Extrusion screw, extruder and method for manufacturing an electrical cable by extruding a propylene polymer-based composition with an improved flow extrusion screw

Abstract

The invention relates to an extrusion screw for an extruder for extruding at least one extruded thermoplastic layer surrounding an elongated electrically conductive element of an electrical cable, the extrusion screw having an outside diameter denoted D and comprising a screw body (24) extending along a longitudinal axis (A), the extrusion screw further comprising along this longitudinal axis (A): - a feeding zone (30) for at least one thermoplastic polymer in solid form, the feeding zone comprising a feeding thread (50) extending around the screw body, - a compression zone (32) for the extrusion composition comprising a compression thread (52) extending around the screw body, - a barrier zone (34) comprising a primary thread and a secondary thread for the gradual melting of the polymer into an extrusion composition, the distance separating the primary and secondary threads varying along the longitudinal axis (A),in which the feed (30) and compression (32) zones respectively comprise a single feed thread and a single compression thread, each of the feed and compression threads having a thread depth (Pa, Pc) taken perpendicular to the longitudinal axis (A) between a thread end and the screw body (24) between 0.11D and 0.16D. Figure for the abbreviation: Figure 3,

Description

Title of the invention: Extrusion screw, extruder and method for manufacturing an electrical cable by extruding a propylene polymer-based composition with an improved flow rate extrusion screw. Technical field

[0001] The present invention relates to an extrusion screw, an extruder comprising such an extrusion screw and a method for manufacturing an electrical cable, in particular of the power cable type, using such an extruder.

[0002] This electrical cable comprises an extruded thermoplastic layer obtained from at least one thermoplastic polymer selected from a homopolymer and a propylene copolymer. The thermoplastic layer may also be obtained by means of an extrusion composition comprising said at least one thermoplastic polymer and at least one dielectric liquid. Technological background

[0003] This invention typically but not exclusively applies to electrical cables intended for the transport of energy, in particular to 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.

[0004] 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 inner semiconductor layer; - an outer semiconductor layer surrounding said insulating layer; and - possibly an electrically insulating protective sheath surrounding said external semiconductor layer.

[0005] For extruding the insulating layer of an electrical cable, it is common to use an extrusion screw of the barrier screw or barrier profile screw type, i.e., a screw comprising a zone called the "barrier zone". This barrier zone includes, in particular, a primary thread and a secondary thread with a slightly larger pitch (sweeping). The channel width gradually increases, separating the molten polymer from the still-solid polymer, as if it were the boundary between the two phases. This barrier zone compresses the solid bed, thereby improving the melting capacity of the extrusion screw.

[0006] These barrier screws comprise a feed zone located at a proximal end of the extrusion screw, intended to be positioned at the extrusion screw's feed point, e.g., a hopper. This feed zone is followed by a compression zone and a barrier zone, allowing the polymer to gradually melt and be transported to the extruder outlet for application around the elongated electrically conductive element.

[0007] The feed zone generally has a constant thread depth over all or part of its length. The compression zone is characterized by a decreasing thread depth over all or part of its length to compress the extrusion composition. The boundary between the feed and compression zones thus generally corresponds to the section of the extrusion screw at which the thread depth begins to decrease.

[0008] To obtain improved extrusion rates, it is known to provide the feed zone of the extrusion screw with at least two threads angularly offset by 180° around the extrusion screw so as to increase polymer compression. It is also known to locally increase the diameter of the screw body at the feed zone, in the form of a bulb, to further compress the material and thus increase the extrusion rate. This extrusion screw profile is generally coupled with a sleeve surrounding the extrusion screw, which has a plurality of helical grooves at this bulb.

[0009] It is also common for the thread depth, taken perpendicular to the longitudinal axis of the extrusion screw between the screw body and the end of the thread, at the level of the feed zone, to be equal to or less than 0.10D, i.e. 8mm for an extrusion screw with a diameter of 80mm.

[0010] A major drawback of this type of extrusion screw is that, despite the specific attributes given to the extrusion screw, the extrusion flow rate is insufficient. Therefore, it is generally necessary to invest in a new extruder with a larger diameter to increase the extrusion flow rate, which is very expensive.

[0011] Moreover, when the extrusion composition incorporates a dielectric liquid, the latter tends to be rejected towards the proximal end of the extrusion screw, which impairs the quality of the extrusion composition.

[0012] There is therefore a need for an improved extrusion screw allowing an increase in the extrusion rate for the same barrel, particularly when the extrusion composition includes a thermoplastic polymer having one of a homopolymer and a propylene copolymer. There is also a need for an extrusion screw that reduces, or even eliminates, the rejection of the dielectric fluid; the extrusion composition incorporates such a dielectric fluid. Summary of the invention

[0013] To this end, the invention proposes an extrusion screw for an extruder for extruding at least one extruded thermoplastic layer surrounding an elongated electrically conductive element of an electrical cable, the extrusion screw having an outside diameter, denoted D, and comprising a screw body extending along a longitudinal axis, the extrusion screw further comprising along this longitudinal axis: - a feeding zone for at least one thermoplastic polymer in solid form, the feeding zone comprising a feeding thread extending around the screw body, - a compression zone for the extrusion composition comprising a compression thread extending around the screw body, - a barrier zone comprising a primary thread and a secondary thread for the gradual melting of the polymer into an extrusion composition, the distance separating the primary and secondary threads varying along the longitudinal axis,in which the feed and compression zones comprise respectively a single feed thread and a single compression thread, each of the feed and compression threads having a thread depth taken perpendicular to the longitudinal axis between a thread end and the screw body between 0.11D and 0.16D.

