EP2184744B1 - Electric wire with PTFE low dieletric constant sheath, manufacturing process and tool - Google Patents

Description

The invention relates to an electrical wire comprising at least one single-stranded or stranded electrical conductor, in a sheath based on PTFE (polytetrafluoroethylene), and a method and a manufacturing tool that make it possible to manufacture it. It also relates to the coaxial cable that can be formed around such an electric wire.

The term conductor here denotes a bare conductor, but may optionally include a conductive outer plating, for example a silver plating or other. The conductor can be single-stranded or stranded.

By "PTFE-based sheath" is meant here a sheath predominantly composed of PTFE, which may further comprise other fillers such as dyes, etc.

The electrical wire targeted by the invention is a wire intended for the transmission of a signal, in particular at high or very high frequency. Insulation plays a very important role in the electrical performance of an electrical cable, and particularly in the field of high frequency signal transmission. The speed of propagation of electromagnetic waves depends on the dielectric constant ε of the insulator, and is inversely proportional to the square root of the latter. In addition the attenuation of the transmitted signals is a function involving the frequency and the dielectric constant of the insulating medium. Materials which have the lowest possible dielectric constant for insulation are thus sought, so as to produce low attenuation cables. Fluorinated polymers, PTFE in particular, are often chosen for this application because they have dielectric constants ε very low, from 2.0 to 2.1.

To further reduce the dielectric constant, with respect to these materials, a first solution is to choose an insulating material further comprising the porous material. This brings a considerable improvement in performance because the dielectric constant of the air is only 1; the presence of air in the insulation thus makes it possible to lower the dielectric constant. For example, cables insulated with expanded PTFE may have a dielectric constant between 1.7 and 1.3 depending on the porosity of the PTFE.

Another solution to lower the dielectric constant is to develop in the insulation continuous longitudinal cells filled with air.

The patent WO 2005/066979 to EI Dupont de Nemours thus describes a method of manufacturing electrical wires comprising one or more conductors, protected by a PTFE sheath, the sheath comprising continuous longitudinal cells.

In the method of manufacturing electrical wires disclosed in this document, the cells of the electric wire obtained are separated from the conductor by a layer of PTFE.

A cross-section of such an electrical wire is illustrated by the figure 1 . This electrical wire 10 comprises a central conductor 12, and a sheath 14. The conductor 12 is a single-strand copper conductor of circular section. The sheath 14 is a hollow sheath of substantially cylindrical shape (on a substantially straight section of wire), inside which are formed four continuous longitudinal cells 16. The cells 16 are separated in pairs by separating partitions or spacers 18 The conductor is housed in a sheath 20 which is directly in contact with its outer surface, on which the radially inner ends of the struts 18 rest. The presence of the sheath 20 causes the cells 16 to be at a distance from the conductor 12. which limits their ability to reduce the dielectric constant of the electric wire.

A first object of the invention is therefore to propose an electrical wire comprising at least one conductor held by at least one spacer in a PTFE-based sheath, the sheath and said at least one spacer further forming at least one substantially longitudinal cell. continuous, said at least one cell being at least partially defined by the conductor, the electric wire having high performance, that is to say a dielectric constant ε very low, and having one or more cells having a high resistance to 'crushing.

This objective is achieved by the fact that seen in a section perpendicular to an axis of the wire, the PTFE fibers of said at least one spacer are mainly directed in a substantially radial direction.

As said at least one cell (it may be several cells) is formed in contact with the driver, its efficiency, in terms of reducing the dielectric constant E of the driver, is maximum. In fact, the closer the cells are to the conductor, the greater their effectiveness in reducing the dielectric constant of the insulator. Advantageously, the dielectric constant of this electric wire may be less than 1.7.

Furthermore, the spacer or spacers ensure the relative positioning of the conductor in the sheath. Because the PTFE fibers of this or these spacers are predominantly (and in fact in general, essentially) directed in the radial direction, the spacers or spacers have a high compressive strength and therefore the cells themselves. have a very good resistance to compression. It follows that the electric wire has a high durability, with a risk of crushing cells and therefore loss of dielectric properties very low.

In the preceding sentence, the direction 'radial' designates the direction in which are formed the spacer or spacers, that is to say, usually the radial direction. However, this direction may possibly not be radial; in this case, the radial direction designates the direction in which the spacers extend, according to the section of the electric wire.

According to one embodiment, a single cell is formed. There is therefore only one spacer for the maintenance of the driver. Preferably in this case, the angular position of the cell and the spacer vary along the wire. Note that it may also be useful to provide this same arrangement (angular position of the variable cell along the wire) in the case where the wire is formed with several cells.

The electrical wire according to the invention can in particular be formed with two to four cells, or more; two to four drivers, or more.

