RU2336586C1 - Method of cable production - Google Patents

Abstract

FIELD: electrical engineering.

SUBSTANCE: invention relates to the method of producing the cable composed of a conductor, an inner semi-conducting layer with thickness smaller than or equal to 0.4 mm, an insulating layer enveloping the said inner semi-conducting layer. The proposed method includes the stage of co-extrusion of the inner semi-conducting and insulating layers. The aforesaid stage comprises making the first circular flux of the inner semi-conducting layer and the second circular flux of the insulating layer, bringing the said first circular flux outer surface in contact with the said circular flux inner surface at a certain axial distance from the contact point of the aforesaid surfaces. The method comprises also the combination of the preset feed rate and the said contact point in the function of semi-conducting and insulating material dynamic viscosity (η) allowing the interrelation between the tangential strain of the inner semi-conducting layer nearby the radial inner wall of the extrusion die and the tangential strain of the insulating layer nearby the extrusion die radial outer wall makes from 0.5 to 4. The method includes also the pressure extrusion of the insulating and outer semi-conducting layers onto the conductor.

EFFECT: miniature cable conductor retaining good general mechanical and electrical properties.

22 cl, 5 dwg, 5 tbl, 5 ex

Description

The present invention relates to a method for manufacturing a cable.

In particular, the present invention relates to a method for manufacturing an electric cable for transmitting or distributing electric energy at medium or high voltage.

More specifically, the present invention relates to a method for manufacturing an electric cable having a structure with a very compact structure.

As used herein, the term “medium voltage” is used to refer to voltages typically from about 10 to about 60 kV, and the term “high voltage” refers to voltages above 60 kV. Typically, the term "low voltage" refers to a voltage below 10 kV, typically more than 100 V. The term "very high voltage" is also sometimes used in the art to define voltages higher than about 150 or 220 kV, up to 500 kV or more .

Cables for the transmission or distribution of electricity at medium or high voltage usually have a metal conductor, which is respectively surrounded by an inner semiconducting layer, an insulating layer and an outer semiconducting layer. In the following description, said predetermined sequence of elements will be denoted by the term “cable core”.

In a position radially external with respect to said core, the cable is provided with a metal sheath (or shield), usually made of aluminum, lead or copper. Typically, the metal sheath consists of a continuous tube or metal tape, rolled up in accordance with the tubular shape and welded or sealed to ensure tightness. Alternatively, a metal sheath is formed of a plurality of metal wires.

The metal sheath performs an electrical function by creating inside the cable, as a result of direct contact between the metal sheath and the outer semiconducting layer of the cable core, a uniform electric field of the radial type, while eliminating the external electric field of the cable.

The metal sheath can also provide tightness against the external environment of the cable by establishing a barrier to the penetration of water in the radial direction.

An additional function of the metal shell is to withstand short circuit currents.

In a unipolar type configuration, the cable is provided with a polymer outer sheath in a position radially external to the metal sheath mentioned above.

In addition, cables for transmitting and distributing electricity are typically provided with one or more layers to protect said cables from accidental impacts that may occur on their outer surface.

Accidental hits on the cable can occur, for example, during its transportation or during the stage of laying the cable in a trench in the ground. Mentioned random shocks can cause a number of structural damage to the cable, including deformation of the insulating layer and peeling off of the insulating layer from the semiconducting layers, damage that can cause changes in the voltage gradient in the insulating layer, followed by a decrease in the insulating ability of said layer.

Cables that are currently available on the market, for example, cables for the transmission and distribution of electricity at low and medium voltage, usually include metal armor covers capable of withstanding the impacts to protect said cables from possible damage caused by accidental impacts. Typically, said armor covers are in the form of tapes or wires (preferably made of steel), or, alternatively, in the form of metal shells (preferably made of lead or aluminum). An example of such a cable design is described in US Pat. No. 5,153,381.

European Patent No. 981821 in the name of the Applicant describes a cable that is provided with a layer of foamed polymer material in order to give said cable a high resistance to accidental impact, said layer of foamed polymer material being preferably applied radially external to the cable core. This proposed technical solution eliminates the use of traditional metal armor covers, thereby reducing the weight of the cable, as well as making the method of its manufacture more simple.

The applicant realized the need to provide a cable with a compact cable core, i.e. with the construction of a cable core having a reduced thickness of the semiconducting layers and the insulating layer compared to conventional cables, in order to reduce the size and weight of the cable to primarily facilitate handling, improve its flexibility and transportation without compromising the overall electrical and mechanical properties of the cable.

However, the Applicant has determined that it is not possible to produce such a compact cable core — at the desired production speed — by using manufacturing methods known in the art that are not suitable to achieve the desired results.

In order to obtain a compact cable core that is provided with a very thin inner semiconducting layer (i.e., with a thickness less than or equal to 0.4 mm), the Applicant has established that there are known extrusion technologies, according to which the flows of various materials forming the layers comprising the cable core supported separately from each other and separately extruded onto the formed cable core, cause many disadvantages that do not allow to obtain the desired cable core at a reasonable speed.

For example, in the case where it is desirable to obtain a significant decrease in the thickness of the inner semiconducting layer, known methods for manufacturing a cable lead to the formation of an inhomogeneous thickness of the inner semiconducting layer in either the longitudinal or radial directions, as well as its ruptures during extrusion of the inner semiconducting layer onto the cable conductor. This is due to the fact that when moving along the extrusion head, the conductor exerts a pulling force on the very thin extruded inner semiconducting layer, thereby causing the above defects. This feature is even enhanced when the cable conductor moves along the extrusion head with a predetermined feed rate that is sufficiently high (for example, with a typical feed speed of about 30 m / min) in order to achieve industrial productivity. Therefore, the combination of the relatively high feed rate of the cable conductor with a very thin extrudable inner semiconducting layer usually results in a defective cable core that is unacceptable and thus discarded.

In addition, in accordance with known methods for manufacturing a cable in the case where it is desired to obtain a reduced thickness of the inner semiconducting layer, the length of the extrusion channel that is used to extrude the inner semiconducting layer is noticeably greater than its average height (the height of the channel is measured in a plane perpendicular to longitudinal walls of the channel). This feature causes a noticeable increase in the extrusion pressure inside the extrusion head, which is due to a decrease in the cross section of the extrusion channel and, as a result, an increase in the speed of movement of the extrudable material along the extrusion channel. Therefore, in order to reduce the pressure in the extrusion head, the extruder performance with respect to the inner semiconducting layer is set to a lower value in order to reduce the speed of the inner semiconducting material in the extrusion channel, thereby adversely affecting the performance of the cable manufacturing process as a whole.

In addition, in accordance with known methods for manufacturing a cable in the case where it is desired to obtain a reduced thickness of the inner semiconducting layer, the precise manufacture and / or assembly of the dies that form the extrusion channel of the inner semiconducting layer significantly affects the stability of the flows of the extrudable material. As a result, a non-uniform distribution of the extruded material and a non-uniform thickness of the inner semiconducting layer on the cable conductor can occur.

