Cable and method of manufacturing a cable

Abstract

A cable (102, 510) has a functional core extending in a longitudinal direction and a sheath (524) surrounding said core such that an outer surface of said sheath forms an outer surface (532) of the cable. The outer sheath is formed around the core by extrusion (722) from a polymer melt. An outer part (726) of the extrusion tooling is maintained at a temperature lower than the melting point of the polymer by such an amount that an irregular texture is continuously imparted to the outer surface (532) of the cable. A miniature fibre optic drop cable made in this way can be delivered more than 250 m through a microduct by pushing alone, without the need for blowing equipment.

Description

[0001] CABLE AND METHOD OF MANUFACTURING A CABLE

[0002] FIELD OF THE INVENTION

[0003] The present invention relates to cables, including but not limited to a fibre optic drop cable. The invention further relates to methods of manufacturing such cables and methods of installation of cables in a duct.

[0004] BACKGROUND TO THE INVENTION

[0005] Optical fibre transmission lines can be installed through the ground, through ducts, and through service spaces within buildings by a variety of methods, including direct burying (trenching), pulling through ducts, pushing through ducts, blowing through ducts, and combinations of these. Fibre to the home (FTTH) is the generic term for broadband network architecture that uses optical fibre technology to carry data to a residential dwelling from a broadband service provider via a telecommunications cabinet located near the residential dwelling. More generally, not only homes, but also office premises, are increasingly connected by optical fibres to the wider telecommunications network. Ducts and cables can also be used, for example, to connect individual offices or apartments to a communications room in a larger premises. A major area of application nowadays is delivery of broadband data to wireless access points, such as 5G antennae (FTTA). The abbreviation (FTTx) is regularly used to encompass this wide range of use cases.

[0006] Lightweight air-blown fibre units, also known as micro cables, have proven extremely successful in delivering optical fibre connections to individual premises in an economic manner. On the other hand, installation by blowing still requires highly specialised equipment and training. It will be understood that pulling a cable through a duct using a pre-installed pulling cord is relatively simpler. However, unless the distances of pulling are limited to a few metres, a much more robust and therefore larger, heavier and more expensive cable is required to protect the optical fibres from the stresses of pulling. Installation by pushing is also relatively straightforward, requiring a more substantial cable design, but saving on the expensive equipment and training required for blowing. At the same time, blowing is normally the only technique that can deliver the compact drop cable several hundred metres, often over a thousand metres, through a duct without opening the duct to perform the installation in stages. Minicables are known that can be installed by pushing over longer distances, but these are too big and too expensive to be used as drop cables to connect individual terminals to a network.

[0007] In recent years, so-called nanocables, which are compact and lightweight but incorporate an additional strength member alongside the optical fibres, have enabled installation over distances of tens of metres, in some cases well over 100 m, to be performed by the relatively simple pushing process. An example of such a cable is disclosed in the applicant’s published patent application W02022049057A1 , the contents of which are hereby incorporated by reference. On the other hand, for distances over 200 m, or even over 300 m, compact drop cables that can be installed simply by pushing do not currently exist in the market. Accordingly, installation over those longer distances generally requires more cumbersome and expensive cable designs and / or more sophisticated installation methods such as blowing.

[0008] As further background, it is known to apply a lubricating material into a duct as a preliminary step to facilitate installation of a cable. In installation of cables by blowing, it is also known to apply a lubricating fluid continuously to an outer surface of the cable before it enters the duct, using one or more sponge elements that are pre-loaded with lubricating fluid. Examples of such apparatus are disclosed in W02004008599A1 (Griffioen et al, Plumettaz, 2004), which illustrates arrangements whereby the blowing air can bypass the lubricating sponges. The authors of that patent application have also published evidence that applying lubricant to the duct in advance brings additional benefit in performance (Griffioen, “Cable in Duct Installation: Lubrication Makes the Difference”, Proc IWCS 61 p.54-59 (2012). US2006102430A1 by the same authors describes alternative lubrication devices for use in installation by blowing, wherein lubricant is delivered from an external reservoir into a lubrication space that surrounds a section of the cable between two foam plugs. Air pressure from the blowing tool pressurises the lubrication space and the external reservoir. A venting arrangement is provided so as to relieve this pressure when the blowing operation stops, while allowing the reservoir to be sealed against leaking when not in use.

[0009] US20040096166A1 (Alcatel, 2004) discloses an optical fibre cable for installation by blowing, in which a textured outer surface is obtained by mixing tiny glass fibre fragments into the molten material of an extruded sheath. This rough profile is suggested to improve installation performance by (i) reducing the contact surface between the cable and a duct during installation, so reducing friction and (ii) increasing air drag. It will be appreciated that the inclusion of these glass fibre fragments not only complicates the manufacturing process, but brings safety hazards and complicates waste management and recycling.

[0010] Larger cables commonly have undulating or wavy surfaces, due to their stranded core structure being encased within a thin sheath. Such an approach is not applicable in the much smaller cables such as are considered in the present disclosure.

[0011] SUMMARY OF THE INVENTION

[0012] The appended claims in the present application relate to the fifth and sixth aspects of the invention as set forth below. Another application having the same priority dates and filing date contains claims relating to the first to fourth aspects set forth below.

[0013] According to a first aspect of the invention, there is provided a cable adapted to be installed by pushing into a microduct with bore less than 5 mm, the cable having an outer diameter less than 3.0 mm and comprising a core and a polymer sheath surrounding the core said polymer sheath defining an outer surface of the cable, the core containing one or more optical fibres and least one longitudinal strength member, all extending in parallel with one another in a longitudinal direction of the cable, wherein said outer surface is a textured surface having both short range and longer range irregularities making up an overall surface texture.

[0014] In some embodiments, the polymer sheath is made of a solid, thermoplastic polymer, the textured surface being formed by irregular cooling of an extruded melt. In this way, a highly textured surface can be obtained on a small cable in a simple process. That is to say, the outer surface of the cable can have relatively large-scale features that are unrelated to any underlying structure in the core, and without inclusion of particles or foamed structures within the polymer material.

[0015] In some embodiments, the polymer sheath is made of a thermoplastic polymer material having a structure that is at least partially crosslinked after extrusion. In other embodiments, the textured surface of the polymer sheath is a result of a foamed structure within at least a layer of the polymer sheath. The polymer sheath can be entirely foamed, or foamed only in one layer of a multilayer structure.

[0016] Alternative methods may of course be deployed to impart the texture.

[0017] The cable may be for example a fibre optic cable, such as a drop cable for connecting a premises to a telecommunications network.

[0018] According to a second aspect of the invention, there is provided a pushing tool for installing a cable into a duct while applying lubricating material to an outer surface of the cable while the cable is fed into a duct, the device operating without assistance by blowing and comprising: a pushing mechanism adapted to receive cable from a cable supply, to grip an outer surface of the cable and to apply a longitudinal driving force to push the cable progressively through a duct; a device body surrounding a bore that extends from an input port to an output port, said output port being adapted for connection to a first end of the duct, said pushing mechanism being housed within the device body, the device body further providing fluid communication between a reservoir of lubricating fluid and a central section of said bore between the input port and the output port; a first seal arranged in a first section of the bore so as to allow passage of a cable from the input port to the central section while substantially preventing leakage of said lubricating fluid; and a second seal arranged in a second section of the bore so as to allow passage of the cable from the central section to the output port while substantially preventing leakage of said lubricating fluid. In some examples, the fluid in the reservoir is maintained at or below ambient pressure and flow of lubricating fluid between the reservoir of lubricating fluid and the central section of said bore is substantially by gravity.

[0019] In some examples, the pushing mechanism includes at least two wheels for engaging each side of the cable, the wheels on each side of the cable being spaced apart in a longitudinal direction of the cable and connected by a flexible drive belt, for gripping the cable over an extended part of its length. The extended contact with the belt allows a relatively high pushing force to be applied without damaging the cable to mechanism.

[0020] In particular examples, the cable has a diameter less than 3.5 mm and in step (c) the cable is pushed along the duct by a distance greater than 200 m, optionally greater than 250 m.

[0021] According to a third aspect of the invention, there is provided a method of installing a cable by pushing into a duct with bore less than 5 mm, the cable being a cable according to the first aspect of the invention as set forth above, the method comprising the steps:

[0022] (a) receiving a length of said cable;

[0023] (b) inserting a leading end of the cable into a first end of the duct;

[0024] (c) transporting the cable progressively through the duct by pushing alone at least partly using a power-operated pushing mechanism until a desired length of cable has been installed in the duct the pushing mechanism being adapted to be receive cable from a cable supply, to grip an outer surface of the cable and to apply a longitudinal driving force to push the cable progressively through the duct. The method of the third aspect of the invention may further comprise the step (d) at least during an initial phase of transporting the cable through the duct, applying a lubricating material along an outer surface of the cable, such that a quantity of lubricating material is carried into the duct by the textured surface of the cable.

[0025] The lubricating material of the third aspect of the invention may be a lubricating fluid applied using a device of the second aspect of the invention or by other means. However, the lubricating material may however take other forms, for example a supply of solid lubricating material pressed against the surface of the cable as it is pushed into the duct.

[0026] In particular examples, the cable has a diameter less than 3.5 mm and in step (c) the cable is pushed along the duct by a distance greater than 200 m, optionally greater than 250 m.

[0027] The invention in a fourth aspect provides a method of installing a cable by pushing into a duct with bore less than 5 mm, the cable being a cable according to the first aspect of the invention as set forth above, the method comprising the steps:

[0028] (a) receiving a length of said cable;

[0029] (b) inserting a leading end of the cable into a first end of the duct; (c) before or after step (b) fitting a lubricating device around the cable at or near the first end of the duct, the lubricating device including a source of lubricating material;

[0030] (d) transporting the cable progressively through the duct by pushing alone until a desired length of cable has been installed in the duct, whereby at least until set source of lubricating material is exhausted, the lubricating device applies said lubricating material continuously along the outer surface of the cable, such that a quantity of said lubricating material is carried into the duct on the surface of the cable. The cable of the fourth aspect of the invention may be a cable having a textured outer surface in accordance with the first aspect of the invention.

[0031] In some embodiments of the third and fourth aspects of the invention, the leading end of said length of cable is directly or indirectly pre-terminated with a partial connector, the method further comprising the step:

[0032] (e) after pushing the desired length of cable through the duct, accessing said partial connector and adding a connector body to said partial connector to form a complete fibreoptic connector.

[0033] The invention in a fifth aspect provides a method of manufacturing a cable, the method comprising the steps:

[0034] (a) providing a functional core extending in a longitudinal direction;

[0035] (b) providing an extruder having tooling for forming a tube of polymer material;

[0036] (c) forming a melt of polymer sheath material in an extruder, the melt having a melt temperature;

[0037] (d) drawing said core through an aperture in said tooling while extruding said melt to form a tube of said polymer sheath material surrounding the core; and

[0038] (e) drawing and cooling the tube and core together to form a fibre optic cable in which said core is surrounded by a polymer sheath, an outer surface of said polymer sheath forming an outer surface of the cable, wherein, during performance of step (c), an outer part of said tooling is maintained at a temperature lower than said melt temperature by such an amount that an irregular texture is continuously imparted to the outer surface of the cable.

[0039] Optionally, said outer part is maintained at a temperature below a melting point of said polymer sheath material, for example more than 15 degrees Celsius, optionally more than 20, 25 or 30 degrees Celsius lower than said melt temperature.

[0040] Fujikura JP2005202310A discloses an optical fibre jumper cable in which an optical fibre core is surrounded by packed aramid yams inside the cable. A sheath of the core is made of PVC or polyolefin and has a certain roughness (Ra no greater than 6 pm). This is done apparently to modify its interaction with the aramid yarns in the vicinity of a terminating connector. The surface roughness of the sheath is said to be controlled by changing the temperature of the thermoplastic resin of the coating during extrusion.