[0014] Such an extrusion screw having feed and compression zones with a single thread having a thread depth between 0.11D and 0.16D presents an improved profile allowing a significant increase in extrusion flow rate.

[0015] It is important to remember that extrusion screws generally have a standard diameter, which can vary from 45 to 175 mm. For example, this diameter could be 80 mm, 120 mm, or 150 mm. This diameter D corresponds to the outer diameter of the extrusion screw, as well as the inner diameter of the associated sleeve. Thus, when the thread depth of an extrusion screw is increased, the diameter of the screw body is correspondingly decreased by the same amount. Therefore, an increase in the thread depth reduces the diameter of the screw body and thus its mechanical strength. Even small variations in thread depth can lead to very significant changes in the mechanical characteristics of an extrusion screw.

[0016] Here, it has been observed by the inventor of this invention that the combination of a single thread at the level of the feeding and compression zones with an increase in the thread depth between 0.11D and 0.16D allows a significant increase in the extrusion flow rate.

[0017] Furthermore, such a profile of the feed and compression zones prevents any rejection of the dielectric fluid when it is included in the extrusion composition. It has been observed that reducing the compression force of the composition in the feed and compression zones allows the dielectric fluid to advance along the extrusion screw without being rejected towards the rear of the screw. Indeed, once the dielectric fluid is sufficiently far from the feed zone, the thermoplastic polymer feeding the extruder forms a barrier to the dielectric fluid, preventing it from flowing back towards the rear of the extrusion screw.

[0018] By the fact that each of the feed and compression threads has a thread depth measured perpendicular to the longitudinal axis between a thread end and the screw body between 0.11D and 0.16D, it is understood that each of these feed and compression threads has at least a local thread depth within this range. One or both of the feed and compression threads may thus locally have a value outside this range without departing from the scope of the invention.

[0019] According to one embodiment, the feed and compression threads have a thread depth between 0.12D and 0.14D, preferably between 0.13D and 0.14D.

[0020] According to one embodiment, the compression zone comprises a proximal compression portion disposed at the level of the supply zone and a distal compression portion disposed at the level of the barrier zone, the compression net having a variable net depth P between the first and second compression portions.

[0021] According to one embodiment, the compression net has a first net depth at the proximal compression portion, equal to the net depth of the feeding zone, and a second net depth at the distal compression portion, the second net depth being less than the first net depth. This reduction in net depth along the compression zone generates compression of the material since the internal diameter of the sheath is constant over this portion. Thus, the space available for the material is reduced and induces said compression.

[0022] According to one embodiment, the thread depth of the compression zone varies linearly between the first and second thread depths.

[0023] According to one embodiment, the compression zone has a length along the longitudinal axis of between 6D and 11D.

[0024] According to one embodiment, the extrusion screw further comprises a dispersive mixing zone after the barrier zone.

[0025] According to one embodiment, the dispersive mixing zone defines at least one inlet channel of the extrusion composition into the dispersive mixing zone and at least one outlet channel of the extrusion composition from the dispersive mixing zone, said at least one inlet channel being separated from said at least one outlet channel by a passage thread configured to permit the passage of the extrusion composition from an inlet channel to an outlet channel and to shear said composition during said passage.

[0026] The invention also provides an extruder for extruding 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 thermoplastic polymer feed element in solid form, - a sheath fed by the feed element, - an extrusion screw as described above, disposed inside the sleeve and allowing the progressive melting of the polymer to form an extrusion composition and the transport of this extrusion composition along the extrusion screw to a distal end of the extrusion screw, - an extrusion head disposed at the distal end of the extrusion screw and configured to apply the composition around an elongated electrically conductive element.

[0027] According to one embodiment, the sleeve has a feed opening formed opposite the feed zone of the extrusion screw, the sleeve further comprising, at the level of an inner wall and following the feed opening, a plurality of grooves extending along the longitudinal axis over a distance between 0.5D and 1.5D, preferably between 1D and 1.5D.

[0028] The grooves serve to increase friction between the inner wall of the barrel and the elements of the extrusion composition. This increased friction helps to improve the extrusion flow rate.

[0029] According to one embodiment, the grooves are straight along the longitudinal axis.

[0030] According to one embodiment, the grooves have a depth of 0.5 to 2mm, preferably 1mm, along an axis radial to the longitudinal axis.

[0031] According to one embodiment, the extrusion screw includes a pumping zone between the barrier zone and the distal end of the extrusion screw. This zone of The pumping function is to stabilize the extrusion composition, particularly after it passes through the dispersive mixing zone when present.

[0032] According to one embodiment, said pumping zone comprises a pumping thread extending around the screw body, the pumping thread having a thread depth taken perpendicular to the longitudinal axis between a thread end and the screw body between 0.06D and 0.11D.

[0033] According to one embodiment, the pumping zone net has a net depth between 0.07D and 0.10D, preferably between 0.08D and 0.09D

[0034] According to one embodiment, the extrusion screw has a compression ratio between 1.2 and 2, preferably between 1.5 and 1.7, the compression ratio being determined with the following equation: CR = Pa / Pp, CR being the compression ratio, Pa being the thread depth of the feed zone and Pp being the thread depth of the pumping zone.