Conventional cable fabrication methods are also known in which a multilayer cable element is coextruded onto a cable conductor, forcing the single layers of said multilayer element to contact each other before extrusion onto the conductor, so that the multilayer element is formed in a position that is upstream of the contact point between the conductor cable and multi-layer cable element.

For example, US Pat. No. 3,737,490 describes a method for manufacturing an extruded composite cable coating of an electric cable on a continuously moving conductor by means of a “floating” (from the English float-down) method, said coating comprising two or more layers of different coating materials. This method comprises passing a core through a core tube of an extrusion machine that feeds extrudable, peripherally continuous layers of coating materials simultaneously towards the outlet end of the extrusion machine; bringing the extruded layers into full and close boundary contact upstream relative to the output end of the extrusion machine; performing continuous processing of the composite coating thus formed by passing the coated core through a chamber sealed to the outlet end of the extrusion machine and containing a fluid with a pressure above atmospheric; and at the same time, forcing the fluid under pressure inside the core tube and maintaining the fluid under pressure that is less than the fluid pressure by such a value that the pressure difference across the extruded composite coating at the extrusion nozzle is sufficient to force the extruded composite coating to firmly sit on the core as it emerges from the extrusion machine, but is not sufficient to drive the extruded composite coating back along the core tube. Furthermore, said document describes a transverse head of an extrusion machine having at its output end an annular extrusion nozzle bounded by an external die and an internal die, which is attached to the front end of the core tube passing through the head. An upstream die is located upstream of the extrusion nozzle. Semiconducting polyethylene in a plastic state is fed into the annular space between the inner die and the intermediate die through the feed channel, and insulating polyethylene in the plastic state is fed into the annular space between the intermediate die and the outer die through the feed channel. The intermediate die is positioned relative to the outer die and the inner die so that the extruded layers of semiconducting polyethylene and insulating polyethylene come into full and close boundary contact upstream of the extrusion nozzle. Using this method, a composite coating may be applied to a sector conductor containing an internal semiconductible crosslinkable polyethylene layer with a radial thickness of 0.5 mm and an external insulating crosslinkable polyethylene layer with a radial thickness of 2.8 mm.

No. 4,093,414 describes a die with which thermoplastic insulating compositions can be coextruded to apply foam / film insulation over a cable conductor, in particular in a method for manufacturing a telephone wire. According to the aforementioned document, only one tip and one extrusion die are used to apply two layers of insulating material (the first cellular insulating layer and the second continuous insulating layer on top of the said cellular layer) with a melt flow separator between the insulation leads as they approach the end of the tip through which passes conductor. The melt flow separator prevents insulating materials from “fusing” before they approach the outlet end of a single tip, said melt flow separator ending at some distance behind the tip end, so that the cellular structure of the inner layer can be destroyed.

EP-534208 describes an extrusion die for coextrusion of at least two different plastic materials that are provided by two feed channels that open into a common outlet nozzle and into a slit-shaped homogenization zone, which serves to homogenize the material flow. The homogenization zone of the internal material extends essentially in the axial direction, while the homogenization zone of the external material extends essentially in the radial direction. Mentioned at least two plastic materials may be coated elongated part.

Applicant has found that when the extruded inner semiconducting layer is in contact with the extruded insulating layer in a position that is located upstream of the contact point between the cable conductor and the inner semiconducting layer, the pulling effect exerted on the inner semiconducting layer (during extrusion) of the cable conductor (when moving along the extrusion head) mainly spreads over the entire thickness, which consists of the sum of the thicknesses of the inner semiconducting layer and insulating layer, respectively.

Therefore, due to the fact that upon contact with the conductor, the inner semiconducting layer is limited — at its radially outer part — by the presence of an insulating layer already extruded onto it, rather than by a fixed die wall suitable for extruding the inner semiconducting layer (as in the case when the inner semiconducting layer and the insulating layer are separately extruded onto the cable conductor being formed), the tensile effect exerted by the cable conductor on the inner semiconducting layer, is already connected to the insulating layer does not cause thickness inhomogeneities and / or gaps at the conductor / inner semiconducting layer.

The applicant further found that in order to produce a compact cable core that is provided with a very thin inner semiconducting layer, it is necessary to force the inner semiconducting layer to contact the insulating layer in a position that is upstream of the contact point between the cable conductor and the inner semiconducting layer, and, in addition, to carry out the step of closing the internal semiconducting material with an insulating material - at the point of contact - in such a way that in the extrudate uemyh materials during their contact and closure does not occur instability flows along the die.

In other words, the Applicant has determined that the extrusion conditions of the two materials (process conditions, for example, material flow, as well as geometric conditions, such as the mutual distance and configuration of the dies) affect the rheological properties at the interface between the inner semiconducting layer and the insulating layer.

Therefore, in order to ensure that the desired thickness of the inner semiconducting layer and its longitudinal uniformity are achieved, the Applicant has found that near the contact point of the inner semiconducting layer and the insulating layer, the difference in the shear stress of the inner semiconducting material and the shear stress of the insulating material should be as small as possible in order to avoid instabilities in the flow of extrudable materials, or at least significantly reduced, and the closure of the two materials should not led to the formation of deformation in two contacting layers.

More specifically, the Applicant has found that, near the contact point of the inner semiconducting layer and the insulating layer, the ratio between the shear stress of the inner semiconducting layer and the shear stress of the insulating layer should be between 0.5 and 4.

In one aspect, the present invention relates to a method for manufacturing a cable, comprising:

- conductor;

an inner semiconducting layer surrounding said conductor and having a thickness less than or equal to 0.4 mm, and

an insulating layer surrounding said semiconducting layer, said method comprising the steps of:

- feeding the conductor with a predetermined feed rate to the extrusion head containing the first extrusion channel for extruding the inner semiconducting layer and the second extrusion channel for extruding the insulating layer, said first extrusion channel and said second extrusion channel being separated from each other by the first extrusion die, and

- coextrusion of the inner semiconducting layer and the insulating layer, and the said stage of coextrusion includes:

• providing a first annular flow of internal semiconducting material and a second annular flow of insulating material;

• contacting the outer surface of said first annular flow and the inner surface of said second annular flow at an axial distance from the contact point, where the inner surface of said first annular flow is in contact with a conductor;

• selection in combination of said predetermined speed and said contact point as a function of the dynamic viscosity of the inner semiconducting material and the insulating material so that the ratio between the shear stress of the inner semiconducting layer at the radially inner wall of said first extrusion die and the shear stress of the insulating layer at the radially outer wall of said the first extrusion die near said contact point is from about 0.5 to 4;

- compression extrusion of the insulating layer and the inner semiconducting layer on the conductor.

Preferably, said axial distance, which is measured along the direction of advancement of the cable conductor, is greater than or equal to 0.5 of the diameter of the conductor. More preferably, said distance is in the range of from about 0.6 to about 10 diameters of the conductor.

