Process and apparatus for manufacturing blown optical fibre units

Abstract

A process for manufacturing an optical fibre unit for air-blown installations includes: providing a deposition chamber for applying particulate material, the deposition chamber having an inlet end and an outlet end and a longitudinal axis; passing through the deposition chamber an optical fibre assembly including at least one optical fibre embedded in an inner layer of cured resin material, and having an outer layer around the inner layer, the outer layer including uncured resin material; injecting a flow of fluid and particle material in the chamber in a direction substantially parallel to the chamber longitudinal axis, at an injection speed of 5 m/s at most; perturbing the flow when in the chamber, thus causing the particle material to impact and partially embed into the outer layer of the optical fibre assembly; and curing the outer layer.

Claims

1. A process for manufacturing an optical fibre unit for air-blown installations comprising: providing a deposition chamber for applying particulate material, the deposition chamber having an inlet end and an outlet end, a longitudinal axis, and a chamber pipe with a central part having a longitudinally changing cross section, wherein the central part of the chamber pipe does not have sharp edges; passing through the chamber pipe an optical fibre assembly comprising at least one optical fibre embedded in an inner layer of cured resin material, and having an outer layer around the inner layer, the outer layer comprising uncured resin material; injecting a flow of fluid and particle material in the deposition chamber in a direction substantially parallel to the chamber longitudinal axis, at an injection speed of below 4 m/s; perturbing the flow when in the chamber pipe, thus causing the particle material to impact and partially embed into the outer layer of the optical fibre assembly; and curing the outer layer.

2. The process according to claim 1, comprising protecting a portion of uncured resin of the optical fibre assembly from injection of the flow.

3. The process according to claim 1, wherein the injection speed of the flow is below 3 m/s.

4. The process according to claim 1, wherein the optical fibre assembly passed through the chamber pipe has an outer layer with a thickness from 40 m to 100 m.

5. The process according to claim 4, wherein the thickness of the outer layer is from 50 m to 70 m.

6. The process according to claim 1, having a process speed of 250 m/min to 300 m/min.

7. The process according to claim 1, wherein the particulate material is embedded with an embedding of the particle material from 20% to 70% to provide a surface roughness of from 80 to 150 m.

8. The process according to claim 1, wherein the longitudinally changing cross section of the central part of the chamber pipe has the form of a sinusoid.

9. The process according to claim 1, wherein the central part of the chamber pipe comprises a tubular body having a number of narrowings of its cross section.

10. The process according to claim 9, wherein the narrowings are arranged according to a pitch that is substantially constant, wherein the pitch is substantially equal to the inner diameter of an inlet and an outlet of the chamber pipe.

11. A process for manufacturing an optical fibre unit for air-blown installations comprising: providing a deposition chamber for applying particulate material, the deposition chamber having an inlet end and an outlet end, a longitudinal axis, a frusto-conical section at the inlet end, and a chamber pipe with a central part having a longitudinally changing cross section, wherein the longitudinally changing cross section of the central part of the chamber pipe has the form of a sinusoid; passing through the chamber pipe an optical fibre assembly comprising at least one optical fibre embedded in an inner layer of cured resin material, and having an outer layer around the inner layer, the outer layer comprising uncured resin material; injecting a flow of fluid and particle material in the deposition chamber in a direction substantially parallel to the chamber longitudinal axis, at an injection speed of below 4 m/s; perturbing the flow when in the chamber pipe, thus causing the particle material to impact and partially embed into the outer layer of the optical fibre assembly; and curing the outer layer.

12. The process according to claim 11, comprising protecting a portion of uncured resin of the optical fibre assembly from injection of the flow.

13. The process according to claim 11, wherein the injection speed of the flow is below 3 m/s.

14. The process according to claim 11, wherein the optical fibre assembly passed through the chamber pipe has an outer layer with a thickness from 40 m to 100 m.

15. The process according to claim 14, wherein the thickness of the outer layer is from 50 m to 70 m.

16. The process according to claim 11, having a process speed of 250 m/m to 300 m/min.

17. The process according to claim 11, wherein the particulate material is embedded with an embedding of the particle material from 20% to 70% to provide a surface roughness of from 80 to 150 m.

18. The process according to claim 11, wherein the central part of the chamber pipe comprises a tubular body having a number of narrowings of its cross section.

19. The process according to claim 18, wherein the narrowings are arranged according to a pitch that is substantially constant, wherein the pitch is substantially equal to the inner diameter of an inlet and an outlet of the chamber pipe.

