Extruder for extruding an electrically insulating layer comprising an extrusion screw having a liquid injection channel

Abstract

The invention relates to an extruder (5) for extruding an electrical cable comprising at least one elongate electrically conductive element and at least one extruded thermoplastic layer surrounding said elongate electrically conductive element, said extruder comprising: - a feed member (4) for polymer in solid form, - a sheath (6) fed by the feed member, - an extrusion screw (7) arranged inside the sheath (6) and allowing the progressive melting of the polymer to form an extrusion composition and the transport of this composition along the extrusion screw (7) to a distal end (21) of the extrusion screw (7), said extrusion screw (7) extending along a longitudinal axis (A), - an extrusion head (8) arranged at a distal end (21) of the extrusion screw (7) and configured to apply the composition around an elongate electrically conductive element,wherein at least one liquid injection channel (22) is formed in the extrusion screw (7), said at least one injection channel (22) opening inside the barrel (7) at at least one outlet orifice (30) formed on an outer surface of the extrusion screw (7). Figure for abstract: Figure 3,

Description

Title of the invention: Extruder for extruding an electrically insulating layer comprising an extrusion screw having a liquid injection channel Technical field

[0001] The present invention relates to an extruder for extruding an electric cable, in particular of the power cable type, comprising a thermoplastic layer extruded around at least one elongated electrically conductive element. The invention also provides a method for manufacturing an electric cable with such an extruder.

[0002] It typically but not exclusively applies to electric cables intended for the transport of energy, in particular to medium voltage (in particular from 6 to 45-60 kV) or high voltage (in particular greater than 60 kV, and up to 400 kV) energy cables, whether in direct or alternating current, in the fields of aerial, underwater, terrestrial electricity transport, or even aeronautics.

[0003] A medium or high voltage power transmission cable generally comprises from the inside to the outside: - an elongated electrically conductive element, in particular made of copper or aluminum; - an internal semiconductor layer surrounding said elongated electrically conductive element; - an electrically insulating layer surrounding said internal semiconductor layer; - an external semiconducting layer surrounding said insulating layer; and - possibly an electrically insulating protective sheath surrounding said external semi-conductor layer. Technological background

[0004] The extruded composition forming the electrically insulating layer may require the addition of a liquid to the thermoplastic polymer. It is known to inject this liquid before inserting the thermoplastic polymer into the sheath. The liquid is thus mixed with the thermoplastic polymer before or at the time of its insertion into the sheath, in particular at the level of a hopper. However, it has been observed that this method of injecting the liquid could lead to manufacturing defects in the electrically insulating layer, linked in particular to a sliding phenomenon. This sliding phenomenon at the wall linked to the lubricating effect of the dielectric liquid (well known by the Anglicism "slippage / sliding phenomenon") can lead to a de gradation of the mechanical and / or electrical properties of the thermoplastic layer obtained at the extrusion head (structural defects of the layer), in particular a variation in the diameter of the cable.

[0005] A solution known from document WO 02 / 47092 A1 consists of providing holes formed through the barrel of the extruder to inject a liquid between the extrusion screw and the barrel, in contact with the thermoplastic polymer in the molten state. Such a solution has the disadvantage, for existing extruders, of requiring the barrel to be pierced and in particular its internal surface. However, these pierced holes deteriorate the surface treatment carried out on this internal surface of the barrel. Injecting the liquid into an area of ​​the barrel where the polymer is in the molten state also requires taking into account a higher pressure than in an area where the polymer is in the solid or partially solid state.