[0035] The compression ratio is preferably between 1.6 and 1.7.

[0036] A compression ratio between 1.5 and 1.7 makes it possible to obtain a very high extrusion flow rate Improved for the extrusion of a composition comprising a thermoplastic polymer in solid form selected from a propylene homopolymer and a propylene copolymer. Thus, such an extrusion screw is particularly well suited for the extrusion of compositions comprising one or more of a propylene homopolymer and a propylene copolymer.

[0037] Generally speaking, the extrusion screw is preferably such that: - the feeding zone has a length along the longitudinal axis between 1D and 4D, preferably between 2D and 3D, and even more preferably 2D; and / or - the compression zone has a length along the longitudinal axis between 6D and 11D, preferably between 8D and 10D; and / or - the barrier zone has a length along the longitudinal axis between 6D and 11D, preferably between 8D and 10D; and / or - the dispersive mixing zone has a length along the longitudinal axis between 2D and 5D, preferably between 3D and 4D; and / or - the pumping zone has a length along the longitudinal axis between 0.5D and 3D, preferably between 1D and 2D.

[0038] By way of preferred example, an extrusion screw of length 24D has a feeding zone of length 2D, a compression zone of 8D, a barrier zone of length 8D, a dispersive mixing zone of 3D and a pumping zone of length 1D.

[0039] By way of preferred example, an extrusion screw with a length of 30D has a feed zone with a length of 2D, a compression zone of 10D, a barrier zone of length 10D, a dispersive mixing zone of 4D and a pumping zone of length 2D.

[0040] The term "extrusion screw length" refers to the functional length of the extrusion screw between the beginning of the feed zone and the end of the pumping zone, i.e., the distal end of the extrusion screw. This length corresponds to the length of the extrusion screw intended to be in contact with the extrusion composition.

[0041] 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 at least one thermoplastic polymer in solid form selected from a propylene homopolymer and a propylene copolymer, into the feed zone of the extrusion screw, (ii) a step in which said at least one thermoplastic polymer is brought from the feed zone to the compression zone and the barrier zone allowing the gradual melting of the thermoplastic polymer into an extrusion composition and the transport of the extrusion composition to the extrusion head located at the exit of the extruder, and iii) an application step at the extrusion head of the extrusion composition from step ii) around the elongated electrically conductive element.

[0042] According to one embodiment, the introduction step involves introducing at least one dielectric liquid into the feed zone of the extrusion screw.

[0043] According to one embodiment, the process further includes upstream of the application step a dispersive mixing step of the extrusion composition by means of the dispersive mixing zone of the extrusion screw.

[0044] In the present invention, the expression "ambient temperature" means a temperature ranging from approximately 15 to 30°C, and preferably ranging from approximately 20 to 25°C.

[0045] According to one embodiment of the invention, the extruder implementing the process of the invention is a single-screw extruder. It therefore comprises a single screw.

[0046] The propylene copolymer of the composition of step i) can be a heterophase propylene copolymer, a statistical propylene copolymer or a mixture thereof.

[0047] The heterophase propylene copolymer generally comprises a propylene-type thermoplastic phase and an elastomer phase of the ethylene-an olefin a1 copolymer type.

[0048] The elastomeric phase of the heterophase copolymer can represent at least 20% by mass approximately, and preferably at least 45% by mass, relative to the total mass of the heterophase copolymer.

[0049] The olefin a1 of the elastomer phase of the heterophase copolymer can be propylene.

[0050] As an example of this type of copolymer, we can mention the heterophase copolymer marketed by Basell Polyolefins under the reference Adflex® Q 200 F.

[0051] The statistical propylene copolymer can be a propylene and olefin copolymer, the olefin being in particular chosen from ethylene and an α2 olefin different from propylene.

[0052] The olefin a2 other than propylene may correspond to the formula CH2=CH RI, in which RI is a linear or branched alkyl group having from 2 to 10 carbon atoms, in particular selected from the following olefins: 1-butene, 1-pentene; 4-methyl-1-pentene, 1-hexene, 1-octene, 1-decene, 1-dodecene, and a mixture thereof.

[0053] The olefin a2 preferably represents at most about 15% by mole and even more preferably at most about 10% by mole of the copolymer.

[0054] A propylene and ethylene copolymer is preferred as a statistical propylene copolymer.

[0055] As an example, the statistical propylene copolymer marketed by Borealis under the reference Bormed® RB 845 MO is particularly preferred.

[0056] The statistical propylene copolymers usable according to the invention preferably have an elastic modulus ranging from approximately 600 to 1200 MPa.

[0057] The propylene homopolymers usable according to the invention preferably have an elastic modulus ranging from approximately 1250 to 1600 MPa.

[0058] The homopolymer (respectively the statistical copolymer of propylene) can have a melting temperature above about 130°C, preferably above about 140°C, and preferably still ranging from about 140 to 165°C.

[0059] The homopolymer (respectively the statistical copolymer of propylene) can have an enthalpy of fusion ranging from approximately 30 to 100 J / g.