[0041] The above aspects of the invention can be used in combination as described, or independently of one another. For example, a cable according to the first aspect of the invention can have the textured outer surface produced by a method according to the fifth aspect of the invention, but other methods of providing a textured outer surface are known, and some of these a discussed below.

[0042] These and other features of the invention will be understood from consideration of the examples described below and the dependent claims, illustrated with the appended drawings.

[0043] BRIEF DESCRIPTION OF THE DRAWINGS

[0044] Embodiments of the present invention are described below, by way of example only, with reference to the accompanying drawings, in which:

[0045] Figure 1 illustrates the installation of a fibre optic cable into a duct to connect a domestic or business premises with a broadband access point;

[0046] Figure 2 illustrates the use of a power operated pushing tool in an installation method of the type shown in Figure 1 ;

[0047] Figures 3 (a) and (b) are schematic external views of a pushing tool, which can be for example the pushing tool of Figure 2 or Figure 10;

[0048] Figure 4 shows in schematic cross-section a known example of a miniature fibre optic cable which is designed for installation by pushing;

[0049] Figure 5 is a schematic cross-section of a modified pushable cable having a textured outer surface;

[0050] Figure 6 shows three photographs of an example cable made with this textured outer surface as illustrated in Figure 5;

[0051] Figure 7 illustrates schematically an apparatus and method for manufacturing the pushable cable of Figure 4 or Figure 5;

[0052] Figure 8 shows schematically the use of a lubricating device to apply lubricating material to the outer surface of a cable during installation into the duct;

[0053] Figure 9 shows an enlarged view of the lubricating device and associated accessories;

[0054] Figure 10 is a schematic, cross-sectional view of an enhanced pushing tool, with integrated lubricating arrangement;

[0055] Figure 11 illustrates a standard test route for testing installation performance of optical fibre cables and cable assemblies made according to the present disclosure;

[0056] Figure 12 presents in graphical form values for three surface profile parameters Rz, Ra and Rq for a number of example cables, alongside corresponding installation performance values Dmax; Figure 13 (a) and (b) illustrates particular examples of pre-terminated cable assemblies, incorporating an optical fibre cable such as the cable of Figure 5;

[0057] Figure 14 (a) and (b) illustrates approximate measurements of feature height by inspection of the photographs of Figure 6 (b) and (c), respectively, the photographs having been scaled anisotropically to accentuate the surface height variations;

[0058] Figure 15 illustrates a more detailed height profile measured from the photograph of Figure 6 (c);

[0059] Figure 16 shows reconstructed images from 3-D profiles measured on three sample areas on the surface of the example cable, obtained by electron microscopy.

[0060] DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

[0061] FTTX INSTALLATION - INTRODUCTION

[0062] Figure 1 is a schematic representation of a Fibre to the Home (FTTH) installation 100 of optical fibres, using a length of cable 102, such as one of the pushable cables to be described later with reference to Figure 4 or 5. It will be understood that terms such as “consumer” and “home” are used by way of example only, and the products and techniques described herein may equally be applied in commercial and industrial installations. Optionally one or more ends of the fibre unit has been terminated with a partial connector 104 typically a blowable optical ferrule with a ferrule body. The ferrule is optically connected to an individual fibre, with any other fibre(s) in the cable being spare for future use. In the illustrated example, a fibre unit is provided wound on a reel 106. When unwound from this reel, the cable 102 can connect user equipment in premises 108 to an access point 110, for example a telecommunications cabinet or comms room in a larger premises. Instead of being wound on a reel 106, the pre-terminated cable assembly may be provided in other forms, for example in a coil, in a fibre pan etc.

[0063] In the illustrated scenario, the cable is fed into the duct from the user premises end and pushed towards the access point. In other scenarios, it may be equally convenient or more convenient, to push the cable through the duct from the access point towards the user premises. The installation is performed by passing a leading end of the fibre unit 102 into a pre-installed duct 112. Other ducts 112’ etc, lead from the same access point 110 to other premises, so that this installation method may be repeated many times in a neighbourhood.

[0064] Referring also to Figure 2, where the duct 112 is of substantial length, it will be usual for installation by pushing the cable 102 along the duct 104 to be performed with the aid of a power-operated pushing tool 200. As illustrated schematically, a pushing mechanism is housed within a casing 202, which also provides a mounting base 204. The pushing tool 200 has an input port 206 for receiving the cable from the reel 106 or 208 for delivering cable into the duct 112. In order to allow the cable to be delivered with sufficient pushing force to overcome friction along its length, that output port 208 is associated with a coupling 210 which grips and / or clamps the first end of the duct to prevent the duct end moving away from the pushing tool.

[0065] This example and further examples of pushing tools will be described further below with reference to Figure 3 and Figure 10.

[0066] The leading end of the cable 102, which includes a partial connector 104, leads the installation of the optical fibre or fibres through the duct 112. The leading end passes through the duct 112 and is fed from the reel 106 until the partial connector 104 and a length of the cable assembly 102 exits the duct at the access point 110. To assist the passage and protect the optical ferrule, partial connector 104 may be protected by a dust cap or other accessory with a rounded end. In the case of a cable that is pushed without being pre-terminated, a round-nosed or bullet-nosed accessory can be fitted to the leading end of the cable, to avoid snagging on bends and joints between sections of duct.

[0067] Particular forms of pre-terminated fibre optic cable assembly and methods of installation are disclosed in our earlier patent applications WO2018146470A1 and WO2023016835A1. The cables newly disclosed herein can be used as part of those assemblies and methods, and the contents of both of those published patent applications are hereby incorporated herein by reference. The cables newly disclosed herein can also be used in assemblies of the type disclosed in patent applications GB2619109A and WO2023227384A1 (not published at the present priority date). An illustration of such an assembly will be described briefly below, with reference to Figure 13.

[0068] The cable designed primarily for pushing may also be adapted for blowing and / or pulling, when the need arises. An alternative or supplementary installation process involves physically pulling the leading end of the cable 102 through the duct. A pulling accessory designed to engage a partial connector already fitted to the cable leading end is illustrated in W02022049057A1. Pulling socks and other pulling accessories are known in the art. In addition to pre-terminated cable assemblies, multiple lengths of cable can be installed loosely arranged within an outer tube, to form a so-called pullback cable, for example as also described in W02022049057A1.

[0069] Figure 3 (a) is a schematic external view of a pushing tool 300, which could be, for example, the pushing tool 200 of Figure 2 or could be the pushing tool 1000 described later with reference to Figure 10. The tool has a body made in two halves 302a and 302b. The lower body half 302b has a mounting foot 304. This tool is viewed from a direction looking along the cable axis, into an output port 308. An input port 306 is at the opposite end, unseen from the viewpoint of Figure 3. The coupling that grips the duct end in use is shown schematically at 310.

[0070] As illustrated schematically in Figure 2, a simple form of pushing mechanism comprises a pair of opposing wheels 220a and 220b, which grip the cable between them. By rotation of the wheels in opposite directions as illustrated by arrows in Figure 2, rotary motive force (torque) of the wheels is translated by friction into a longitudinal pushing force on the cable 102. Tyres of rubber or rubber-like material may be provided on the wheels to ensure an effective grip, and also to prevent damage to the cable.

[0071] Returning to Figure 3, an input driveshaft 322 is illustrated, which receives torque from an external motor, which may conveniently be a regular hand-held drill 324. By opposing wheels or other pushing mechanism within the body, the cable is propelled through the pushing tool. While motive power is delivered to just one of the body halves, as shown, a mechanism of gears, belts or equivalent may be provided to provide synchronised driving of the wheels in both halves. Alternatively, rotation of wheels in the other half of the pushing mechanism may be caused passively, by transfer of friction between the tyres on the opposed wheels and the movement of the cable.

[0072] Referring now also to Figure 3 (b), it will be seen that the body of pushing tool 300 is made in two halves 302a and 302b so that it may be opened to allow access to the channel through which cable 102 must pass. In the example shown, the halves are connected by a hinge 328 to allow this opening. Not shown in the drawing is a locking mechanism to hold the halves tightly closed around the cable. Such a locking mechanism would be provided on the opposite side of the body hinge 328. In other examples, instead of being hinged, the halves may be completely separable and clamped together with locking mechanisms are both sides. The gripping and clamping of a duct end by coupling 310 may occur as part of closing the body, or a separate clamping mechanism may be provided.

[0073] PUSHABLE CABLE EXAMPLES - INTRODUCTION

[0074] Figure 4 shows in schematic cross-section an example fibre optic cable 410 which is optimised for installation by pushing as well as being installable by blowing and / or pulling. This type of cable, sometimes referred to in the market as “nanocable” is of a type described in our prior patent application W02022049057A1 , mentioned above (see specifically Figure 19). One or more optical fibres 406 are embedded alongside one another (without stranding) in a solid resin material 420 to form a coated fibre bundle having an outer surface 422. A polymer sheath 424 of extruded polymer material surrounds the coated fibre bundle. The optical fibres 406 are so-called primary coated optical fibres, in which a glass body 426 (typically comprising a core and cladding layer, or a graded index core) is coated with two or three layers of resin 428, to provide buffering and to protect the surface against damage. The diameter of the glass core is commonly on the order of 100 pm, for example 125 pm. The diameter of the primary coated optical fibre 506 is conventionally either 250 pm or nowadays 200 pm. The number of optical fibres in the illustrated is two, but similar cables could be made with one optical fibre, or more than two optical fibres. The expression “coated fibre bundle” should be understood as including one or more optical fibres embedded in resin, with or without one or more longitudinal strength members. The presence of further functional or structural elements, including potentially electrical conductors, is not excluded.

[0075] To make the cable suitable for pushing installation, the coated fibre bundle in this example also includes a longitudinal strength member 430 lying closely alongside the optical fibres. The strength member may be made, for example, of fibre reinforced plastic (FRP). As is known, such a strength member provides a cable with a degree of stiffness against bending, as well as strength against tensile and compressive forces. This strength, embedded with the optical fibres in the resin, protects the optical fibres from experiencing such forces and so allows them to survive the installation process with good optical performance.

[0076] While the illustrated example includes only one strength member embedded within the resin alongside the optical fibre(s), examples with two or more strength members embedded in the resin may also be envisaged. The fibre reinforced plastic (FRP) type of strength member is only one type of strength member that may be embedded in the resin alongside the optical fibres. Other types of strength member may be envisaged.

[0077] Additional sleeving may be added to portions of the cable that will be located outside the duct. Otherwise, and in particular for the purposes of installation through a microduct, an outer surface 432 of the polymer sheath forms also the outer surface of the cable. In the known cables of this design, the outer surface 432 of the sheath is smooth to the touch and without discernible texture.

[0078] PUSHABLE CABLE EXAMPLES - MATERIALS

[0079] The resin 420 of the coated fibre bundle may in particular be a radiation-cured resin, for example UV cured resin, for example an acrylate. Suitable resins are readily available, and similar to the second layer of a typical primary coating 428. The selected resin has a relatively high glass transition temperature, so that it is not rubbery, but rather solid as it encases the fibres 406 and locks them into a unitary structure. The elastic modulus of the resin material 420 is greater than 100 MPa, for example in the range 300 to 900 MPa. For the purposes of installation and operation, resin material 420 has a hardness (modulus) and tensile strength such that the individual optical fibre or fibres 406 and any included strength member 430 are locked in a bundle, meaning that they are substantially prevented from moving relative to one another, and / or relative to the resin material 420. This coated fibre bundle therefore has a unitary structure and stiffness much greater than that of any individual optical fibre. On the other hand, the resin material 420 is not so hard and strong that it cannot be broken away from the optical fibres 406, when access to the individual fibres is required for termination and / or splicing.

