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

Abstract

The invention relates to an extruder (5) for extruding an electrical cable, said extruder comprising: - an extrusion screw (7) disposed inside a barrel (6) and enabling the progressive melting of the polymer to form an extrusion composition and the transport of this composition along the extrusion screw (7), - an extrusion head (8) disposed at a distal end (21) of the extrusion screw (7) and configured to apply the composition around an elongated electrically conductive element, wherein at least one injection channel (22) for a liquid is formed through the barrel (7), said at least one injection channel (22) comprising at least one outlet orifice (30) opening into a region of the extrusion screw (7) where the thermoplastic polymer is at least partially in the solid state. Figure 3

Description

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

[0001] The present invention relates to an extruder for extruding an electrical cable, particularly a power cable, comprising a thermoplastic layer extruded around at least one elongated electrically conductive element. The invention also provides a method for manufacturing an electrical cable using such an extruder.

[0002] It typically but not exclusively applies to electrical cables intended for the transport of energy, in particular medium voltage power cables (in particular from 6 to 45-60 kV) or high voltage power cables (in particular above 60 kV, and up to 400 kV), whether they are direct current or alternating current, in the fields of aerial, submarine, terrestrial, or aeronautical electricity transport.

[0003] A medium or high voltage power transmission cable generally comprises, from the inside out: - an elongated electrically conductive element, in particular made of copper or aluminium; - an internal semiconducting layer surrounding said elongated electrically conductive element; - an electrically insulating layer surrounding said internal semiconductor layer; - an external semiconductor layer surrounding said insulating layer; and - optionally an electrically insulating protective sheath surrounding said external semiconductor 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 sleeve, for example, inside a hopper communicating with the sleeve. The liquid is thus mixed with the thermoplastic polymer before or at the time of its insertion into the sleeve. However, it has been observed that this method of injecting the liquid can lead to manufacturing defects in the electrically insulating layer, particularly related to a sliding phenomenon. This wall sliding phenomenon, linked to the lubricating effect of the dielectric liquid (well known by the English term "slippage / sliding phenomenon"), can lead to a degradation of the mechanical properties. and / or electrical defects of the thermoplastic layer obtained at the extrusion head (structural defects of the layer), including a variation in the diameter of the cable.

[0005] In addition, the injection of liquid at the hopper level can cause a leak of liquid from the proximal end of the extrusion screw, opposite the extrusion head.

[0006] It was also observed that the mixing of the liquid with the thermoplastic polymer in solid form was not optimal when the liquid is injected at the hopper.

[0007] A solution known from document WO 02 / 47092 A1 consists of providing holes formed through the extruder barrel to inject a liquid between the extrusion screw and the barrel, in contact with the molten thermoplastic polymer. Such a solution has the disadvantage of being difficult to implement because the composition is subjected to high pressure at this point on the extrusion screw.

[0008] There is therefore a need for an extruder configuration that promotes the mixing of the liquid with the thermoplastic polymer while avoiding the risk of liquid leaking back from the extruder and limiting the degradation of the mechanical and / or electrical properties of the thermoplastic layer. Summary of the invention

[0009] To this end, the invention proposes, for the extrusion of an electrical 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 power supply unit in solid form, - a sheath fed by the feeding organ, - an extrusion screw disposed 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, the extrusion screw comprising a feeding zone extending from the feeding member along the longitudinal axis at which the thermoplastic polymer is at least partially in a solid state when the extruder is in operation, - an extrusion head disposed at a distal end of the extrusion screw and configured to apply the composition around an elongated electrically conductive element, in which at least one liquid injection orifice is formed through the sleeve, said at least one injection orifice being formed opposite the feed zone of the extrusion screw.

[0010] The composition is subjected to a much lower pressure in the feed zone than near the distal end of the extrusion screw, close to the extrusion head. Injecting the liquid in the feed zone, where the thermoplastic polymer is still at least partially solid, reduces the stresses that must be considered for liquid injection. This makes injection easier at this point in the extruder.

