Foam insulated wire and its manufacturing method

Abstract

To provide a foamed insulated wire that improves the adhesion between an insulating layer and a core wire, while reducing the capacitance.SOLUTION: A foamed insulated wire 4 is provided with a core wire 10 comprising a single wire, and a foamed insulating layer 11 covering the outer periphery of the core wire 10. A foamed core consisting of the core wire 10 and the foamed insulating layer 11 has a capacitance of 50 pF / m or more and 70 pF / m or less. A bond strength indicating a force of drawing the core wire 10 from the foamed insulating layer 11 is 0.1 N or more when the foamed insulated wire 4 is 50 mm long and 1.0 N or less when the foamed insulated wire 4 is 10 mm long.SELECTED DRAWING: Figure 10

Description

[Technical Field]

[0001] The present invention relates to an electric wire, and more particularly to a foam insulated electric wire having a foam insulation layer covering the outer periphery of a core wire made of a single wire, and to a technique that is effectively applicable to a manufacturing method thereof. [Background technology]

[0002] Japanese Patent Application Laid-Open No. 2011-162721 (Patent Document 1) describes a technique relating to a foam insulated electric wire including a foam insulation that covers the outer periphery of an internal conductor. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Laid-Open No. 2011-162721 Summary of the Invention [Problem to be solved by the invention]

[0004] In order to achieve high-speed signal transmission through an electric wire, it is effective to reduce the capacitance of the electric wire. For example, one method for reducing the capacitance of an electric wire is to reduce the dielectric constant of the insulating layer that covers the outer periphery of the core wire made of a conductor. Specifically, the dielectric constant of the insulating layer is reduced by making the insulating layer from a foamed insulating material.

[0005] In addition, it is also necessary to improve the adhesion between the core wire and the insulating layer of an electric wire. In this regard, even if an attempt is made to improve the adhesion between the insulating layer and the core wire, for example, when an extruder is used to manufacture a foam-insulated electric wire, it is difficult to improve the adhesion between the insulating layer and the core wire. The present inventors have newly discovered that it is particularly difficult to improve the adhesion between the insulating layer and the core wire when the core wire is composed of a solid wire.

[0006] Therefore, there is a need for a method for improving the adhesion between the insulating layer and the core wire while reducing the capacitance of the foam-insulated electric wire.

[0007] An object of the present invention is to improve the adhesion between the insulating layer and the core wire while reducing the electrostatic capacitance of a foam-insulated electric wire.

[0008] Other objects and novel features will become apparent from the description of this specification and the accompanying drawings. [Means for solving the problem]

[0009] In one embodiment, the foam-insulated electric wire comprises a core wire made of a single wire and a foam insulation layer provided around the core wire, wherein the capacitance of the foam core including the core wire and the foam insulation layer is 50 pF / m or more and 70 pF / m or less, and the bond strength, which indicates the force required to pull the core wire out of the foam insulation layer, is 0.1 N or more when the length of the foam-insulated electric wire is 50 mm and 1.0 N or less when the length of the foam-insulated electric wire is 10 mm.

[0010] In another embodiment, a method for manufacturing a foam-insulated electric wire includes moving a core wire consisting of a single wire at a predetermined linear speed while extruding a foam insulation material from an extruder at a predetermined discharge flow rate so as to cover the outer periphery of the core wire, thereby forming a foam insulation layer made of the foam insulation material and covering the outer periphery of the core wire, wherein the discharge flow rate is greater than the linear speed. [Effects of the Invention]

[0011] According to one embodiment, it is possible to improve the adhesion between the insulating layer and the core wire while reducing the electrostatic capacitance of the foam-insulated electric wire. [Brief explanation of the drawings]

[0012] [Figure 1] 1 is a diagram showing a schematic configuration of a foam-insulated electric wire. FIG. [Figure 2] FIG. 1 is a diagram showing an example of a foam insulation layer manufacturing system. [Figure 3] 10 is a diagram showing how a foamed insulation layer is formed around the outer periphery of a core wire using an extruder. FIG. [Figure 4] 1 is a diagram schematically illustrating a foam-insulated electric wire having a core wire made of a stranded wire; [Figure 5] 1 is a diagram schematically illustrating the longitudinal direction of a foam-insulated electric wire having a core wire made of twisted wires. FIG. [Figure 6] 1 is a diagram schematically illustrating a foam-insulated electric wire having a core composed of a single wire. [Figure 7] 1 is a diagram schematically illustrating the longitudinal direction of a foam-insulated electric wire having a core composed of a single wire; [Figure 8] FIG. 10 is a diagram illustrating the protrusion of a core wire. [Figure 9] 10A and 10B are diagrams illustrating the core wire being displaced from the center position of the foam-insulated electric wire. [Figure 10] 1 is a cross-sectional view showing a schematic configuration of a foam-insulated electric wire according to an embodiment. [Figure 11] 10 is a diagram showing how a foamed insulation layer is formed around the outer periphery of a core wire using an extruder. FIG. [Figure 12] 3 is a photograph showing the cross-sectional structure of a foamed insulation layer in an embodiment. [Figure 13] 3 is a photograph showing the cross-sectional structure of a foamed insulation layer in an embodiment. [Figure 14] 1 is a photograph showing a cross-sectional structure of a core wire covered with a foam insulation layer. [Figure 15] 14. (a) is a cross-sectional photograph taken along line AA in FIG. 14, (b) is a cross-sectional photograph taken along line BB in FIG. 14, and (c) is a cross-sectional photograph taken along line CC in FIG. [Figure 16] FIG. 1 is a diagram illustrating a method for evaluating bond strength. [Figure 17] FIG. 10 is a diagram illustrating a method for measuring capacitance. DETAILED DESCRIPTION OF THE INVENTION