In the present description and the following claims, the term “near the contact point” means that the shear stress of two extrudable layers (i.e., the inner semiconducting layer and the insulating layer) is calculated directly from the corresponding wall of the extrusion die (radially inner wall and radially outer wall, respectively) directly before the mentioned two layers come into mutual contact, i.e. just before the two layers mentioned leave the corresponding walls of the extrusion die.

Preferably, the ratio between the shear stress of the inner semiconducting layer at the radially inner wall of the first extrusion die and the shear stress of the insulating layer at the radially outer wall of the first extrusion die near the contact point is from about 0.7 to about 3.0.

More preferably, said ratio is about 1, i.e. the shear stress of the inner semiconducting layer at the radially inner wall of the first extrusion die is essentially equal to the shear stress of the insulating layer at the radially outer wall of the first extrusion die.

In the present description and the following claims, the term "compression extrusion" of a layer made of a polymeric material means that the pressure of said material inside the extrusion die is higher than atmospheric pressure, and that at the exit of the extrusion die the diameter of the extruded polymer layer is greater than or equal to the inner diameter extrusion die, responsible for the formation of the aforementioned polymer layer (this means that at the exit of the extrusion head the diameter of the extruded polymer layer is not significantly reduced).

According to the present invention, the cable manufacturing method allows a very thin inner semiconducting layer to be obtained, i.e. inner semiconducting layer with a thickness less than or equal to 0.4 mm

Preferably, the thickness of the inner semiconducting layer is in the range of from about 0.05 mm to about 0.4 mm.

More preferably, the thickness of the inner semiconducting layer is in the range of from about 0.2 mm to about 0.3 mm.

As described in document WO 04/003940, the Applicant further observed that by providing a cable with a protective element containing a foamed polymer layer suitable for imparting a predetermined resistance to accidental shock to the cable, the cable structure can be made more compact than a conventional cable.

In fact, the Applicant has found that by providing the cable with a protective element containing a foamed polymer layer, it is possible to advantageously reduce the thickness of the insulating layer of the cable to an electrical load comparable to the electric strength of the insulating material. In addition, it is possible to reduce the thickness of the inner and outer semiconducting layers surrounding the insulating layer, obtaining a more compact design of the cable core without reducing the properties of its electrical and mechanical resistance.

Preferably, the manufacturing method of the present invention is suitable for producing an electric cable that is provided with a solid rod conductor, the profile of the outer contour of which has a regular round section.

Alternatively, the cable conductor may be made of twisted metal wires, provided that the profile of the outer contour of the twisted structure does not contain inhomogeneities due to the composite structure of the conductor, the combination of which cannot be smoothed from an electrical point of view by an internal semiconducting layer.

Preferably, the cable manufacturing method of the present invention is a continuous method, i.e. the cable is produced in the absence of intermediate phases of rest or storage.

In the present description and the following claims, “continuous method” means a method in which the time required to produce a given cable length is inversely proportional to the speed of cable advancement in the line, so that intermediate phases of rest between the conductor feed and the finished cable exit are eliminated.

Preferably, the line speed in the method according to the present invention is from about 30 to about 100 m / min.

The method of the present invention further comprises the step of providing an external semiconducting layer surrounding the insulating layer to form a cable core.

Then, the method of the present invention further comprises the step of cooling the cable core, for example, by passing the cable through an elongated open channel in which the cooling fluid flows. Water is a preferred example of such a cooling fluid.

Preferably, the method of the present invention further comprises the step of drying the chilled cable core so as to remove residual cooling fluid, such as moisture or water droplets, especially when such residues are detrimental to the overall performance of the cable.

In addition, the method of the present invention further comprises the step of providing a metal sheath around the cable core, which can be formed from a folded length of metal sheet or from spirally wound wires or tapes.

In addition, preferably, the method of the present invention comprises the step of applying a shockproof element around a metal sheath. Preferably, said shockproof element is applied by extrusion. Preferably, said shockproof element comprises an unfoamed polymer layer and a foamed polymer layer. Preferably, the foamed polymer layer extends radially outward with respect to the non-foamed polymer layer. Preferably, the non-foamed polymer layer and the foamed polymer layer are applied by coextrusion.

Typically, the method of the invention further comprises the step of applying an outer shell around the metal shell. Preferably, the outer shell is applied by extrusion.

Finally, the method of the present invention comprises an additional cooling step and then a winding step to collect the finished cable to a reel.

The present invention is advantageously applicable not only to electric cables for transmitting or distributing electricity, but also to cables of mixed power / telecommunication type, which include an optical fiber core. In this sense, therefore, in the remainder of the present description and the following claims, the term “conductor” means a metal conductor or a mixed electric / optical type conductor.

Additional details will be illustrated in the following detailed description with reference to the accompanying drawings, in which:

• Figure 1 is a perspective view of an electric cable obtained using the manufacturing method of the present invention;

• FIG. 2 is a partial longitudinal sectional view of an extrusion head according to a conventional method for manufacturing a unipolar electrical cable;

• FIG. 3 is a partial longitudinal sectional view of an extrusion die according to the manufacturing method of the present invention, and

• Figures 4 and 5 are schematic partial views of the distribution of velocities and the distribution of shear stresses of Newtonian and non-Newtonian fluids, respectively, flowing through an annular channel.

Figure 1 shows a perspective view, partially in cross section, of an electric cable 1, which is advantageously obtained by the manufacturing method of the present invention, typically intended for use in the medium and high voltage range.

Cable 1 has a cable core that contains a conductor 2, an inner semiconducting layer 3, an insulating layer 4, and an outer semiconducting layer 5.

According to the preferred embodiment shown in FIG. 1, conductor 2 is a metal rod, preferably made of copper or aluminum. Alternatively (not shown in FIG. 1), conductor 2 comprises at least two metal wires, preferably of copper or aluminum, which are twisted together according to any conventional core forming technique.

The cross-sectional area of conductor 2 is determined in relation to the transmitted power at the selected voltage. Preferred cross-sectional areas for compact cables produced by the method of the present invention are in the range of 16 to 1000 mm ² .

Typically, the insulating layer 4 is made of a crosslinked or non-crosslinked polymer composition, for example, selected from: polyolefins (homopolymers or copolymers of different olefins), copolymers of olefin / ethylenically unsaturated ester, polyesters, polyethers, copolymers of polyester / polyester and mixtures thereof. Examples of said polymers are: polyethylene (PE), in particular linear low density PE (LLDPE); polypropylene (PP); thermoplastic propylene / ethylene copolymers; ethylene-propylene rubbers (EPA) or rubbers based on a copolymer of ethylene, propylene and diene monomer (EPDM); natural rubbers; butyl rubbers; ethylene / vinyl acetate copolymers (EVA); ethylene / methyl acrylate copolymers (EMA); ethylene / ethyl acrylate (EEA) copolymers; ethylene / butyl acrylate copolymers (EBA); ethylene / α-olefin copolymers and the like.

Preferably, said insulating layer 4 is made of a non-crosslinked base polymer material.