[0011] Since the injection is carried out through the sleeve and no longer through the hopper, or feeding device, the phenomenon of slippage and the risks of leaks are limited or even eliminated.

[0012] According to one embodiment of the extruder, the distance separating said at least one outlet orifice from the feeding member, projected onto the longitudinal axis, is less than or equal to 8D, D being the nominal diameter of the extrusion screw.

[0013] According to one embodiment of the extruder, the distance separating said at least one outlet orifice from the feeding member, projected onto the longitudinal axis, is greater than or equal to 2D, D being the nominal diameter of the extrusion screw.

[0014] According to one embodiment of the extruder, the extrusion screw comprises: - a feeding zone extending from the feeding member along the longitudinal axis, at which point the thermoplastic polymer is in a solid state when the extruder is in operation, - a first barrier zone extending along the longitudinal axis from the feeding zone towards the extrusion head, at which point the thermoplastic polymer is in a partially solid state when the extruder is in operation, - a second barrier zone extending along the longitudinal axis from the first barrier zone towards the extrusion head, at which the thermoplastic polymer is in a molten state when the extruder is in operation, said having at least one outlet opening opposite the feed zone and / or the first barrier zone of the extrusion screw.

[0015] According to one embodiment of the extruder, said at least injection channel comprises at least one inlet orifice formed on an external surface of the barrel, said at least one outlet orifice opening at the level of an internal surface of the barrel, said at least one injection channel extending through the barrel between said at least one inlet orifice and said at least one outlet orifice.

[0016] According to one embodiment of the extruder, said at least one injection channel extends perpendicularly to the longitudinal axis.

[0017] According to one embodiment of the extruder, the latter comprises a plurality of injection channels formed through the barrel and distributed around the longitudinal axis.

[0018] According to one embodiment of the extruder, the injection ports are positioned in the same plane perpendicular to the longitudinal axis.

[0019] According to one embodiment of the extruder, the latter further comprises a liquid supply element communicating with said at least one injection channel.

[0020] The invention further proposes 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, said method employing 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 an area of ​​the extrusion screw, called the feed area, located at the inlet of the extruder, ii) a step in which the composition from step i) is brought from the feeding zone to one or more intermediate zones of the extrusion screw allowing the transport of the composition to the extrusion head located at the exit of the extruder and the gradual melting of the thermoplastic polymer, iii) an application step at the extruder head of the composition from step ii) around the elongated electrically conductive element, in which at least one liquid is injected, at the level of the feed zone between the barrel and the extrusion screw, by said at least one injection channel formed through the barrel.

[0021] According to one embodiment of the process, 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

[0022] The following description, with reference to the accompanying drawings, given by way of non-limiting examples, will clearly explain what the invention consists of and how it can be implemented. In the accompanying figures:

[0023] Fig. 1 schematically represents a partially cross-sectional view of an extruder including in particular an extrusion screw and a barrel.

[0024] [Fig.2] Fig.2 schematically represents a partially cross-sectional and perspective view of an electrical cable obtained with the extruder of Fig.1.

[0025] [Fig. 3] Figure 3 schematically represents a partially cross-sectional view of an extruder according to the invention in which a liquid injection orifice is formed through the sleeve to inject a liquid between the sleeve and the extrusion screw at the level of a feeding zone of the extrusion screw.

[0026] [Fig.4] Fig.4 is a front view of the extruder of Fig.3 highlighting a plurality of injection ports. Description of method(s) of implementation

[0027] For reasons of clarity, only the essential elements for understanding the invention have been represented schematically in these figures, without regard to scale.

[0028] The concept of the invention is described more fully below with reference to the accompanying drawings, in which embodiments of the concept of the invention are shown. In the drawings, the size and relative sizes of the elements may be exaggerated for clarity. Similar numbers refer to similar elements in all the drawings. However, this concept of the invention can be implemented in many different forms and should not be interpreted as being limited to the embodiments set forth herein. Rather, these embodiments are offered so as to make this description complete and to communicate the scope of the concept of the invention to those skilled in the art.

