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 electric cable by extruding a composition based on a propylene polymer with an extrusion screw with improved flow rate Technical field

[0001] The present invention relates to an extrusion screw, an extruder comprising such an extrusion screw and a method for manufacturing an electric 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 chosen from a propylene homopolymer and a copolymer. The thermoplastic layer can 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 applies typically but not exclusively 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.

[0004] 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 semiconducting layer.

[0005] For the extrusion of an insulating layer of an electric cable, it is common to use an extrusion screw of the barrier screw type or barrier profile screw, i.e. a screw comprising a zone called a "barrier zone". This barrier zone comprises in particular a main thread and 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. The barrier zone allows compression of the solid bed and consequently an improvement in 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 arranged at the feed of the extrusion screw, e.g. a hopper. This feed zone is followed by a compression zone and a barrier zone allowing the gradual melting of the polymer and transport towards the outlet of the extruder to be applied 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 the fact that the thread depth decreases 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 flow 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 the compression of the polymer. 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 to increase the extrusion flow rate. This extrusion screw profile is generally coupled with a sleeve surrounding the extrusion screw provided with a plurality of helical grooves at this bulb.

[0009] It is also usual 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 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 disadvantage of this type of extrusion screw is that despite the specific attributes provided to the extrusion screw, the extrusion flow rate is not sufficient. Thus, 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] Furthermore, when the extrusion composition incorporates a dielectric liquid, the latter tends to be rejected towards the proximal end of the extrusion screw, which is detrimental to the quality of the extrusion composition.

[0012] There is therefore a need for an improved extrusion screw allowing an increase in the extrusion flow rate for the same barrel, in particular when the com extrusion position comprises a thermoplastic polymer having one of a homopolymer and a propylene copolymer. There is also a need for an extrusion screw capable of reducing or even eliminating the discharge of the dielectric liquid; the extrusion composition incorporates such a dielectric liquid. Summary of the invention

[0013] For this purpose, the invention proposes an extrusion screw for an extruder for the extrusion of at least one extruded thermoplastic layer surrounding an elongated electrically conductive element of an electric 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 feed zone for at least one thermoplastic polymer in solid form, the feed zone comprising a feed thread extending around the screw body, - a compression zone of the extrusion composition comprising a compression thread extending around the screw body, - a barrier zone comprising a main thread and a secondary thread for the gradual melting of the polymer into an extrusion composition, the distance separating the main and secondary threads varying along the longitudinal axis, wherein the feed and compression zones respectively comprise 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 of between 0.11D and 0.16D has an improved profile allowing a significant increase in the extrusion flow rate.

[0015] It is important to remember that extrusion screws generally have a standard diameter, which can vary from 45 to 175mm. For example, this diameter can be 80mm, 120mm or 150mm. This diameter D corresponds to the outside diameter of the extrusion screw, as well as to the inside diameter of the associated barrel. Thus, when the thread depth of an extrusion screw is increased, the diameter of the screw body is correspondingly reduced by the same value. Therefore, an increase in the thread depth reduces the diameter of the screw body and therefore its mechanical strength. The variation in the thread depth, even in minimal values, can lead to very large variations 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 feed and compression zones with an increase in thread depth between 0.11D and 0.16D allows a significant increase in extrusion throughput.

[0017] In addition, such a profile of the feed and compression zones makes it possible to avoid any rejection of the dielectric liquid when it is included in the extrusion composition. It has in fact been observed that the reduction in the compression force of the composition at the feed and compression zones allows the dielectric liquid to advance along the extrusion screw without being rejected towards the rear of the extrusion screw. Indeed, once the dielectric liquid is sufficiently far from the feed zone, the thermoplastic polymer feeding the extruder forms a barrier to the dielectric liquid which can no longer return towards the rear of the extrusion screw.

[0018] The fact that each of the feed and compression threads has a thread depth taken perpendicular to the longitudinal axis between a thread end and the screw body between 0.11D and 0.16D means that each of these feed and compression threads has at least locally a thread depth in this value range. One or both of the feed and compression threads may thus locally have a value outside this value 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 arranged at the feed zone and a distal compression portion arranged at the barrier zone, the compression thread having a thread depth P which varies between the first and second compression portions.

[0021] According to one embodiment, the compression thread has a first thread depth at the proximal compression portion, equal to the thread depth of the feed zone, and a second thread depth at the distal compression portion, the second thread depth being less than the first thread depth. This reduction in the thread depth along the compression zone generates a compression of the material given that 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 of the longitudinal axis 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 for the extrusion composition in the dispersive mixing zone and at least one outlet channel for 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 net 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.