[0080] Concerning the material of the extruded polymer sheath 424, a variety of materials could be chosen, including blends and including various additives mixed into a base polymer. One known product of the design shown in Figure 4 design is made of HDPE (high density polyethylene) with a friction reducing additive and, optionally, antistatic additives, colour etc. By suitable control of the extrusion process, and selection of materials, the extruded outer sheath 424 can be prevented from bonding to the coated fibre bundle. This allows it to be cut and removed by sliding over the outer surface 422 of the resin material, or else peeled after being cut longitudinally. This is necessary to access the individual optical fibres when terminating the cable.

[0081] The outer sheath in one known cable of the general form shown in Figure 4 is made of HDPE, with a friction reducing additive and, optionally, antistatic additives, colour etc. Such a cable has been found to have good blowing performance and pushing performance. For example, with a partial connector (ferrule sub-assembly) pre-fitted on the end, a 2-fibre example has been pushed over 90 m through buried micro-duct of 7 / 3.5 mm dimensions with no difficulty. (The skilled person will understand that the designation “7 / 3.5” refers to the outer diameter of the duct being 7 mm, and the inside diameter being 3.5 mm.) Cross-linked polyethylene would be another material that has been used in blowable micro-cables, and could be used for the polymer sheath in cables according to the present disclosure.

[0082] In the following examples, the material of the extruded polymer sheath 424 of each fibre unit in the cable of present Figure 4 is based on polybutylene terephthalate (PBT) polymer instead of HDPE. However, this is only one possible material, used by way of example.

[0083] The base polymer of the polymer sheath 424 may comprise a commercially available PBT material such as a grade of BASF Ultradur® 6550. Samples described herein have been made using BASF® Ultradur® B 6550 LN in particular. This PBT material is designed for loose tube optical fibre applications and is believed already to contain a very small amount of friction reducing material to assist normal processing (“lubricant” in the manufacturer’s terminology). Other grades of PBT may be used with suitable adaptation. PBT is of course available from manufacturers other than BASF. The preferred grade will combine desirable properties for processing, finished product performance and cost. Certain grades may allow a thinner sheath, or easier processing, but at greater cost. For example, BASF Ultradur® B6550LNX is a high viscosity extrusion grade for microtubes in fibre optical cable applications, potentially allowing a thinner sheath.

[0084] PBT material is conventionally used in “loose tube” cable designs, but those do not require low friction. For the present purpose, therefore, the PBT is augmented with additives to reduce friction and change other properties, as disclosed in patent application W02022049057A1 , mentioned above. The additional friction reducing additive may comprise a polydimethylsiloxane material, PDMS, in a carrier material. The carrier material may be a polyacrylate material, for example a copolymer of ethylene and methyl acrylate, EMA. In other examples the carrier is a polyolefin, such as low-density polyethylene (LPDE). Alternatively, or in addition, forms of carbon including carbon nanotubes, erucamide and / or oleamide materials may be used for improving slip and reducing friction.

[0085] The amount of additive including a carrier material may be between 1% and 5% by weight of the material of the extruded sheath, for example 3%. As an example of a polyacrylate (EMA-based) dimethyl siloxane, Dow Coming® HMB-1103 Masterbatch has been tried. However, it has been found that this additive can cause excessive flowing of the PBT melt during the extrusion process. As disclosed in W02022049057A1 , it has been found that a higher percentage of PDMS additive can be accommodated in the PBT sheath material, without causing problems in extrusion, if a polyolefin-based carrier material is used. An example of this class is Dow Coming® MB 50-002 Masterbatch, which is available commercially as a formulation containing 50% of an ultra-high molecular weight (UHMW) siloxane polymer dispersed in low-density polyethylene (LDPE).

[0086] Accordingly, in all the examples presented below, this polyolefin-based additive is used to lower the friction coefficient of the PBT-based polymer sheath. The masterbatch MB50- 002 has a loading of PDMS of 50%. Based on the value of 50% and the inclusion of 2.65% of the additive as a whole, it will be seen that the overall siloxane content of the sheath material is around 1 .3%, i.e., greater than 1 %. Up to 3% or even 4% additive can be used without extrusion problems.

[0087] EXAMPLE CABLES - DIMENSIONS

[0088] As mentioned in the introduction, a particular concern of the present disclosure is to enable to provision of a miniature cable or nanocable with a superior performance when installed by pushing. For the purposes of this disclosure, the cables in question may be characterised by a diameter less than 4 mm, and less than 3 mm. Using the example of Figure 4 with two optical fibres and any PBT-based sheath, an outer diameter Db of the coated fibre bundle may be in the range 1.3 to 1.5 mm, for example around 1.4 to 1.45 mm. The strength member in this example has a diameter Dm of approximately 0.8 mm. The outer diameter Ds of the cable 410 may be for example in the range of 1.2 to 2.5 mm, for example in the range 1.2 to 2.1 mm, for example, 1.5 to 2.0 mm. The polymer sheath 424 in designs with HDPE-based sheaths may have a thickness in the range 0.25 to 0.4 mm, for example 0.3 to 0.35 mm. However, in the case of a sheath based on the stronger and stiffer polymer PBT, it may be preferred to reduce the sheath thickness to less than 0.3 mm or less than 0.25 mm, for example, in the range 0.1 to 0.25 mm. On the other hand, these are dimensions suited to a small cable for installation in (for example) ducts of 3.5 mm inside diameter. Where larger ducts are provided, for example up to 4.0, 4.5 mm or more, larger diameters of cable can be accommodated.

[0089] The size and composition of the strength member or strength members should be selected with regard to the other components and the desired dimensions and properties of the cable. In versions with more fibres, the additional strength member or members may be smaller, or fewer, or even omitted altogether, if the optical fibres themselves provide adequate stiffness for pushing. For example, a coated fibre bundle of 12 optical fibres may be suitable for blowing and for pushing, without having the additional strength member 426. A 12 -fibre example with PBT-based sheath material and 1.8 mm outer diameter Ds has been pushed 100 m through a micro-duct of 6 / 3.2 mm size. A 24-fibre blowable microcable with bundle size similar to the bundle of Figure 4 is known.

[0090] EXAMPLE CABLES - TEXTURED SHEATH

[0091] Figure 5 is a schematic cross-section of a modified pushable cable. For the purposes of this description, the modified cable 510 is identical in structure and material composition to the cable 410 of Figure 4. Corresponding features of cable 510 are indicated with similar reference numbers to the ones of Figure 4, but with prefix ‘5’ instead of ‘4’.

[0092] All of the above descriptions of the Figure 4 cable and its variants apply equally to the cable 510, with the single exception that the outer surface 532 of the polymer sheath in cable 510 is textured, rather than smooth. Moreover, the texture in these examples is irregular, and is applied not by any change in tooling, and not by mixing any particulate material such as glass fibre fragments with the polymer of the sheath. In these examples, the irregular textures is obtained more simply by material selection and / or control of parameters of the extrusion process. Specific examples will be presented and discussed below. Additionally, applying a texture by modifying the sheath outer surface after extrusion would be within the scope of the present disclosure. In that case, additional equipment would likely be required.

[0093] Figure 6 shows three photographs of an example cable 510 made with this textured sheath. In photograph (a) we see an end of the cable in which the polymer sheath 524 has been cut away, exposing the coated fibre bundle. The resin coating has been broken away, to break out the coloured optical fibres 506, resin layer 520 and strength member 530. An irregular, striated texture can be seen in the surface 532 of polymer sheath 524 at the right-hand end of the photograph. Photograph (b) is a view of a representative section of cable with texture clearly visible on the surface 532 of the polymer sheath 524. Photograph (c) is a close-up detail of part of the polymer sheath surface 532, showing both short range and longer-range irregularities making up the overall surface texture.

[0094] Techniques for quantitative measurement of surface roughness on different scales, as well as surface waviness, will be described further below, with reference to the experimental examples.

[0095] PUSHABLE CABLE EXAMPLES- METHOD OF MANUFACTURE

[0096] Figure 7 illustrates schematically the apparatus 700 and processing steps used to manufacture the pushable cable 410 or 510 in one example of a method of manufacture. Except when specifically mentioned, the apparatus and method used to produce either the smooth cable 410 or the textured cable 510 will be the same. Whether a smooth surface or a textured surface is produced on the polymer sheath depends on the way the process is controlled, particularly with regard to certain temperatures at the point of extruding the polymer sheath. This will be explained further below. The skilled reader will appreciate the attraction of producing such functional textures, without the need for additional equipment or additional materials and pre-processing.

[0097] For an example of pushable cable 410 or 510 having two optical fibres 406 / 506, two bobbins 702 are provided, each delivering a different coloured optical fibre. A larger bobbin 704 provides a supply of FRP strength member 430 / 530. The bobbins 702, 704 are controlled to apply a certain back tension while their payload is drawn off progressively to form the cable, the optical fibres and strength member are gathered into a guide plate 706 which has an aperture designed to guide these components into a well-defined bundle 708 prior to coating.

[0098] In a coating station 710, a cup 712 holds liquid resin and has an aperture for passage of the bundle, sized so that the desired thickness of resin coating is applied uniformly as the bundle passes through. Resin is supplied at a suitable rate to replenish the cup, from a reservoir 714. The liquid-coated fibre bundle then passes into a curing stage 716 where radiation, such as UV radiation, is applied to cure and solidify the resin before it encounters any further mechanical contact. This process, involving liquid resin, can be performed in a vertical column, if necessary, to maintain uniformity of the liquid coating prior to curing. The coating and curing process just described is well established for the production of air blown fibre units and pushable cables of the known types already described. It does not need to be described further in detail.

[0099] Once cured, the coated fibre bundle 720 is diverted from its vertical orientation into a horizontal orientation for entry into an extrusion station 722. Within the extrusion station 722, an extrusion head 724 is shown only as a block in the middle of the drawing Figure 7, with an enlarged schematic cross-section in the dashed oval at the upper right portion of the drawing. Extrusion head 724 includes an extrusion die 726 and an extrusion tip 728, which define between them an annular passage for hot melted polymer material to be extruded in the form of a hot polymer tube 732, which eventually will form the polymer sheath 424 / 524 of the cable 410 / 510. Extrusion tip 728 is provided with an internal bore 734 large enough for the coated fibre bundle 720 to pass through synchronously with extrusion of the hot polymer tube 732. These parts are again conventional for the production of the known nanocable and microcables.

[0100] To deliver molten polymer 736 to extrusion head 724, a multistage heater and compression / mixing unit 740 is provided within extrusion station 722. This is fed by a hopper 742 which receives polymer in the form of pellets in a conventional manner. These pellets may already be pre-mixed with the desired combination of additives to form the material of the polymer sheath 424 / 524. Alternatively, one or more additives 744 may be supplied to the hopper in pellet form along with pellets of base polymer 746. An example where mixing of an additive is deferred until this stage would be where a cross-linked polymer sheath is desired. For other types of additives, it may be a matter of choice whether premixing in the pellets is desired, or mixing of pellets into the hopper.

[0101] The skilled person will understand how to adjust the machine settings, particularly temperatures and pressures according to the melting and flow properties of different polymer sheath materials such as PBT, HDPE, polypropylene (PP) or polyamide (PA; nylon). Process conditions for a PBT-based sheath and examples having a PBT-based sheath will be given. Melting and processing temperatures for HDPE, for example, will typically be lower by 40 to 60°C. Extrusion rate and pressure on the molten polymer material supplied to the extrusion head 724 can be controlled by the design and operating speed of the extruder screw, which typically both mixes and compacts the molten polymer before forcing it through the extrusion head.