According to one embodiment, the sheath is kept at a distance from the conductor by said at least one spacer, the latter comprising a substantially radial junction portion, and a holding portion enlarged in contact with the conductor. The term 'enlarged' indicates here that the holding portion is wider than the joining portion. Advantageously, the enlarged part ensures a better support of the conductor by the spacer (compared to a spacer which does not include an enlarged part), without significantly penalizing neither the dielectric constant nor the weight of the cable, the junction part of the remaining spacer relatively thin.

The invention also relates to a coaxial cable comprising a central conductor, an insulating intermediate sheath based on PTFE, an outer conductor, an outer protective sheath and whose central conductor considered with the intermediate sheath is an electric wire as defined above according to the invention.

A second object of the invention is to define a method for manufacturing an electrical wire comprising at least one conductor in a PTFE-based sheath, the sheath and said at least one spacer forming at least one continuous longitudinal cell, which is passed a PTFE-based extrudate in an extrusion passage around said at least one conductor so as to form the sheath, and which allows the formation of a cell delimited at least partially by the driver, and thus the obtaining a very low dielectric constant electric wire E.

• sur une première partie du passage d'extrusion, en amont d'une section d'expansion, au moins une partie de la gaine est formée sensiblement à sa forme définitive à distance du conducteur par un guide, celui-ci maintenant l'extrudat à distance d'au moins une surface, dite surface libre, du conducteur ;
• en aval de la section d'expansion, ladite au moins une alvéole est formée sur ladite surface libre par au moins un profilé s'étendant dans le prolongement d'une partie du guide, la section de ladite alvéole correspondant à celle dudit profilé, au moins une entretoise de maintien de la partie de gaine sur le conducteur étant formée par remplissage d'au moins un canal d'expansion s'ouvrant entre des parois dudit au moins un profilé.

This goal is achieved thanks to the fact that

• on a first portion of the extrusion passage, upstream of an expansion section, at least a portion of the sheath is substantially formed to its final shape away from the conductor by a guide, the latter now the extrudate to distance from at least one surface, called the free surface, of the conductor;
• downstream of the expansion section, said at least one cell is formed on said free surface by at least one profile extending in the extension of a portion of the guide, the section of said cell corresponding to that of said profile, to at least one spacer for holding the sheath portion on the conductor being formed by filling at least one expansion channel opening between walls of said at least one profile.

The process thus comprises two successive steps. In the first stage, the shaping takes place for the most part, during which the sheath generally takes on its final shape (in particular, with the shrinkage). Naturally, this formatting requires machine settings known for the implementation of the PTFE, in particular to obtain fibrillation of PTFE. At the end of this first step, immediately upstream of the expansion section, the sheath is formed, at least a portion of the sheath being formed at a distance from the conductor, leaving a space between a free surface of the conductor, which remains exposed, and the sheath portion intended to become the outer wall of the cell. This sheath portion is formed to its definitive shape, which means that it is formed with the shape that it will retain in the manufactured electrical wire.

In the second step, the cells will be formed, forming connecting struts between the aforementioned sheath portion and the conductor. These spacers are formed because during the progression of the extrudate in the tooling, from the expansion section, the guide which separated the outer portion of the sheath, the driver, is replaced by one or more profiles. This profile (It may be several profiles) extends downstream in the extension that a portion of the guide, and at least one expansion channel is formed between the walls of the profile (s) ).

As the extrudate progresses in the extrusion passage, as soon as the extrudate reaches the expansion section, it fills this expansion channel, and thus forms one or more struts connecting the outer sheath to the conductor. . The material of the spacers therefore comes from the material of the outer sheath, which flows radially towards the conductor or conductors to form the spacers. These spacers form the walls of the cell or alveoli. However, the formation of the spacers is not accompanied by the covering of the free surfaces of the conductor by the extrudate, for reasons indicated below. Also, the free surface (s) remain free, thus making it possible to obtain an electrical wire of particularly low dielectric constant.

The originality of the method thus lies in the fact that although the sheath is essentially shaped during the first stage (upstream of the expansion section), the extrudate retains a residual capacity of expansion, advantageously used in the process to form from the expansion section or spacers connecting the sheath to the conductor and constituting the radial or lateral walls of the cells. These spacers ensure the correct positioning of the conductor relative to the sheath.

The formation of the spacers only after the forming of the sheath prevents the appearance of PTFE on the free surface of the conductor, and keeps it naked within the cell.