[0006] There is therefore a need for an extruder configuration allowing injection of a liquid into contact with the thermoplastic polymer without risking damage to the internal surface of the sheath. Summary of the invention

[0007] For this, the invention proposes an extruder for extruding an electric cable comprising at least one elongated electrically conductive element and at least one extruded thermoplastic layer surrounding said elongated electrically conductive element, said extruder comprising: - a polymer feed member in solid form, - a sheath supplied by the feed member, - an extrusion screw arranged inside the barrel and allowing the progressive melting of the polymer to form an extrusion composition and the transport of this composition along the extrusion screw to a distal end of the extrusion screw, said extrusion screw extending along a longitudinal axis, - an extrusion head disposed at a distal end of the extrusion screw at the pumping zone and configured to apply the composition around an elongated electrically conductive element, wherein at least one liquid injection channel is formed in the extrusion screw, said at least one injection channel opening inside the barrel at at least one outlet orifice formed on an outer surface of the extrusion screw.

[0008] Injecting a liquid through the extrusion screw eliminates the need to pierce the sheath and therefore eliminates the need to damage the surface treatment of the inner surface of the sheath.

[0009] According to one embodiment of the extruder, said extrusion screw is a screw barrier comprising a feed zone at which the polymer is in solid form when the extruder is in operation and at least one intermediate zone at which the polymer is partially solid or in the molten state when the extruder is in operation, said at least one outlet orifice opening at said at least one intermediate zone. Alternatively, said at least one orifice may open at the feed zone or at a zone of the extrusion screw where the polymer is in the molten state.

[0010] According to one embodiment of the extruder, said at least one injection channel is shaped so that the distance separating said at least one outlet orifice from the feed member, in projection on the longitudinal axis, is greater than or equal to 3D, D being the nominal diameter of the extrusion screw.

[0011] According to one embodiment of the extruder, said at least one injection channel comprises at least one inlet orifice formed at a feed end of the extrusion screw opposite the extrusion end, said at least one injection channel extending inside said extrusion screw between said at least one inlet orifice and said at least one outlet orifice.

[0012] It is thus possible to connect a pump to the extrusion screw, in particular to the inlet orifice, to supply said at least one injection channel with liquid.

[0013] According to one embodiment of the extruder, said at least one injection channel comprises at least a first channel portion extending mainly along the longitudinal axis at a central portion of the extrusion screw and at least a second channel portion extending transversely to the longitudinal axis from said at least one first channel portion and said at least one outlet orifice.

[0014] According to one embodiment of the extruder, said at least one injection channel comprises a plurality of outlet orifices distributed around the extrusion screw.

[0015] According to one embodiment of the extruder, the outlet orifices are equally distributed around the extrusion screw.

[0016] According to one embodiment of the extruder, the outlet orifices are positioned in the same plane perpendicular to the longitudinal axis.

[0017] According to one embodiment of the extruder, the plurality of outlet orifices comprises an even number of outlet orifices, said at least one second channel portion being produced by a single bore extending between two diametrically opposite outlet orifices.

[0018] According to one embodiment of the extruder, the extrusion screw comprises an elongated body and at least one thread extending around and along said elongated body, said at least one outlet orifice opening at an outer surface of the elongated body.

[0019] The invention also relates to a method of manufacturing an electric cable comprising at least one elongated electrically conductive element and at least one extruded thermoplastic layer surrounding said elongated electrically conductive element, said method using an extruder as described above, characterized in that it comprises at least the following steps: i) a step of introducing a composition comprising at least one thermoplastic polymer in solid form into a zone of the extrusion screw, called the feed zone, and located at the entrance to the extruder, ii) a step during which the composition resulting from step i) is brought from the feed zone to one or more intermediate zones of the extrusion screw allowing the transport of the composition to the extrusion head located at the outlet of the extruder and the gradual melting of the thermoplastic polymer, iii) a step of applying at the head of the extruder the composition resulting from step ii) around the elongated electrically conductive element, in which at least one liquid is injected between the barrel and the extrusion screw through said at least one injection channel formed in the extrusion screw.

[0020] According to one embodiment of the manufacturing method, said at least one thermoplastic polymer in solid form is chosen from a propylene homopolymer and a propylene copolymer, the liquid being a dielectric liquid. Brief description of the figures

[0021] The description which follows with reference to the appended drawings, given as non-limiting examples, will make it clear what the invention consists of and how it can be implemented. In the appended figures:

[0022] [Fig-1] [Fig.l] schematically represents a partially sectional view of a extruder including in particular an extrusion screw and a barrel.

