Methods and Apparatus for Coating a Filament

1. Field of the Invention

This invention relates to the coating of filamentary articles. In particular continuous filaments are uniformly and concentrically coated.

2. Description of the Prior Art

In the field of optical glass fibers, it is desirable to coat a continuous glass fiber filament with a coating material in order to protect and strengthen the filament. In the prior art such continuous glass fiber filaments are coated by the extrusion of plastic as set forth in U.S. Pat. No. 3,960,530, which issued to R. Iyengar on June 1, 1976. That patent discloses a closed vertical cylinder charged with plastic coating material. The cylinder has a die aperture in the lower end thereof and an axial core tube terminating in the die aperture to form an annular orifice, the plastic being liquified to pass through the die orifice. The glass fiber filament is drawn from a source of molten glass coaxial with the core tube, continuously through the core tube. Pressurized gas is introduced into the upper portion of the cylinder to force the liquified plastic through the annular orifice and onto the filament as it leaves the axial core tube.

Such extrusion application of the plastic coating induces undesirable stresses in the coating material as it passes through the die orifice and, in addition, the drawing speed of the glass fiber filament is limited due to the relatively slow application speed of the extrusion process.

The instant invention overcomes the foregoing problems with a method and apparatus for uniformly and concentrically distributing flowable material on a continuous filament by forming a vortex of the material and drawing the filament through the center of the vortex, in contact with the material, to uniformly and concentrically distribute the material thereon.

The vortex of flowable material is formed by drawing the filament through a material distributor means comprised of an open-ended cage having the geometry of a hyperboloid of rotation.

The filament does not make physical contact with the distributor means, but only contacts the flowable material. Advantageously, this precludes undesirable scraping or abrading of the filament.

Also, by using an open ended cage having the shape of a hyperboloid of rotation to distribute the previously applied material, a vortex is formed which centers the filament in the throat portion of the cage. Also, by using a cage there is substantially no back pressure at the throat portion resulting in substantially no stress being applied to the coating material.

Furthermore, the open ended cage has a variable throat diameter which, advantageously, can accommodate a wide range of filament thicknesses and can control the thickness of the flowable material applied thereto.

FIG. 1 depicts a coating material being applied to a filament in accordance with the instant invention;

FIG. 2 is a partial view of the filament passing through a rotating body of the coating material;

FIG. 3 is a side view of the rotating body of coating material with the filament being drawn therethrough in the direction of rotation of the material;

FIG. 4 depicts a filament being drawn through the rotating material and through an open ended material distribution cage to distribute the material on the filament;

FIG. 5 is a cross-sectional view of the open ended distribution cage mounted within a housing;

FIG. 6 is an isometric view of the distribution cage;

FIG. 7 is an isometric view of the distribution cage wherein one end has been rotated to form a hyperbolic cross section;

FIG. 8 is a cross-sectional view through the throat of the distribution cage in a plane perpendicular to the axis thereof;

FIG. 9 is an isometric view of a modified distribution cage;

FIG. 10 is an isometric view of a modified distribution cage housing; and

FIG. 11 is a cross-sectional view of the instant material applicator.

The instant invention is related to a method and apparatus for distributing a flowable material on a filament and is herein described in relation to the application of a protective coating of material to a glass fiber filament. However, it should be understood that such description is exemplary only and is for the purposes of exposition and not for purposes of limitation. It will be readily appreciated that the inventive concept as described is equally applicable for applying and distributing flowable material to any filament such as metallic wire, string, yarn, or the like.

The instant coating apparatus is generally designated by the numeral 10 in FIG. 1. A laser 11 or any other kind of heat source such as resistance or induction furnace is used to melt a portion of a glass preform 12 and a continuous glass fiber filament 13 is then drawn from the melt zone onto a rotatable take-up reel 14. The filament 13 passes through a substantially planar rotating body of flowable material 16 which is formed by an applicator, generally designated by the numeral 17.

The applicator 17 is comprised of a rotatable conical member 18 having an apex 19 in juxtaposition to a planar plate 26. The flowable material 16 is fed into the volume bounded by the surface of the plate 26 and the rotatable conical member 18 through a conduit 27. The apex 19 projects slightly into, but does not make contact with the end of the conduit 27, as can best be seen in the enlarged partial cross-sectional view of FIG. 11. The rotatable conical member 18 is rotated under the control of a variable speed motor 28 via a shaft 29.