In fact, in general, the viscosity of the PTFE extrudate is very high, and the extrudate only flows under a very strong pressure. If necessary, depending on the viscosity of the PTFE, it may be useful to reduce this viscosity by increasing the proportion of lubricant in the extrudate (that is to say in the mixture to be extruded at the time of preparation of the latter). this). Increasing the proportion of lubricant modulates the creep of the extrudate occurring during extrusion. According to the invention, the extrudate flows from and downstream of the expansion section, releasing the residual stresses that are in it and whose relaxation is critical. By adequately adjusting the percentage of lubricant in the mixture to be extruded, the size of the spacer, and the gap between the guide and the conductor, it is possible to act if necessary on the amount of PTFE that comes into contact with the or the free surface (s) of the driver.

According to one embodiment, the reduction ratio immediately downstream of the expansion section is smaller than the reduction ratio of a so-called 'maximum reduction' section located at or upstream of the expansion section in the extrusion passage. The reduction ratio is equal to the ratio between the area of a section of the compression chamber at the upstream portion of the tooling and the area of the section considered of the die. In this embodiment, the pressure in the extrusion passage is less at the (or immediately downstream of) the expansion section than the level of the maximum reduction section. Because of this, the filling of the expansion channel (s) with PTFE is measured, and the invasion of the volume of the cells by the extrudate is avoided. For this purpose, it can be provided in particular that according to a variant of this embodiment, the reduction ratio immediately downstream of the expansion section is 10% or more lower than that of the maximum reduction section.

According to one embodiment, the expansion section is located at a distance downstream of an extrusion orifice of the tool located downstream of an extrusion chamber of decreasing section. The extrudate shaped at the the vicinity of the extrusion orifice thus has a period of time allowing it to stabilize during its progression between the extrusion orifice and the expansion section.

According to one embodiment, the contact between the extrudate and the conductor takes place substantially at the level of the expansion section, at which point the respective axial speeds of the extrudate and the conductor are substantially equal. In this way, the shear forces applying to the interface between the extrudate and the conductor are minimized, which avoids the risk of breakage of the conductor during manufacture.

A third objective of the invention is to define a tool for manufacturing an electrical wire comprising at least one conductor in a PTFE-based sheath, the sheath and said at least one spacer forming at least one continuous longitudinal cell delimited at least partially by the conductor, and which allows the manufacture of an electric wire of very low dielectric constant.

• en amont d'une section d'expansion, un guide apte à former une partie extérieure de gaine sensiblement à sa forme définitive autour du conducteur, ce guide étant apte à maintenir l'extrudat à distance d'au moins une surface, dite surface libre, du conducteur ;
• en aval de la section d'expansion, dans le prolongement d'une première partie du guide, au moins un profilé, ce profilé étant apte à former ladite au moins une alvéole sur ladite surface libre ; des parois dudit au moins un profilé définissant au moins un canal d'expansion, dont le remplissage permet la formation d'au moins une entretoise de maintien de la partie de gaine extérieure sur le conducteur,

ledit au moins un profilé s'étendant radialement sensiblement jusqu'à la surface interne de diamètre minimal du guide, afin d'empêcher l'irruption d'extrudat sur la surface libre du conducteur.This objective is achieved thanks to the fact that

• upstream of an expansion section, a guide capable of forming an outer portion of sheath substantially in its definitive shape around the conductor, this guide being able to hold the extrudate at a distance from at least one surface, said free surface , the driver;
• downstream of the expansion section, in the extension of a first portion of the guide, at least one profile, this profile being able to form said at least one cell on said free surface; walls of said at least one section defining at least one expansion channel, the filling of which allows the formation of at least one spacer for holding the outer sheath portion on the conductor,

said at least one profile extending radially substantially to the internal surface of minimum diameter of the guide, to prevent bursting of extrudate on the free surface of the conductor.

The inner diameter of the minimum diameter of the guide - or the extension thereof - in fact substantially corresponds to the diameter that must have the driver, to allow effective guidance of the driver without unnecessary play. When the profile extends substantially to this surface, during the manufacture of the electric wire, the surface internal profile is in the vicinity of the driver. Thanks to this, the risk of invasion by the PTFE of the space between the conductor and the profile remains limited: PTFE having a high viscosity, when this space is reduced, the PTFE does not penetrate, allowing to preserve the free surface or surfaces of the driver.

Naturally, the machine in which the tooling is mounted must include means for using the extrudate, which allow the formation and fibrillation of the PTFE contained in the extrudate, when the extrudate is shaped in the passage extrusion.

According to one embodiment, the tooling immediately downstream of the expansion section, a passage surface of the extrudate greater than that of a so-called 'maximum reduction' section of the tool located further upstream. In particular, the passage area of the extrudate immediately downstream of the expansion section may be 10% greater than that of the maximum reduction section.

According to one embodiment, the expansion section is located downstream of an extrusion orifice of the tool located downstream of an extrusion chamber of decreasing section.