[0023] [Fig.2] [Fig.2] schematically represents a partially sectional and perspective view of an electric cable obtained with the extruder of [Fig.l].

[0024] [Fig.3] [Fig.3] schematically represents a partially sectional view of an extruder according to the invention in which a liquid injection channel is formed through the extrusion screw to inject a liquid between the barrel and the extrusion screw, the injection channel comprising a first longitudinal channel portion and a second transverse channel portion.

[0025] [Fig.4] [Fig.4] a front view of the extrusion screw of [Fig.3] highlighting the second transverse channel portion of the injection channel. Description of embodiment(s)

[0026] For reasons of clarity, only the elements essential for understanding the invention have been shown schematically in these figures, and this without respecting the scale.

[0027] The inventive concept is described more fully below with reference to the accompanying drawings, in which embodiments of the inventive concept are shown. In the drawings, the size and relative sizes of elements may be exaggerated for clarity. Like numerals refer to like elements throughout the drawings. However, this inventive concept may be embodied in many different forms and should not be construed as being limited to the embodiments set forth herein. Instead, these embodiments are provided so that this description is complete, and communicates the scope of the inventive concept to those skilled in the art.

[0028] A reference throughout the specification to "an embodiment" means that a particular feature, structure, or characteristic described in connection with an embodiment is included in at least one embodiment of the present invention. Thus, the occurrence of the phrase "in an embodiment" at various locations throughout the specification does not necessarily refer to the same embodiment. Furthermore, the particular features, structures, or characteristics may be combined in any suitable manner in one or more embodiments. Furthermore, the term "comprising" does not exclude other elements or steps.

[0029] In [Fig. 1], the device 1 comprises a container 2 which can be fed with granules of a thermoplastic polymer, a feed hopper 4 which can be fed at room temperature with the granules of the thermoplastic polymer contained in the container 2 and an extruder 5 comprising for example a barrel 6, an extrusion screw 7 and an extrusion head 8.

[0030] The sheath 6 forms an internal cavity in which the extrusion screw 7 is arranged. The sheath 6 is supplied with thermoplastic polymer by a supply member, here the supply hopper 4.

[0031] The thermoplastic polymer in solid form may be a crosslinked polyolefin such as crosslinked polyethylene (XLPE), a crosslinked ethylene-propylene or ethylene propylene-diene elastomer. In a particular application, the thermoplastic polymer comprises a propylene-based thermoplastic such as a propylene homopolymer or a propylene copolymer.

[0032] The extrusion screw 7 extends along a longitudinal axis A. The extrusion screw 7 is configured to be moved in rotation about the longitudinal axis A. The extrusion screw 7 comprises at least one thread extending around and along said elongated body 24.

[0033] The extrusion screw 7 comprises an elongated body 24 and at least one thread 26 (see [Fig. 3]). The extrusion screw 7 makes it possible to generate pressure at the end of the extrusion screw 7, the transport of the thermoplastic polymer and, optionally, its mixing to the extrusion head 8 where the composition formed by the molten thermoplastic polymer is applied around an elongated electrically conductive element.

[0034] The extruder 5 may be a single screw extruder as illustrated in [Fig.l].

[0035] The extrusion screw 7 is preferably a barrier screw or barrier profile screw, i.e. a screw comprising a zone called the "barrier zone". This barrier zone includes in particular a secondary thread with a slightly larger pitch (progressively sweeping the width of the channel) which allows the molten polymer to be separated from the still solid polymer, as if it materialized the boundary between the two phases. It thus allows compression of the solid bed and consequently an improvement in the melting capacity of the screw.