[0060] The homopolymer (respectively the statistical copolymer of propylene) can have a melt flow index ranging from approximately 0.5 to 3 g / 10 min, measured at approximately 230°C with a charge of approximately 2.16 kg according to ASTM D1238-00.

[0061] The composition of step i) may further include polyethylene in solid form.

[0062] Said dielectric liquid represents an amount less than 15% by mass, relative to the total mass of the composition of step i).

[0063] The polyethylene is preferably a high-density polyethylene, a medium-density polyethylene or a linear low-density polyethylene.

[0064] A polyethylene called “high density” or HDPE according to ISO 1183A (at a temperature of 23 °C) has a density varying from 0.930 to 0.970 g / cm3 approximately, and even more preferably from 0.940 to 0.965 g / cm3 approximately.

[0065] A “medium density” polyethylene or MDPE according to the standard according to ISO 1183A (at a temperature of 23°C) has a density ranging from approximately 0.925 to 0.930 g / cm3.

[0066] A linear polyethylene referred to as "low-density" or LLDPE according to ISO standard 1183A (at a temperature of 23°C) has a density ranging from approximately 0.91 to 0.925 g / cm³.

[0067] The propylene homopolymer may represent approximately 40% to 90% by mass, and preferably 40 to 70% by mass, approximately, relative to the total mass of the composition of step i).

[0068] The statistical propylene copolymer can represent from about 40% to 90% by mass, and preferably from about 40% to 70% by mass, relative to the total mass of the composition of step i).

[0069] The heterophase propylene copolymer can represent from 5% to 60% by mass, and preferably from about 5% to 50% by mass, relative to the total mass of the composition of step i).

[0070] Polyethylene can represent from 20% to 60% by mass approximately, and preferably from 20% to 50% by mass, relative to the total mass of the composition of step i).

[0071] According to a preferred embodiment of the invention, the composition comprises, as polymers: - a random copolymer of propylene (e.g., approximately 50-70% by mass, relative to the total mass of the composition), a heterophase copolymer of propylene (e.g., approximately 5-30% by mass, relative to the total mass of the composition) and linear low-density polyethylene (e.g., approximately 20-40% by mass, relative to the total mass of the composition), or - a heterophase copolymer of propylene (e.g., approximately 35-55% by mass, relative to the total mass of the composition) and high-density polyethylene (e.g., approximately 35-55% by mass, relative to the total mass of the composition).

[0072] Such combinations of polymers in association with the dielectric liquid make it possible to obtain a thermoplastic layer, in particular of the electrically insulating layer type, exhibiting good mechanical properties, in particular in terms of elastic modulus, and electrical properties.

[0073] Since the polymers of the composition are in solid form, they can be in the form of pellets or granules.

[0074] The composition of step i) may further include one or more additives.

[0075] The additives are well known to those skilled in the art and can be chosen from antioxidants, anti-UV agents, flame retardants, colorants, anti-copper agents, anti-water treeing agents and mixtures thereof.

[0076] The composition can typically comprise from 0.01 to about 5% by mass, and preferably from 0.1 to about 2% by mass of additives, relative to the total mass of the composition of step i).

[0077] More specifically, antioxidants help to protect the composition from thermal stresses generated during the cable manufacturing or cable operation stages.

[0078] The antioxidants are preferably chosen from among hindered phenols, thioesters, sulfur-based antioxidants, phosphorus-based antioxidants, amine-type antioxidants and a mixture thereof.

[0079] Examples of hindered phenols include pentaerythritol tetrakis(3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate) (Irganox® 1010), octadecyl 3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate (Irganox® 1076), 1,3,5-trimethyl-2,4,6-tris(3,5-di-tert-butyl-4-hydroxybenzyl)benzene (Irganox® 1330), 4,6-bis(octylthiomethyl)-o-cresol (Irgastab® KV10), 2,2'-thiobis(6 tert-butyl-4-methylphenol) (Irganox® 1081), and 2,2'-thiodiethylene bis[3-(3,5-di-tert-butyl-4-hydroxyphenyl) propionate] (Irganox® 1035), 2,2' methylenebis(6-tert-butyl-4-methylphenol), 1,2 bis(3,5-di-tert-butyl-4-hydroxyhydrocinnamoyl) hydrazine (Irganox® MD 1024), and 2,2'-oxamido-bis(ethyl-3(3,5-di-tert-butyl-4-hydroxyphenyl)propionate).

[0080] Examples of thioesters include didodecyl-3,3'-thiodipropionate (Irganox® PS800), distearylthiodipropionate (Irganox® PS802) and 4,6-bis(octylthiomethyl)-o-cresol (Irganox® 1520).

[0081] Examples of sulfur-based antioxidants include dioctadecyl-3,3'-thiodipropionate and didodecyl-3,3'-thiodipropionate.

[0082] Examples of phosphorus-based antioxidants include tris(2,4-di-tert-butyl-phenyl)phosphite (Irgafos® 168) and bis(2,4-di-tert-butylphenyl)pentaerythritoldiphosphite (Ultranox® 626).

[0083] Examples of amine-type antioxidants include phenylenediamines (e.g., 1PPD or 6PPD), diphenylamine styrene, diphenylamines, mercaptobenzimidazoles and polymerized 2,2,4-trimethyl-1,2-dihydroquinoline (TMQ).

[0084] Examples of antioxidant mixtures include Irganox B 225, which comprises an equimolar mixture of Irgafos 168 and Irganox 1010 as described above.