[0029] A reference throughout the specification to "an embodiment" means that a particular feature, structure, or characteristic described in relation to an embodiment is included in at least one embodiment of the present invention. Thus, the appearance of the phrase "in an embodiment" in various places 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. Moreover, the term "including" does not exclude other elements or steps.

[0030] In [Fig. 1], the device 1 includes 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.

[0031] The sleeve 6 forms an internal cavity in which the extrusion screw 7 is disposed. The sleeve 6 is supplied with thermoplastic polymer by a feeding member, here the feeding hopper 4.

[0032] 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. Depending on a particular preferred application, the polymer thermoplastic includes a propylene-based thermoplastic such as a propylene homopolymer or a propylene copolymer.

[0033] 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.

[0034] The extrusion screw 7 comprises an elongated body 24 and at least one thread 26 (see [Fig. 3]). The extrusion screw 7 allows for an increase in the pressure of the thermoplastic polymer, the transport of the thermoplastic polymer and, optionally, its mixing up to the extrusion head 8 where the composition formed by the molten thermoplastic polymer is applied around an elongated electrically conductive element.

[0035] The extruder 5 can be a single-screw extruder as illustrated in [Fig. 1] or a twin-screw extruder. Thus, the extruder 5 comprises at least one extrusion screw 7.

[0036] The extrusion screw 7 is preferably a barrier screw or screw with a barrier profile, 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 separates the molten polymer from the still-solid polymer, as if it were the boundary between the two phases. It thus allows compression of the solid bed and consequently improves the melting capacity of the screw.

[0037] These barrier screws include a feed zone 9 located at a proximal end 20 of the extrusion screw 7, intended to be positioned at the feed point of the extrusion screw 7, e.g., the feed hopper 4. The thermoplastic polymer is in a solid state at this feed zone 9 when the extruder 5 is in operation. The extrusion screw 7 also includes a distal end 21 positioned opposite the proximal end 20.

[0038] This feed zone 9 is followed by a barrier or intermediate zone 10 allowing the gradual melting of the polymer and the transport towards the exit of the extruder ensured by a terminal zone of the screw called the pumping zone 18.

[0039] The barrier or intermediate zone 10 extends to the pumping zone 18 of the extrusion screw 7. The barrier or intermediate zone 10 comprises a first barrier zone 40 and a second barrier zone 42.

[0040] The first barrier zone 40 extends along the longitudinal axis A from the feed zone 9 to the extrusion head 8. The thermoplastic polymer is in a partially solid state at the level of this first barrier zone 40 when the extruder 5 is in operation.

[0041] The second barrier zone 42 extends along the longitudinal axis A from the first barrier zone 40 towards the extrusion head 8 or the pumping zone 18. The thermoplastic polymer is in a molten state at the level of this second barrier zone 42 when the extruder 5 is in operation.

[0042] The pumping zone 18 can have a length of up to twice the diameter of the extrusion screw. The composition exiting the barrier or intermediate zone 10 after passing through the pumping zone 18 is directly transported into the extruder head 8 to be applied around the elongated electrically conductive element.

[0043] A specific sleeve 6 (i.e., a grooved sleeve) can be used. This makes it possible, particularly 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 resulting thermoplastic layer, in particular of the electrically insulating layer type.

[0044] In [Fig.2], the medium or high voltage power cable 11 obtained according to the process of the invention comprises an elongated central electrically conductive element 12, in particular made of copper or aluminum, and, successively and coaxially, comprises around this element 12, a first semiconducting layer 13 called the "internal semiconducting layer", an electrically insulating layer 14, a second semiconducting layer 15 called the "external semiconducting layer", a metallic screen 16 of the cylindrical tube type, and an outer protective sheath 17.

[0045] Layers 13 and 15 are layers extruded by processes well known to those skilled in the art.

[0046] The presence of the metal screen 16 and the outer protective sheath 17 is preferential, but not essential.