[0102] Downstream of extrusion head 724, a series of cooling tanks 750, 752 are provided, from which the cable 410 / 510 emerges more or less in its finished form. Followed optionally by a monitoring station 754 and / or printing station 756. A puller 758 of caterpillar or similar design applies the tension to draw all the elements of the cable from the bobbins 702, 704, through all of the process steps until the finished cable 410 / 510 is wound on a drum 760 mounted in a take-up unit 762.

[0103] It will be understood that the hot polymer tube 732 that emerges from the extrusion head surrounding the coated fibre bundle 720 has inner and outer diameters larger than those of the polymer sheath 424 / 524 in the finished cable. In a manner well known, the process parameters of all the illustrated units are controlled to “draw down” and cool the polymer tube, giving it the interior and exterior dimensions of the polymer sheath 424 / 524 desired in the finished cable.

[0104] A computerised control system 770 is illustrated schematically, which receives many sensory inputs, for example from temperature sensors (e.g. thermocouples), pressure sensors and the like, and controls power to the several heaters, coolers that are distributed throughout the apparatus. It is a matter of design choice to what extent automated feedback control is provided, and to what extent manual adjustment is relied upon. Many local feedback control loops will be implemented, for example to control the rotation of the bobbins 702, 704, the puller 758, and the take-up unit 762. Operation of certain parts such as the coating station 710 may be controlled by dedicated subsystems within the overall control system 770.

[0105] At some point between the second cooling tank 752 and the take-up unit 762, such as monitoring station 754, parameters such as the outer diameter of the produced cable 410 / 510 are measured, to ensure that the product is within specification. If the diameter looks like exceeding the maximum specified value, measures can be taken, for example, to accelerate the drawing of the cable by puller 758, and / or to reduce the flow of polymer material into the hot polymer tube 732, or a combination of these. With particular regard to the production of the textured sheath of the cable 510, additional monitoring may optionally be performed, for example by optical sensors, to identify increases or decreases in the scale or quality of the texture on the outer surface 532 of the cable, as well as the average cable diameter. These measurements likewise can be used to control temperatures in the extrusion head 724. Such feedback control can be automated, if desired, of course with regard to the substantial time lag between any adjustment being made and the effect of that adjustment being apparent in the finished product.

[0106] It goes without saying that any cable produced by these methods will also be tested to confirm satisfactory optical performance of the cable under a range of environmental and mechanical conditions. The optical fibres used in the examples disclosed herein were single mode fibres compliant with G.657.A2 (ITU-T). Optical performance was satisfactory in all examples. These and many other quality-control measurements can be performed off-line, once the drum 760 has been unloaded.

[0107] INSTALLATION WITH LUBRICANT APPLIED TO SHEATH

[0108] In addition to novel types of cable with textured sheath, another aspect of the present disclosure concerns the systematic application of lubricant to a cable during installation by pushing, blowing or pulling. Installation by pushing will be described, by way of example, while installation by pulling may also benefit from this application of lubricant.

[0109] Figure 8 shows schematically the use of a lubricating device 800 for applying lubricant (lubricating material) to the outer surface of a cable 102 during installation into the duct 112. This may be used as a simple addition within the installation apparatus and method described above with reference to Figures 1 , 2 and 3. Figure 9 shows an enlarged view of lubricating device 800 and associated accessories. The cable 102 may for example be cable 410 or 510, as described above, with outer surface 432, 532 respectively.

[0110] The lubricating device 800 in this example is a stand-alone device which is inserted between a pushing tool 200 and the duct 112. A short section of duct 112’ couples the device 800 to the body of the pushing tool. The device has a body 802 through which passes a bore that extends from an input port 806 to an output port 808. (The design of the device may or may not allow the roles of these ports to be reversed.) A fluid reservoir 810 is formed in the device body, the device body further providing fluid communication between the reservoir and a central section 814 of the bore 804 via a duct 812. In use, a supply of lubricating fluid is loaded into the reservoir. While the term “fluid” naturally includes a lubricating liquid such as an oil, it should be understood that lubricant in the form of a free-flowing powder may also be regarded as a fluid. While the term “fluid” may also extend to gases, gases are not considered as effective lubricating fluids for the present purpose. A built-in 810 reservoir is just one convenient option for providing the supply of lubricating fluid. Other arrangements are of course possible, including but not limited to a detachable reservoir.

[0111] A first seal 816, for example an O-ring, is arranged in a first section of the bore 804 so as to allow passage of a cable from the input port 806 to the central section 804 while substantially inhibiting leakage of the lubricating fluid back towards the supply of cable. A second seal 818 is similarly arranged in a second section of the bore so as to allow passage of the cable from the central section to the output port 808 while substantially inhibiting leakage of said fluid. Depending on the character of the outer surface 432 / 532 of the cable, as well as the character of the seal and the lubricant material, a greater or lesser quantity of the lubricating fluid may be carried by the cable as it leaves the lubricating device 800 and enters the duct 112. The O-rings in the examples presented herein can be made of a solid but elastomeric material. This is in contrast for example to the sponge material used in the lubricating devices known from W02004008599A1 , mentioned in the introduction.

[0112] To facilitate coupling of the input port 806 and output port 808 to the duct sections 112’ and 112, respectively, each port in this example is the form of a spigot, having external and internal diameters similar to a standard microduct. In this way, standard microduct connectors 820 and 822 can be used to couple the device 800 securely to the incoming and outgoing sections of duct. In the illustrated example, the spigot dimensions are the same as those of the ducts. In such a case, it may also be convenient to insert the spigot of the lubricating device input port directly into the output port of the pushing device without the need of an intermediate duct section 112’. Connectors allowing an interface between different dimensions of duct are also readily available. In alternative examples, coupling of the input port and / or output port to a duct may be implemented in the form of a socket with internal gripping / clamping features, similar to the output port of the pushing tool, described above with reference to Figures 2 and 3.

[0113] Where the desired quantity of lubricant material is finite, material in the reservoir may last throughout the entire installation process, or may be exhausted after an initial phase of the installation.

[0114] It is not necessary to regard this as the only measure to reduce friction, of course. As in the known blowing techniques, applying lubricating material to the sheath can be performed in addition to additional lubricating material being applied, in advance of installation, to the duct lining in the known manner.

[0115] It will further be understood that ducts such as are used for blowing or pushing cable long distances, are generally provided with a low-friction lining, containing friction reducing additives as well as base polymer. All ducts used in the examples herein are made of high-density polyethylene, HDPE, with an inner layer comprising additional friction reducing additives, and additionally with longitudinal ribs to minimise surface contact area between the cable and the duct. Likewise, friction reducing additives can be incorporated in the material of the polymer sheath.

[0116] Instead of the device with seals presented herein, in alternative methods, lubricating material can be applied to the cable surface using one or more sponges soaked in lubricant, as described in W02004008599A1 , mentioned above. While the devices and methods illustrated in these examples involve the application of a fluid lubricant (liquid or powder) using a suitable device, it may also be envisaged to arrange a supply of a solid lubricating material that is soft enough to be eroded gradually by the cable surface as it moves towards the duct. Methods disclosed and claimed herein encompass the use of such solid lubricant, except where the context requires otherwise. ENHANCED PUSHING TOOL WITH INTEGRATED LUBRICATING DEVICE

[0117] Figure 10 is a schematic, cross-sectional view of a pushing tool 1000 that has been developed and tested for pushing miniature cables of the type described above, including cables having a textured outer surface 532 as described herein. In this diagram, parts are labelled with reference numbers similar to those used in Figures 2 and 3, with prefix ‘10’ instead of ‘2’ or ‘3’. Pushing tool 1000 also has integrated within it the operative features of the lubricating device 800, described above for the continuous application of lubricating material to an outer surface of a cable, while it is being pushed. Reference numbers with prefix ‘18’ indicate features in the pushing tool 1000 that correspond to like-numbered features of the lubricating device 800.

[0118] As with the pushing tool 300, the enhanced pushing tool 1000 has a central bore extending from an input port 1006 to an output port 1008. The tool has a body made in two halves 1002a and 1002b which are hinged together or otherwise separable to allow the tool to be fitted around a length of cable. An input driveshaft similar to shaft 322 can be provided. Again, the wheel opposing a driven wheel in the pushing mechanism can optionally be driven actively in synchronism with the driven wheel, for example by gears that engage when the body halves 1002a, 1002b are brought together.

[0119] A pushing mechanism in the enhanced pushing tool has not only one pair of opposed, counter-rotating wheels 1002a / 1020b but also a second pair of opposed, counter-rotating wheels 1002c / 1002d, these pairs of wheels being spaced apart from one another along the axis of the central bore. Two or more pairs of wheels with individual tyres can be spaced along the central bore, to allow a greater contact area for transmission of pushing forces to the cable. Furthermore, as is done in this enhanced pushing tool 1000, two or more pairs of wheels can be connected by opposing drive belts 1030a and 1030b for an even more effective transfer of longitudinal pushing forces to the cable. In order to maintain and adjust tension on the drive belts, for example, as they stretch with age, corresponding tension wheels 1034a and 1034b are provided with adjustable tensioning controls 1036a and 1036b in the casing to deform the belt where it returns from its engagement with the cable. Adjustable positioning and / or tensioning of the individual wheel axles relative to the bore can be provided, if required.

[0120] By applying the driving force to the cable over an extended length, a higher pushing force can be applied than with a single pair of wheels. This, along with a suitably robust construction of the cable, allows the miniature cable to be driven a greater distance through a duct route, without risk of damage caused by the tyres skidding and potentially damaging themselves and / or the cable.

[0121] As already mentioned, aside from the enhanced pushing mechanism, the pushing tool 1000 is also distinguished by having lubricating device functionality integrated within it. The reader will recognise that, in the example of Figure 10, a lubricating arrangement 1800 similar in form to that of the stand-alone lubricating device 800 is housed in the same body 1002a, 1002b surrounding a portion of the central bore between the pushing mechanism and the output port 1008. Effectively, an input port 1806 of the lubricating arrangement 1800 is arranged to receive a cable directly from the pushing mechanism, without the need for a separate body, couplings and intermediate duct 112’. The form of the reservoir 1810, the duct 1812 that provides fluid communication between the reservoir and the central section 1814 of the central bore, are all the same as seen in the example of Figures 8 and 9. One addition is an adjusting mechanism 1820 for regulating flow of fluid if desired. This can be, for example, a simple grubscrew constricting the duct 1812 in an adjustable manner. Alternatively, or in addition, flow can be regulated by restricting communication between the upper part of the reservoir and the surrounding atmosphere. In a simple arrangement, a lid of the reservoir 1810 can be opened slightly, to allow air to replace the lubricating fluid, as it is dispensed. The lid can be closed to prevent leakage when not in use, and / or to regulate the flow of lubricating fluid when in use. The same can be said of the example of Figure 8.

[0122] In contrast to the lubricating device of US2006 / 102430A1 , mentioned in the introduction, the loading and applying of lubricating fluid in the described apparatus occurs under gravity, and at atmospheric pressure. This avoids the need for complicated arrangements to manage a pressurisation and depressurisation of the interior spaces 1801-1814 of the device, of the type disclosed in that prior art. The same can be said of the example of Figure 8, of course.

[0123] Another modification associated with integration of the lubricating arrangement 1800 into the pushing tool 1000 relates to the first seal 1816 and the second seal 1818 of the lubricating arrangement. As illustrated in an inset detail, in the illustrated example, these seals are formed as a pair of split O-rings. The split or divided structure allows the seals to be opened as the body halves 1002a 1002b are separated to allow insertion of a cable 102 / 410 / 510. In this way, the seals can engage the outer surface of a cable to be pushed into a duct, regardless of the diameter of any leading portion of the cable, and any accessory such as partial optical connectors. The leading end of the cable only needs to be small enough to travel successfully through the duct 112. Once the cable is safely inserted, and the body halves are brought together to grip the cable between the opposing drive belts, screw bolts 1040 or other clamping devices are applied to ensure the appropriate grip for driving, and for sealing against free flow of lubricating fluid. In the same action, or by a separate mechanism, the coupling 1010 attaches the pushing unit 1000 securely to a duct, ready for installation of the cable.