In operation, the motor 28 is activated to rotate the conical member 18 as the flowable material 16 is introduced between the conical member and the plate 26, via the conduit 27, to fill the volume therebetween. As the conical member 18 rotates, the viscous force of the flowable material 16, balanced with the centrifugal and gravitational forces, holds the flowable material between the conical member and the plate 26. For a small angle between the surface of the conical member 18 and the plate 26, of 3 degrees or less, the rotating body of flowable material 16 is substantially planar in shape and in a specific embodiment has been operated in a vertical plane. Although the vertical positioning of the flowable material 16 has been found to be most advantageous it is contemplated that any plane including the horizontal may be used depending upon the particular application.

Once the flowable material 16 has been rotated to form the substantially planar body of material the laser 11 is activated and directed at the optical glass preform 12 is form a melt zone from which the optical fiber filament 13 is drawn. The glass fiber filament 13 passes through the substantially planar body of flowable material 16 without making contact with either the plate 26 or the rotatable conical member 18 as can best be seen in the enlarged partial plan view of FIG. 2.

FIG. 3 depicts a cross-sectional view in elevation of the rotating body of flowable material 16 with arrows indicating the direction of movement of the material. The filament 13 is shown passing through the flowable material 16 in the same direction as the movement of the material. Advantageously, this permits application of the material 16 to the filament 13 at high drawing speeds since the material can be made to move at substantially the same velocity as the filament. This was not possible using the prior art extrusion techniques.

In a particular working embodiment of the instant invention, the glass fiber filament 13 was an optical fiber having a diameter ranging from approximately 70 to 200 micrometers and the coating applied was approximately 50-100 micrometers thick.

The material 16 used was a "hot melt" plastic which is raised to its melting temperature prior to being fed to the applicator 17 via the conduit 27. The use of such "hot melt" plastics may require that the conduit 27 and/or the plate 26 be heated during operation. In particular, the "hot melt" plastic used was Ethylene-Vinyl Acetate (EVA) having a melting temperature of 180.degree. C. and a viscosity in the neighborhood of 150 poise. Once the material 16 has been applied to the filament 13, it becomes hardened as it travels between the applicator 17 and the take-up reel 14 due to cooling of the material to ambient temperature.

The plate 26 was circular in shape with a diameter of 10 centimeters and made of aluminum. The surface of the conical member 18 formed an angle of 3 degrees with the plate 26 and was rotated at a tangential velocity of approximately 0.2 meters per second as the optical fiber filament 13 was drawn at a speed of approximately 0.75 meters per second for the EVA material. The viscosity level of the coating material used is one of the governing factors determining the maximum drawing speed.

FIG. 4 depicts the coating apparatus 10 shown in FIG. 1 with the addition of distribution apparatus 41 and a curing apparatus 42 positioned in tandem with the applicator 17.

It should be noted that although the applicator 17 advantageously applies the coating material 16 to glass fiber filament 13, the coating may be non-uniform in thickness. Thus, the purpose of the distribution apparatus 41 is to uniformly and concentrically distribute the flowable material 16 which was previously applied to the glass fiber filament 13. Although FIG. 4 depicts an embodiment in which the flowable material 16 was applied by the applicator 17, such apparatus is not required. Any apparatus which can apply the flowable material 16 such as that shown in the above-referred to Iyengar patent or the like may be used. All that is required is that the flowable material 16 be, in some fashion, applied to the fiber 13 prior to passing through the distribution apparatus 41.

The distribution apparatus 41 schematically shown in FIG. 4 is shown in detail in FIGS. 5, 6 and 7. FIG. 5 is a cross-sectional view of the distribution apparatus 41 fully assembled and which comprises a cylindrical housing 43 having a distribution cage 44 therein which can best be seen in FIGS. 6 and 7. The cylindrical housing 43 has upper and lower shoulders 46 and 47, respectively. The upper shoulder 46 has a plurality of upwardly projecting spring biased pins 48--48.