According to one embodiment, the tooling comprises at least one adjustable portion, for axially varying the position of the expansion section in the tooling. Indeed, depending on the operating conditions and the raw material used, the viscosity of the extrudate can vary. These variations lead to more or less filling of the expansion channels. By varying along the axis of the extrusion passage, the position of the expansion channels, it is possible to trigger the filling of these channels more or less long after forming the sheath, and thus to control the importance of filling these channels.

According to one embodiment, said at least one profile is a solid section. Advantageously, such a section is easy to achieve, and the process requires neither vacuum, nor air injection for the formation of cells.

• la figure 1 est une section, perpendiculairement à son axe, d'un fil électrique connu comportant un conducteur central dans une gaine en PTFE, le conducteur comportant quatre alvéoles longitudinales continues ;
• la figure 2 présente un fil électrique selon l'invention comportant un conducteur dans une gaine à base de PTFE, le fil comportant quatre alvéoles longitudinales continues, chacune de ces alvéoles étant partiellement délimitée par un conducteur, le fil étant représenté en cours de fabrication sur l'outillage de fabrication;
• la figure 3 présente une section perpendiculairement à son axe d'un fil électrique selon l'invention comportant deux conducteurs dans une gaine commune à base de PTFE, chaque conducteur étant entouré par quatre alvéoles partiellement délimitées par un conducteur ;
• la figure 4 présente un fil électrique comportant une alvéole unique, formée en hélice autour d'un conducteur unique ;
• la figure 5 présente une vue en perspective schématique d'un câble coaxial comportant un fil électrique selon l'invention ;
• la figure 6 est une vue schématique en perspective d'un outillage de fabrication de fils électriques selon l'invention ;
• la figure 7 représente une coupe longitudinale d'un outillage de fabrication d'un fil électrique selon l'invention ;
• la figure 8 est un diagramme représentant l'évolution du rapport de réduction dans l'outillage de fabrication représenté sur la figure 6.

The invention will be better understood and its advantages will appear better on reading the following detailed description of embodiments. represented as non-limiting examples. The description refers to the accompanying drawings, in which:

• the figure 1 is a section, perpendicular to its axis, of a known electrical wire having a central conductor in a PTFE sheath, the conductor having four continuous longitudinal cells;
• the figure 2 presents an electrical wire according to the invention comprising a conductor in a PTFE-based sheath, the wire comprising four continuous longitudinal cells, each of these cells being partially delimited by a conductor, the wire being represented during manufacture on the tooling Manufacturing;
• the figure 3 has a section perpendicular to its axis of an electric wire according to the invention comprising two conductors in a common sheath based on PTFE, each conductor being surrounded by four cells partially delimited by a conductor;
• the figure 4 has an electrical wire having a single cell, formed helically around a single conductor;
• the figure 5 shows a schematic perspective view of a coaxial cable comprising an electric wire according to the invention;
• the figure 6 is a schematic perspective view of a tool for manufacturing electrical wires according to the invention;
• the figure 7 represents a longitudinal section of a tool for manufacturing an electric wire according to the invention;
• the figure 8 is a diagram representing the evolution of the reduction ratio in the manufacturing tooling represented on the figure 6 .

Referring to the figure 2 , an electrical wire according to the invention will now be described.

The electrical wire 22 comprises a central conductor 24, and a sheath 26 based on PTFE. The central conductor 24 is constituted by a copper core 23, having a thin silver plating 27. For ease of understanding, the silver plating 27 has been shown with an exaggerated thickness on the figure 2 . The sheath 26 is an outer sheath, substantially cylindrical and tubular.

Inside the sheath 26 are formed four continuous longitudinal cells 28. These cells 28 are separated in pairs by four separating walls or spacers 30. The sheath 26 has a generally tubular shape; the spacers 30 make it possible to center the conductor 24 inside the sheath 26. The struts 30 extend from the inner wall of the sheath 26 to the conductor 24 in a radial direction. The spacers 30 comprise a substantially radial junction portion 32, which forms a relatively thin or thin partition wall between two adjacent cells 28, and an enlarged portion 34 formed at the point of contact between the spacer 30 and the conductor 24. Each of the cells 28 is partially delimited by the conductor 24 at a free surface 25, which is not covered with PTFE and remains 'naked' inside the cell 28.

It should be noted that on the figure 2 , the electric wire 22 is shown during manufacture. In the manufacturing stage shown, the cavities 28 are partly occupied by profiled tooling portions 29 (or sections 29, shown hatched) serving to form the cavities 28. The cells 28 occupy the volume of these sections 29, as well as the volumes voids 31 which each extend between two spacers 30, the conductor 24 and a profile 29.