[0047] With reference to Figures 3 and 4, a liquid injection channel 22 is formed through the sleeve 6 to inject liquid between the sleeve 6 and the extrusion screw 7. In particular, the injection channel 22 is formed at an inner surface 23 of the sleeve 6 facing the extrusion screw 7. The inner surface 23 of the sleeve defines a cavity 25 in which the extrusion screw is disposed. Several injection channels 22 can be formed through the sleeve 6.

[0048] The liquid can be a dielectric liquid.

[0049] The extruder 5 may include a liquid supply element (not shown) communicating with the injection channel 22.

[0050] The injection channel 22 is preferably located at the level of an upper portion of the sheath 6.

[0051] The injection channel 22 extends inside the sleeve 6 between an inlet orifice 28 and an outlet orifice 30.

[0052] The inlet port 28 allows the injection channel 22 to be supplied with liquid. The inlet port 28 is formed on an outer surface 27 of the sleeve 6. A connection device for the liquid supply member can be attached to the inlet port 28.

[0053] A plurality of inlet ports 28 may be provided, for example, to inject a liquid from several separate sources or to inject different liquids. It is also possible to provide a plurality of injection channels 22 so that the sleeve 6 has a plurality of inlet ports 28.

[0054] The outlet orifice 30 opens into the inside of the sleeve 6 so as to inject the liquid between the sleeve 6 and the extrusion screw 7. In particular, the outlet orifice 30 opens at the level of the inner surface 23 of the sleeve 6 facing the extrusion screw 7.

[0055] The outlet orifice 30 can 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 can be shaped so that the distance separating the outlet orifice 30 from the feed member 4, projected onto the longitudinal axis A, is greater than or equal to 3D, preferably greater than or equal to 4D. D is the nominal diameter of the extrusion screw. This positioning of the outlet orifice 30 allows the liquid to be injected at a sufficient distance to limit, or even avoid, the slippage phenomenon.

[0056] The injection channel 22 can also be shaped so that the distance separating the outlet orifice 30 from the feed member 4, projected onto the longitudinal axis A, is less than or equal to 8D, preferably less than or equal to 7D, and even more preferably less than or equal to 6D. Thus, the outlet orifice 30 is opposite a region of the extrusion screw 7 where the thermoplastic polymer is at least partially solid. Beyond this distance of 8D, the thermoplastic polymer is in a molten state.

[0057] The outlet orifice 30 is located in a region of the extrusion screw 7 where the thermoplastic polymer is at least partially solid, i.e., not yet completely melted, so as to limit the pressure stresses of the composition. Thus, the liquid is injected into the thermoplastic polymer either in a solid state, as granules, or in a partially solid state. For this purpose, the outlet orifice 30 preferably opens into the feed zone 9 or the first barrier zone 40.

[0058] The different positions of the outlet orifices 30 mentioned above can also be combined by providing a plurality of outlet orifices 30

[0059] According to a preferred embodiment, the injection channel 22 comprises a plurality of outlet ports 32 distributed around the extrusion screw 7. Thus, a single channel The extrusion 22 allows liquid to be injected at several injection points to obtain better liquid diffusion. The outlet ports 30 can open at different positions along the longitudinal axis A.

[0060] 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 or at least at several injection points to obtain better distribution of the liquid in the composition. The injection points are, for example, spaced 1D apart along the longitudinal axis A.

[0061] To facilitate the manufacture of the injection channel 22, the injection channel 22 extends along a straight line between the inner 23 and outer 27 surfaces of the sleeve 6. Thus, it is possible to make the injection channel 22 by a single drilling extending between the inner 23 and outer 27 surfaces. Preferably, the injection channel 22 extends perpendicularly to the longitudinal axis A, i.e. radially with respect to this longitudinal axis A.

[0062] Figure 4 shows a cross-sectional view to the longitudinal axis A showing two injection channels 22 opening at the level of an upper portion of the sleeve 6. Each injection channel 22 is connected to a liquid supply conduit 46 communicating with a liquid supply element.