[0013] In all the drawings for explaining the embodiments, the same components are generally designated by the same reference numerals, and repeated explanations thereof will be omitted. In addition, hatching may be used even in plan views to make the drawings easier to understand.

[0014] <Composition of foam insulated wire> FIG. 1 is a diagram showing a schematic configuration of a foam insulated wire.

[0015] In particular, FIG. 1 shows a schematic connection structure in which the end of the foam insulated wire is processed, so the configuration of the foam insulated wire will be described based on this connection structure.

[0016] As shown in FIG. 1, the foam-insulated electric wire 1 has a core wire 10. This core wire 10 is composed of an inner conductor. For example, the inner conductor constituting the core wire 10 may be made of any material, including copper, copper alloy, aluminum, and aluminum alloy. The surface of this inner conductor may be plated. The diameter of the core wire 10 varies depending on the product size, 42 AWG to 46 AWG (American Wire Gauge, hereinafter abbreviated as AWG), but is, for example, 0.04 mm to 0.06 mm.

[0017] Next, the foam insulated wire 1 has a foam insulation layer 11 provided around the core wire 10. This foam insulation layer 11 is made of an insulating material containing bubbles inside. This makes the dielectric constant of the foam insulation layer 11 lower than that of a material containing no bubbles. The foam insulation layer 11 can be made of, for example, a fluororesin containing a foam nucleating agent. Here, the foam nucleating agent functions as a nucleus for bubble growth, and since it is dispersed and mixed into the fluororesin, it has the function of dispersing bubbles formed inside the fluororesin.

[0018] For example, the fluororesin may include tetrafluoroethylene-perfluoroalkoxyethylene copolymer or tetrafluoroethylene-hexafluoropropylene copolymer, while the foam nucleating agent may be composed of, for example, boron nitride.

[0019] In this specification, the component consisting of the core wire 10 and the foamed insulation layer 11 is referred to as a foamed core.

[0020] Next, the foamed insulated wire 1 has a skin tape 12 provided on the outer periphery of the foamed insulation layer 11, and an outer conductor 13 provided on the outer periphery of this skin tape 12. The skin tape 12 can be made of, for example, polyester or polyethylene terephthalate, and the thickness of the skin layer can be, for example, 0.01 mm or more and 0.02 mm or less.

[0021] The outer conductor 13 has, for example, a shielded braid structure or a shielded spiral winding structure, and has the function of shielding against external electromagnetic noise. The conductor used for the outer conductor 13 can be, for example, copper, copper alloy, aluminum, or aluminum alloy. The diameter of the conductor used for the outer conductor 13 can be, for example, 0.02 mm or more and 0.03 mm or less. The surface of this conductor may be plated.

[0022] Furthermore, the foam insulated wire 1 has a sheath 14 provided around the outer periphery of the outer conductor 13. This sheath 14 is made of an insulating resin material such as polyethylene resin, fluororesin, polyvinyl chloride resin, urethane rubber, or silicone rubber. The outer diameter of the sheath 14 (foam insulated wire 1) can be, for example, 0.21 mm or more and 0.37 mm or less.

[0023] The foam insulated electric wire 1 configured as above is required to have the following characteristics, for example.

[0024] <Characteristics required for foam-insulated wires> Since the foam insulated wire 1 is used, for example, as a high-speed transmission wire, it is required to have a small capacitance to suppress signal delay. For example, the capacitance required for the foam core of the foam insulated wire 1 is 50 pF / m or more and 70 pF / m or less. More specifically, there are two types of required specifications, as shown in Table 1.