The formation of the spacers 30 is illustrated in particular by the detail left in the upper left on the figure 2 . In a first step of manufacturing the electrical wire 22, the sheath 26 is formed. In a second step, a portion of the extrudate constituting the sheath 26 flows radially inwards, along the arrows B, in the space provided between the sections 29, thus forming the spacers 30 between the walls of the profiles 29. flow is stopped by the driver 24; however, in contact with the latter at the end of the struts 30 the extrudate flow divides, and partially fills the annular space between the conductor 24 and the profiles 29, thereby forming one or more enlarged portions 34 to the end of the spacers 30. The manufacturing tool is arranged and adjusted to limit this expansion. In particular, the high viscosity of the extrudate can be taken into account if necessary to reduce the extrudate entry into the space available between the conductor 24 and the profiles 29.

Each of the spacers 30 has a joining portion 32, and an enlarged portion 34. On the detail out of the figure 2 the radial axis of the spacer 30 is represented by the dashed line A. The width 38 of the enlarged portion 34 is greater than the width 36 of the joining portion 32 of the spacer 30, these two widths being measured perpendicularly to the radial axis A. As will be specified in the following, the portion 34 may be more or less wide depending on the setting of the manufacturing tool, thus allowing a arbitration between an electrical wire having a very low dielectric constant and wherein the spacers 30 provide less maintenance of the conductor 24 (because their enlarged portion 34 is little or not widened), or the opposite.

Moreover, usually, the section of the electric wire as represented on the figure 2 is invariant along the axis of it. However, it is possible to manufacture the electrical wire according to the invention by rotating the position of the cells inside the sheath during manufacture, thus obtaining an electrical wire in which the cells are formed helically. .

Finally, figure 2 shows the direction in which the PTFE fibers 33 are formed in the sheath and spacers, during the manufacture of the electrical wire. As shown in this figure, the PTFE fibers in the spacers are mainly directed in a substantially radial direction, since this is the direction in which the spacers 30 extend to connect the outer sheath to the central conductor 24. This orientation of the fibers 33 PTFE is due to the fact that during the manufacture of the wire, after the formation of the outer sheath of the wire, the formation of the spacers is done using the material of the outer sheath, the material of the outer sheath fluent in direction of the central electrical wire 24 so as to form the spacers 30. This direction of extension of PTFE fibers 33 in the spacers gives them excellent crush resistance.

The figure 3 represents another electrical wire according to the invention, comprising two conductors 40, each conductor being surrounded by four cells 42. An outer sheath 44 based on PTFE is formed around the conductors and their cells. Advantageously, each of the cells 42 is partially delimited by one of the conductors 40.

In this electrical wire, the cells have been formed so as to minimize the dielectric constant of the wire. For this purpose, the spacers 46, formed between adjacent cells, do not have an enlarged portion in contact with the conductor. In a section perpendicular to the longitudinal axis of the wire (for example the section of the figure 3 ), the cells 42 have a shape of angular sector, relatively large center angle β, for example 80 degrees; the spacers 46 also have a substantially angular sector shape but their central angle α is reduced to about 10 to 20 degrees.

Referring to the figure 4 another embodiment of electric wire according to the invention will now be described. This electrical wire 50 comprises a single-strand electric conductor 52 disposed inside a sheath 54 made of PTFE-based material. The sheath 54 is tubular. Inside the latter, the free space 56 between the central conductor 52 and the sheath 54 is only occupied by a helicoidal spacer 58 which connects the outer surface of the conductor 52 to the inner surface of the sheath 54. With this configuration, the electrical wire 50 has an extremely low dielectric constant E.

The figure 5 has a coaxial cable incorporating an electrical wire according to the invention. This coaxial cable 60 integrates, successively, a central electrical conductor 62, a tubular sheath 64 made of PTFE material, separated from the central conductor 62 by four cavities 66, a conductive layer 68 and finally an outer sheath 70. Thanks to the presence longitudinal cells 66, each being partially delimited by the conductor 62, the dielectric constant E of the coaxial cable is very small.

Referring to figures 6 , 7 and 8 , a tool and a manufacturing method according to the invention will now be presented.

The figure 6 presents a production tooling 100 adapted for the implementation of the method according to the invention. It also has a portion of electrical wire 150 made by this tool.

The tool 100 mainly comprises a die 102 and a guide 104. The die 102 has an internal passage 106 within which the sheathing of PTFE-based material takes place. This passage 106 has a general shape of revolution around an extrusion axis B. The axis B is a vertical axis and the extrusion takes place in the downward direction. The guide 104 also has a general shape of revolution and is arranged coaxially with the axis B of the die 102, inside the internal passage 106 thereof.