Description

(1) The present invention will become more clear from the detailed following description, given by way of example and not of limitation, with reference to the following figures, wherein:

(2) FIG. 1 is a schematic perspective view of a portion of an exemplifying optical fibre unit for air-blown installations;

(3) FIG. 2 is a schematic representation of a portion of a manufacturing apparatus for manufacturing an optical fibre unit for air-blown installations;

(4) FIG. 3 is a schematic cross-section of a chamber where particulate material is partially embedded into the outer layer according to one embodiment of the present invention;

(5) FIG. 4 is an enlarged schematic representation of a chamber core according to one embodiment of the invention; and

(6) FIG. 5 shows a particular of the frusto-conical section at the inlet end of an apparatus according to the present invention.

DESCRIPTION OF EMBODIMENTS

(7) FIG. 1 is a schematic perspective view of a portion of an optical fibre unit FU for air-blown installations. The optical fibre unit FU comprises a number of optical fibres F1, F2, F3, F4, an inner layer IL which is radially outer to the optical fibres and an outer layer OL which is radially outer to the inner layer IL.

(8) The term optical fibre is meant to indicate an optical glass core surrounded by a glass cladding and a coating system comprising one or two layers of cured resins, for example acrylate resins.

(9) The outer layer OL comprises particulate material PM which is partially embedded into the outer layer.

(10) Particulate material can comprise any material which provides low friction when the optical fibre unit FU is blown in a duct.

(11) For example, the particulate material could be selected among beads of glass, of ceramic, of polytetrafluoroethylene (PTFE) or of high-density polyethylene (HDPE). The beads can be either hollow or solid.

(12) The particles of the optical fibre unit manufactured with the process of the invention preferably have a diameter of from 0.070 mm to 0.150 mm, more preferably of from 0.115 mm to 0.125 mm.

(13) The particle material coveragei.e. the amount of beads per unit surface area of the productin the optical fibre unit manufactured according to the present process can be of from 15 to 35 beads/mm.sup.2.

(14) The embedding is the amount of sinking of the particles into the optical fibre outer layer, expressed as percentage of the particle dimension which is embedded into the outer layer. In the optical fibre unit manufactured with the process of the invention, the embedding of the particle material is of from 20% to 70%, preferably from 20% to 60%. The embedding should be high enough to maintain the particle fixed into the outer layer, but not too much so as to compromise the surface roughness parameter. As a matter of fact, a relatively high surface roughness reduces the friction between the optical fiber unit and the duct during the blowing procedure and increases the ability of the optical fiber unit to be entrained by the air blowing.

(15) The surface roughness (Rz) of the optical fibre unit should be suitable to provide the sought friction for enabling the unit to be installed by air-blown techniques. Surface roughness is connected to the embedding parameter in that the less the particles are embedded, the more the surface of the optical fibre unit is rough and vice versa. The surface roughness can be evaluated by SJ400 surface roughness tester (Mitutoyo Ltd).

(16) Rz (DIN) is the average distance between the highest peak and lowest valley in each sampling length, (ASME Y14.36M1996 Surface Texture Symbols).

(17) The optical fibre unit manufactured with the process of the invention has a surface roughness Rz of from 80 m to 150 m.

(18) FIG. 2 is a schematic representation of a portion of a manufacturing apparatus 100 for manufacturing an optical fibre unit FU for air-blown installations according to the present invention.

(19) Four coated optical fibres F1 to F4 are unwound from corresponding reels 101-1 to 101-4 and then grouped at a grouping unit 102.

(20) The apparatus 100 further comprises: an inner layer applicator unit 103 which applies inner layer IL to the grouped optical fibres; a first curing unit 104 where inner layer is cured; an outer layer applicator unit 105 which applies outer layer OL on cured inner layer IL; a particulate material applicator unit 106 which applies particulate material PM to the outer layer OL; a second curing unit 107 where outer layer OL, provided with particulate material PM is cured.

(21) Profitably, the apparatus 100 allows manufacturing an optical fibre unit FU according to a continuous process.

(22) In embodiments of the invention, the first curing unit 104 operates a curing by ultraviolet radiation. Ultraviolet exposure could be carried out by a D lamp type and ultraviolet exposure power could be from about 0.15 to about 0.22 watt/cm.sup.2. In embodiments of the invention, it could be approximately 0.19 watt/cm.sup.2.

(23) Also second curing unit 107 can operate by ultraviolet radiation. Ultraviolet exposure could be D lamp type and ultraviolet exposure power could be from about 0.15 watt/cm.sup.2 to about 0.22 watt/cm.sup.2. In embodiments of the invention, it could be approximately 0.19 watt/cm.sup.2.

