Method and apparatus for making a loose tube fiber optic cable component

#The invention deals with method and apparatus for making a loose tube buffered fiber optic cable component.

#Fiber optic cables usually come in either the loose tube or tight buffered variety and their chief component is made by horizontal or vertically disposed apparatus. This invention concerns a horizontally disposed improved apparatus and method of making a loose tube cable component, which comprises a grease or hydrocarbon jelly like substance coated optical fibers loosely disposed inside of a tube made from some kind of suitable polymer, such as polycarbonate. The grease or jelly coated fibers are so disposed so that they may move relative to one another and to the tube itself. To make a loose tube fiber optic cable component, the prior art teaches one to assemble one or more optical fibers or a bundle of optical fiber ribbons in an elongated stream, a petroleum grease or jelly is applied thereto and then a plastic tube is extruded over the previously coated fibers. Such components are assembled into a core over which a plastic jacket is formed to complete a cable assembly. See, for example, U.S. Pat. No. 4,828,352, the contents of which are herein incorporated by reference.

#Horizontally disposed prior art loose tube component making apparatus employ a long needle that extends from a grease injection head (where optical fibers, or ribbons, are coated with a jelly like hydrocarbon and then introduced into the needle) and a polymer extrusion crosshead, which forms a plastic tube about the coated fibers as they exit the polymer crosshead. Such apparatus requires that the needle have a centering device to manually center the needle in the X-Y position. Satisfactory results can be achieved using this type of apparatus if the polymer crosshead has a plastic flow passageway that is less than six (6) inches. Unsatisfactory results have been observed with crossheads having a plastic flow passage in the neighborhood of 12 inches or more. The longer the crosshead the longer the needles must be to extend through it to a point (outside of the crosshead) where molten polymer is caused to also exit (through a die) to form a tubular like body circumscribing the grease coated optical fibers. Long needles tend to produce higher tensions on the grease coated optical fibers because of the increased drag inside the needle, i.e., the grease comes into and stays in contact with the fibers or ribbons a longer time giving rise to stress on the fibers or ribbons. The present invention has as its objectives three things: (1) decreased tension on the fiber, short needles can be used; (2) prior art X-Y positioning apparatus is eliminated, the disclosed apparatus is self centering; and, (3) elimination of a tendency to waste jelly or grease like hydrocarbon fill material experienced in prior art apparatus arising out of grease back flow and/or prewetting.

#FIG. 1 is a cross sectional view of a loose tube fiber optic cable component.

#FIG. 2 is a schematic representation of prior art horizontal extrusion and grease injection apparatus used to manufacture the loose tube component of FIG. 1.

#FIG. 3 is a schematic cross sectional view of the apparatus of the present invention.

#Shown by element 1 of FIG. 1 is a loose tube component of a fiber optic cable. Plastic tube 2 is made from any suitable polymer, such as polycarbonate, formed about grease coated optical fibers 3-4. Optical fibers 4 can be either single fibers and/or optical fiber ribbons like that shown in U.S. Pat. No. 4,900,126, the contents of which are herein incorporated by reference. Plastic tube 2 can be either a single wall type or a multiwall type (not shown). For example it may be composed of two walls, both walls being formed from thermoplastic materials.

#Loose tube element 1 is made using the prior art apparatus shown as element 5 in FIG. 2. Such apparatus includes a grease injection unit 9 and a plastic extrusion crosshead 13. Grease injection unit 9 also contains a small diameter tube (or a needle) 10 that extends all the way through crosshead 13 to the other side where, at its terminal free surface, a plastic tube 2 is formed over grease coated optical fibers 3-4. Crosshead 13 has inlet port 6 and exit port (die) 8. Between these two ports there is passageway 7 (flow channel) through which there is flowed a suitable polymer in a manner well known to the prior art, e.g, extrusion. It is of interest to note that at a point denoted by element 11 (inside of grease injection device 9) fibers 4 come into contact with the grease and stay in such contact all the way through the needle 10 until plastic tube is formed over the grease coated fibers 3-4.

#In contrast to the prior art apparatus 5, the invention is shown as element 12 in FIG. 3. Comparing the structures of FIGS. 2 and 3, the contact point 17 of the invention, where optical fibers 4 first contact grease, is further to the right than contact point 11 of FIG. 2 and the needle of FIG. 3 is much shorter than the needle 10 of FIG. 2. As mentioned above, additional apparatus is needed to manually center the needle in the X-Y axis. Such is not needed within the invention of FIG. 3.