The internal passage 106 of the die has three main parts, which correspond to the three main phases of the extrusion:

In the upper part, the internal passage 106 comprises a cylindrical chamber 108 or upstream compression chamber. During manufacture, this cylindrical chamber is filled with lubricated material ready for extrusion, forming the extrudate. The extrudate is pushed down by a slide 110 sliding inside the chamber 108 around the guide 104. Under the effect of the pressure exerted by the slide or piston 110, the extrudate is pushed down to the interior of the cylindrical chamber 108, then in a convergent conical 112. At the bottom of the convergent 112, the inner passage 106 has a third portion which is the extrusion passage 114. The extrusion passage is of substantially cylindrical and extends over a certain height below the outlet orifice 116 of the convergent 112.

Correlatively, the guide 104 also has an outer shape in three parts. Its cylindrical upper part 118 is adapted to allow the piston 110 to slide around it; this upper portion 118 is extended downwards by a conical portion 120. The cone of this portion 120 is adapted relative to the convergent 112 to allow a progressive pressure increase of the extrudate and an increase in the reduction ratio, as and when as the extrudate is lowered into the internal passage 106 of the die. The guide 104 is finally extended by a downstream cylindrical portion 122. In the downward extension of this downstream cylindrical portion 122, four sections 124 extend, which serve to form the cells in the electrical wire manufactured by the tool. 100. The four sections 124 each have the same shape; they extend inside the cylindrical envelope of the downstream cylindrical portion 122 of the guide 104, each occupying an angular sector within this volume. Between each pair of adjacent profiles 124 is formed an expansion channel 126. The expansion channels 126 thus form four radial slits 0.8 mm wide, separating the profiles 124 at the end of the guide 104.

In addition, the guide 104 serves to guide a conductor 128, which is the central conductor of the electrical wire 150 manufactured with the tooling 100. In the tool shown in FIG. figure 6 , the four sections 124 extend radially inwardly substantially to the contact of the central conductor 128. The inner diameter of the guide 104 is 2.8 mm, while the conductor 128 has an outer diameter of 2.27 mm . So, a light game is provided between the guide 104 and the conductor 128, which clearance is sufficient to allow the substantially frictionless circulation of the conductor in the tooling, but is sufficiently small to prevent any extrudate rising within the guide 104 in the passage provided for the driver 128.

In general, depending on the embodiment and depending on the viscosity of the extrudate, more or less clearance can be maintained between the radially inner surface of the profiles 124 and the conductor 128.

The operation of the tool 100, and the method of manufacturing an electric wire according to the invention, thanks to this tool, will now be presented.

In this exemplary embodiment of the invention, the extrudate consists of a mixture consisting essentially of PTFE and lubricant. As lubricant, a solvent based on aliphatic hydrocarbons may be used, for example in a proportion of 10 to 35%, and preferably 15 to 25%.

For the manufacture of an electrical wire with the tool 100, a conductor 128 is set up inside the guide 104, and the cylindrical chamber 108 is filled with extrudate ready for extrusion. The amount of lubricant present in the extrudate is adjusted to allow sufficient creep thereof during extrusion.

The piston 110 descends gradually and, at the same time, the conductor 128 is driven downwards. The extrudate is compressed in the cylindrical chamber 108, then inside the convergent 112 and engulfs via the outlet orifice 116 of the convergent 112 in the extrusion passage 114. The extrusion passage 114 comprises a first upstream portion 101 for forming the sheath of the electrical wire, this upstream portion extending from the outlet of the convergent 116 to the downstream section of the guide said expansion section 130. During its passage in this first upstream part of the extrusion passage 114, the extrudate is subjected to fibrillation which makes it possible to form the sheath 160 of the electrical wire 150 manufactured, between the outer wall of the guide 104, of diameter 5 mm, and the inner wall of the die 102, with a diameter of 6 mm. It is therefore for the tooling a sheath formation part, in which the shape of the extrudate is stabilized in enjoy the game. The extrudate is introduced into this first upstream portion, at the outlet orifice 116 of the internal passage 106, through a passage section which has the shape of the section of the outer sheath of the formed wire.

From the expansion section 130 which is located at the downstream end of the first upstream portion, the sheath material expands and fills the four expansion channels 126. By filling these channels, the material of the sheath forms the spacers 158 joining and holding between the central conductor 128 and the sheath 160.