[0124] In the illustrated example, the larger wheels 1020a and 1020b are nearer to the inlet port 1006 of the pushing tool 1000, and one of those larger wheels is the driven wheel. In another examples, the larger wheels may be the ones nearer to the outlet port 1008. In other examples the driven wheel may be one of the smaller wheels. In other examples, or all the wheels may be the same size. In other examples, more than two pairs of wheels may be provided.

[0125] MANUFACTURE AND TESTING OF EXAMPLE CABLES - INTRODUCTION

[0126] There will now be described various examples of cables 510 made with a textured outer surface 532 according to the design of Figure 5, as well as a comparative example cable 410 with a smooth outer surface 432 according to the known design of Figure 4.

[0127] In order to test the performance of cables under different installation conditions, it is customary to use a test route consisting of a length of duct of the desired inner and outer diameters, material properties etc.. To reflect real-world conditions, the route is normally convoluted. A length of duct wound tightly on a drum is known as a challenging route to test air-blown fibre units. However, that is rather more extreme than real-world situations, and so more realistic routes are usually employed.

[0128] Figure 11 illustrates an ISO standard test route that comprises a series of straight sections of duct connected to one another by 180° bends of a defined bend radius rB. The bend radius is specified as a performance parameter of the specific cable, and will naturally be larger for larger, stiffer cables, and smaller for lighter, more flexible cables. For the tests reported herein with a 7 / 3.5 mm duct, the bend radius rB was 170 mm. In accordance with international standards, the default value for length L of each straight section is 100 m, in which case the test is known as the “Nx100” test. Test routes comprising 10 or 20 times this length L may be required to assess the performance of a lightweight, air-blown microcable. On the other hand, the example shows only four lengths L, which is enough to accommodate the maximum pushing distance that can be expected for the cables 410 and 510 of these examples.

[0129] Different methods may be used to produce a textured surface 532 on the sheath 530 of a cable 510 as disclosed herein. Processing after extrusion to add texture could be performed by addition or removal of material, or imprinting a pattern into the surface. This could be done by mechanical and / or chemical means. Below we describe three methods for imparting the surface texture without the need for such post-extrusion steps.

[0130] ADDING SURFACE TEXTURE BY EXTRUSION PARAMETERS

[0131] Referring to the tables below, we report test results for five example cables, labelled Example 0, 1 , 2, 3, 4. Their manufacturing parameters will be presented, as well as their measured surface roughness, and their performance measured in the pushing test. Performance of cables with and without textured surfaces, and with and without the application of lubricating material is also compared.

[0132] Apart from a variation in the temperatures in the final stages of the extrusion of the polymer sheath, the design and manufacturing of these five cables Example 0 to Example 4 is identical. A coated fibre bundle of the form illustrated in Figures 4 and 5 was made, incorporating two primary-coated optical fibres of type G657A2 and a 0.8 mm glass reinforced plastic rod as strength member 430 / 530. The resin coating for 420 / 520 was applied with identical dimensions and processing in the five examples, the coated fibre bundle having a diameter control to be between 1 .43 and 1 .45 mm. The base polymer for exclusion of the polymer sheath 424 / 524 is the extrusion grade PBT polymer BASF Ultradur® B 6550 LN, mentioned already above. A PDMS friction reducing material in a PE carrier (DOW CORNING® MB50-002) was added at 2.65% by weight. Also added was a UV-protective additive. Using the control parameters described above with reference to Figure 7, a line speed during production was 80 m / min, implying that (after setup and stabilisation) 1 km of cable would be produced every 12% minutes. The outer diameter of the polymer sheath 424 / 524 is controlled to be around 1 .8 mm, for example by keeping it with the range 1 .79 mm to 1.85 mm. As part of this control process, numerous parameters are defined and monitored by feedback control, according to long-established principles.

[0133] For the present purpose, and specifically with regard to the differences between the different examples, the only parameters of interest are the temperatures in different parts within the extrusion station 722.

[0134] T able 1 shows the different temperatures set for different zones within the extrusion station 722 (Figure 7), which are actively heated to melt, mix, press and extrude the polymer mix that has been fed into the hopper 742. An extrusion screw in a barrel is divided in to three zones of roughly equal length, that subject the material to a sequence of melting, compression and metering. Barrel zone temperatures Z1 , Z2, Z3, Z4 support this processing sequence, with Z1 being closest to the hopper and Z4 being closest to the head clamp. Column C represents the head clamp stage, where material is transferred into the specific extrusion head 724 required for this product. It will be seen from the table that, throughout these zones, the temperature for melting and processing in all five example cables is identical. After the clamp, there is a connector zone, which may or may not need to be actively heated. The last two columns H1 , H2 represent temperatures within the extrusion head 724 itself, including the die 726 and tip 728.

[0135] A melting point of the PBT polymer is nominally 223°C, so that the processing at temperatures as high as 260° ensures that the product is fully molten and liquid. The melting point of the PE carrier in the friction reducing additive is well below 200°C. As will be seen, controlling the temperature of the extrusion head relative to the melting point of the base polymer and / or the carrier of the additive is what allows a texture to be imparted to outer surface 532 of the finished cable.

[0136] TABLE 1

[0137] Example 0 is the known cable 410, manufactured with a polymer sheath having a smooth outer surface. As can be seen in the table, the extrusion head is held throughout at a temperature of 250° C, that is well above the melting point of the polymer mix. Accordingly, as may be expected, the material forming the hot polymer tube 732 as it emerges from the extrusion head 724 is fully molten, and solidifies into a smooth-surfaced sheath with no surface texture.

[0138] Examples 1 to 4 are different examples of the novel cable 510, having a textured outer surface, of the form illustrated in Figure 5 and the photographs of Figure 6.

[0139] For the cable of Example 1 , zone temperatures during production are not given, for the reason that authoritative values are not available. The production of the textured outer surface in Example 1 was not done by design, but rather due to error or a technical problem in the temperature control system suspected as a failure of one or more thermocouples in the extrusion head. On the other hand, the skilled person experienced in extrusion recognises that the type of texture produced, known colloquially as a “sharkskin effect”, is characteristic of a situation in which the extrusion head temperature is too low for normal purposes. To understand the reason why this texture arises, it is necessary only to consider what must happen, if the temperature of the extrusion head falls below the melting point of the polymer material being extruded. At least a surface layer of the material will begin to solidify, where it contacts the metal that is below its melting point. As molten material continues to flow freely within the bulk of the material, this skin of solid or semi-solid material will be pulled into an irregular profile, as it is dragged out of the extruder by the flowing bulk material. The skin layer will not re-melt, but rather this irregularity will become solidified in the finish surface.

[0140] Having produced a quantity of this “faulty” cable Example 1 , the product was nevertheless tested to see whether the texture impacted its installation and optical performance, with surprising results as detailed below.

[0141] The cables of Examples 2, 3 and 4 were produced with progressively different parameters so as to replicate and optimise the textured surface effect, for maximum installation performance. In each of these examples, temperatures within the successive zones of the extrusion head 724 are controlled so as to fall progressively from the Clamp temperature of 250°C to a value somewhere below the melting point of the PBT base polymer. As will be seen from the table, Example 2 has the steepest fall, ending at 190°C in the final zone. Example 3 falls progressively to 225°C in zone H1 , and then falls steeply to a final temperature of 205°C in zone H2. Example 4 falls more steeply to zone H1 , and then more gradually to the same file temperature 205°C. All of these cables exhibit greater and lesser degrees of texture.

[0142] ADDING SURFACE TEXTURE BY CROSSLINKING

[0143] Another method that may be useful to produce a finely textured surface on the sheath is to form the sheath using a polymer material that is at least partially crosslinked. A partially crosslinked polyethylene material (PE-PEX), and the method by which the PE-PEX sheath can be made, are disclosed in WO2019053146A1 , the contents of which are hereby incorporated by reference. As demonstrated by measurements of Example 5 below, a known example of such a sheath has a small-scale, irregular surface roughness comparable to the small-scale roughness of the Examples 1 to 4 produced by control of extrusion parameters.

[0144] For the avoidance of doubt, the cable of Example 5 is not a pushable cable having a strength member 430 / 530 as part of the core, but rather a commercially available blowable microcable. Nevertheless, it is evident that such a sheath material would be equally applicable to a to make a further example of pushable cable 510 in which one or more optical fibres 506 and a longitudinal strength member 530 are embedded alongside one another in a solid resin material 520. A polymer sheath 524 is applied by extrusion of partially-crosslinked polymer material. The crosslinking is achieved by mixing cross- linkable base polymer with an initiator material in the extruder 722, so that cross-linking will begin, and then complete over a curing period, after extrusion. Storage conditions (temperature and humidity) of the cable 510 on the drum 706 are controlled during that curing period, to promote the crosslinking. It is observed that, as a consequence of structure introduced within the material in the cross-linking process, a degree of small- scale surface roughness arises that may not be present in the base polymer alone.

[0145] To limit the degree of cross-linking, the cross-linkable base polymer may be mixed with a non-cross-linkable base polymer in the extruder. Considering as an example the PE-PEX material of WO2019053146A1 , a non-cross-linkable HDPE can be mixed with a cross- linkable PE of lower density (LDPE) to make the PE-PEX polymer material of the sheath 524. The material may have a degree of crosslinking of at least 15%, preferably at least 30%, as determined according to ISO 10147:2011 degree of cross-linking can be for example less than 50%. In particular examples, the crosslinked base polymer is a low- density polyethylene or medium density polyethylene, and the non-crosslinked base polymer is HDPE at a content of from 20% to 50% by weight of the polymer material of the sheath.

[0146] The PE / PEX mix appropriate to a pushable cable can be the same as the known blowable cable, or it can be optimised for the specific application by experiment. The optimum thickness for the sheath 524 likewise can be determined by experiments. In the case of PE-PEX material, the fact that it has a lower stiffness and strength than the PBT polymer of Examples 1 to 4 suggests that a greater thickness may be desirable, but this can be determined again by experiment, and tailored for any particular field of use.

[0147] For the avoidance of doubt, it should be understood that the sheath of the known cable of Example 5 lacks completely the larger-scale features that are seen in the photographs of Figure 6, and in Examples 1 to 4. However, it is to be expected that larger-scale features may be included in the textured surface of the cross-linked sheath, in particular by controlling the extrusion parameters so that parts of the extrusion head are below the melting point of the extruded material, or at least below the melting point of one of its component polymers. ADDING SURFACE TEXTURE BY FOAMING AND OTHER METHODS

[0148] Yet another method that may be useful to produce a textured surface on the sheath is adapted from earlier designs of air-blown fibre units, or microcables. For air blown microcables, prior to the invention of fibre units based on a solid resin core, there was motivation to provide microcables with a sheath that was bulky but did not increase the weight of the cable. A solution for this was to make some or all of the polymer sheath a foamed polymer. A foamed structure can be imparted to a polymer such as polyethylene or PVC in a number of ways. For example, pressurised gas (such as nitrogen) can be injected into the melt prior to extrusion, which then expands to form bubbles in the finished product. Other methods of foaming involve the use of additives that generate gas bubbles by chemical action.

[0149] As a by-product of foaming, typically a textured outer surface is produced. This is therefore another way that could be used to impart a textured outer surface to the polymer sheath of an optical fibre cable. The entire thickness of the cable could be foamed, or the foamed polymer could be thin layer on the outside of an otherwise solid polymer sheath. Multilayer sheaths can be formed for example by co-extrusion, in a known manner.