(24) Process speed is the speed at which the optical fibres enter the grouping unit 102, which substantially corresponds to the speed at which the optical fibre unit FU exits the apparatus.

(25) The process speed is higher than 100 m/min and can exceed 300 m/min (possibly up to 500 m/min). Preferably, the process speed is between 250 m/min and 300 m/min.

(26) Particulate material applicator unit 106 comprises a deposition chamber through which the optical fibre assembly with uncured outer layer is caused to travel. In the chamber, particulate material is caused to impinge against uncured outer layer and to partially become embedded therein.

(27) FIG. 3 shows a deposition chamber 10 according to the present invention. The optical fibre assembly enters the chamber by a tubing 15 inserted into an axially converging frusto-conical section 13 and travels substantially along the chamber longitudinal axis A-B in the direction of the arrow a, which is the drawing direction. As it will be explained in more details, at least one through-passage 12 is provided for the injection of a flow of fluid, for example air, and particulate material into a chamber pipe 14. In the embodiment of FIG. 3, two opposite through-passages 12 offset in the horizontally plane perpendicular to the longitudinal axis A-B are connected to the frusto-conical section 13. In other embodiment (not shown) further flow admitting ducts could be provided.

(28) An extension 16 is preferably provided to the end of the tubing 15 protruding into the frusto-conical section 13. The extension 16 can help to protect the uncured resin matrix at the point of injection where the fluid entraining the particulate material enters into the chamber pipe 14. The extension 16 could be made of metal, such as stainless steel; ceramic or glass.

(29) The chamber 10 according to the present invention further preferably comprises a discharge exit 18 for dismissing the fraction of particulate material which failed to embed into the uncured outer layer. In the embodiment shown in FIG. 3, the discharge exit 18 is a frusto-conical section axially diverging from the drawing direction, having one or more outgoing ducts 18a connected thereto (for example three). Preferably, the discharge exit 18 has surface inclined toward the outlet end by an angle of 35 to 45, more preferably by 40 with respect to the longitudinal axis A-B.

(30) Profitably, the chamber pipe 14 is made in a single piece.

(31) The chamber pipe 14 comprises a tubular body having a substantially circular cross-section. This cross section is preferably substantially constant in proximity of the inlet and the outlet ends a, b. Preferably, in its central part, the tubular body has a number of narrowings 14 of its cross section.

(32) In the example shown in FIG. 4, the chamber pipe 14 is shown with five annular narrowings 14.

(33) The narrowings 14 are arranged according to a pitch 141 which is substantially constant. The pitch 141 could be substantially equal to the inner diameter of chamber pipe at its inlet and outlet ends.

(34) For example, the inner diameters at the inlet and outlet ends are of 0.26 mm and the pitch 141 is of 0.27 mm.

(35) The profile of the central part of chamber pipe of FIG. 4, when seen in longitudinal cross-section, is shaped in form of sinusoid, where sharp edges are preferably avoided.

(36) Having reference to FIG. 5, the configuration of the fluid entrance portion will be further detailed. Each through-passage 12 has a respective longitudinal axis C, D substantially perpendicular to the longitudinal axis A-B. The frusto-conical section 13 has a surface inclined by angle of from 15 to 45, preferably of about 36 with respect to longitudinal axis A-B.

(37) The frusto-conical section 13 comprises an upper portion 13a and a lower portion 13b. The upper portion 13a is a substantially toroidal volume defined by the outer surface of the frusto-conical section 13 and the surface of the conical insert 15a coaxially inserted therein.

(38) In the depicted embodiment, the tubing 15 protrudes of about 10.5 mm into the frusto-conical section 13, and it is provided with a 35 mm-long extension 16, protruding of 5.5 mm beyond the bottom of the frusto-conical section 13.

(39) Preferably, the two through-passages 12 are diametrically offset, so as to impart a rotational motion to the fluid.

(40) When the fluid entraining the particulate material flows from the through-passages 12 in to the toroidal upper portion 13a of the frusto-conical section 13, it enters in a zone having increased transversal area and, thereafter, the fluid speed is progressively accelerated while flowing downstream.

(41) Such a speed change is conducive to produce a fluid vortex in the direction of the fibre unit movement.

(42) The Applicant carried out production tests on diverse deposition chambers to evaluate process and product parameters. In all of the test runs the particulate material was solid glass beads having an average diameter of 0.12 mm. The results are set forth in Table 1.