As used herein, the term “insulating material” is used to mean a material having a dielectric strength of at least 5 kV / mm, preferably greater than 10 kV / mm. For medium-high voltage power transmission cables, the insulating material has a dielectric strength greater than 40 kV / mm.

Preferably, the insulating material of the insulating layer 4 is an unfoamed polymeric material. In the present invention, the term “non-foamed” polymer material is used to mean a material that is substantially free of empty space within its structure, i.e. material having essentially zero degree of expansion, as best explained in the next part of the present description. In particular, said insulating material has a density of 0.85 g / cm ³ or more.

Typically, the insulating layer of power transmission cables has a dielectric constant (K) of greater than 2.

The inner semiconducting layer 3 and the outer semiconducting layer 5, both non-foamed, are prepared according to the method of the present invention (as described in detail in the next part of the present description), the main polymer material and carbon black (the latter being used to make the said layers electrically semiconducting) selected from those indicated in the next part of the present description.

In a preferred embodiment of the present invention, the inner and outer semiconductor layers 3, 5 comprise a non-crosslinked base polymer material, more preferably a polypropylene compound.

In addition, the cable 1 further comprises a metal sheath 6 that surrounds the cable core. According to the embodiment shown in FIG. 1, the metal sheath 6 is made of a continuous metal sheet that is shaped like a tube. Preferably, the metal sheath is made of aluminum or, alternatively, of copper. In some cases, lead can also be used.

The metal sheath 6 is folded around the outer semiconducting layer 5, the overlapping edges having padded sealing material so as to make the metal sheath waterproof. Alternatively, the metal shell is welded.

Alternatively, the metal sheath 6 is made of spirally wound metal wires or strips located around said outer semiconducting layer 5.

Typically, the metal shell is coated with an outer shell (not shown in FIG. 1) consisting of a crosslinked or non-crosslinked polymeric material, for example, polyvinyl chloride (PVC) or polyethylene (PE).

According to the embodiment shown in FIG. 1, in a position radially external to the metal sheath 6, the cable 1 is provided with a protective element 7. According to the said embodiment, the protective element 7 comprises a foamed polymer layer 9 that is sandwiched between two non-foamed polymer layers, external (first) non-foamed polymer layer 10 and internal (second) non-foamed polymer layer 8, respectively. The protective element 7 performs the function of protecting the cable from any external impact that occurs with the cable by at least partially absorbing said impact.

According to European patent No. 981821 in the name of the Applicant, the polymer material constituting the foamed polymer layer 9 can be any type of expandable polymer, such as, for example: polyolefins, copolymers of different olefins, copolymers of olefin with ethylenically unsaturated ester, polyesters, polycarbonates, polysulfones, phenolic resins, urea resins and mixtures thereof. Examples of suitable polymers are: polyethylene (PE), in particular low density PE (LDPE), medium density PE (MESP), high density PE (HDPE), linear low density PE (LLDPE), ultra low density polyethylene (PESNP; polypropylene ( PP); elastomeric ethylene / propylene copolymers (EPR) or triple copolymers of ethylene / propylene / diene monomer (EPDM); natural rubber; butyl rubber; ethylene / vinyl ester copolymers, e.g. ethylene / vinyl acetate (EVA); ethylene / acrylate copolymers, in particular ethylene / meth lacrylate (EMA), ethylene / ethyl acrylate (EEA) and ethylene / butyl acrylate (EBA); thermoplastic copolymers of ethylene / α-olefin; polystyrene; copolymers of acrylonitrile / butadiene / styrene (ABS); halogenated polymers, in particular, polyvinyl chloride; polyurethane (PUR); polyamides; aromatic polyesters such as polyethylene terephthalate (PET) or polybutylene terephthalate (PBT); and their copolymers or mechanical mixtures thereof.

For the purposes of the present invention, the term "foamed" polymer refers to a polymer in the structure of which the percentage of "empty" volume (that is, the space occupied not by the polymer, but by gas or air) is usually more than 10% of the total volume of said polymer.

Typically, the percentage of free space in a foamed polymer is expressed in units of expansion ratio (G). In the present description, the term "degree of expansion of the polymer" refers to the expansion of the polymer, defined as follows:

G (degree of expansion) = (d ₀ / d _e -1) · 100,

where d ₀ denotes the density of the non-foamed polymer (i.e., a polymer with a structure that essentially has no empty volume), and d _e denotes the observed density measured for the foamed polymer.

Preferably, the expansion ratio of said foamed polymer layer 9 is selected in the range from 25% to 160%, more preferably from 40% to 140%.

Preferably, the two non-foamed polymer layers 8, 10 of said security element 7 are made of polyolefin materials.

Two non-foamed polymer layers 8, 10 can be made of a polymer material selected from the group consisting of: low density polyethylene (LDPE) (d = 0.910-0.926 g / cm ³ ); copolymers of ethylene with α-olefins; polypropylene (PP); rubbers based on ethylene / α-olefin copolymer, in particular ethylene-propylene rubbers (EPR), rubbers based on ethylene / propylene / diene monomer copolymer (EPDM); natural rubber; butyl rubber and mixtures thereof.

Preferably, the two non-foamed polymer layers 8, 10 are made of a thermoplastic material, preferably a polyolefin, such as non-crosslinked polyethylene (PE); alternatively, polyvinyl chloride (PVC) can be used.

In the embodiment shown in FIG. 1, cable 1 is further provided with a water blocking layer 11 located between the outer semiconducting layer 5 and the metal sheath 6.

According to a preferred embodiment of the invention, the water blocking layer 11 is a foamed water-swelling semiconducting layer, as described in WO 01/46965 in the name of the Applicant.

Preferably, said water blocking layer 11 is made of a foamed polymeric material into which a water-swellable material is embedded or dispersed.

Preferably, the expandable polymer of said water blocking layer 11 is selected from the above polymeric materials.

Said water blocking layer 11 is intended to provide an effective barrier to the longitudinal penetration of water into the cable.

A water-swellable material usually consists of a homopolymer or copolymer having hydrophilic groups along the polymer chain, for example: crosslinked and at least partially salt-converted polyacrylic acid (for example, Cabloc® products from C. F. Stockhausen GmbH or Waterlock® from Grain Processing Co.); starch or its derivatives mixed with copolymers of acrylamide and sodium acrylate (for example, SGP Absorbent Polymer® products from Henkel AG); sodium carboxymethyl cellulose (e.g., Blanose® products from Hercules Inc.).

In addition, the foamed polymeric material of the water blocking layer 11 can be modified to be semi-conductive.

The amount of carbon black added to the polymer matrix may vary depending on the type of polymer and carbon black used, the degree of expansion to be obtained, a blowing agent, etc. Thus, the amount of carbon black should be such as to impart sufficient semiconducting properties to the foamed material, in particular so as to obtain a volume resistivity of the foamed material at room temperature of less than 500 Ohm · m, preferably less than 20 Ohm · m. Typically, the amount of carbon black can vary from 1 to 50% by weight, preferably from 3 to 30% by weight relative to the weight of the polymer.

