GB2158963A

GB2158963A - Manufacture of optical fibre cable

Info

Publication number: GB2158963A
Application number: GB08511957A
Authority: GB
Inventors: John Derek Smithies Hinchliffe; Clifford Heywood
Original assignee: Telephone Cables Ltd
Current assignee: Telephone Cables Ltd
Priority date: 1984-05-14
Filing date: 1985-05-10
Publication date: 1985-11-20
Also published as: GB8511957D0; GB2158963B; GB8412276D0

Abstract

The manufacture of an optical fibre cable of the kind in which a plastics extrudate (2,21) is formed with a plurality of closed channels (6) to accommodate optical fibres (1,2), a slot (10) being formed in the outer wall of the extrudate over each channel to provide access thereto. The fibres may be fed into the respective channels during the extrusion process, or subsequently through the formed slots. <IMAGE>

Description

SPECIFICATION Manufacture of optical fibre cable This invention relates to a process for producing optical fibre cables, that is to say cables of the kind incorporating one or more optical fibres accommodated in a protective cover, and to optical fibre cables produced by such a process.

The invention is more especially concerned with the manufacture of cables of the kind referred to in which the protective cover is formed of extruded plastics material and comprises a central core, an outer wall surrounding the core and a plurality of fins, ribs or the like extending between the core and the outer wall to define between them a plurality of longitudinally-extending channels at least one of which provides accommodation for one or more optical fibres.

In accordance with the invention in the manufacture of such a cable the cover is initially extruded with closed channels, and a slot is subsequently formed in the outer wall of the cover over each channel accommodating or required to accommodate one or more optical fibres.

The presence of the slot or slots provides access to the interior of the respective channel or channels, and facilitates the removal from the formed cable of a fibre without cutting into the outer wall of the respective channel with a knife or like tool.

The fibre or fibres may be fed into the respective channel or channels either during the extrusion process or through the formed slots during a subsequent stage of manufacture. The feeding of each fibre into its respective channel is preferably effected at a rate greater than the rate of which the cover is extruded such that it has a natural length, i.e.

when straightened out, which is greater than the length of the channel in which it is located this avoiding undue stress being placed on the fibre or fibres during subsequent manufacture or use of the cable. Preferably each fibre is loosely accommodated in its respective channel so that it is free to move transversely within the channel.

Preferably a tape and sheath are applied in tandem around the extrudate at some stage after the fibres have been introduced into the respective channels. This sheath may also be formed of plastics material and may be extruded around the slotted extrudate. If desired a moisture barrier, which is conveniently in the form of a longitudinal metal/polymer tape, may be applied around the assembly before the application of the sheath.

One or more strength members are preferably incorporated into the extrudate during the extrusion process.

The extrudate may be twisted while it is still soft so that, on cooling, the channels extend helically around the cover axis. The direction of twist is conveniently reversed periodically at not more than half a turn, so that each channel has a substantially sinusoidal configuration projected on to a cylindrical surface coaxial with the cover.

Where the fibres are subsequently introduced into helical or periodically-reversed helical channels through the formed slots, the means for feeding the fibres into the channels is conveniently mounted on a carriage which is capable of rotation or partial rotation about the cover so that it can follow the lay of the slots.

In an alternative arrangement the slots are formed close to the extruder head and the fibres introduced into the channels through the slots whilst they are straight, thereby avoiding the need for a rotating carriage; the cover in such a case may be subsequently given a helical or a periodically reversed helical twist with the fibres contained within it.

The invention will now be described by way of example with reference to Figs. 1 to 4 of the accompanying schematic drawings, in which Figure 1 illusrates diagramatically, and not to scale, apparatus for manufacturing optical fibre cable in accordance with the invention, Figure 2 represents part of the apparatus on a larger scale, Figure 3 and 4 represent transverse crosssections of two cables manufactured in accordance with the invention and employing different forms of fibre cover.

Referring first to Fig. 1, a suitable plastics material at an appropriately high temperature is extruded from an extruder 1 to form a cover 2 which is drawn through a water bath 3 by a traction device 4.

As the molten material is forced through the extruder 1 a central reinforcing member 5, of any suitable material, is simultaneously drawn through a central hole in the extruder head, and the latter incorporates a plurality of channel forming elements (not shown) disposed uniformly around it, such as to form channels 6 in the extrudate having a cross-section determined by the shape of the elements.

Downline of the water bath 3 torsional gripping means 7 engage the cooled extrudate 2, and are torsionally oscillated at a frequency determined by the rate of travel of the extrudate. The section of the extrudate lying within the water bath 3 is too cool for significant plastic deformation to occur, but a pronounced permanent periodically-reversing twist is imparted to the hot section of the extrudate between the extruder 1 and the water bath 3. As a result the channels 6 follow sinusoidal paths as indicated at 6'.

Annular seals 8 of any convenient form are disposed in the walls of the water bath and provide the necessary longitudinal and torsional freedom of movement of the extrudate.

In accordance with the invention means, shown diagrammatically at 9, are provided downstream of the traction device 4 for forming slots 10 (Figs. 3 and 4) in the cover 2, over the centre of each of the channels 6. The slotting means can be of any suitable form.

Optical fibres 1 2 are arranged to be fed into the channels 6, by means of narrow-bore tubes 14 extending into the channels through the formed slots 10.