[0150] Other methods not mentioned above may include modulating parameters of the extrusion process. For example, a pulsing component could be added in the extrusion pressure, and / or in the haul-off speed. A vibration could be induced in the extrusion tip or die. In the normal way of working, any such pulsing would be considered an undesirable behaviour, leading to imperfection in the sheath dimensions. In the context of the present disclosure, however, such imperfections, if suitably controlled, can be exploited as a beneficial surface texture.

[0151] Alternatively, or in addition, texture could be introduced by incorporating particles of different material in the polymer mix, such as the glass fibre fragments described in US2004 / 096166A1 , mentioned in the introduction.

[0152] Any of the methods described herein can be combined, and Multilayer sheath structures can be provided in any of the example methods, if needed to meet specific design goals and preferences. MEASUREMENTS OF SURFACE ROUGHNESS

[0153] For products of the type illustrated in Figures 5 and 6, qualitative and quantitative measurements of surface roughness can be made in a variety of ways. The roughness of the surface can be measured for example by running a mechanical stylus along a surface of interest. A number of recognised parameters have been defined, for representing in different ways the roughness of the surface. These parameters are obtained by aggregating height measurements taken by the probe at different points on the surface, over a defined sample length. Three such parameters are indicated in Figure 5, namely:

[0154] • Rz - defined as the maximum peak to valley height of the profile, within a single sampling length.

[0155] • Ra - defined as the average (arithmetic average) of profile height deviations from a calculated mean line.

[0156] • Rq - defined as the quadratic mean, or root mean square average of profile height deviations from the mean line.

[0157] Throughout this document, parameters Ra etc. are written as abbreviated terms with lower-case suffixes rather than as symbols with subscripts. This is for ease of reproduction only.

[0158] The unit of measurement of each of these parameters is a distance, which may conveniently be expressed in microns (pm) for the surfaces of interest here. There are limitations as to how completely a single parameter such as Ra or Rq can characterise a surface. However, for the purposes of comparing examples made according to the present disclosure, a single value Ra or Rq can be used. Each of these parameters is conventionally defined on the basis that the roughness profile has been filtered from the raw profile data and the mean line has been calculated. The roughness profile contains ordered, equally spaced points along a sample trace, and is the vertical distance from the mean line to the data point. Height is assumed to be positive in the up direction, away from the bulk material. To obtain a representative value for surface roughness, multiple traces may be taken along a sample length of cable, and optionally at positions spaced circumferentially around the cable. Instruments for taking a desired number of sample measurements and aggregating them to obtain the above and / or other surface roughness parameters are readily available. Measurements in the examples presented below are taken with a Surftest SJ-210 surface roughness tester, made by Mitutoyo Corporation of Japan and available in the UK from Mitutoyo UK Ltd. This surface tester has a diamond contact stylus and can measure many surface points automatically over a range of several millimetres. The stylus has a tip of radius 2 pm and a contact force of 0.75 mN. Of course, other suitable machines are available from this and other manufacturers. Sensing of surface roughness may be by mechanical probing and / or optical probing of the surface of interest.

[0159] Before testing is carried out the equipment is calibrated using a calibration plate with a known Ra value. The machine has a number of customisable settings, listed below. In this list, an asterisk * indicates the settings used in the measurements presented here.

[0160] Roughness standard: EN ISO (*), VDA, JIS, ANSI

[0161] Sampling length (L): 0.25 mm, 0.8 mm (*), 2.5 mm (*)

[0162] No. of sampling lengths (L): x 1 , x 3, x 5(*), x L

[0163] Digital filter: Gauss (*), 2CR75, PC75

[0164] Cut-off length: Ac : 0.08 mm; 0.25 mm; 0.8 mm (*); 2.5 mm (*)

[0165] As : 2.5 pm (*); 8 pm

[0166] The cut-off lengths represent the longest (Ac) and shortest (As) spatial wavelengths of the texture that will be measured. Where any of the appended claims refers to a specific value of a surface parameter such as Ra or Rq, the values marked (*) above can be applied. The ISO standard applied in this instrument is understood to be ISO 4287:1997. We note that 1997 standard has recently been superseded by standard ISO 21920-2:2021. It is not expected that the measured values will differ significantly, if at all. In case of conflict, the version of 1997 shall be authoritative for interpretation of the present claims.

[0167] For interest, a white-dotted arrow 602 on the photograph of Figure 6 (c) represents the cut-off length Ac of 0.8 mm, roughly to scale with the cable diameter of just under 2 mm. This is the value used for all of the measurement results reported for the examples below. It will be seen that the measurements are performed so as to reflect the finer details of the texture (referred to above as ‘striations’), without necessarily picking up the larger-scale features at all. Of course, such larger-scale features can have an impact on different parameters of performance, and measurements can be made with cut-off lengths of 2.5 mm and / or varying other settings, to investigate the effect of features of specific sizes. A second arrow 604 indicates a scale of 2.5 mm. Likewise, it may be of interest for example to measure roughness in a circumferential direction, as well as or instead of a longitudinal position, and / or to obtained area roughness measurements. The scale of features will be discussed again below, in connection with the application of lubricating material during installation.

[0168] NOTE: For the purpose of interpreting the appended claims, and in particular the limitations expressed in terms of surface roughness, it shall be sufficient if either value Ra or Rq satisfies the claimed threshold, unless otherwise specified. Unless otherwise specified, surface roughness can be measured with a cut-off length of either 0.8 mm or 2.5 mm. In other words, to meet the parameter limitations in the claims, it is not necessary that both values Ra and Rq satisfy the claimed threshold. It is not necessary that the claimed threshold is satisfied at both 0.8 mm and 2.5 mm cut-off lengths. Also note discussion further below of surface waviness, representing specifically those larger features visible on the surface of the example cables. Such features may not be detected within roughness measurements, which filters out features longer than the cut off length Lc.

[0169] Unless otherwise specified, all surface roughness measurements referenced in the test results presented below have been taken with the parameters indicated by an asterisk above, with the value of 0.8 mm chosen for the cut-off length, Ac. Measurements of roughness taken with longer cut off lengths will be presented further below, as well as measurements of ‘waviness’. Measurements of waviness are similar to roughness measurements but filter out features shorter than the cut off length. The Surftest SJ-210 apparatus is not so well suited to measurement of waviness, due to its limited sampling length. Techniques for measuring waviness are discussed further below. Macroscopic features characterised by waviness are also discernible with the human eye or by feel.

[0170] In order to distinguish from purely local irregularities in an otherwise smoother surface, measurements can be taken at a number of locations spaced longitudinally along a length of cable, and at two or more positions around the circumference of the cable. To measure all points along several kilometres of an already-manufactured cable may not be practical in high volume production. However, “in-line” measurements can be implemented as a function of monitoring station 754 in the production process of Figure 7. Non-contact, for example optical, surface-monitoring techniques would be preferred for this. Table 2 presents selected measurements of average surface roughness Ra using the Surftest SJ-210 apparatus in the manner described above. Linear profiles were measured using the stylus along several sample lengths taken from a length of the manufactured cable. For the purposes of the present preliminary investigation, measurements have been taken from 10 m lengths cut from the example cables. Each column corresponds to a different one of the five sample products Example 0 to Example 4. The rows of the table correspond to ten test locations spaced longitudinally along the sample length of cable, with two measurements being performed at each longitudinal location, spaced 180° around the circumference of the cable.

[0171] TABLE 2

[0172] A final row of the table presents the average of the values obtained for each example cable, over the 20 longitudinal and circumferential test locations. (Of course, each reported value Ra is itself an average of raw profile heights measured at many sampled locations within a short sampling interval, or rather an average of the deviations of those heights from a mean line.)

[0173] With regard to the smooth surface 432 of cable 410 of Example 0, it can be seen that the Ra values are clustered in a relatively narrow range around the average of 0.19 pm. As another reference, measurements on a similar-sized cable having a sheath based on HDPE as the base polymer showed surface roughness values of Ra around 0.37 pm and Rq around 0.47 pm. By contrast, the Ra values for the surfaces of the cables of Examples 1 , 2, 3 and 4 confirm quantitatively that significant texture has been added by control of the extrusion temperatures as discussed above. The average Ra values for those textured cables are around 5 to 8 times larger than for the smooth cable. Additionally, the cables of Example 1 and Example 2 exhibit very wide variations between Ra values, even between neighbouring sample locations and between opposite sides of the cable at the same longitudinal location. Example 3 and Example 4 show a narrower range of variation in that regard.

[0174] While each parameter Rz, Ra or Rq is only one number, and no single number can completely characterise the cable surface, the contrast between the smooth surface of Example 0 and the textured surfaces is very clear. Figure 12 presents in graphical form values for the three profile parameters, Rz, Ra and Rq, for each of the textured cables Example 1 to Example 4. The right-hand vertical scale corresponds to the profile height in microns. In addition to presenting an average value for each parameter, as presented for parameter Ra in the last row of Table 2 above, the graph of Figure 12 presents also a maximum value and a minimum value for the same parameter, over the 20 sample locations. As could be observed already from the numerical values in Table 2, the graph confirms that in Example 1 and Example 2, there is a wide variation of individual R values either side of their averages average.

[0175] For Example 3 and Example 4, the minimum and maximum R values are proportionately closer to the average R values. The values obtained for Example 3 are, at least at first sight, closer to those of Example 1 than either Example 2 or Example 4. Example 3 is used for additional tests.

[0176] Although the cable of Example 0 is not plotted on the same graph, it will be appreciated that the extremely low surface roughness Ra value of less than 0.2 pm would barely register on the same scale.

[0177] Table 3 below presents all three values Rz, Ra and Rq for the cable of Example 2. TABLE 3

[0178] Also presented in Table 3 are values Rz, Ra and Rq measured on the outer surface of the blowable microcable, introduced above as Example 5. This is the one having a sheath made of partially crosslinked polymer material PE-PEX. It is not a pushable cable having a strength member 430 / 530, but rather a commercially available blowable microcable having only optical fibres within the resin-coated core. Nevertheless, it is to be expected that the outer surface 532 of an example having the same sheath and an embedded strength member would have the same texture as the outer surface of the known blowable microcable. A crosslinked PE-PEX material can be used instead of the PBT-based material described above. On the other hand, the known PE-PEX sheath of Example 5 has only short-range, microscopic texture. Larger features discernible by sight or feel are not present. As already mentioned, larger scale texture features can be applied to the cross-linked sheath, with greater benefits in installation performance.

[0179] MEASUREMENTS OF LARGER FEATURES & SURFACE WAVINESS

[0180] In addition to short-scale (microscopic) texture characterised by conventional surface roughness measurements, larger scale (macroscopic) features can be seen on the specific example cables illustrated for example in Figure 6. The dimensions and the characteristics of these features can be characterised by number of parameters, including parameters Rz, Ra and Rq, but measured using cut-off length greater than the value of 0.8 mm used in the tests above. At these longer scales, the term “waviness” is conventionally used to distinguish longer-scale textures from small-scale surface roughness. For the purposes of the present disclosure, longer-scale features with an average wavelength of 1 mm or more may be characterised by waviness.

[0181] For the measurement of these longer-scale, larger-amplitude features, automated measuring apparatus can be used. As mentioned already, the example test apparatus can be set to measure roughness using a cut off length as long as 2.5 mm. Such a dimension is comparable with the length of the larger scale features in the illustrated cables, and begins to provide a measurement of the larger-scale structures, as well as small-scale roughness.