[0063] The extrusion screw 7 may also include a cooling channel 44 extending inside the elongated body 24. A coolant, for example water, may be circulated inside this cooling channel. This coolant is not intended to be injected into contact with the thermoplastic polymer. Thus, this cooling channel 44 is in a closed loop inside the extrusion screw 7. In other words, the cooling channel 44 does not include an opening into the barrel 6.

[0064] 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.

[0065] In a first step i), a composition comprising at least one thermoplastic polymer in solid form is introduced into an area of ​​the extrusion screw, called the feed area, and located at the inlet of the extruder.

[0066] In a second step ii), the composition from step i) is brought from the feeding 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.

[0067] A third step iii) consists of applying the composition from the second step ii) around the elongated electrically conductive element at the extruder head 8.

[0068] During the process, at least one liquid is injected between the barrel 6 and the extrusion screw 7 through the injection channel 22.

Claims

Demands

1. An extruder (5) for extruding 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 extruder comprising: - a polymer feed member (4) in solid form, - a barrel (6) fed by the feed member, - an extrusion screw (7) disposed inside the barrel (6) and enabling 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) disposed at a distal end (21) of the extrusion screw (7) and configured to apply the composition around an electrically conductive element lying down,in which at least one injection channel (22) for a liquid is formed through the barrel (7), said at least one injection channel (22) comprising at least one outlet orifice (30) opening into a region of the extrusion screw (7) at which the thermoplastic polymer is at least partially in the solid state, in which, D being the nominal diameter of the extrusion screw (7), the distance separating said at least one outlet orifice (30) from the feed member (4), projected onto the longitudinal axis (A), is less than or equal to 8D, and / or in which the distance separating said at least one outlet orifice (30) from the feed member (4), projected onto the longitudinal axis (A), is greater than or equal to 2D.

2. Extruder (5) according to claim 1, wherein the extrusion screw (7) comprises: - a feed zone (9) extending from the feed member (4) along the longitudinal axis (A), at which point the thermoplastic polymer is in a solid state when the extruder is in operation, - a first barrier zone (40) extending along the longitudinal axis (A) from the feed zone (9) towards the head extrusion (8), at which the thermoplastic polymer is in a partially solid state when the extruder is in operation, - a second barrier zone (42) extending along the longitudinal axis (A) from the first barrier zone (40) towards the extrusion head (8), at which the thermoplastic polymer is in a molten state when the extruder is in operation, said at least one outlet orifice (30) opening opposite the feed zone (9) and / or the first barrier zone (40) of the extrusion screw (7).

3. Extruder (5) according to any one of claims 1 to 2, wherein said at least injection channel (22) comprises at least one inlet orifice (28) formed on an outer surface of the barrel (6), said at least one outlet orifice (30) opening at the level of an inner surface of the barrel (6), said at least one injection channel (22) extending through the barrel (6) between said at least one inlet orifice (28) and said at least one outlet orifice (30).

4. Extruder according to any one of the preceding claims, wherein said at least one injection channel (22) extends perpendicularly to the longitudinal axis (A).

5. Extruder (5) according to any one of the preceding claims, comprising a plurality of injection channels (22) formed through the barrel (6) and distributed around the longitudinal axis (A).

6. Extruder (5) according to claim 5, wherein the injection ports (22) are positioned in at least one plane perpendicular to the longitudinal axis (A).

7. Extruder (5) according to any one of the preceding claims, further comprising a liquid feed member communicating with said at least one injection channel (22).

8. A method for 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 employing an extruder (5) according to any 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

9. extrusion screw (7), referred to as the feed zone (9), and located at the inlet of the extruder (5), ii) a step in which the composition 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) an application step at the level of the extruder head (5) of the composition from step ii) around the elongated electrically conductive element (12), in which at least one liquid is injected between the barrel and the extrusion screw (7) through said at least one injection channel (22) formed through the barrel (6), at the level of a zone of the extrusion screw where the thermoplastic polymer is in at least partially solid form. A method according to claim 8, 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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