[0085] The dielectric liquid can represent approximately 3 to 10% by mass. Brief description of the figures

[0086] 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:

[0087] [Fig-1] Fig. 1 schematically represents a partially cross-sectional view of a extruder including in particular an extrusion screw and a barrel;

[0088] [Fig.2] The [Fig.2] schematically represents a partially cross-sectional and perspective view of an electrical cable obtained with the extruder of the [Fig.1];

[0089] [Fig.3] Fig.3 represents a side view of an extrusion screw comprising a feed zone, a compression zone, a barrier zone and a dispersive mixing zone;

[0090] [Fig.4] Fig.4 represents a detailed side view of the feed and compression areas of an extrusion screw;

[0091] [Fig.5] The [Fig.5] represents a detailed perspective view of the dispersive mixing zone of the extrusion screw of the [Fig.3];

[0092] [Fig.6] Fig.6 represents a detailed cross-sectional view of an extruder sleeve having a plurality of straight grooves intended to extend along the longitudinal axis of the extrusion screw. Description of method(s) of implementation

[0093] For reasons of clarity, only the essential elements for understanding the invention have been represented schematically in these figures, without regard to scale.

[0094] 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 construed 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.

[0095] A reference throughout the specification to "an embodiment" means that a particular functionality, structure, or feature described in relation to An embodiment is included in at least one other embodiment of the present invention. Therefore, the appearance of the phrase "in an embodiment" in various places throughout the specification does not necessarily refer to the same embodiment. Furthermore, functionalities, structures, or particular features may be combined in any suitable manner in one or more embodiments. Moreover, the term "including" does not exclude other elements or steps.

[0096] In [Fig. 1], the device 1 includes a container 2 that can be fed with granules of a thermoplastic polymer, a feed hopper 4 that can be fed 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.

[0097] 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.

[0098] A container or reservoir 3 can also be put in place to supply the feed hopper 4, or more generally the extruder 5, with dielectric liquid.

[0099] The thermoplastic polymer in solid form preferably comprises a propylene-based thermoplastic such as a propylene homopolymer or a propylene copolymer.

[0100] The extrusion screw 7 extends along a longitudinal axis A. The extrusion screw 7 is configured to be moved in rotation around the longitudinal axis A.

[0101] The extrusion screw 7 comprises a screw body 24 extending along the longitudinal axis A and one or more threads extending around the screw body 24. The extrusion screw 7 extends between a proximal end 20, close to the feeding member, and a distal end 21, close to the extrusion head 8. The extrusion screw 7 allows an increase in the pressure of the thermoplastic polymer to achieve a gradual melting of this thermoplastic polymer, the transport thereof 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.

[0102] With reference to [Fig. 2], the medium or high voltage power cable 11 obtained according to the process 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 semiconducting layer 13 called the "inner semiconducting layer", an electrically insulating layer 14, a second semiconducting layer 15 called the "outer semiconducting layer" external”, a metallic screen 16 of the cylindrical tube type, and an external protective sheath 17.

[0103] The electrically insulating layer 14 is extruded by means of the extruder 5 and the extrusion screw 7.

[0104] Layers 13 and 15 are layers extruded by processes well known to those skilled in the art.

[0105] The presence of the metal screen 16 and the outer protective sheath 17 is preferable, but not essential.

[0106] 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.

[0107] With reference to [Fig.3], the extrusion screw 7 comprises successively along the longitudinal axis A a feeding zone 30, a compression zone 32 and a barrier zone 34.

[0108] The feed zone 30 of the extrusion screw 7 is intended to be aligned with a feed orifice 40 of the sleeve 6 for feeding the extrusion screw 7 and the sleeve 6 with at least one thermoplastic polymer in solid form. The feed orifice 40 has an opening along the longitudinal axis A, preferably of 1D.

[0109] Preferably, the feed orifice 40 is disposed opposite a proximal portion of the feed zone 30, closer to the proximal end 20 of the extrusion screw 7. The feed zone 30 also includes a distal portion disposed between the proximal portion of the feed zone 30 and the compression zone 32.

[0110] The feed zone 30 comprises a single feed thread 50 extending around the screw body 24, along the longitudinal axis A. The term "single feed thread" means that only one thread runs through the feed zone 30.

[0111] The feed zone 30 has a thread depth Pa, taken perpendicular to the longitudinal axis A between one end of the feed thread 50 and the screw body 24.

[0112] The feeding zone 30 has a length along the longitudinal axis A between 1D and 4D, preferably between 2 and 3D.

[0113] The compression zone 32 of the extrusion composition is disposed between the feed zone 30 and the barrier zone 34. The compression zone 32 has the function of compressing the thermoplastic polymer and transporting it towards the barrier zone 34.

[0114] The compression zone 32 comprises a single compression thread 52 extending around the screw body 24, along the longitudinal axis A. A "single compression thread" is understood to mean that only one thread runs through the compression zone 32. The threads supply 50 and compression 52 preferably extend continuously one after the other so that they form a single thread extending along the supply 30 and compression 32 zones.

[0115] The compression zone 32 has a thread depth Pc, taken perpendicular to the longitudinal axis A between one end of the compression thread 52 and the screw body 24.