A preferred range for the degree of expansion of the water blocking layer 11 is from 10% to 50%.

In addition, by supplying the cable 1 with a semiconducting water-blocking layer 11, the thickness of the outer semiconducting layer 5 can be advantageously reduced, since the electrical properties of the outer semiconducting layer 5 are partially realized by said water-blocking layer. Therefore, this feature contributes favorably to reducing the thickness of the outer semiconducting layer and, thus, the mass of the entire cable.

Figure 2 shows a partial view in longitudinal section of a conventional extrusion head for the manufacture of a core of a unipolar electric cable.

The cable conductor 2 is unwound from the feed coil and fed to extrusion equipment equipped with a three-layer extrusion head, said equipment comprising three separate extruders feeding into a common extrusion head so as to deposit an internal semiconducting layer 3 ′, an insulating layer 4 ′, and an external semiconducting layer 5 'to conductor 2.

Figure 2 shows a partial view in longitudinal section of a three-layer extrusion head 20 of extrusion equipment, which is known per se and therefore not shown in its entirety.

The extrusion head 20 comprises a male die 21, a first intermediate die 22, a second intermediate die 23 and a female die 24. Said die are arranged in the above sequence, concentrically overlapping each other and radially moving away from the Z-Z axis of conductor 2.

More specifically, arrow A indicates the direction of advancement of the cable conductor 2, in a radially external position with respect to which the inner semiconducting layer 3 ′ is extruded through a channel 25 provided between the male die 31 and the first intermediate die 32. The insulating layer 4 ′ is extruded in a radially position external to the inner semiconducting layer 3 ', through a channel 26, which is located between the first intermediate die 22 and the second intermediate die 23. The external semiconductor layer 5' ex rudiruetsya in a position radially external to the insulating layer 4 ', through the passageway 27 which is located between the intermediate die 23 and female die 24.

Arrow B shows the direction of exit of the cable core from the extrusion head 20.

According to a conventional manufacturing method known in the art and provided by an extrusion head 20, as partially and schematically shown in FIG. 2, as the conductor 2 is unwound from a feed coil (not shown), the polymer compositions of the inner semiconducting layer 3 ′ insulating layer 4 'and the outer semiconductor layer 5' respectively separately fed to the input of each extruder (not shown), for example, using separate bins that are located upstream relative to the channels 25, 26 and 27 co respectively.

The preparation of said polymer compositions may require a step of pre-mixing the base polymer material with other components (e.g., fillers or additives), said step of pre-mixing being carried out in equipment located upstream of the extrusion process, such as, for example, a closed tangential rotary mixer (Banbury) or with interpenetrating rotors, or in a continuous mixer type Ko-Kneader (for example, manufactured by Buss) or a type having two screws, rotating in the same or opposite directions.

Each polymer composition is usually fed into the corresponding extruder in the form of granules and plasticized, that is, it is transferred to the molten state by supplying heat (through the outer cylinder of the extruder) and the mechanical action of the screw, which processes the polymer material and extrudes it into the corresponding extrusion channel in the exit direction of each channel, forming the desired coating layer.

According to the conventional extrusion die 20 shown in FIG. 2, the flows of various materials forming the layers of the cable core (i.e., the inner semiconducting layer 3 ', the insulating layer 4' and the outer semiconducting layer 5 ') are kept separate from each other and separately extruded onto a molded cable core.

In more detail, as clearly shown in FIG. 2, first, the inner semiconducting layer 3 ′ is extruded directly onto the cable conductor 2; then, the insulating layer 4 ′ is extruded onto the inner semiconducting layer 3 ′ (the latter has already connected to the conductor 2), and finally, the outer semiconducting layer 5 ′ is extruded to the insulating layer 4 ′ when the latter has already joined to the inner semiconducting layer 3 ′.

Such a specific extrusion sequence is obtained by means of a set of dies, schematically and partially shown in FIG. 2, according to which the first intermediate die 22 has an axial extension (along the direction of advancement of the conductor) that is greater than the corresponding length of the male die 21, so that the inner semiconducting material is directly extruded onto conductor 2 cables.

In more detail, the first intermediate die 22 is provided with an elongated portion 28 that is coaxially (coaxially) with respect to the cable conductor 2, said elongated portion performing the function of directing the inner semiconducting layer 3 ′ to the conductor 2 and holding said semiconducting layer pressed against the conductor for a sufficiently long length distance and, therefore, a sufficiently long period of time in order to obtain a uniform and homogeneous thickness of the inner semiconducting layer.

FIG. 3 is a partial longitudinal sectional view of an extrusion die 40 for use in the manufacturing method of the present invention, wherein the structural elements of FIG. 3, which are similar or identical to the corresponding structural elements of FIG. 2, are denoted in this description by the same reference numbers.

According to the manufacturing method of the present invention, the inner semiconducting layer 3 is in contact with the insulating layer 4 in position C, which is located upstream relative to the contact point D, where the cable conductor 2 and the inner semiconducting layer 3 come into mutual contact.

In more detail, in the extrusion head 40 of the present invention, the first intermediate die 22 is shifted backward (along the direction of advancement of the conductor) relative to the axial end of the male die 21, so that the annular flow of internal semiconducting material (which extrudes along the channel 25) and the annular flow of insulating material (which extruded along the channel 26) are in contact with each other before drawing on the conductor 2 of the cable. Thus, the inner semiconducting layer is allowed to come into contact with the conductor 2 when the inner semiconducting layer flows together with the insulating layer.

Preferably, the contact point C, i.e. the point at which the outer surface of the annular flow of internal semiconducting material and the inner surface of the annular flow of insulating material come into contact with each other is located at an axial distance x relative to the contact point D, i.e. the point at which the inner semiconducting layer comes into contact with the conductor 2.

Preferably, said distance x is greater than or equal to 0.5 of the diameter of the conductor.

As used herein, the term “axial distance” means a distance that is calculated along an axial direction, i.e. along the direction of advancement of the conductor.

Preferably, said distance x is from about 0.6 to about 10 diameters of the conductor, more preferably from about 1 to about 4 diameters of the conductor.

Preferably, the contact point C is located on the diameter Ф relative to the longitudinal axis Z-Z of the cable conductor, greater than or equal to 1.5 diameters of the conductor.

Preferably, said diameter Φ is from about 1.8 to about 4 diameters of the conductor, more preferably from about 2 to about 3 diameters of the conductor.

Figure 4 is a schematic and partial view of the distribution of 50 speeds and distribution 60 of the shear stresses of the material of the inner semiconducting layer flowing in the annular channel formed by dies 21 and 22 in the extrusion head 20 according to the prior art, and accordingly, the distribution of 50 'speeds and distribution 60 'tangential stresses of the material of the insulating layer flowing in the annular channel formed by the dies 22 and 23 in the same extrusion die according to the prior art, zyatyh in a certain longitudinal position indicated by the plane x1.

