, such as a torch, is used to heat the end of the exhaust tube to be formed to a ductile state and the mold members are moved into the mold position, embracing the end of the exhaust tube, and the mold plug is moved into place within the end of the exhaust tube. Water is applied through the conduits to the elongated bores, and through the channels to the surface of the rotating exhaust tube, thereby lubricating the mold-glass interface. The application in this manner of a water lubricant materially reduces the long term wear of the contoured inner surface of the mold members and also results in higher surface quality of the exhaust tube end. 
     The water or other lubricant will, when it contacts the hot glass, vaporize and the water vapor thus created will escape through the vent bores to the outside. The steady flow of water gives continuous lubrication by continuously replacing the vaporized water. The rate of such replacement is governed by the settings of the valves in the conduits, which can be accomplished manually or automatically. As the shaping step is completed, the water supply may be turned off. 
    
    
     From the foregoing, it can be seen that the use of a lubricant in the molding process is readily accomplished with the apparatus of the present invention, the various features and principles of which will be more readily apparent from the following detailed description, read in conjunction with the accompanying drawings. 
     DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view of a combined starter tube and exhaust tube for the formation of which the present invention is used; 
     FIG. 1 a  is a cross-sectional view of the combined tubes of FIG. 1; 
     FIG. 2 is a partially diagrammatic, partial cross-sectional elevation view of the apparatus of the present invention; 
     FIG. 3 is a cross-sectional view along the line A—A of a portion of the mold FIG. 2; 
     FIG. 4 is an exploded perspective view, in cross-section, of the mold of the present invention; and 
     FIG. 5 shows the mold of FIG. 4 in its operational configuration. 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 is a perspective view of a glass preform tube  11  which is treated, as discussed hereinbefore, with injection in the direction of the arrow of a gaseous mixture containing reactants, i.e., soot, for deposition on the inner walls. Preform tube  11  comprises a glass starter tube  12  and a glass exhaust tube  13  which are fused together at their ends, as shown. FIG. 1 a , which is a cross-sectional elevation view of the preform tube  11  shows more clearly the relationship of tubes  12  and  13 , which are butt-fused at the junction  14  of their ends. During the deposition process, the tube  11  is rotated on the central axis  16  which is common to both tubes  12  and  13 . Tube  13  is, as discussed hereinbefore, of larger internal diameter than tube  12 , so that the remanant gas-reactant mixture exiting tube  12  is not impeded in any way, such as by deposition build-up on the interior wall of tube  13 , thereby assuring a uniform deposition in tube  12 . It is also desirable that the transition at the fusion region  14  be smooth so as not to disrupt or impede the gaseous flow thereacross. A reduction in turbulence or the prevention thereof helps insure that the layers of soot deposited on the inner wall of the tube  12  have a uniform build-up. Uniformity of deposition is also aided by the centerline or central axis  16 , which is composed of the centerlines of the tubes  12  and  13 , being straight, i.e., the centerlines being collinear and coextensive. Both of these desiderata are achieved with the present invention, as will be apparent hereinafter. 
     FIG. 2 is a diagrammatic view in partial cross-section of the principal components of an apparatus assembly  21  for forming the end of an exhaust tube  13  being held and rotated by, for example, a chuck  15 , and which embodies the principles of the present invention. FIG. 3 is a cross-sectional view taken along the line A—A of the apparatus assembly  21  of FIG.  2 . Apparatus  21  comprises an external mold  22  of suitable material, such as carbon, graphite, or the like and which comprises two movable mold members  23  and  24 . As shown in FIG. 3, mold members  23  and  24 , when in the mold position, have surfaces that butt against each other, at  26 , thereby forming a complete mold. Each of the mold members  23 ,  24  is mounted on a positioning arm  27  or  28  which may be threaded or may be the shaft of a piston, for example, for moving each member  23 ,  24  toward or away from the mold position. 
     When in the mold position, as shown in FIGS. 2 and 3, mold  22  has a central bore  29  adapted to receive the rotating exhaust tube  13 . As can be seen in FIG. 2, bore  29 , at the end  31  thereof has a rounded (or tapered) portion  32  for producing a smooth reduction of the diameter of the end of tube  23  when it has been heated to a ductile state by suitable means, such as a torch  33 . It is to be understood that torch  33  is intended to be indicative of any of a number of heating elements available in the art. The end  31  of mold  22  extends beyond the end of tube  13  and has a stepped recess  34  for receiving a movable plug  36  mounted on a positioning arm  37 . Plug  36  has a reduced diameter portion  38  which fits into a reduced diameter portion of recess  34  and, in the mold position, extends into the interior of mold  22 , as shown. Portion  38  has a diameter substantially equal to the diameter of starter tube  12 , and functions to limit the molded end of tube  13  to that diameter. Thus, when molding is completed, the end of tube  13  is of the proper size for butt fusing to the end of starter tube  13 . Axial travel of plug  36  is limited by the shoulder formed by portion  38  butting against a shoulder in stepped recess  34 , as shown. 