A second portion 103 of the extrusion passage, or strut forming part, extends from the expansion section 130 to the downstream section of the profiles 124. In this part, the cell separation struts are formed. , and the extrudate constituting the spacers is shaped and stabilized. The formation of the spacers is by creep of the material of the outer sheath, taking advantage of the residual expansion capabilities that the PTFE constituting the sheath at this stage of the extrusion. In this part 103, the tool is arranged in such a way that the material of the spacers can come only from the outer sheath. Indeed, upstream of each spacer 30 is the downstream cylindrical portion 122 of the guide 104. The latter makes it possible to form the inner surface of the outer sheath inside the first upstream portion 101, but thereby prevents the forming the spacers 30 in this part 101.

Downstream of the downstream section of the profiles 124, over a short distance, a downstream cylindrical portion 105 of the die 102 allows stabilization and ultimate maintenance of the sheath 160 before the output of the wire 150 to the outside of the tool 100. In known manner, the electrical wire 150 thus shaped then requires sintering to stabilize the structure of the sheath 160 and spacers 158 by thermal coalescence.

The cell manufacturing process can be better understood in relation to the Figures 7 and 8 . The process of forming the electrical wire jacket is a continuous process that takes place during the progression of the extrudate along the B axis in the production tool 100. As it progresses, the extrudate crosses certain points on the axis B at which the shape of the tool 100 varies. The extrudate progresses firstly in the compression chamber, to an X ₀ abscissa on the B axis from which the guide 104 is of minimum diameter and is extended by a cylindrical portion 122. Further downstream, to the X1 abscissa, the extrudate reached, at the downstream end of the compression chamber, the orifice 116 or convergent outlet orifice 112 of the die 102. Downstream of this orifice 116, the extrudate progresses in a cylindrical portion of the die 102, around the guide 104.

In this downstream part of the die 102, the guide 104 terminates at the expansion section 130 at an abscissa X ₂ . The guide 104 is extended downstream of this expansion section 130 by the profiles 124, which serve to form the four cells of the electrical wire. The profiles 124 extend to a section X ₃ inside the die 102, and the die 102 extends downstream of these profiles to an abscissa X ₄ (downstream of X ₃ ).

The figure 8 presents the evolution of the reduction ratio in the manufacturing tool as the extrudate advances in it. From the injection of the extrudate upstream of the compression chamber, in a first step, the reduction ratio increases when the extrudate is compressed inside the convergent 112. From the abscissa X ₀ , from which the guide adopts a cylindrical shape 122, the reduction ratio increases even faster to reach a maximum value R ₁ from X ₁ . This high value of the reduction ratio R ₁ is chosen so as to allow fibrillation of the extrudate in the first portion 101 of the extrusion passage 114. Thus, this first portion 101 of the extrusion passage allows the forming of the sheath 160 of the electrical wire 150.

From the expansion section 130, the extrudate passage space is increased due to the opening of the expansion passages 126 between the profiles 124. The extrudate fills the expansion channels 126 and forms the separating walls between the cells, namely the spacers 158 for holding the sheath 160 of the electrical wire 150. The reduction ratio decreases to a second value R2 less than R1. In this second portion 103 of the extrusion passage, the extrudate has taken its final form. The length of this second portion 103 of the extrusion passage is chosen to be sufficient to allow the stabilization of the extrudate, which takes in this part 103 its substantially definitive form.

When this stabilization has been sufficiently effected to prevent sagging, the profiles 124 terminate at abscissa X ₃ , leaving the cells 158 filling with air. This filling takes place inside the third portion 105 of the extrusion passage, in which the guide of the sheath by the die 102 promotes the maintenance of the shape of the sheath and prevents the collapse thereof on the alveoli.

In known manner, after the extrusion takes place a sintering step of the cable sheath, to stabilize the structure of the sheath. After sintering, the cable has an outside diameter of 5.3 mm. The degree of vacuum in the dielectric is estimated at 28%, and the dielectric constant ε is measured equal to 1.47, which confirms the efficiency of the structure chosen for the production of electric cables of low dielectric constant. An important parameter for the setting of the tool and the development of the manufactured electrical wire is the position of the expansion section 130 (abscissa X2) in the tool. Indeed, the further the expansion section 130 is upstream, near the outlet port 116 of the convergent 112, the less the PTFE of the extrudate is stabilized in the sheath. Therefore, the closer the expansion section 130 is to this outlet port 116, the further the extrudate can enter the expansion channels 126 and, therefore, the spacers 158 will have an enlarged portion in contact with each other. of the driver.

To enable this adjustment, the tooling 100 comprises an adjustable part comprising the cylindrical portion 122 of the guide associated with the profiles 124, this part being able to be displaced axially vertically in the direction of extrusion (arrow F) by an actuator 140, as a function of the greater or lesser importance that the widened part of the struts 158 of the wire 150 has to have. By raising the adjustable part upwards (in the direction opposite to the arrow F, in figure 6 ), the length of the portion 101 of the extrusion passage 114 is reduced, thus the spacers 158 are allowed to be formed shortly after the formation of the sheath 160. The sheath then being unstabilized, an increased amount of of PTFE moves radially between the profiles 124 to form the spacers 158, and accordingly these have a larger enlarged portion.

