GB1560750A

GB1560750A - Coating optical fibres

Info

Publication number: GB1560750A
Application number: GB53384/76A
Authority: GB
Original assignee: Standard Telephone and Cables PLC
Current assignee: STC PLC
Priority date: 1976-12-21
Filing date: 1976-12-21
Publication date: 1980-02-06
Also published as: AU507571B2; AU3171577A; ZA776714B

Description

(54) COATING OPTICAL FIBRES (71) We, STANDARD TELE PHONES AND CABLES LIMITED, a British Company. of 190 Strand, London, WC2, England, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:- This invention relates to plastics coating of optical fibres.

Glass or silica optical fibres for optical transmission systems are generally fragile and difficult to handle thus making them unsuitable for the stranding and twisting operations involved in forming a multicore light guide cable. Attempts to prepare fibres from tougher materials are somewhat restricted as the fibre material must be selected for its optical rather than its mechanical properties.

In the past glass optical fibres have been protected by melt extruding a layer of plastics material, e.g. polyethylene or polypropvlene, on to the fibres. This method is satisfactory for bare fibres, but cannot be readily applied to the coating of clad fibres having a precoating, e.g. of silicone resin for mechanical protection and/or optical cladding purposes, as the conventional melt extrusion process often damages this precoating laver.

According to the invention there is provided a method of coating a glass or silica optical fibre with polypropylene, including passing the fibre through the head of a cross-head plastics extruder, extruding at a temperature of 190 to 240 C a polypropylene tube around but not in contact with the fibre, and applying suction within the tube to collapse it into contact with the fibre whilst the plastics material is in a deformable condition.

An embodiment of the invention will now be described with reference to the accompanying drawing in which Fig. 1 is a part schematic sectional view of the head of a cross-head extruder, and Fig. 2 shows a die and point assembly for use in the extruder of Fig. 1.

Referring to Fig. 1, which for clarity omits some of the features of the extruder head, molten plastics material is forced from the extruder barrel 11 by a screw 12. The plastics material is filtered to remove solid particles by filter plate 13 and is then forced through a breaker plate 14 into bore 15 which bore communicates with the extruder cross-head 16. From the bore 15 the plastics flows over the profiled surface 17 of torpedo 18 and is extruded as a tube from the annular gap 19 between a tubing die 20 and point 21. The extruder cross head 16 is fitted with an upstream extension portion 22 for receiving an optical fibre (not shown) to be coated. and via which suction is applied to the plastics tube extruded from the die.

An optical fibre, e.g. a silicone resin clad silica fibre, to be plastics coated is fed through gland 23 into the extension 22, through an axial bore 24 along the torpedo 18 and through bore 25 of the point 21. The fibre is a sufficiently close fit in the gland 23 to prevent excessive loss of vacuum therebetween when coating is in progress.

The fibre is a loose fit in the bore 25 of the point 21 to allow suction to be applied to the extruded material adjacent the exit of the point.

To effect coating, the fibre is drawn at a constant velocity through the cross-head and plastics material is extruded in tubular form from the die. Suction is applied via the side tube 26 of the extension 22 of the head to collapse the plastics tube down on to the fibre a short distance downstream of the outlet of the annular gap whilst the plastics material is still molten or in a deformable condition. The coating is then annealed and cooled before the coated fibre is wound onto a drum.

The material employed for coating optical fibres is polypropylene, for example that sold under the name PROPATHENE (R.T.M.) grade number GWE 105. A suitable die and point for this material are shown in Fig. 2. The die 31 advantageously has an entry cone angle of 120 C, a land 32 of 1.5 mm and an exit diameter of 2.5 mm.

The corresponding point 33 has a 70 cone angle, an entry bore 34 of 2.4 mm reducing to 0.46 mm at the exit, and a land 35 of 1 mm whose outer diameter is 0.9 mm. This arrangement is adapted for coating a fibre of 0.25 mm diameter.

Using such a die and point of 0.25 mm clad silica fibre may be coated with polypropylene to a diameter of 1.0 mm at an extrusion temperature between 190 and 240 C and at a rate of 20 metres/min.

WHAT WE CLAIM IS: 1. A method of coating a glass or silica optical fibre with polypropylene, including passing the fibre through the head of a cross-head plastics extruder, extruding at a temperature of 190 to 240 C a polypropylene tube around but not in contact with the fibre, and applying suction within the tube to collapse it into contact with the fibre whilst the plastics material is in a deformable condition.

2. A method as claimed in claim 1, and in which said fibre is provided with a protective precoating.

3. A method as claimed in claim 2 and in which said fibre is of the silicone resin clad silica type.

4. A method of plastics coating an optical fibre substantially as described herein with reference to the accompanying drawings.

5. An optical fibre plastics coated by a method as claimed in any one of claims 1 to 4.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. shown in Fig. 2. The die 31 advantageously has an entry cone angle of 120 C, a land 32 of 1.5 mm and an exit diameter of 2.5 mm. The corresponding point 33 has a 70 cone angle, an entry bore 34 of 2.4 mm reducing to 0.46 mm at the exit, and a land 35 of 1 mm whose outer diameter is 0.9 mm. This arrangement is adapted for coating a fibre of 0.25 mm diameter. Using such a die and point of 0.25 mm clad silica fibre may be coated with polypropylene to a diameter of 1.0 mm at an extrusion temperature between 190 and 240 C and at a rate of 20 metres/min. WHAT WE CLAIM IS:

1. A method of coating a glass or silica optical fibre with polypropylene, including passing the fibre through the head of a cross-head plastics extruder, extruding at a temperature of 190 to 240 C a polypropylene tube around but not in contact with the fibre, and applying suction within the tube to collapse it into contact with the fibre whilst the plastics material is in a deformable condition.