     In accordance with the present invention, each member  23 ,  24  has a longitudinal bore  41 ,  42 , respectively, extending from end  31  rearwardly to a point adjacent, but not through, the end  43  of mold  22 . Bores  41  and  42  are preferably, but not necessarily, parallel to central axis  16 . Extending from bore  41  are a plurality of transversely oriented radial bores  44 ,  46 ,  47 , and  48  extending to the wall of bore  29  and opening therein. In like manner, bores  49 ,  51 ,  52 , and  53  connect bore  42  to the interior of bore  29 . It is to be understood that more or fewer radial bores may be used, as well as more than two longitudinal bores. Extending from the outer surface of each of members  23  and  24  to the interior of bore  29  are a plurality of vent holes  54 , which do not intersect either bore  41  or  42 , or any of the transverse bores. 
     A lubricating fluid, e.g., water, supply tank  56  is connected, by means of conduits  57  and  58  and suitable coupling members  59  and  61  to bores  41  and  42 , for supplying the water lubricant to the bores  41  and  42  and, through transverse or radial bores  44 ,  46 ,  47 ,  48 ,  49 ,  51 ,  52 , and  53  to the surface of tube  13  which, in operation, is mounted in and rotated by a suitable fixture such as a lathe chuck, not shown. Each of conduits  57  and  58  has a control valve,  62  and  63 , respectively, for control of the amount of water or other lubricant supplied to mold  22 . Valves  62  and  63  may be manually operated, or may be under control of a suitable automatic control arrangement, not shown. Also not shown is a means for pumping the water, if necessary, to insure that there is a uniform delivery of lubricant to the surface of tube  13 . 
     In operation, tube  13  which is being rotated by chuck  15  about the central axis or centerline  16  is heated to ductility and positioned as shown in FIG.  2  and the mold members  23  and  24  are closed thereabout by means of arms  27  and  28 . At the same time, plug member  36  is moved into the position shown to limit the diameter of the end of molded tube  13  as explained hereinbefore. The lubricant is supplied to the mold while the end of tube  13  is being shaped, and the vaporized lubricant or water is vented to the outside through vents  54 . The heating element  33  is preferably able to maintain the heat of the tube end sufficient to maintain ductility until the molding process is completed after which the lubricant fluid can have a cooling effect on the end of tube  13 . The plug  36  insures that the axis of the shaped end of tube  13 , i.e., the diameter, is collinear with the tube axis so that it will, in turn, be collinear with the starter tube  12  when they are fused together. During the molding process, the tube  13  can be heated to a point where it is so ductile that it tends to collapse during the molding process, depending, at least in some cases, on the material of the tube. This collapse can be prevented by injecting pressurized air into the tube while it is being molded. Also, plug  36  restrains the end of the tube from collapsing. In the aforementioned U.S. patent application of Jason Kay et al., there is shown an arrangement for preventing collapse of the tube without the need for pressurized air. 
     FIG. 4 is a perspective exploded view, in cross-section of the mold  22  of the invention and FIG. 5 depicts the mold  22  of FIG. 4 in its operative configuration. As discussed hereinbefore, mold  22  comprises two members  23  and  24  of suitable material such as graphite or carbon or other material commonly used in glass molding, such as ceramic material. Member  23  is semi-circular in shape and has an arcuate inner surface  66  and member  24  has an arcuate inner surface  67 . When the two halves  23  and  24  are brought together to form mold  22 , surfaces  66  and  67  form bore  29  which receives tube  13 , as shown in FIG.  5 . Surfaces  66  and  67  have, adjacent end  31  of mold  22 , tapers  32  for imparting to tube  13  a tapered end  68 , as best seen in FIG.  4 . In accordance with one aspect of the invention, member  23  has a longitudinal bore  41  therein which ends just short of end  43  thereof from which extend transverse (or radial) bores  44  and  47  which connect bore  41  to the interior of bore  29  through surface  66 . Although only two such radial bores  44  and  47  are shown, a greater number of such bores may be formed in member  23 , depending upon such factors as the material of tube  13 , the heat required to impart ductility thereto, and the type and amount of lubricant to be used. Extending from surface  66  to the external surface of member  23 , thereby connecting the interior of bore  29 , to the outside, are one or more radial vent holes  54  which do not intersect any of the bores  41 ,  44 , and  47 . In like manner, member  24  has a longitudinal bore  42 , radial bores  49 ,  51 , and  53 , and one or more vent holes  54 . 
     The front  31  of mold  22  has, when members  23  and  24  are in operative position as shown in FIG. 5, a stepped recess  34  having a reduced diameter portion which receives the reduced diameter portion  38  of plug  36  as discussed hereinbefore. The shoulder  71  formed by the step in recess  34  serves to locate plug  36  longitudinally relative to the end of tube  13  by engaging the large diameter portion of plug  36 , as shown in FIG.  5 . 
     It is to be understood that the various features of the present invention are susceptible to inclusion in any of a number of possible embodiments, and that various modifications or adaptations might occur to workers in the art. All such embodiments, variations, or adaptations are intended to be included herein as being within the scope of the present invention as set forth. Further, in the claims hereinafter, the corresponding structures, materials, acts, and equivalents of all means or step plus function elements are intended to include any structure, material, or acts for performing the functions in combination with other elements as specifically claimed.