Means are also provided for ensuring that the slotting means 9 and the fibre feed tubes 14 follow the paths of the channels 6, and for this purpose they are conveniently carried by a head, represented by the broken line 13 which is capable of oscillating around the cover 2, the engagement of at least one locating member in a respective slot or channel ensuring that the head 1 3 oscillates in the required manner to conform to the periodically reversing twist of the channels.

Following the insertion of the fibres 1 2 a plastics tape 1 5 is wrapped longitudinally, or lapped helically around the cover, followed by a moisture barrier in the form of a metal tape 16 which is wrapped longitudinally around the cover 2 over the tape plastics tape 1 5.

The wrapped cover is then passed through a further extruder 1 7 which surrounds it with a plastics sheath 18, and is finally fed to a takeup spool (not shown).

In a modification (not illustrated) the fibres 1 2 are fed into the cover 2 during the extrusion process as described, for example in copending Patent Application No. 8331255, the slots 10 being formed in the cover wall with the fibres 1 2 already located within the channels 6. Care should then be taken to prevent damage to the fibres 1 2 during the cutting of the slots 1 0.

In a further modification the slotting means 9 is located just downstream of the extruder 1.

One form of optical fibre cable manufactured by the process described is shown in Fig. 3, the cable having a cover 2 comprising a central core 20 surrounding the strength member 5, and having a plurality, in this case four, longitudinally-extending fins 21 forming between them approximately triangularlyshaped channels 6, each loosely accommodating a number of optical fibres 1 2. The outer wall of each channel has a centrally-disposed longitudinally-extending slot 1 0, the fibres either being fed into the channels 6 during the extrusion process or subsequent to the formation of the slots as above described.

An alternative form of cable is illustrated in Fig. 4, the channels 6 in this case being substationally circular in cross-section, although it will be realised that the cover may be formed with various other shapes of channel.

Similarly with either form of cable illustrated more or less than four channels may be provided.

One or more conductive wires may replace the optical fibres in one or more of the channels, the wire or wires being fed into the respective channel or channels either simultaneously with the fibres or at a different stage.

In the case where the fibres are fed into the channnels 6 in the cover 2 during the extrusion process the slotting means may be associated with guards 24, for example as shown in Fig. 2, to protect the fibres 1 2 during the cutting of the slots 10, the guards 24 extending into the channels through the slots just downstream of the slotting means 9 and extending between the latter and the fibres, being shaped to ensure that the fibres are kept away from the outer wall of the cover 2 as they are carried past the slotting means.

Claims

1. The manufacture of an optical fibre cable comprising a protective cover formed of extruded plastics material and having a central core, an outer wall surrounding the core and a plurality of fins, ribs or the like extending between the core and the outer wall to define between them a plurality of longitudinallyextending channels at least one of which provides accommodation for one or more optical fibres, wherein the cover is initially extruded with closed channels, and a slot is subsequently formed in the outer wall of cover over each channel required to accommodate one or more optical fibres.

2. The manufacture according to Claim 1 wherein the fibre or fibres is/are fed into the respective channel or channels as they are formed during the extrusion process.

3. The manufacture according to Claim 1 wherein the fibre or fibres is/are fed into the respective channel or channels through the formed slots.

4. The manufacture according to any preceding claim wherein the fibre or fibres is/are fed into the respective channel or channels at a rate greater than the rate at which the cover is extruded.

5. The manufacture according to any preceding claim wherein at least one tape and a surrounding sheath are applied around the cover following the introduction of the fibre or fibres into into the respective channel or channels.

6. The manufacture according to Claim 5 having one said tape of plastics material wrapped longitudinally or lapped helically around the cover and a moisture barrier is applied around the cover and plastics tape assembly before the application of the sheath.

7. The manufacture according to Claim 6 wheren the moisture barrier is in the form of a metal/polymer tape wrapped longitudinally around the cover and plastics tape assembly.

8. The manufacture according to any preceding claim wherein one or more strength members are incorporated into the cover during the extrusion process.

9. The manufacture according to any preceding claim wherein the extruded cover is twisted while it is still soft such that, on cooling, the channels extend helically around the cover axis.

10. The manufacture according to Claim 9 wherein the direction of twist is periodically reversed at not more than half a turn so that each channel has a substantially sinusoidal configuration projected on to a cylindrical surface coaxial with the cover.

11. The manufacture according to Claim 3 in which the extruded cover is twisted while it is still soft such that on cooling, the channels extend around the cover axis in a helical or a periodically reversed helical fashion, wherein the fibres are introduced into the helical or reversed helical channels through the formed slots by means capable of rotation or partial rotation about the cover so as to follow the lay of the slots.

1 2. The manufacture according to Claim 3 in which the extruded cover is twisted while it is still soft such that on cooling, the channels extend around the cover axis in a periodically reversed helical fashion, wherein the slots are formed in the cover close to the extruder head, the fibres are introduced into the channels through the sots while they are still straight, and the cover is given said helical or periodically reversed helical twist with the fibres contained within it.

1 3. The manufacture of an optical fibre cable carried out substantially as shown in and as hereinbefore described with reference to Figs. 1 and 3 or Figs. 1, 2 and 4 of the accompanying drawing.

14. An optical fibre cable made in accordance with any preceding claim.