[0182] Because the longer-scale features in the illustrated example are also larger in height, easily visible in the photographs and to the naked eye, higher values of Rz, Ra and Rq may be reported as a result, when roughness is measured with a sufficiently long cutoff length. This is confirmed in Table 4, which presents values measured by the same apparatus as in Table 2, but with the cut off length 2.5 mm selected. The cable in this example was produced under extrusion conditions similar to Example 3 above. TABLE 4

[0183] Additionally, or alternatively, measurement of roughness and waviness can be made by analysing profiles captured in images such as the images presented in Figure 6.

[0184] Figure 14 (a) illustrates the amplitude of some of the longer-scale features in the captured image of Figure 6 (b). An anisotropic scaling has been applied so that the profile can be appreciated more easily by eye, and by measurement from the images without special analytic equipment. In particular, the photograph in Figure 14 (a) has been scaled down in the longitudinal (horizontal) direction while being scaled up in the transverse (vertical) direction. Over a total length of around 150 mm, three representative segments of the surface are bounded by dotted-line boxes, with the height of each bounding box enclosing roughly the peaks and troughs of the surface profile over that segment. These bounding boxes, when measured to scale, have heights that corresponding to surface variations Rz of around 0.07 mm, 0.1 mm and 0.06 mm, as marked in Figure 14 (a). Similarly, Figure 14 (b) shows the captured image of Figure 6 (c), scaled up in the transverse direction and scaled down in the longitudinal direction. The height of the bounding box over a cable length of around 15 mm is around 0.14 mm, or 140 pm.

[0185] Figure 15 illustrates a trace of the surface height in the photograph in Figure 6 (c), longitudinal position on the horizontal axis and height in millimetres relative to a calculated mean height along the vertical axis. Over a length of around 10 mm, the height was measured at 8 samples per millimetre. By applying the formulae defined in the ISO standard method mentioned above, a value Rz of around 90 pm (0.09 mm) was been measured for the example cable. Average value Ra was measured at approximately 23 pm (0.023 mm). These values are an order of magnitude greater than the microscopic roughness. Periodicity values for the surface features can also be calculated by looking at the number of cycles (peak-trough-peak) in a given length along the surface. Analysis of part of Figure 6 (c) established that there were roughly 4 cycles in a 9 mm length, implying a wavelength of 2.25 mm. A more detailed characterisation of periodicity can be obtained, if desired, by Fourier analysis of the measured profiles.

[0186] As seen in Figure 16, yet more detailed 3-D surface profiles can be obtained and analysed. Figure 16 shows images reconstructed from 3-D electron micrograph profiles of three samples on the cable of Example 1 , mentioned above. Each profile represents an area of the sheath surface that is approximately 3mm in the longitudinal direction of the cable and 1 mm in the circumferential direction. These can be subjected to yet further profile analysis in both longitudinal and circumferential directions. Measurements of a surface profile can be used simultaneously to derive waviness values Wz, Wa, Wq, (filtering out height deviations shorter than a cut off length Lc), and roughness values Rz, Ra, Rq (filtering out height deviations longer than the cut off length). Table 5 presents roughness values Rz, Ra, Rq and waviness values Wz, Wa, Wq calculated from the three measured surface profiles. A cut off length Lc of 250 pm was used to distinguish roughness features (shorter than Lc) and waviness features (longer than Lc). The same formulae can be used as in the ISO standard method mentioned above.

[0187] TABLE 5

[0188] All this being said, the benefits of the invention are likely to be achieved using whenever the overall quality of the surface has macroscopic features (waviness) recognisable by sight and feel, and optionally microscopic roughness as well. Precise quantitative characterisation is likely to be relevant mainly for optimising the performance, and optimising the production process (quality control).

[0189] Particular embodiments of the cable may have waviness values Wz greater than 20 pm (0.02 mm), optionally greater than 30 pm or 40 pm or 50 pm or 60 pm. The average amplitude measurements of waviness such as Wa and Wq may be greater than 15 pm (0.015 mm), optionally greater than 20 pm or 25 pm. As in the case of the fine scale surface roughness, waviness can be measured at multiple locations along a sample cable, with individual measurements being averaged together to achieve a representative measurement. The average wavelength of the macroscopic features in the longitudinal direction may be longer than 1 mm, optionally longer than 2 mm or 3 mm.

[0190] The waviness values stated in the preceding paragraph are based on the assumption that a sampling lengths is sufficient to capture multiple peaks and troughs of a surface height profile. In case the available instrument can only measure roughness using a shorter cut off length, meaningful results can still be obtained. For example, particular embodiments of the cables may have average roughness values Rz greater than 10 pm, optionally greater than 15 pm, when measured with a cutoff length of 2.5 mm. MEASUREMENTS OF INSTALLATION PERFORMANCE

[0191] Also visible in Figure 12 are values Dmax, representing installation performance. Measurements of installation performance have been obtained by testing using the “Nx100” test route illustrated in Figure 11. The duct for these tests is a microduct of inner diameter 3.5 mm and 7 mm outer diameter. The duct had a low friction lining and this was also provided with a texture of longitudinal ribs, in a commercially available form.

[0192] Table 6 presents the result of four tests of installation performance, using a pushing tool having drive belts of the general type illustrated in Figure 10. The tool used for Example 1 was an earlier prototype of this tool, one without the adjustable belt tensioners. In other words, it should be noted that the tests on Examples 2, 3 and 4 were performed using slightly different tools. For completeness, the test was repeated on the cable of Example 1 using the same pushing tool as the other examples yielded a similar pushing distance Dmax of 364 m. The lubricating material in all of these tests was, a commercially available lubricating material Pre-Lube 5000. This material was added in a measured quantity (15 drops) to the reservoir of the lubricating arrangement 1800 in the pushing tool. Each test on Examples 2, 3 and 4 was conducted with fresh tyres on the wheels of the pushing tool, to ensure that the results are fairly comparable.

[0193] Starting the pushing operation, a timer measured the progress to milestones at 200 m installed and 300 m installed. If the distance of 300 m was not reached within 20 minutes, the test was stopped.

[0194] TABLE 6

[0195] As mentioned already, the cable of Example 1 exhibited a surprising performance when tested. In fact, as recorded in Table 6, this first example of the cable with textured surface cable travelled a distance of 377 m before stopping. Such a distance would not be remarkable for a cable installed by blowing, but for pushing such a thin cable, in a convoluted test route, distances like 300 m are not normally achievable. As seen in the table, the cables of Example 2 and Example 3 both managed to exceed 300 m, taking only 7m15s and 7m25s respectively, to reach the 300 m mark. For commercially viable installation performance, progress at a rate slower than 20 m / min might be considered unproductive. By comparison, speeds up to 50 m / min are commonly achieved when blowing cable through a duct. For the purposes of these trials, however, pushing was continued even after the cable slowed to speeds as low as 1 m / min. Eventually, after 10m52s the cable of Example 2 reached 307 m, and the test was stopped. Similarly, after 15m03s the cable of Example 3 stopped at 315 m.

[0196] The cable of Example 4 reached a distance of 288 m before stopping. On the other hand, all three of the examples reached 200 m in only 1 m30s-1m35s. That indicates an installation speed in excess of 100 m / min. The additional time required to cover the distance between 200 m and 300 m was less than in under five minutes, so that the speed remained above the mentioned 20 m / s for both Example 2 and Example 3.

[0197] As already mentioned, distances in excess of 200 m are already considered remarkable for a nano cable installed by pushing into a duct with a bore of only 3.5 mm. Distances up to and beyond 300 m even more remarkable.

[0198] T able 7 presents a further set of tests, designed to evaluate independently (i) the influence of the surface texture and (ii) the influence of applying a lubricant material to the sheath during installation, using the lubricating arrangement 1800 of the lubricating device 1000. Two of the example cables were selected for these comparative tests, as indicated in the leftmost column of the table. Example 0 is the known cable 410 with a smooth outer surface 432. Example 3 is the novel cable 510 with a textured outer surface 532, and the one whose surface profile, at least as represented by the parameters Rz, Ra, Rq, most closely resembles the profile of the textured surface of Example 1. The second column of the table records the presence or absence of texture accordingly. TABLE 7

[0199] As indicated in the third columns of the table, each of these two cables was subjected to pushing test in which lubricating material was not added, and a pushing test in which lubricating material was added. (It will be understood that the same piece of cable was not used twice. Rather, representative lengths of cable were cut from a longer production run made by the continuous process described above with reference to Figure 7.)

[0200] In the test of the smooth cable Example 0 without lubricant, a maximum distance of 220 m was achieved, before the time limit of 20 minutes expired. This is already a very good performance compared with conventional cables. On the other hand, textured cable of Example 3 progressed to 284 m before the test was stopped, even without lubricant.

[0201] The smooth cable of Example 0 did not in this test perform any better with application of lubricating material than without. However, when the textured cable of Example 3 was installed with the application of lubricating material, again a distance comfortably beyond 300 m was achieved.

[0202] In conclusion, although further experimentation and refinement is always possible to yield optimal and consistent performance, the tests already conducted appear to confirm that the textured surface, originally obtained by a fault in the manufacturing process, actually offers a level of installation performance beyond what is conventionally possible. Moreover, the application of lubricating material to a cable having such texture brings a further benefit not seen with smooth cables.

[0203] It can be postulated that, due to the smoothness of the surface, the cable 410 of Example 0 actually carries very little lubricating material into the duct, due to the efficient action of the seal 816 / 1816. On the other hand, the textured cable 510, even as it passes through the seal 816 / 1816, is able to carry a quantity of lubricating material within the ‘valleys’ of its textured surface 532. This has been confirmed by observation during the performance of the above tests. In the case of the textured cables, the measured quantity of lubricating fluid was drawn from the reservoir within the first 30 seconds or so of pushing. The same carrying away of the lubricating material did not happen with the smooth sheath of Example 0.

[0204] In other words, in order to obtain a significant improvement in installation performance, it appears to be sufficient that lubricating material is applied continuously onto the surface of the cable during at least an initial phase of the installation by blowing. On the other hand, further investigations may reveal that the greater supply of lubricating material, and / or a more effective regulation of the flow of the lubricating material, will yield the optimum performance by allowing lubricating material to be conveyed further through the duct.

[0205] Furthermore, the rate of transfer of lubricating material per unit length of cable will also depend on the interaction of the seal with the textured surface, and the properties of the lubricant material itself. A function of the seals is to substantially prevent leakage of the lubricating fluid, meaning that the fluid will not flow freely from the input and / or output ports of the lubricating device so as to be wasted. It is particularly important, of course, to prevent flow of lubricating material in the reverse direction, so as to contaminate the belts and tyres of the pushing mechanism. At the same time, it is only necessary to prevent flow of a significant quantity of lubricating fluid in the interval between the lubricating fluid being loaded into the reservoir 802 / 1802 and the pushing of the cable for installation into the duct. The seal need not be perfect to meet this aim.

[0206] To achieve these dual goals, the implementer can pay attention the dimensions and properties of the seals, relative to the dimensions of features in the textured surface 532^ For example, if all valleys or troughs in the textured surface are much smaller than the contact length of the seal, it is unlikely that fluid can pass freely If on the other hand, there is in the surface 532 a narrow valley or trough that is narrow enough and long enough to extend from one side of the seal to the other, that might allow leakage, subject to properties of the fluid and its interaction with materials of the sheath and or seal. Conversely, if all the features of the textured surface were so large and smooth that the elastic material of the seal could conform perfectly with the cable outer surface as the cable passes through, then the surface will not be able to carry much lubricant. In the example lubricating devices used in the presented tests, the contact length of the seals 802 / 1802 is 3 mm, while the minimum contact circumference (inside diameter) of the seals is 6.9 mm). For different cables, different dimensions and / or elasticities may be specified. These may be fitted interchangeably within the same body, though of course cables above a certain diameter would require a tool with a larger bore diameter. For present purposes, it is envisaged that the same device will be adaptable for coupling to ducts of 7, 8, 10 and 12 mm diameter.