[0036] These barrier screws comprise a feed zone 9 located at a proximal end 20 of the extrusion screw 7 intended to be arranged at the feed of the extrusion screw 7, eg of the feed hopper 4. This feed zone is followed by a barrier or intermediate zone 10 allowing the gradual melting of the polymer and the transport towards the outlet of the extruder ensured by a terminal zone of the screw called the pumping zone 18. This pumping zone can have a length of at least twice the diameter of the extrusion screw. The barrier or intermediate zone 10 extends to the pumping zone of the extrusion screw. The composition leaving the barrier or intermediate zone 10 after passing through the pumping zone 18 is directly transported into the head of the extruder 8 to be applied around the elongated electrically conductive element.The extrusion screw 7 also comprises a distal end 21 disposed opposite the proximal end 20.

[0037] A specific sheath 6 (i.e. grooved sheath) can be used. This makes it possible, in particular in combination with a barrier screw, to obtain a homogeneous composition that is easy to extrude, while avoiding or limiting the formation of structural defects in the thermoplastic layer obtained, in particular of the electrically insulating layer type.

[0038] In [Fig.2], the medium or high voltage power cable 11 obtained according to the method of the invention, comprises a central elongated electrically conductive element 12, in particular made of copper or aluminum, and, successively and coaxially comprises around this element 12, a first semi-conductive layer 13 called "internal semi-conductive layer", an electrically insulating layer 14, a second semi-conductive layer 15 called "external semi-conductive layer", a metal screen 16 of the cylindrical tube type, and an external protective sheath 17.

[0039] Layers 13 and 15 are layers extruded by methods well known to those skilled in the art.

[0040] The presence of the metal screen 16 and the outer protective sheath 17 is preferential, but not essential.

[0041] With reference to Figures 3 and 4, the extrusion screw 7 comprises a liquid injection channel 22 inside the barrel 6. The injection channel 22 is formed in the extrusion screw 7. In particular, the injection channel 22 is formed in the elongated body 24 of the extrusion screw 7.

[0042] The liquid may be a dielectric liquid, a reactive liquid, an areactive liquid, a silane cocktail. The liquid may further comprise antioxidants or even lubricant.

[0043] The injection channel 22 extends inside the extrusion screw 7 between an inlet orifice 28 and an outlet orifice 30.

[0044] The inlet orifice 28 allows the injection channel 22 to be supplied with liquid. The inlet orifice 28 is preferably formed at the proximal end 20 of the extrusion screw 7 so as to facilitate access to the injection channel 22. Thus, the inlet orifice 28 is preferably formed in a plane perpendicular to the longitudinal axis A. A device for connection to a liquid supply member can be fixed to the inlet orifice 28.

[0045] A plurality of inlet orifices 28 may be provided, for example to inject a liquid from several distinct sources or to carry out an injection of different liquids. It is also possible to provide a plurality of injection channels 22 so that the extrusion screw 7 comprises a plurality of inlet orifices 28. Alternatively, it is also possible for a single inlet orifice 28 to supply liquid to a plurality of injection channels 22.

[0046] The outlet orifice 30 opens into the interior of the sleeve 6 so as to inject the liquid between the sleeve 6 and the extrusion screw 7. The liquid can thus be injected downstream of the thermoplastic polymer supply zone 9. The sliding phenomenon can thus be avoided.

[0047] The outlet orifice 30 may be formed at a predetermined distance from the feed member or feed hopper 4, in particular along the longitudinal axis A. Thus, the injection channel 22 may be shaped so that the distance separating the outlet orifice 30 from the feed member 4, in projection on the longitudinal axis A, is greater than or equal to 3D. D is the nominal diameter of the extrusion screw. This positioning of the outlet orifice 30 makes it possible to inject the liquid at a sufficient distance to limit, or even avoid, the slip phenomenon.

[0048] The outlet orifice 30 may be arranged in a zone of the extrusion screw 7 at which the thermoplastic polymer is in the solid state, i.e. not yet melted, so as to limit the stresses linked to the pressure of the composition. In this case, the outlet orifice 30 opens at the feed zone 9.

[0049] Alternatively, the outlet orifice 30 may be arranged in an area of ​​the screw extrusion 7 at which the polymer is partially melted and partially solid. In this case, the outlet orifice 30 opens at the barrier or intermediate zone 10.