[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 feed member made of thermoplastic polymer in solid form, - a sheath supplied by the feed member, - an extrusion screw as described above arranged inside the barrel 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 sheath has a feed orifice formed opposite the feed zone of the extrusion screw, the sheath further comprising, at an inner wall and following the feed orifice, 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 function of the grooves is to increase the friction between the inner wall of the sheath and the elements of the extrusion composition. The increase in this friction helps to improve the extrusion throughput.

[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 2 mm, preferably 1 mm, along a radial axis to the longitudinal axis.

[0031] According to one embodiment, the extrusion screw comprises a pumping zone between the barrier zone and the distal end of the extrusion screw. This pumping zone has the function of stabilizing the extrusion composition, in particular after its passage through the dispersive mixing zone when it is 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 thread of the pumping zone has a thread 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 of 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 to 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 feed zone has a length along the longitudinal axis between 1D and 4D, preferably between 2D and 3D, 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] As a preferred example, an extrusion screw with a length of 24D has a feed zone with a length of 2D, a compression zone of 8D, a barrier zone with a length of 8D, a dispersive mixing zone of 3D and a pumping zone with a length of 1D.

[0039] As a 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 with a length of 10D, a dispersive mixing zone of 4D and a pumping zone with a length of 2D.

[0040] The term "length of the extrusion screw" means the functional length of the extrusion screw between the start 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 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, said method using 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 chosen from a propylene homopolymer and a propylene copolymer, into the feed zone of the extrusion screw, ii) a step during 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 as well as the transport of the extrusion composition to the extrusion head located at the outlet of the extruder, and iii) a step of applying at the extrusion head the extrusion composition resulting from step ii) around the elongated electrically conductive element.

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

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

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

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

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

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

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

[0049] The olefin a1 of the elastomeric phase of the heterophase copolymer may be propylene.

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

[0051] The statistical copolymer of propylene may be a copolymer of propylene and olefin, the olefin being chosen in particular from ethylene and an α2 olefin other than propylene.

[0052] The olefin a2 other than propylene can correspond to the formula CH2=CH RI, in which RI is a linear or branched alkyl group having from 2 to 10 carbon atoms, notably chosen 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 approximately 15 mol% and more preferably at most approximately 10 mol% of the copolymer.

[0054] A copolymer of propylene and ethylene is preferred as the random propylene copolymer.

[0055] By way of example, the statistical propylene copolymer marketed by the company Borealis under the reference Bormed® RB 845 MO is particularly preferred.

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

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

[0058] The homopolymer (respectively the propylene random copolymer) may have a melting temperature greater than approximately 130°C, preferably greater than approximately 140°C, and more preferably ranging from approximately 140 to 165°C.

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

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

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

[0062] Said dielectric liquid represents a quantity of 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 so-called “high density” polyethylene or HDPE according to the ISO 1183A standard (at a temperature of 23°C) has a density varying from approximately 0.930 to 0.970 g / cm3, and even more preferably from approximately 0.940 to 0.965 g / cm3.

[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 known as “low density” or LLDPE according to the ISO standard 1183A (at a temperature of 23°C) has a density ranging from about 0.91 to about 0.925 g / cm3.

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

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

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

[0070] The polyethylene may represent from 20% to 60% by mass approximately, and preferably from 20 to 50% by mass approximately, 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 statistical copolymer of propylene (eg approximately 50-70% by mass, relative to the total mass of the composition), a heterophase copolymer of propylene (eg approximately 5-30% by mass, relative to the total mass of the composition) and a linear low density polyethylene (eg approximately 20-40% by mass, relative to the total mass of the composition), or - a heterophase copolymer of propylene (eg approximately 35-55% by mass, relative to the total mass of the composition) and a high-density polyethylene (eg 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, having good mechanical properties, in particular in terms of elastic modulus, and electrical properties.

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

[0074] The composition of step i) may further comprise 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 tree agents and a mixture thereof.

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

[0077] More particularly, the antioxidants make it possible to protect the composition from the thermal stresses generated during the cable manufacturing steps or during the cable operation.

[0078] The antioxidants are preferably chosen from bulky 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), 2,2'-thiodiethylene bis[3-(3,5-di-tert-butyl-4-hydroxyphenyl) propionate] (Irganox® 1035), 2,2' m-ethylenebis(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-butylphenyl)phosphite (Irgafos® 168) and bis(2,4-di-tert-butylphenyl)pentaerythritoldiphosphite (Ultranox® 626).

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

[0084] As examples of mixtures of antioxidants, mention may be made of Irganox B 225 which comprises an equimolar mixture of Irgafos 168 and Irganox 1010 as described above.