[0116] The compression zone 32 has a length along the longitudinal axis A between 6D and 11D. The variation in length of the compression zone 32 depends, for example, on the length of the extrusion screw 7 for a given diameter D. By way of example, the compression zone 32 can be 6D to 8D long for an extrusion screw 7 with a total length of 24D or 25D, but only 10D long for an extrusion screw 7 with a total length of 30D. The principle of the invention and its associated advantages are maintained regardless of the total screw length chosen.

[0117] The feed threads 50 and compression threads 52 have, respectively, a thread depth Pa and a thread depth Pc, measured perpendicular to the longitudinal axis A between a thread end and the screw body 24, between 0.11D and 0.16D, preferably between 0.12D and 0.14D, and even more preferably between 0.13D and 0.14D. This thread depth maximizes the extrusion flow rate of the composition while ensuring sufficient mechanical strength of the extrusion screw 7. This balance is even more optimal between 0.13D and 0.14D.

[0118] The thread depth Pa of the feed thread 50 is preferably constant along the feed zone 30. Thus, the diameter of the screw body 24 is also constant along this feed zone 30.

[0119] The mesh depth Pc of the compression mesh 52 is preferably variable along the longitudinal axis A. In particular, the compression zone 32 comprises a proximal compression portion disposed at the level of the feed zone 30 and a distal compression portion disposed at the level of the barrier zone 34. The mesh depth Pc of the compression mesh 52 has a variable mesh depth Pc between the proximal and distal compression portions.

[0120] The net depth Pc of the compression net 52 is preferably decreasing along the longitudinal axis A. In particular, the net depth Pc of the compression net 52 has a net depth Pc decreasing from the proximal compression portion to the distal compression portion.

[0121] The thread depth Pc of the compression thread 52 preferably has a thread depth Pc that is linearly decreasing from the proximal compression portion to the distal compression portion.

[0122] Thus, the compression net 52 has a first net depth Pci at the level of the proximal compression portion and a second net depth Pc2 at the distal compression portion. The second thread depth Pc2 is less than the first thread depth Pci. The first thread depth Pci is preferably equal to the thread depth Pa of the supply thread 50. Therefore, the first thread depth Pci is between 0.11D and 0.16D, preferably between 0.12D and 0.14D, and even more preferably between 0.13D and 0.14D.

[0123] The second thread depth Pc2 is preferably between 0.10D and 0.13D, preferably between 0.11D and 0.12D.

[0124] The diameter of the screw body 24 therefore preferably decreases along the longitudinal axis A between the proximal and distal compression portions. This reduction in the thread depth Pc allows the extrusion composition to be compressed.

[0125] The barrier zone 34 is arranged between the compression zone 32 and the distal end 21 of the extrusion screw 7. The barrier zone 34 comprises a primary thread 54 and a secondary thread 56 for the gradual melting of the polymer into an extrusion composition. The secondary thread 56 has a larger pitch than the primary thread 54 to allow separation of the molten polymer from the still-solid polymer. The secondary thread 56 thus sweeps across the width of the channel formed by the space between two turns of the primary thread 54, as if it were the boundary between the two phases. The barrier zone 34 allows compression of the solid bed and consequently improves the melting capacity of the extrusion screw 7.

[0126] The dimensions of the secondary thread 56, in particular the thread depth taken perpendicular to the longitudinal axis A, allow only the polymer in molten form to pass through the secondary thread 56.

[0127] The extrusion screw 7 may further include a dispersive mixing zone 36 after the barrier zone 34, in particular between the barrier zone 34 and the distal end 21 of the extrusion screw 7.

[0128] The dispersive mixing zone 36 has the function of generating a significant shear force on the extrusion composition to improve its mixing, particularly when the composition includes a dielectric liquid.

[0129] The dispersive mixing zone 36 defines at least one inlet channel of the extrusion composition into the dispersive mixing zone and at least one outlet channel of the extrusion composition from the dispersive mixing zone. Said at least one inlet channel is separated from said at least one outlet channel by a flow guide configured to allow the passage of the extrusion composition from an inlet channel to an outlet channel and to shear said composition during said passage.

[0130] With reference to [Fig. 5], the dispersive mixing zone 36 comprises a succession of flow-through fillets 67 and scraper fillets 68 defining between them a plurality of inlet channels 70 and outlet channels 73. In particular, the flow-through fillets 67 and scraping threads 68 alternate around the screw body 24 so as to form an alternation of inlet channels 70 and outlet channels 73. Thus, each inlet channel 70 or outlet channel 73 is formed by a through thread 67 and a scraping thread 68. In the example of [Fig.4], the dispersion zone comprises 4 inlet channels 70 and 4 outlet channels.

[0131] The term "screw body" means a portion of the body of the extrusion screw 7. In other words, the screw body 24 is a section of the extrusion screw 7 corresponding to the dispersive mixing zone 36.

[0132] The through threads 67 and scraping threads 68 preferably extend along helical paths around and along the screw body 24. The through and scraping threads 68 are preferably equidistant around this screw body 24.

[0133] Each inlet channel 70 forms an inlet opening 69 at a proximal end 76 of the dispersive mixing zone 76 to allow the composition to enter the inlet channel 70. Each inlet channel 70 includes an obstruction element 74 closing the inlet channel 70 at a distal end 78 of the dispersive mixing zone 36. This obstruction element 74 is preferably configured to allow the passage of the composition while shearing it by a predetermined value. Thus, the composition can exit the inlet channel by passing over the obstruction element 74 but is sheared.