Claims (13)

1. An electric wire (22,50) comprising at least one conductor (24,40,52) held by at least one spacer (30,46,58) in a covering (26,44,54) based on PTFE, the covering and said at least one spacer also forming at least one longitudinal and continuous cell (28,42,56), said at least one cell being defined at least partially by the conductor;
   the electric wire being characterized in that, as seen in a section perpendicular to an axis of the wire, the majority of the PTFE fibers of said at least one spacer point in a substantially radial direction.
2. An electric wire according to claim 1, having its covering held at a distance from the conductor by said at least one spacer (30), said spacer having a substantially radial junction portion (32), and a broader holding portion (34) in contact with the conductor.
3. A coaxial cable (60) comprising a central conductor (62), an intermediate insulating covering (64) based on PTFE, an outer conductor (68), and a protective outer covering (70), characterized in that the central conductor (62) considered together with the intermediate covering (64) forms an electric wire according to claim 1 or 2.
4. A method of manufacturing an electric wire comprising at least one conductor (24,40) held by at least one spacer in a covering (26,44,54) based on PTFE, the covering and said at least one spacer (30,46,58) forming at least one substantially longitudinal and continuous cell (28,42,56);
   in which method an extrudate based on PTFE is caused to pass through an extrusion passage (114) around said at least one conductor in such a manner as to form the covering;
   characterized in that:

   • over a first portion (101) of the extrusion passage, upstream an expansion section (130), at least a portion of the covering is formed substantially to its final shape at a distance from the conductor by a guide, which guide keeps the extrudate at a distance from a free surface of the conductor (25); and

   • downstream from the expansion section, said at least one cell is formed on said free surface (25) by at least one section member (124) extending following on from a portion of the guide (104), the section of said cell corresponding to the section of said section member, at least one spacer (158) for holding the covering portion on the conductor being formed by filling at least one expansion channel (126) opening between walls of said at least one section member.
5. A manufacturing method according to claim 4, in which the reduction ratio (R) immediately downstream from the expansion section (130) is less than the reduction ratio of a "maximum reduction" section (X1) situated at or upstream from the expansion section in the extrusion passage (114).
6. A manufacturing method according to claim 5, in which the reduction ratio (R) immediately downstream from the expansion section (130) is less by 10% or more than the reduction ratio of the maximum reduction section.
7. A manufacturing method according to any one of claims 4 to 6, in which the expansion section (130) is situated at a distance downstream from an extrusion orifice (116) of the tool (100) that is situated downstream from an extrusion chamber of decreasing section (112).
8. A manufacturing method according to any one of claims 4 to 7, in which the contact between the extrudate and the conductor takes place substantially at the expansion section (130), whereat the respective axial speeds of the extrudate and of the conductor (128) are substantially equal.
9. Tooling (100) for manufacturing an electric wire (150) that has at least one conductor (128) held by at least one spacer (30,46,58) in a covering (160) based on PTFE, the covering and said at least one spacer forming at least one substantially longitudinal and continuous cell (28,42,56) that is defined at least partially by the conductor;
   the tool being characterized in that it comprises:

   ■ upstream from an expansion section (130), a guide (104) suitable for forming an outer portion of covering substantially to its final shape around the conductor, which guide is designed to keep the extrudate at a distance from at least one free surface (25) of the conductor, which free surface is not to be covered with extrudate; and

   ■ downstream from the expansion section, following on from a first portion of the guide, at least one section member (124), which section member is suitable for forming said at least one cell on said free surface; walls of said at least one section member defining at least one expansion channel (126), filling of which makes it possible to form at least one spacer for holding the portion of outer covering on the conductor;

   said at least one section member extending radially substantially to the inside surface of minimum diameter of the guide, in order to prevent extrudate from spreading over the free surface (25) of the conductor (128).
10. Manufacturing tooling (100) according to claim 9, that, immediately downstream from the expansion section, has a through area for passing the extrudate that is greater than the through area of a "maximum reduction" section of the tool that is situated further upstream.
11. Manufacturing tooling according to claim 9 or 10, having its expansion section (130) situated downstream from an extrusion orifice (116) of the tool that is situated downstream from an extrusion chamber of decreasing section (112).
12. Manufacturing tooling according to any one of claims 9 to 11, having at least one adjustable portion, making it possible to cause the position of the expansion section in the tooling to vary axially.
13. Manufacturing tooling according to any one of claims 9 to 12, in which said at least one section member is a solid section member.

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