As is known in the art, the shear stress τ represents the friction force that a single fluid layer must overcome to slide along an adjacent fluid layer. At a given temperature, the tangential stress τ of the fluid is related to the shear rate γ˙ (i.e., γ˙ = dγ / · dt ),

Shear rate is the slope of the velocity distribution profile.

In the simplest version, for Newtonian fluid, this ratio is:

τ = η · γ,

where η is the dynamic viscosity of the fluid in question.

In a non-Newtonian fluid, and this is just the case of the materials under consideration, this mathematical expression takes a more complex form:

τ = f (T, γ ).

Typically, the velocity distribution profile 50, 50 ′ of the fluid materials flowing through the respective annular channels of the extrusion head is such that the maximum fluid velocity is obtained in the flow volume, i.e. in the most remote place from the fixed surfaces bounding these channels, while the velocity of the flowing materials becomes zero at the outer diameter of the channel and at the inner diameter of the channel, i.e. at r = R 'and r = R ", as shown in the profile 50 of the distribution of speeds in figure 4.

In accordance with such velocity distribution profiles, shear stress distribution profiles 60, 60 ′ take place.

As shown in FIG. 4, the shear stress τ becomes zero in the flow volume, i.e. in the most remote place from the fixed surfaces bounding the annular channels, while the shear stress τ takes its maximum absolute value at the outer diameter of the channel and the inner diameter of the channel (i.e., near the fixed surfaces).

Based on such a profile of the distribution of shear stresses, it can be shown that for layers of small thickness, such as the inner semiconducting layer 3, the shear stress is quite high, and it additionally increases with a decrease in the channel cross section, in particular because the annular channel converges to the conductor, which leads to an increase in the speed of the current material when moving in the direction of the contact point D.

This situation remains the same until the inner semiconducting material comes into contact with the conductor and reaches its final outer diameter.

In the extrusion die 40 according to the present invention, as shown in FIG. 5, in the x1 position just before the end point C of the die 22, the velocity distribution profiles 50 ", 50" "and the shear distribution profiles 60", 60 "" are essentially the same as similar profiles described with reference to Fig.4.

However, in position x2, immediately upstream of the end point C, where the flows of the inner semiconducting layer and the insulating layer have already come into contact, the situation changes.

In fact, although the speeds of the two materials near the point of contact with the internal (21) and external (23) dies still remain close to or equal to zero, at the interface between the two current materials, the two speeds V1 and V2 become essentially the same.

As a result of this, since the speeds make it possible to change least sharply from zero to its maximum value, and such a change in speed occurs along the entire thickness of the inner semiconducting layer and the insulating layer, the corresponding shear rate reaches lower maximum values and, in particular, the shear rate is close to zero at the interface between two materials.

The convergence of the flows in the direction of the conductor causes the overall flow rate to increase due to a decrease in the flow cross section, but since the inner semiconducting layer and the insulating layer flow together, the maximum shear rate is kept low until the inner semiconducting layer comes into contact with the conductor 2.

Preferably, the longitudinal position where the flows of the inner semiconducting layer and the insulating layer come into contact (i.e., the position of the die end C) is such that the ratio between the shear stress of the inner semiconducting layer 3 at the radially inner wall of the first intermediate die 22 and the shear stress of the insulating layer 4 at the radially outer wall of the first intermediate die 22 near the contact point C is from about 0.5 to about 4.

To further describe the invention, some illustrative examples are given below.

Example 1 (invention)

Received unipolar cable medium voltage shown in figure 1 type.

For the purposes of the present invention, only the cable core (i.e., the inner semiconducting layer 3, the insulating layer 4 and the outer semiconducting layer 5) was manufactured by means of the extrusion head 40 shown in FIG.

The cable conductor was a solid rod made of aluminum and having a cross section of 150 mm ² . The speed of advancement of the conductor was set to approximately 60 m / min.

An inner semiconducting layer (HFDA-0801 polyethylene copolymer is a commercial product from Dow Chemicals) having a thickness of 0.2 mm, an insulating layer (crosslinked polypropylene under the brand name LE 4201 manufactured by Borealis) having a thickness of 4.0 mm, and an outer semiconducting layer (HFDA-0801 polyethylene copolymer is a commercial product from Dow Chemicals) having a thickness of 0.2 mm was obtained using the manufacturing method of the present invention.

We used a 90 mm extruder (from Costruzioni Meccaniche Luigi Bandera SpA - Busto Arsizio (VA) - Italy) in a 25 D configuration (i.e., with a screw 25 times longer than its diameter), a 160 mm Bandera single screw extruder in configuration 30 D and a 90 mm Bandera extruder in a 25 D configuration for applying respectively an inner semiconducting layer, an insulating layer and an outer semiconducting layer.

Contact point C, i.e. the point at which the outer surface of the annular flow of internal semiconducting material and the inner surface of the annular flow of insulating material come into contact with each other, was located at an axial distance x measured from the point of contact D along the longitudinal direction Z-Z of the cable and equal to 0.69 diameter of the conductor.

The extruder productivity of the internal semiconducting material was set to 33.5 kg / h, while the extruder performance of the insulating material was set to 720 kg / h.

The inner diameter of the annular flow of the internal semiconducting material at the point of contact C (i.e., the inner diameter of the male die 21 at point E, obtained at the intersection of the male die with a plane YY perpendicular to the axis ZZ of the conductor and touching the first intermediate die 22 at point C) was 25 , 2 mm.

The outer diameter of the annular flow of the inner semiconducting material (i.e., the diameter of the first intermediate die 22 at point C) was 26.4 mm.

The inner diameter of the annular flow of insulating material at the contact point C was 26.4 mm.

The outer diameter of the annular flow of insulating material (i.e., the outer diameter of the second intermediate die 23 at point F obtained at the intersection of the second intermediate die with the Y-Y plane) was 36.8 mm.

The extrusion temperature of the inner semiconducting material and the insulating material was set to 130 ° C.

Having established the performance of the extruders of the inner semiconducting material and the insulating material, the temperature of their extrusion, and also the geometry of the dies, the ratio between the tangential stress of the inner semiconducting layer at the radially inner wall of the first intermediate die and the tangent stress of the insulating layer at the radially outer wall of the first intermediate die near the contact point C (i.e. before coming into mutual contact), taking into account the dynamic viscosities of said materials (measured by capillary rheological analysis using a Göttfert Laboratory rheometer Rheograph 2001 equipped with a capillary having an internal diameter of 1 mm and a length of 20 mm) as a function of extrusion temperature and shear rate.

In more detail, Tables 1 and 2 show dynamic viscosities as a function of extrusion temperature and shear rate for the internal semiconducting material and the insulating material, respectively.