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

[0086] 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:

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

[0088] [Fig.2] [Fig.2] schematically represents a partially sectioned and partially perspective of an electric cable obtained with the extruder of [Fig.l];

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

[0090] [Fig.4] [Fig.4] shows a detailed side view of the feeding and compression of an extrusion screw;

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

[0092] [Fig.6] [Fig.6] shows a detailed sectional view of an extruder barrel having a plurality of rectilinear grooves intended to extend along the longitudinal axis of the extrusion screw. Description of embodiment(s)

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

[0094] 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.

[0095] 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.

[0096] 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 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 sheath 6 forms an internal cavity in which the extrusion screw 7 is arranged. The sheath 6 is supplied with thermoplastic polymer by a feed member, here the feed 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 about the longitudinal axis A.

[0101] The extrusion screw 7 comprises a screw body 24 extending along the longitudinal axis A as well as one or more threads extending around the screw body 24. The extrusion screw 7 extends between a proximal end 20, close to the feed 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 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 the “internal semi-conductive layer”, an electrically insulating layer 14, a second semi-conductive layer 15 called the “external semi-conductive layer”, a metal 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 methods well known to those skilled in the art.

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

[0106] The extruder 5 may be a single-screw extruder as illustrated in [Fig.l] 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 successively comprises along the longitudinal axis A a feed zone 30, a compression zone 32 and a barrier zone 34.

[0108] The feed zone 30 of the extrusion screw 7 is intended to come opposite a feed orifice 40 of the sleeve 6 for feeding the extrusion screw 7 and the sleeve 6 with said 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 arranged 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 comprises a distal portion arranged 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 a single 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 feed 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 arranged 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. The term "single compression thread" means that a single thread runs through the compression zone 32. The feed 50 and compression 52 threads preferably extend continuously one after the other so that they form a single thread extending along the feed 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 the same diameter D. For example, the compression zone 32 may be of a length of 6D to 8D for an extrusion screw 7 with a total length of 24D or 25D but of a length of 10D for an extrusion screw 7 with a total length of 30D. The principle of the invention and the associated advantages are retained regardless of the total screw length chosen.

[0117] The feed 50 and compression 52 threads respectively have a thread depth Pa and a thread depth Pc, taken 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, more preferably between 0.13D and 0.14D. This thread depth makes it possible to maximize the extrusion flow rate of the composition while guaranteeing sufficient mechanical strength of the extrusion screw 7. This balance is all the 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 thread depth Pc of the compression thread 52 is preferably variable along the longitudinal axis A. In particular, the compression zone 32 comprises a proximal compression portion arranged at the feed zone 30 and a distal compression portion arranged at the barrier zone 34. The thread depth Pc of the compression thread 52 has a variable thread depth Pc between the proximal and distal compression portions.

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

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

[0122] Thus, the compression thread 52 has a first thread depth Pci at the proximal compression portion and a second thread 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 feed thread 50. Therefore, the first thread depth Pci is between 0.11D and 0.16D, preferably between 0.12D and 0.14D, 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 main thread 54 and a secondary thread 56 for the gradual melting of the polymer into an extrusion composition. The secondary thread 56 has a pitch greater than the main thread 54 to allow the molten polymer to be separated from the still solid polymer. The secondary thread 56 therefore sweeps the width of the channel formed by the space between two turns of the main thread 54, as if it materialized the boundary between the two phases. The barrier zone 34 allows compression of the solid bed and consequently an improvement in 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 comprise 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, in particular when the composition comprises a dielectric liquid.

[0129] The dispersive mixing zone 36 defines at least one inlet channel for the extrusion composition in the dispersive mixing zone and at least one outlet channel for 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 passage net 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 passage threads 67 and scraper threads 68 defining between them a plurality of inlet channels 70 and outlet channels 73. In particular, the passage threads 67 and scraper 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 passage thread 67 and a scraper 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 passage threads 67 and scraping threads 68 preferably extend along tra helical jectories around and along the screw body 24. The passage and scraping threads 68 are preferably equally distributed 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 entry of the composition into 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 passage of the composition while shearing it by a predetermined amount. 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 disposed at the proximal end 76 of the dispersive mixing zone 36 so as to prevent, at least partially, the flow of the composition towards the outlet channels 73 from the barrier zone. The flow of the composition is therefore facilitated towards the inlet channels 70. Each outlet channel 73 further forms an outlet opening 79 formed 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, 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. For this, the height of the passage threads 67 is chosen to generate this shear value. Thus, the composition can pass from an inlet channel 70 to an outlet channel 73 by passing over a passage thread 67 but is sheared.

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

[0137] At the exit from the barrier zone 34, the extrusion composition therefore passes through the dispersive mixing zone 36 by passing through an inlet channel 70 and / or an outlet channel 73 while 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 70 and outlet 73 channels extend around and along the screw body 24. The inlet 70 and outlet 73 channels 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 threads 67 and scraper threads 68 preferably have a width different from each other. This width is defined perpendicular to the extension path of the threads. Preferably, a ratio of 2 is defined between the width of a passage thread 67 and a scraper thread 68.