[0207] As also mentioned, different forms of lubricant, including liquid, powder and solid can be envisaged. In addition, lubricant applied continuously to be outer surface of the cable may be used in combination with lubricant pre-loaded into the duct in the conventional manner.

[0208] Also, as noted above, the tests already indicate that the textured surface by itself, even in the absence of added lubricant, offers a level of installation performance beyond what is conventionally possible. One can speculate as to possible mechanisms by which this benefit is obtained, without limiting the present disclosure to any particular mechanism. For example, one may speculate that the overall friction between the cable and the inside of a duct is reduced, by reducing the surface area of the cable sheath that is in contact with the duct wall. That is to say, only the peaks of the profile are in contact with the duct, not the whole surface area. This may be the case for a textured surface having only fine texture, but is particularly the case for a textured surface having longer-scale, larger amplitude features characterised as waviness.

[0209] Compared with known methods for applying texture to a cable sheath, particularly a large- scale texture, the methods disclosed herein require little or no special equipment and avoid the need for complex material preparations.

[0210] While the present disclosure focuses primarily on examples having only optical fibres as functional elements within the cable, parts of the disclosure are applicable also to cables including electrical conductors as well as or instead of optical fibres. Hybrid cables may carry optical communications signals via optical fibres in parallel with electrical power via conductors of copper or other metal. A miniature hybrid cable intended for installation by blowing is disclosed for example in EP3327732A1. The textured surface and its method of manufacture can be applied in any size of cable, whether optical, hybrid or electrical only. PRETERMINATED CABLE ASSEMBLY EXAMPLES

[0211] Finally, it may be recalled from the above discussion that a common application of the cable 510 with textured outer surface will be in the provision of cables, in which at least a partial connector is factory-fitted to a leading end of the cable, so that an optical data connection can be completed by simple steps and the need for cutting into the cable and fitting an optical ferrule and / or splicing to a another length of optical fibre in the field can be avoided.

[0212] As mentioned already, the cables newly disclosed herein can be employed to make the kinds of assemblies disclosed in previously published patent applications WO2018146470A1 and WO2023016835A1 . WO2023016835A1 in particular discloses PBT sheath of a pushable cable of the type illustrated above in Figure 4. The same type of assembly can be made with the cable 510 having textured sheath.

[0213] Furthermore, as illustrated in Figure 13 (a), it is not necessary that a partial connector be applied directly to the end of the cable with the textured outer surface. The drawing illustrates an assembly based on a length of cable 1310 similar to cable 510 of the above examples. This cable is delivered on a reel 1312 or in a ‘pan’. A first end of the cable 1310 is terminated with a partial connector 1324a, for example a ferrule subassembly of the type illustrated in WO2023016835A1. However, rather than being applied directly to the end of the cable 1310, this partial connector has been factory fitted with a “tail” 1328a of another type of cable. This tail, in turn is spliced to one of the optical fibres within the cable 1310, to complete the optical connection at that end of the cable. The spliced joint, which would otherwise be vulnerable to damage during installation and operation, is protected by a splice protector accessory 1332a, as disclosed in the patent application GB2619109A, published at the present priority date. The splice protector accessory is strong enough to protect the splice joint, including transferring axial forces between the sheath and strength element of the cable 1310 and the sheaths of the tail 1328a.

[0214] The tail can be of a more compact and flexible design, in particular a microcable not involving a strength member 530. The length Lt can be anything from a few centimetres to a metre or two. Usefully, the length of the tail can be such that the spliced joint remains protected within a duct, after the tail and collector have emerged and been connected to terminating equipment.

[0215] In the illustrated example, a second end of the cable 1310 is similarly terminated with a partial connector 1324b, a tail 1328b and a spliced joint with splice protector accessory 1332b. For installation using a pushing tool 1000 or similar, the partial connector and tail at whichever end is the leading end of the cable assembly, can be fed into the duct, along with the spliced joint and splice protector accessory, prior to engagement of the pushing tool over the cable 1310.

[0216] Referring to Figure 13(b), this shows that the termination of the pre-terminated cable assembly 1300 need not be the same at both ends. In this example, a partial connector 1324b is fitted within a ruggedised or “hardened” connector body 1340. An additional protective sheath 1342 also extends from the hardened connector body some distance along the cable. By installing the opposite end of the cable through a duct, and then coupling the leading end of the additional protective sheath 1342b to a trailing end of the duct, a hardened connection can be made, while the cable 1310 itself remains protected within the duct and protective sheath.

[0217] CONCLUSION

[0218] The present disclosure provides a number of novel products, tools and processes, each of which may be used independently and / or in combination with one another.

[0219] The disclosure includes methods and apparatus by which a miniature cable can be installed reliably over 200 m, optionally over 300 m into a microduct, by pushing alone. The cable may be an optical fibre cable, including a miniature hybrid cable.

[0220] The disclosure enables cables having a highly textured outer surface to be produced by a simple extrusion of a solid polymer, without reliance on internal structure, foreign particles or foaming. Cables of any kind can be provided with such an outer surface, including miniature optical fibre cables capable of being installed great distances by pushing alone. While specific embodiments of the present invention have been described above, it will be appreciated that departures from the described embodiments may still fall within the scope of the present invention, defined by the appended claims and their equivalents.

Claims

CLAIMS1 . A method of manufacturing a cable, the method comprising the steps:(a) providing a functional core extending in a longitudinal direction;(b) providing an extruder having tooling for forming a tube of polymer material;(c) forming a melt of polymer sheath material in an extruder, the melt having a melt temperature;(d) drawing said core through an aperture in said tooling while extruding said melt to form a tube of said polymer sheath material surrounding the core; and(e) drawing and cooling the tube and core together to form a fibre optic cable in which said core is surrounded by a polymer sheath, an outer surface of said polymer sheath forming an outer surface of the cable, wherein, during performance of step (c), an outer part of said tooling is maintained at a temperature lower than said melt temperature by such an amount that an irregular texture is continuously imparted to the outer surface of the cable.

2. A method as claimed in claim 1 wherein said outer part is maintained at a temperature below a melting point of said polymer sheath material.

3. A method as claimed in claim 1 or 2 wherein said outer part is maintained at a temperature more than 15 degrees Celsius, optionally more than 20, 25 or 30 degrees Celsius lower than said melt temperature.

4. A method as claimed in any of claims 1 , 2 or 3 wherein said irregular texture includes macroscopic features measurable on a scale longer than 1 mm, optionally longer than 2.5 mm or 3 mm, for example having an average wavelength in the longitudinal direction that is longer than 1 mm, optionally longer than 2 mm or 3 mm5. A method as claimed in any of claims 1 , 2, 3 or 4 wherein said irregular texture includes macroscopic features discernible individually by the human eye and by touch.

6. A method as claimed in any of claims 1 to 5 wherein said macroscopic features exhibit height variation greater than greater than 20 pm, optionally greater than 30 pm, over a sampling length sufficient to capture multiple peaks and troughs of a surface height profile.

7. A fibre optic cable as claimed in any of claims 1 to 6 wherein an average waviness Wa or Wq of said outer surface measured over a sampling length greater than 1.0 mm, optionally greater than 2 mm or 3 mm, is greater than 10 microns, optionally greater than 15 or 20 microns.

8. A fibre optic cable as claimed in any of claims 1 to 7 wherein an average roughness value Rz measured with a cut off length of 2.5 mm, is greater than 10 microns, optionally greater than 15 microns.

9. A method as claimed in any of claims 1 to 8 wherein a base polymer of said polymer sheath material is polybutylene terephthalate PBT.

10. A method as claimed in any of claims 1 to 9 wherein said polymer sheath material comprises a base polymer material mixed with at least one friction reducing additive.

11. A method as claimed in claim 9 or 10 wherein said friction reducing additive comprises a polydimethylsiloxane material, PDMS in a carrier material.

12. A method as claimed in claim 11 wherein said carrier material is polyethylene.

13. A method as claimed in any of claims 9 to 12 wherein the amount of friction reducing additive excluding any carrier material is between 1 % and 5%, optionally between 1.2% and 3% by weight of the material of the extruded sheath.

14. A method as claimed in any of claims 1 to 13 wherein said core comprises a coated fibre bundle wherein said one or more optical fibres are embedded with said strength member in a solid resin material.

15. A method as claimed in any of claims 1 to 14 wherein said cable is adapted to be installed by pushing into a microduct with bore less than 5 mm, and wherein an outer diameter of the cable is in the range of 1.2 to 3.0 mm, optionally greater than 1.4 or 1.6 mm and optionally less than 2.8 mm or 2.5 mm.

14. A method as claimed in any of claims 1 to 15 wherein the polymer sheath has an average thickness between 0.05 mm and 0.25 mm, optionally between 0.15 mm and 0.25 mm.

15. A method as claimed in any of claims 1 to 14 wherein said functional core includes at least one longitudinal strength member.

16. A method as claimed in any of claims 1 to 15 wherein said functional core comprises a coated fibre bundle wherein one or more optical fibres are embedded in a solid resin material.

17. A method as claimed in claim 16 wherein at least one strength member, for example an FRP (fibre reinforced plastic) strength member, is embedded alongside the optical fibre(s) within said solid resin material.

18. A method as claimed in any of claims 16 to 17 further comprising the steps (e) cutting the cable into a number of lengths and pre-terminating at least one said optical fibre with at least a partial optical connector and (f) packaging the pre-terminated lengths for subsequent use at an installation site.

19. A cable obtainable by a method as claimed in any of claims 1 to 18, the cable having a functional core extending in a longitudinal direction and a sheath surrounding said core such that an outer surface of said sheath forms an outer surface of the cable, said sheath being formed of a solid polymer material, wherein the outer surface of said sheath has an irregular texture as formed by irregular cooling of the surface during extrusion of the polymer material from a melt.

20. A cable as claimed in claim 19 wherein said irregular texture includes macroscopic features measurable on a scale longer than 1 mm, optionally longer than 2.5 mm or 3 mm, for example having an average wavelength in the longitudinal direction that is longer than 1 mm, optionally longer than 2 mm or 3 mm21. A cable as claimed in claim 19 or 20 wherein said irregular texture includes macroscopic features discernible individually by the human eye and by touch.

22. A cable as claimed in any of claims 19 to 21 wherein said macroscopic features exhibit height variation greater than greater than 20 pm, optionally greater than 30 pm, over a sampling length sufficient to capture multiple peaks and troughs of a surface height profile.

23. A cable as claimed in any of claims 1 to 22 wherein an average waviness Wa or Wq of said outer surface measured over a sampling length greater than 1 .0 mm, optionally greater than 2 mm or 3 mm, is greater than 10 microns, optionally greater than 15 or 20 microns.

24. A cable as claimed in any of claims 1 to 7 wherein an average roughness value Rz measured with a cut off length of 2.5 mm, is greater than 10 microns, optionally greater than 15 microns.

25. A cable as claimed in any of claims 1 to 8 wherein a base polymer of said polymer sheath material is polybutylene terephthalate PBT.

26. A cable as claimed in any of claims 19 to 25 wherein said polymer sheath material comprises a base polymer material mixed with at least one friction reducing additive.

27. A cable as claimed in any of claims 19 to 26 wherein said functional core includes at least one longitudinal strength member.

28. A cable as claimed in any of claims 19 to 27 wherein said functional core includes one or more optical fibres.

29. A cable as claimed in any of claim 28 wherein said functional core comprises a coated fibre bundle wherein said one or more optical fibres are embedded in a solid resin material.

30. A cable as claimed in claim 29 wherein at least one longitudinal strength member is embedded alongside the optical fibre(s) within said solid resin material.

31. A cable as claimed in any of claims 28 to 30 further comprising at least a partial optical connector terminating at least one of said one or more optical fibres.

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