Table 1 Internal semiconducting material
HFDA-0801 from Dow Chemicals Shear rate
(s ^-1 ) Dynamic viscosity
(Pass) Temperature
(° C) 10 26130 110 fifty 10380 110 one hundred 6545 110 1000 1114 110 2500 519 110 10 26370 120 fifty 9404 120 one hundred 5886 120 1000 1038 120 2500 482 120 10 24660 130 fifty 8867 130 one hundred 5568 130 1000 1003 130 2500 469 130

table 2 Insulating material
LE 4201 by Borealis Shear rate
(s ^-1 ) Dynamic viscosity
(Pass) Temperature
(° C) 10 8222 120 fifty 2377 120 one hundred 1498 120 1000 340 120 2500 167 120 10 7327 130 fifty 2051 130 one hundred 1278 130 1000 277 130 2500 2500 130

The ratio between the shear stress of the inner semiconducting layer and the shear stress of the insulating layer was 3.7.

An optical examination was carried out on the elements making up the cable core (for example, by means of a scanning electron microscope), which confirmed the homogeneous and uniform thickness (in the radial and longitudinal directions) of the inner semiconducting layer and the insulating layer, as well as the absence of gaps or defects on their interface.

Example 2 (invention)

Received unipolar cable medium voltage shown in figure 1 and described in example 1 type.

For the purposes of the present invention, only the cable core (i.e., the inner semiconducting layer 3, the insulating layer 4 and the outer semiconducting layer 5) was manufactured using the extrusion head 40 shown in FIG.

The cable conductor was a solid rod made of aluminum and having a cross section of 150 mm ² . The speed of advancement of the conductor was set to approximately 58 m / min.

An inner semiconducting layer (made of semiconducting material shown in table 3) having a thickness of 0.2 mm, an insulating layer (made of semiconducting material shown in table 3) having a thickness of 2.6 mm, and an outer semiconducting layer (made of the semiconducting material shown in table 3), having a thickness of 0.2 mm, was obtained using the manufacturing method of the present invention.

Table 3 Inner and outer semiconducting layers
(% by weight) Insulating layer
(% by weight) Adflex ^® Q 200 F 60,4 - Hifax ^® CA 7320 A - 47 Moplen ^® RP210G - 47 Ensaco ^® 250 G 33 - Jarylec ^® Exp3 6 5,4 Irganox ^® PS 802 0.4 0.4 Irganox ^® 1010 0.2 0.2

Adflex ^® Q 200 F: heterophasic propylene copolymer with a melting point of 165 ° C, a melting enthalpy of 30 J / g, a melt flow index (MFI) of 0.8 dg / min and a bending modulus of 150 MPa (commercial product from Basell);

Hifax ^® CA 7320 A: a thermoplastic polyolefin with a high content of rubber (commercial product of Basell);

Moplen ^® RP210G: polypropylene random copolymer (commercial product of Basell);

Ensaco ^® 250 G: furnace black (commercial product from Erachem Europe);

Jarylec ^® Exp3 (commercial product from Elf Atochem): dibenzyltoluene (DBT);

Irganox ^® PS 802 (antioxidant): distearyl thiodipropionate (commercial product from Ciba Specialty Chemicals);

Irganox ^® 1010 (antioxidant): pentaerythrityl-tetrakis (3- (3,5-di-tertbutyl-4-hydroxyphenyl) propionate (commercial product from Ciba Specialty Chemicals).

A 90 mm Bandera extruder in a 25 D configuration, a 160 mm Bandera single screw extruder in a 30 D configuration, and a 90 mm Bandera extruder in a 25 D configuration were used to deposit an inner semiconducting layer, an insulating layer, and an outer semiconducting layer, respectively.

Contact point C, i.e. the point at which the outer surface of the annular flow of internal semiconducting material and the inner surface of the annular flow of insulating material come into contact with each other, was located at an axial distance x from the contact point D of 0.69 of the diameter of the conductor.

The extruder productivity of the internal semiconducting material was set to 32.6 kg / h, while the extruder performance of the insulating material was set to 450 kg / h.

The inner diameter of the annular flow of the internal semiconducting material at the contact point C was 25.2 mm.

The outer diameter of the annular flow of the inner semiconducting material was 26.4 mm.

The inner diameter of the annular flow of insulating material at the contact point C was 26.4 mm.

The outer diameter of the annular flow of insulating material was 36.8 mm.

The extrusion temperature of the internal semiconducting material and the insulating material was set to 230 ° C.

Having established the performance of the extruders of the inner semiconducting material and the insulating material, the temperature of their extrusion, and also the geometry of the dies, the ratio between the tangential stress of the inner semiconducting layer at the radially inner wall of the first intermediate die and the tangent stress of the insulating layer at the radially outer wall of the first intermediate die near the contact point C (i.e. before coming into mutual contact), taking into account the dynamic viscosities of said materials (measured by capillary rheological analysis using a Göttfert Laboratory rheometer Rheograph 2001 equipped with a capillary having an internal diameter of 1 mm and a length of 20 mm) as a function of extrusion temperature and shear rate.

In more detail, Tables 4 and 5 show dynamic viscosities as a function of extrusion temperature and shear rate for the internal semiconducting material and the insulating material, respectively.

Table 4 Internal semiconducting material
(as indicated in table 3) Shear rate
(s ^-1 ) Dynamic viscosity
(Pass) Temperature
(° C) 10 6106 180 fifty 2735 180 one hundred 1758 180 1000 329 180 2500 167 180 10 5866 200 fifty 2629 200 one hundred 1604 200 1000 - 200 2500 - 200

Table 5 Insulating material
(as indicated in table 3) Shear rate
(s ^-1 ) Dynamic viscosity
(Pass) Temperature
(° C) 10 6024 170 fifty 2328 170 one hundred 1555 170 1000 366 170 2500 224 170 10 4640 190 fifty 1954 190 one hundred 1335 190 1000 267 190 2500 132 190 10 210 fifty 3582 210 one hundred 1628 210 1000 234 210 2500 117 210

The ratio between the shear stress of the inner semiconducting layer and the shear stress of the insulating layer was 3.0.

An optical examination was carried out on the elements making up the cable core (for example, by means of a scanning electron microscope), which confirmed the homogeneous and uniform thickness (in the radial and longitudinal directions) of the inner semiconducting layer and the insulating layer, as well as the absence of gaps or defects on their interface.

Example 3 (invention)

A cable similar to that of Example 2 was obtained, and the only difference was that the extruder performance of the internal semiconducting material was set to 33.5 kg / h, and the performance of the extruder of the insulating material was set to 720 kg / h.

The ratio between the shear stress of the inner semiconducting layer and the shear stress of the insulating layer was 2.6.

An optical examination carried out as described in examples 1 and 2, confirmed the homogeneous and uniform thickness (in the radial and longitudinal directions) of the inner semiconducting layer and the insulating layer, as well as the absence of gaps or defects on their interface.

Example 4 (invention)

Received a cable similar to the cable from example 2, and the only difference was that: 1) the extruder performance of the internal semiconducting material was set to 23.9 kg / h; 2) the performance of the extruder of the insulating material was set at 720 kg / h; 3) the speed of advancement of the conductor was set at approximately 43 m / min, and 4) the thickness of the insulating layer was approximately 5.5 mm.