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

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

[0142] With reference to [Fig. 6], the sheath 6 comprises a feed orifice 40 intended to face the feed zone 30 of the extrusion screw 7 when the extrusion screw 7 is arranged in the sheath 6. The sheath 6 further comprises an initial portion 42 arranged downstream of the feed orifice 40 relative to the direction of circulation of the extrusion composition in the sheath 6. The initial portion 42 of the compression zone 32 is intended to be arranged opposite the feed zone 30 and / or the compression zone 32.

[0143] This initial portion 42 preferably comprises, at an inner wall 23 of the sheath 6, a plurality of grooves 44 extending along the longitudinal axis A of the extrusion screw 7. The grooves are preferably rectilinear. 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 30 and compression 32 zones contributes to increasing the extrusion flow rate.

[0145] The extrusion screw 7 also comprises a pumping zone 38 between the barrier zone 34 and the distal end 21 of the extrusion screw 7. The pumping zone 38 comprises 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 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 of 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. Said method uses an extruder 5 as described above.

[0150] The manufacturing method comprises at least the following steps: i) a step of introducing at least one thermoplastic polymer in solid form chosen from a propylene homopolymer and a propylene copolymer, into the feed zone 30 of the extrusion screw 7, ii) a step during 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 as well as the transport of the extrusion composition to the extrusion head 8 located at the outlet of the extruder 5, and iii) a step of applying at the extrusion head 8 the extrusion composition resulting from step ii) around the elongated electrically conductive element.

[0151] The introduction step preferably provides for 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 may be combined with the introduction of the thermoplastic polymer.

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

Claims

Claims

1. Extrusion screw for an extruder for extruding at least one extruded thermoplastic layer (14) surrounding an elongated electrically conductive element of an electric cable (11), 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) for 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) for the extrusion composition comprising a compression thread (52) extending around the screw body, - a barrier zone (34) comprising a main thread and a secondary thread for the gradual melting of the polymer into an extrusion composition, the distance separating the main and secondary threads varying along the longitudinal axis (HAS),wherein 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.,

2. An extrusion screw 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. An extrusion screw according to claim 1 or 2, wherein the compression zone comprises a proximal compression portion disposed at the feed zone and a distal compression portion disposed at the barrier zone, the compression thread having a thread depth (Pc) varying between the proximal and distal compression portions.

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

5. An extrusion screw 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. An extrusion screw according to any preceding claim, wherein the compression zone has a length along the longitudinal axis (A) of between 6D and 11D.

7. An extrusion screw according to any preceding claim, 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. Extrusion screw according to claim 7, wherein the extrusion screw has a compression ratio (CR) of 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 feed zone and Pp being the thread depth of the pumping zone.

9. An extrusion screw according to any preceding claim, further comprising a dispersive mixing zone after the barrier zone.

10. An extrusion screw according to claim 9, wherein the dispersive mixing zone defines at least one inlet channel for the extrusion composition into the dispersive mixing zone and at least one outlet channel for 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 allow the passage of the extrusion composition from an inlet channel to an outlet channel and to shear said composition during said passage.

11. Extruder (5) for extruding an electric 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 feed member (4) for thermoplastic polymer in solid form, - a sheath (6) fed by the feed member, - an extrusion screw (7) according to any one of claims 1 to 10 arranged inside the sheath (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), - an extrusion head (8) arranged at the distal end (21) of the extrusion screw (7) and configured to apply the composition around an elongated electrically conductive element.

12. An extruder according to claim 11, wherein the barrel has a feed orifice formed opposite the feed zone of the extrusion screw, the barrel further comprising, at 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.

13. An extruder according to claim 12, wherein the grooves are rectilinear along the longitudinal axis (A).

14. A method of 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 using an extruder according to one of claims 11 to 13, characterized in that it comprises at least the following steps: i) a step of introducing at least one thermoplastic polymer in solid form chosen from a propylene homopolymer and a propylene copolymer, into the feed zone of the extrusion screw, ii) a step during 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 as well as the transport of the extrusion composition to the extrusion head located at the outlet of the extruder,and iii) a step of applying at the extrusion head the extrusion composition resulting from step ii) around the elongated electrically conductive element.,

15. The method of claim 14, wherein the introducing step includes introducing at least one dielectric liquid into the area feed of the extrusion screw.

16. Method according to claim 14 or 15, further comprising upstream of the application step a step of dispersive mixing of the extrusion composition by means of the dispersive mixing zone of the extrusion screw.

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