(43) TABLE-US-00001 TABLE 1 Chamber Chamber Chamber Parameter assy 1* assy 2* assy 3 Maximum production 18 km 12 km 150 km length Process speed 150 m/min 300 m/min 300 m/min Fluid injection speed 4 m/s 4 m/s 2.3 m/s Bead embedding <20% 89% 40% Surface roughness >60 m 70 m 120 m (Rz)

(44) Chamber assembly 1 (comparative) included an axially extended passage with a substantially radial fluid inlet, with diameter of 18 mm and length 32 mm, followed by a chamber pipe having inner diameter of about 25 mm and length of about 235 mm and including 5 evenly spaced restrictions with inner diameter of about 15 mm, substantially corresponding to the chamber assembly disclosed in FIG. 3 of the already mentioned EP 0 757 022.

(45) Chamber assembly 2 (comparative) was substantially similar to the one disclosed by the already mentioned U.S. Pat. No. 7,618,676, FIG. 8 and included a frusto-conical input cavity with input diameter of 59 mm+/1 mm, length of 67 mm+/1 mm and output diameter of 26 mm+/1 mm, followed by a cylindrical pipe having a constant diameter of about 25 mm and length of about 235 mm; the fluid inlets had a substantially input radial direction and were curved with a final outlet with axis parallel to the fiber path.

(46) Chamber assembly 3 was according to an embodiment of the invention and included a frusto-conical input cavity with input diameter 59 mm+/1 mm, length 67 mm+/1 mm and output diameter 26 mm+/1 mm, followed by a chamber pipe having inner diameter of about 25 mm and length of about 235 mm and including 5 evenly spaced restrictions with inner diameter of about 15 mm.

(47) As for comparative Chamber assembly 1, a limited length of optical fibre unit was manufactured before stop due to apparatus maintenance (cleaning) requirements, and the process speed had to be limited to 150 m/min. The optical fibre assembly tested had a satisfactory surface roughness at the test speed of 150 m/min with an outer layer 30 m thick.

(48) When the thickness of the outer layer was changed to 60 m while maintaining the process speed at 150 m/min, the resulting fibre unit showed a low surface roughness (<50 m, due to excessive bead embedding), unsuitable for the air blown installation and an excessive bead coverage (>40 beads/mm.sup.2), making the product unacceptably brittle. This negative outcome was independent from the injection speed imparted to fluid.

(49) Making the apparatus to run at 300 m/min with an outer layer 60 m thick, the resulting product showed a bead coverage <10 beads/mm.sup.2, insufficient for the air blown installation, when the fluid injection speed was of 2-3 m/s. When the fluid injection speed was of 4 m/s or more the resulting product showed an excessive bead embedding (>90%), negatively affecting the surface roughness.

(50) Summarizing, an apparatus comprising Chamber assembly 1, besides working at slow process speed and frequent maintenance stops, cannot be used for producing optical fibre unit with an outer layer thickness greater than 30 m.

(51) As for comparative Chamber assembly 2, a stop for apparatus maintenance due to lumps formation was necessary after the production of 12 km of optical fibre unit.

(52) The outer layer of the optical fibre assembly used in this chamber had a thickness of 100 m.

(53) The resulting surface roughness was satisfactory, but the bead embedding was too high (89%) and resulted in a very brittle product not passing the 40 mm bend test according to IEC 60794-1-2 Ed 2.

(54) The deposition process in comparative Chamber assembly 2 was also evaluated by a Computational Fluid Dynamics (CFD) study. A fluid comprising air and particulate material (beads) was injected into the deposition chamber at a 4 m/s speed and reached a speed of 90 m/s in the vicinity of the exit of the cylindrical pipe, causing the most of the bead impacts on the optical fiber assembly to happen in the second half of the cylindrical pipe. This uneven speed profile along the chamber resulted in a poor bead coverage in the chamber portion closer to the inlet. Also a too deep impact was observed.

(55) To improve the local coverage it was tried to increase the fluid injection speed; unfortunately, this caused deeper embedding of the particles.

(56) In addition, fluid flow stagnation occurred near the inlet and outlet ends of the deposition chamber, where the particulate material fell out of fluid suspension and adhered to the optical fibre assembly, causing lumps thereon.

(57) The process of the invention was applied in testing Chamber assembly 3.

(58) Spans of optical fibre units much longer than those produced with the comparative processes and chambers were manufactured before maintenance stop (150 km). The outer layer of the optical fibre assembly used in this chamber had a thickness of 60 m.

(59) The resulting product showed a particle embedding of 40%, such as to provide a surface roughness of 120 m, ensuring the optical fibre unit an efficient behaviour during the blowing procedure.

(60) Also the local bead coverage was satisfying and the product passed the 40 mm bend test.