The ratio between the shear stress of the inner semiconducting layer and the shear stress of the insulating layer was 2.4.

An optical examination carried out as described in examples 1 and 2, confirmed the homogeneous and uniform thickness (in the radial and longitudinal directions) of the inner semiconducting layer and the insulating layer, as well as the absence of gaps or defects on their interface.

Example 5 (comparative)

Received unipolar cable medium voltage shown in figure 1 and described in example 2 type.

The cable conductor was a solid rod made of aluminum and having a cross section of 150 mm ² . The speed of advancement of the conductor was set to approximately 58 m / min.

An inner semiconducting layer (HFDA-0801 polyethylene copolymer is a commercial product from Dow Chemicals) having a thickness of 0.2 mm, an insulating layer (crosslinked polypropylene under the brand name LE 4201 manufactured by Borealis) having a thickness of 2.6 mm, and an outer semiconducting layer (HFDA-0801 polyethylene copolymer is a commercial product from Dow Chemicals) having a thickness of 0.2 mm was obtained using the manufacturing method of the present invention.

A 90 mm Bandera extruder in a 25 D configuration, a 160 mm Bandera single screw extruder in a 30 D configuration, and a 90 mm Bandera extruder in a 25 D configuration were used to deposit an inner semiconducting layer, an insulating layer, and an outer semiconducting layer, respectively.

Contact point C, i.e. the point at which the outer surface of the annular flow of internal semiconducting material and the inner surface of the annular flow of insulating material come into contact with each other, was located at an axial distance x measured from the point of contact D along the longitudinal direction Z-Z of the cable and equal to 0.24 diameter of the conductor.

The extruder productivity of the internal semiconducting material was set to 32.6 kg / h, while the extruder performance of the insulating material was set to 450 kg / h.

The inner diameter of the annular flow of the internal semiconducting material at the contact point C was 18.0 mm.

The outer diameter of the annular flow of the inner semiconducting material was 18.5 mm.

The inner diameter of the annular flow of insulating material at the contact point C was 18.5 mm.

The outer diameter of the annular flow of insulating material was 33.0 mm.

The extrusion temperature of the inner semiconducting material and the insulating material was set to 130 ° C.

Having established the performance of the extruders of the inner semiconducting material and the insulating material, the temperature of their extrusion, and also the geometry of the dies, the ratio between the tangential stress of the inner semiconducting layer at the radially inner wall of the first intermediate die and the tangent stress of the insulating layer at the radially outer wall of the first intermediate die near the contact point C (i.e. before coming into mutual contact), taking into account the dynamic viscosities of said materials (measured by capillary rheological analysis using a Göttfert Laboratory rheometer Rheograph 2001 equipped with a capillary having an internal diameter of 1 mm and a length of 20 mm) as a function of extrusion temperature and shear rate.

In more detail, Tables 1 and 2 show dynamic viscosities as a function of extrusion temperature and shear rate for the internal semiconducting material and the insulating material, respectively.

The ratio between the shear stress of the inner semiconducting layer and the shear stress of the insulating layer was 10.1.

An optical examination was carried out on the elements making up the cable core (for example, by means of a scanning electron microscope) and the mutual penetration of the inner semiconducting layer and the insulating layer (i.e., the inhomogeneous and inhomogeneous thickness of the mentioned layers), as well as ruptures of the inner semiconducting layer in some parts of the interface mentioned layers.

Claims (22)

1. A method of manufacturing a cable (1) containing a conductor (2), an inner semiconducting layer (3) surrounding said conductor and having a thickness less than or equal to 0.4 mm, and an insulating layer (4) surrounding said inner semiconducting layer, wherein said method includes the steps of: feeding the conductor (2) with a predetermined feed rate into the extrusion head (40) containing the first extrusion channel (25) for extruding the inner semiconducting layer and the second extrusion channel (26) for extruding the insulating layer, said first extrusion channel and said second extrusion the channel is separated from each other by a first extrusion die (22), and coextruding the inner semiconducting layer and the insulating layer, wherein said coextrusion step includes: providing a first annular flow of internal semiconducting material and a second annular flow of insulating material; contacting the outer surface of said first annular flow and the inner surface of said second annular flow at an axial distance (x) from the contact point (D), where the inner surface of said first annular flow comes into contact with a conductor; combining said predefined feed rate and said contact point (D) as a function of the dynamic viscosity (n) of the inner semiconducting material and the insulating material such that the ratio between the shear stress of the inner semiconducting layer at the radially inner wall of the first extrusion die and the shear stress of the insulating the layer at the radially outer wall of said first extrusion die near said contact point is from about 0.5 to 4; compression extrusion of the insulating layer and the inner semiconducting layer on the conductor. 2. The method according to claim 1, wherein said axial distance (x) is greater than or equal to 0.5 of the diameter of the conductor. 3. The method according to claim 2, in which said axial distance (x) is in the range from about 0.6 to about 10 diameters of the conductor. 4. The method according to claim 3, in which said axial distance (x) is in the range from about 1 to about 4 diameters of the conductor. 5. The method according to claim 1, wherein said ratio is from about 0.7 to about 3.0. 6. The method according to claim 5, in which said ratio is approximately 1. 7. The method according to claim 1, in which the contact point (C) of the outer surface of the annular flow of internal semiconducting material and the inner surface of the annular flow of insulating material is on a diameter (F) greater than or equal to 1.5 of the diameter of the conductor (2). 8. The method according to claim 7, in which said diameter (Φ) is from about 1.8 to about 4.0 diameters of the conductor (2). 9. The method according to claim 8, in which said diameter (Φ) is from about 2.0 to about 3.0 diameters of the conductor (2). 10. The method according to claim 1, in which the thickness of the inner semiconducting layer (3) is in the range from about 0.1 to about 0.4 mm. 11. The method of claim 10, wherein said thickness is in the range from about 0.2 mm to about 0.3 mm. 12. The method according to claim 11, in which the conductor (2) is a solid rod. 13. The method according to claim 1, further comprising the step of providing an external semiconducting layer (5) surrounding said insulating layer (4) to obtain a cable core. 14. The method according to claim 1, further comprising the step of cooling the cable core. 15. The method according to claim 1, further comprising the step of drying the cable core. 16. The method according to claim 1, further comprising the step of providing a metal sheath (6) around said cable core. 17. The method according to clause 16, further comprising the step of applying a shockproof element (7) around the metal shell (6). 18. The method according to clause 16, further comprising the step of applying the outer shell (10) around the metal shell (6). 19. The method according to claim 1, wherein said predetermined feed rate is from about 30 to about 100 m / min. 20. The method according to claim 1, in which the thickness of the insulating layer (4) is not more than 2.5 mm 21. The method according to claim 1, in which the inner semiconducting layer (3) is made of thermoplastic material. 22. The method according to claim 1, in which the insulating layer (4) is made of thermoplastic material.

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