[0134] Each outlet channel 73 is closed by an obstruction element 74 located at the proximal end 76 of the dispersive mixing zone 36 so as to prevent, at least partially, the flow of the composition from the barrier zone towards the outlet channels 73. The flow of the composition is thus facilitated towards the inlet channels 70. Each outlet channel 73 also forms an outlet opening 79 at the distal end 78 of the dispersive mixing zone 36. This obstruction element 74 is preferably configured to allow the passage of the composition while shearing it by a predetermined value. For this purpose, the height of the obstruction element is preferably chosen to generate this shear value. Thus, the composition can enter the outlet channel 73 by passing over the obstruction element 74 but is sheared.

[0135] The passage threads 67 separating adjacent inlet 70 and outlet 73 channels are configured to allow the composition to pass through while shearing it by a predetermined value. To achieve this, the height of the passage threads 67 is chosen to generate this shearing value. Thus, the composition can pass from an inlet 70 to an outlet 73 by passing over a passage thread 67, but will be sheared.

[0136] The scraper nets 68 are configured to prevent the composition from passing between adjacent inlet 70 and outlet 73 channels at the level of this scraper net 68. The composition is thus guided towards a passage net 67 or an element obstruction 74 to be sheared before exiting the dispersive mixing zone 36.

[0137] At the exit of the barrier zone 34, the extrusion composition therefore crosses the dispersive mixing zone 36 by passing through an inlet channel 70 and / or an outlet channel 73 being sheared by a predetermined value either by at least one obstruction element 74 of an outlet channel 73, or by at least one passage thread 67, or by at least one obstruction element 74 of an inlet channel 70.

[0138] The inlet channels 70 and outlet channels 73 extend around and along the screw body 24. The inlet channels 70 and outlet channels 73 preferably extend over at least 75%, preferably at least 95%, of the length of the dispersive mixing zone 36 along a longitudinal axis A of the extrusion screw 7.

[0139] The passage nets 67 and scraping nets 68 preferably have different widths. This width is defined perpendicular to the extension path of the nets. Preferably, a ratio of 2 is defined between the width of a passage net 67 and a scraping net 68.

[0140] According to an example of an embodiment of the dispersive mixing zone 36, each scraper 68 has a width of 0.1D and each passage 67 has a width of 0.2D.

[0141] The pitch of the through threads 67 and scraper threads 68 is, for example, defined as 4*D, where D is the diameter of the screw body 24 at the level of the dispersive mixing zone 36. The pitch is thus 600 for a diameter of 150 mm. This diameter of 150 mm is considered to be the nominal diameter of the sleeve receiving the extrusion screw or the external diameter of the screw threads, neglecting the operating clearance, which may be 0.1 mm. The dispersive mixing zone 36, for example, has a length of 450 mm along the longitudinal axis A.

[0142] With reference to [Fig. 6], the sleeve 6 includes a feed orifice 40 intended to face the feed zone 30 of the extrusion screw 7 when the extrusion screw 7 is disposed in the sleeve 6. The sleeve 6 further includes an initial portion 42 disposed downstream of the feed orifice 40 with respect to the direction of flow of the extrusion composition in the sleeve 6. The initial portion 42 of the compression zone 32 is intended to be disposed opposite the feed zone 30 and / or the compression zone 32.

[0143] This initial portion 42 preferably comprises, at the level of an inner wall 23 of the sleeve 6, a plurality of grooves 44 extending along the longitudinal axis A of the extrusion screw 7. The grooves are preferably straight. The grooves preferably extend along the longitudinal axis A over a distance between 0.5D and 1.5D, more preferably over a distance of 1D. The grooves 44 are preferably opposite the feed zone 30.

[0144] The combination of these grooves 44 with the profile of the threads of the feed zones 30 and compression zones 32 contributes to increasing the extrusion flow rate.

[0145] The extrusion screw 7 also includes a pumping zone 38 between the barrier zone 34 and the distal end 21 of the extrusion screw 7. The pumping zone 38 includes a pumping thread 57 extending around the screw body 24.

[0146] The pumping thread 57 has a thread depth Pp taken perpendicular to the longitudinal axis A between a thread end and the screw body of between 0.06D and 0.11D. The pumping thread 57 of the pumping zone 38 preferably has a thread depth between 0.07D and 0.10D, more preferably between 0.08D and 0.09D.

[0147] The extrusion screw 7 has a compression ratio CR between 1.2 and 2, preferably between 1.5 and 1.7. The compression ratio CR is determined with the following equation: CR = Pa / Pp, CR being the compression ratio, Pa being the thread depth of the feed zone and Ps being the thread depth of the pumping zone.

[0148] The compression ratio is preferably between 1.6 and 1.7.

[0149] 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 method employs an extruder 5 as described above.

[0150] The manufacturing process comprises at least the following steps: i) a step of introducing at least one thermoplastic polymer in solid form selected from a propylene homopolymer and a propylene copolymer, into the feed zone 30 of the extrusion screw 7, (ii) a step in which said at least one thermoplastic polymer is brought from the feed zone 30 to the compression zone 32 and the barrier zone 34 allowing the gradual melting of the thermoplastic polymer into an extrusion composition and the transport of the extrusion composition to the extrusion head 8 located at the outlet of the extruder 5, and iii) an application step at the extrusion head 8 of the extrusion composition from step ii) around the elongated electrically conductive element.