[0050] Alternatively, the outlet orifice 30 may be arranged in a zone of the extrusion screw 7 at which the polymer is in the molten state. In this case, the outlet orifice 30 opens at the pumping zone 18.

[0051] The different positions of the outlet orifice 30 mentioned above can further be combined by providing a plurality of outlet orifices 30 opening at different positions along the extrusion screw 7.

[0052] The outlet orifice 30 is formed on and opens at an outer surface of the extrusion screw 7. In particular, the outlet orifice 30 is formed on and opens at an outer surface 32 of the elongated body 24.

[0053] The injection channel 22 comprises a first channel portion 34 extending mainly along the longitudinal axis A. This first channel portion 34 preferably extends at a central portion of the extrusion screw 7. The injection channel 22 also comprises a second channel portion 36 extending transversely to the longitudinal axis A between said at least one first channel portion 34 and the outlet orifice 30. This second channel portion 36 thus preferably extends radially relative to a circular section of the extrusion screw 7.

[0054] According to a preferred embodiment, the injection channel 22 comprises a plurality of outlet orifices 32 distributed around the extrusion screw 7. Thus, the same extrusion channel 22 makes it possible to inject a liquid at several injection points so as to obtain better diffusion of the liquid.

[0055] Preferably, the outlet orifices 32 are equally distributed around the extrusion screw 7. In other words, the angle separating two consecutive outlet orifices 32 is identical. For example, the outlet orifices 32 are separated by an angle of 180° when there are two outlet orifices 32, by an angle of 120° when there are three outlet orifices 32 or even by an angle of 90° when there are four outlet orifices 30.

[0056] To allow injection at the same longitudinal position, the outlet orifices 30 are preferably positioned in the same plane perpendicular to the longitudinal axis A. Thus, it is possible to inject the liquid all around the extrusion screw 7.

[0057] To facilitate the manufacture of the injection channel 22, the plurality of outlet orifices 30 preferably comprises an even number of outlet orifices 30. The plurality of outlet orifices 30 can therefore, for example, comprise 2, 4, 6 or 8 outlet orifices 28. Thus, it is possible to produce the second channel portion 36 by a single drilling extending between two diametrically opposite outlet orifices 30. Such a configuration is for example illustrated in Figures 3 and 4.

[0058] When the second channel portion 36 extends between two outlet orifices 30, the first channel portion 34 preferably communicates with the second channel portion 36 at a central portion of this second channel portion 36. The injection channel 22 therefore has mainly the shape of a T, with the first 34 and second 36 channel portions respectively forming the lower and upper bars of this T.

[0059] The extrusion screw 7 may also comprise a cooling channel (not shown) extending inside the elongated body 24. A cooling liquid, for example water, may be circulated inside this cooling channel. This cooling liquid is not intended to be injected into contact with the thermoplastic polymer. Thus, this cooling channel is in a closed loop inside the extrusion screw. In other words, the cooling channel does not comprise an orifice opening inside the barrel 6.

[0060] The invention also provides a method for manufacturing an electrical cable comprising at least one elongated electrically conductive element and at least one extruded thermoplastic layer surrounding said elongated electrically conductive element. This thermoplastic layer is extruded using an extruder 5 as described above.

[0061] During a first step i), a composition comprising at least one thermoplastic polymer in solid form is introduced into a zone of the extrusion screw, called the feed zone, and located at the entrance to the extruder.

[0062] During a second step ii), the composition resulting from step i) is brought from the feed zone to one or more intermediate zones of the extrusion screw 7 allowing the transport of the composition to the extrusion head 8 and the gradual melting of the thermoplastic polymer.

[0063] A third step iii) consists of applying at the head of the extruder 8 the composition resulting from the second step ii) around the elongated electrically conductive element.

[0064] During the process, at least one liquid is injected between the barrel 6 and the extrusion screw 7 through the injection channel 22.