The distribution cage 44 is comprised of a first circular plate 49, having a first central aperture 50, which is spaced from and aligned with a second circular plate 51 having a second central aperture 52. Both plates 49 and 51 have a plurality of holes 53--53 arranged in a circular array about the first and second central apertures 50 and 52, respectively. A strand 54 is threaded through the holes 53--53 of the plates 49 and 51 to form the distribution cage 44 connected by substantially parallel sections of the filament 54 as can best be seen in FIG. 6. The filament 54 may be fine wire, nylon or the like having a melting point above the melting point of the flowable material 16.

The distribution cage 44 is shown assembled within the cylindrical housing 43 in FIG. 5. The second cylindrical plate 51 is seated in the lower shoulder 47 of the housing 43 and is held fixedly in place by a set screw 61. A downwardly extending section 62 of the first circular plate 49 is seated within the upper shoulder 46 on the spring biased pins 48--48.

Prior to the distribution operation, the first circular plate 49 is rotated using a pair of outwardly extending arms 63--63 as shown in FIG. 7. As the first circular plate 49 is rotated, it simultaneously moves downward, urging the spring biased pins 48--48 down into the cylindrical housing 43. As the first circular plate 49 is rotated the normally parallel filaments 54--54 (see FIG. 6) now become skewed as shown in FIG. 7 forming a hyperboloid of rotation with a throat 64 (see FIG. 8) that can be varied as a function of the angular rotation of the first circular plate 49. Once the desired diameter of the throat 64 has been obtained the first circular plate 49 is maintained in the position with the spring biased pins 48--48 which are held in place by the frictional contact between the plate and the pins. Accordingly, the throat 64 can be varied to accommodate a wide variety of filament diameters by rotating the first circular plate 49 to provide the desired diameter in the throat. Additionally, by varying the diameter of the throat 64 the thickness of the coating material 16 on the fiber 13 can be controlled. The wider the diameter of the throat 64 the thicker the coating will be.

FIGS. 9 and 10 depict a modified distribution cage 44' and distribution apparatus 41', respectively. The distribution cage 44' has a first radial slot 66 formed in the first circular plate 49 and is aligned with a second radial slot 67 formed in the second circular plate 51 when in the unoperated position. FIG. 10 shows the distribution apparatus 41' assembled within a modified cylindrical housing 43' having a longitudinal opening 71 therein. Prior to the rotation of the first circular plate 49 as hereinbefore described, the first and second slots 66 and 67 are aligned with the longitudinal opening 71 to provide radial access for the continuous optical fiber filament 13 to the central portion of the distribution cage 44'.

In operation, as can best be seen in FIG. 4, once the flowable material 16 has been applied by the applicator 17 (or other applying means) to the optical fiber filament 13, the filament is drawn through the distribution apparatus 41. The coated filament 13 is threaded axially through the central apertures 50 and 52 when using the distribution cage 44 depicted in FIG. 6 or the filament may be inserted radially through the slots 66 and 67 and the aligned longitudinal opening 71 when using the distribution apparatus 41' shown in FIG. 10.

As the optical fiber filament 13 is drawn through the distribution apparatus 41 the flowable material 16 on the filament 13 contacts the sections of skewed strand sections 54--54 which tends to move the material in a spiral direction, along the path of the filaments (see FIG. 8) forming a vortex of the flowable material terminating at the throat 64 of the cage 44. As the filament 13 is pulled through this vortex, it is automatically centered within the throat 64 of the distribution cage 44 by the movement of the flowing material 16 causing the material to be uniformly distributed to concentrically coat the filament 13. Additionally, by forming the throat 64 with a plurality of strand sections 54--54 there is substantially no back pressure at the throat, resulting in a stress-free coating.

Once the filament has been uniformly and concentrically coated with the flowable material 16 it is wound about take-up reel 14. The curing apparatus 42 is used to cure the flowable material 16 coating the filament 13 prior to take-up when curing is required.

The material 16 can be a material having a low enough viscosity to flow into the volume between the plate 26 and the rotatable conical member 18 and sufficiently high viscosity to be held within that volume as the rotatable conical member is rotated. An example of such a material which can be used at normal room temperatures would be any curable materials such as a resin or epoxy which is flowable at room temperatures. Such materials generally require use of the curing apparatus 42 to cure the material prior to being rolled onto the take-up reel 14. Such curing may be accomplished by heat curing (i.e., infra-red or the like) or by polymerization (i.e., ultra-violet or the like) depending on the selection of materials.