[0151] The introduction step preferably involves the introduction of at least one dielectric liquid into the feed zone 30 of the extrusion screw 7. This introduction of at least one dielectric liquid can be combined with the introduction of the thermoplastic polymer.

[0152] The manufacturing process may also provide, upstream of the application step, for a dispersive mixing step of the extrusion composition by means of the dispersive mixing zone 36 of the extrusion screw 7.

Claims

1. Demands 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 feeding element (4) made of thermoplastic polymer in solid form, - a sheath (6) fed by the feeding organ, - an extrusion screw (7) disposed inside the sleeve (6) and allowing the progressive melting of the polymer to form an extrusion composition and the transport of this extrusion composition along the extrusion screw (7) to a distal end (21) of the extrusion screw (7), the extrusion screw having an outside diameter, denoted D, and comprising a screw body (24) extending along a longitudinal axis (A), the extrusion screw further comprising along this longitudinal axis (A): * a feed zone (30) of at least one thermoplastic polymer in solid form, the feed zone comprising a feed thread (50) extending around the screw body, * a compression zone (32) of the extrusion composition comprising a compression thread (52) extending around the screw body, * a barrier zone (34) comprising a primary thread and a secondary thread for the gradual melting of the polymer into an extrusion composition, the distance separating the primary and secondary threads varying along the longitudinal axis (A), in which the feed (30) and compression (32) zones comprise respectively a single feed thread and a single compression thread, each of the feed and compression threads having a thread depth (Pa, Pc) taken perpendicular to the longitudinal axis (A) between a thread end and the screw body (24) between 0.11D and 0.16D, - an extrusion head (8) disposed at the distal end (21) of the extrusion screw (7) and configured to apply the composition around an elongated electrically conductive element, in which the sleeve has a feed orifice formed opposite the feed zone of the extrusion screw, the sleeve further comprising, at the level of an inner wall and following the feed orifice, a plurality of grooves extending along the longitudinal axis (A) over a distance between 0.5D and 1.5D.

2. Extruder according to claim 1, wherein the feed and compression threads have a thread depth (Pa, Pc) between 0.12D and 0.14D, preferably between 0.13D and 0.14D.

3. Extruder according to claim 1 or 2, wherein the compression zone comprises a proximal compression portion disposed at the level of the feed zone and a distal compression portion disposed at the level of the barrier zone, the compression net having a net depth (Pc) variable between the proximal and distal compression portions.

4. Extruder according to claim 3, wherein the compression thread has a first thread depth (Pci) at the level of the proximal compression portion, equal to the thread depth Pa of the feed zone, and a second thread depth (Pc2) at the level of the distal compression portion, the second thread depth (Pc2) being less than the first thread depth (Pci).

5. Extruder according to claim 4, wherein the thread depth (Pc) of the compression zone varies linearly between the first (Pci) and second (Pc2) thread depths.

6. Extruder according to any one of the preceding claims, wherein the compression zone has a length along the longitudinal axis (A) of between 6D and 11D.

7. Extruder according to any one of the preceding claims, further comprising a pumping zone between the barrier zone and a distal end of the extrusion screw, said pumping zone comprising a pumping thread extending around the screw body, the pumping thread having a thread depth (Pp) taken perpendicular to the longitudinal axis (A) between a thread end and the screw body between 0.06D and 0.11D.

8. Extruder according to claim 7, wherein the extrusion screw has a compression ratio (CR) between 1.2 and 1.7, preferably between 1.5 and 1.7, the compression ratio (CR) being determined with the following equation: CR = Pa / Pp, CR being the compression ratio, Pa being the thread depth of the supply zone and Pp being the thread depth of the pumping zone.

9. Extruder according to any one of the preceding claims, further comprising a dispersive mixing zone after the barrier zone.

10. Extruder according to claim 9, wherein the dispersive mixing zone defines at least one inlet channel of the extrusion composition into the dispersive mixing zone and at least one outlet channel of the extrusion composition from the dispersive mixing zone, said at least one inlet channel being separated from said at least one outlet channel by a passage thread configured to permit the passage of the extrusion composition from an inlet channel to an outlet channel and to shear said composition during said passage.

11. Extruder according to any one of the preceding claims, wherein the grooves are straight along the longitudinal axis (A).

12. 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 according to any one of the preceding claims, characterized in that it comprises at least the following steps: i) a step of introducing at least one thermoplastic polymer in solid form selected from a propylene homopolymer and a propylene copolymer into the feed zone of the extrusion screw, ii) a step in which said at least one thermoplastic polymer is brought from the feed zone to the compression zone and the barrier zone allowing the gradual melting of the thermoplastic polymer into an extrusion composition and the transport of the extrusion composition to the extrusion head located at the outlet of the extruder,and iii) an application step at the extrusion head of the extrusion composition from step ii) around the elongated electrically conductive element. 22

13. A method according to claim 12, wherein the introduction step involves introducing at least one dielectric liquid into the feed zone of the extrusion screw.

14. A method according to claim 12 or 13, further comprising upstream of the application step a dispersive mixing step of the extrusion composition by means of the dispersive mixing zone of the extrusion screw.

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