Claims

Claims

1. Extruder (5) for extruding an electrical cable (11) comprising at least one elongate electrically conductive element (12) and at least one extruded thermoplastic layer (14) surrounding said elongate electrically conductive element, said extruder comprising: - a feed member (4) for polymer in solid form, - a sheath (6) fed by the feed member, - an extrusion screw (7) arranged inside the sheath (6) and allowing the progressive melting of the polymer to form an extrusion composition and the transport of this composition along the extrusion screw (7) to a distal end (21) of the extrusion screw (7), said extrusion screw (7) extending along a longitudinal axis (A), the extrusion screw being a barrier screw comprising a barrier zone,- an extrusion head (8) arranged at a distal end (21) of the extrusion screw (7) and configured to apply the composition around an elongated electrically conductive element, in which at least one liquid injection channel (22) is formed in the extrusion screw (7), said at least one injection channel (22) opening inside the sleeve (7) at at least one outlet orifice (30) formed on an outer surface of the extrusion screw (7).,

2. Extruder (5) according to claim 1, wherein said at least one injection channel (22) is shaped so that the distance separating said at least one outlet orifice (30) from the feed member (4), in projection on the longitudinal axis, is greater than or equal to 3D, D being the nominal diameter of the extrusion screw (7).

3. An extruder (5) according to claim 1 or 2, wherein said at least one injection channel (22) comprises at least one inlet orifice (28) formed at a proximal end (20) of the extrusion screw (7) opposite the distal end (21), said at least one injection channel (22) extending inside said extrusion screw (7) between said at least one inlet orifice and said at least one outlet orifice (30).

4. Extruder (5) according to claim 3, wherein said at least one injection channel (22) comprises at least a first channel portion (34) extending mainly along the longitudinal axis (A) at a central portion of the extrusion screw (7) and at least a second channel portion (36) extending transversely to the longitudinal axis (A) longitudinal (A) from said at least one first channel portion (34) and said at least one outlet orifice (30).

5. Extruder (5) according to one of the preceding claims, wherein said at least one injection channel (22) comprises a plurality of outlet orifices (30) distributed around the extrusion screw (7).

6. Extruder (5) according to claim 5, wherein the outlet orifices (30) are equally distributed around the extrusion screw (7).

7. Extruder (5) according to claim 5 or 6, in which the outlet orifices (30) are positioned in the same plane perpendicular to the longitudinal axis (A).

8. Extruder (5) according to any one of the preceding claims in combination with claims 4 and 5, wherein the plurality of outlet orifices (30) comprises an even number of outlet orifices (30), said at least one second channel portion (36) being produced by a single bore extending between two diametrically opposite outlet orifices (30).

9. Extruder (5) according to one of the preceding claims, wherein the extrusion screw (7) comprises an elongated body (24) and at least one thread (26) extending around and along said elongated body (24), said at least one outlet orifice (30) opening at an outer surface of the elongated body (24).

10. A method of manufacturing an electrical cable (11) comprising at least one elongated electrically conductive element (12) and at least one extruded thermoplastic layer (14) surrounding said elongated electrically conductive element, said method using an extruder (5) according to one of the preceding claims, characterized in that it comprises at least the following steps: i) a step of introducing a composition comprising at least one thermoplastic polymer in solid form into a zone of the extrusion screw (7), called the feed zone (9), and located at the inlet of the extruder (5), ii) a step during which the composition resulting from step i) is brought from the feed zone (9) to one or more intermediate zones (10) of the extrusion screw (7) allowing the transport of the composition to the extrusion head (5) located at the outlet of the extruder (5) and the gradual melting of the thermoplastic polymer,iii) a step of applying at the head of the extruder (5) the composition resulting from step ii) around the electrically element, elongated conductor (12), wherein at least one liquid is injected between the barrel and the extrusion screw (7) through said at least one injection channel (22) formed in the extrusion screw (7).

11. The method of claim 10, wherein said at least one thermoplastic polymer in solid form is selected from a propylene homopolymer and a propylene copolymer, the liquid being a dielectric liquid.

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