[0025] [Table 1]

[0026] The foam insulated wire 1 has air bubbles in the foam insulation layer 11 in order to reduce the capacitance of the foam insulated wire 1. As a result, the presence of the air bubbles reduces the dielectric constant of the foam insulated wire 11, thereby reducing the capacitance of the foam insulated wire 1. In other words, the foam insulated wire 1 has the characteristic of having a small capacitance.

[0027] On the other hand, the foam-insulated electric wire 1 also requires good adhesion between the core wire 10 and the foamed insulation layer 11. This is because, when a connection structure is formed at the end of the foam-insulated electric wire 1, terminal processing is performed, and if the adhesion between the core wire 10 and the foamed insulation layer 11 is poor (there are many air bubbles), the core wire 10 will protrude during terminal processing (when the foamed insulation layer 11 is pressed by a cutting blade to cut it). Also, if the adhesion between the core wire 10 and the foamed insulation layer 11 is poor, there is a high possibility that a problem will occur in which only the core wire 10 protrudes when the foam-insulated electric wire 1 is bent.

[0028] Conversely, if the adhesion between the core wire 10 and the foamed insulation layer 11 is too strong (there are too few bubbles), it becomes difficult to reduce the capacitance and the foamed insulation layer 11 becomes difficult to peel off during termination, which causes problems with termination. Therefore, the foam insulated wire 1 is required to have a bond strength between the core wire 10 and the foamed insulation layer 11 that is neither too weak nor too strong. Specifically, the bond strength, which represents the force required to pull the core wire 10 out of the foamed insulation layer 11, is required to be 0.1 N or more when the length of the foam insulated wire 1 is 50 mm and 1.0 N or less when the length of the foam insulated wire 1 is 10 mm.

[0029] As described above, the foam insulated electric wire 1 is required to have reduced capacitance and moderately improved adhesion. In order to improve the performance of the foam insulated electric wire 1, it is important to find a way to achieve both reduced capacitance and moderately improved adhesion.

[0030] Here, the core wire 10 of a conventional foam-insulated electric wire 1 is often composed of a twisted wire. In this case, a foam-insulated electric wire 1 having a capacitance within the above-mentioned capacitance range and the above-mentioned appropriate adhesion has been achieved. That is, a conventional foam-insulated electric wire 1 having a twisted wire core wire 10 has been able to achieve both reduced capacitance and appropriate improved adhesion. However, in recent years, it has been considered to configure the core wire 10 of the foam-insulated electric wire 1 from a solid wire. The present inventor has newly discovered that it is difficult to achieve both reduced capacitance and appropriate improved adhesion in a foam-insulated electric wire 1 having a solid wire core wire 10. The present inventor's new discovery is described below.

[0031] <Demand for thinner foam-insulated wires> For example, a foam-insulated electric wire 1 with a small capacitance is used in a medical probe cable. More specifically, the foam-insulated electric wire 1 is used as a probe cable for an ultrasound imaging device. In this case, the foam-insulated electric wire 1 is connected to each pixel element of a display device that displays an image of the ultrasound imaging device. Therefore, in order to increase the resolution of the image, more foam-insulated electric wires 1 are required, and for example, a bundle of several hundred foam-insulated electric wires 1 is required.

[0032] On the other hand, since a probe cable made of the foam insulated wire 1 is applied to various parts of a patient by an examiner, the foam insulated wire 1 constituting the probe cable is also required to be flexible. For this reason, it is desirable that the foam insulated wire 1 used in the probe cable be made thinner to increase flexibility while also increasing wire density.

[0033] For these reasons, in recent years, there has been a demand for thinner foam insulated wires 1.

[0034] In order to advance the thinning of the foam-insulated electric wire 1, it is being considered to change the structure of the core wire 10 from a stranded wire structure to a solid wire structure. The reason for this will be explained below.

[0035] For example, the stranded wire constituting the core wire 10 of the foam-insulated electric wire 1 is realized by twisting seven single wires. However, in order to further thin the stranded core wire 10, the diameter of the single wires constituting the stranded wire must simply be reduced (for example, by one-third), which makes it difficult to manufacture the single wires constituting the stranded wire. Furthermore, thinning the single wires is thought to increase the likelihood of wire breakage due to the tension generated when twisting them together. Furthermore, the manufacturing cost of a foam-insulated electric wire 1 using stranded wires is higher than that of a foam-insulated electric wire 1 using single wires due to the twisting process. However, while a stranded wire is more flexible than a single wire, the difference in flexibility between a single wire and a twisted wire becomes smaller as the wires are made thinner.

[0036] For this reason, in order to advance the thinning of the foam insulated electric wire 1, it is being considered to change the structure of the core wire 10 constituting the foam insulated electric wire 1 from a stranded wire structure to a solid wire structure.

[0037] In this regard, the present inventors have newly discovered that it is difficult to achieve both reduced capacitance and adequately improved adhesion in a foam-insulated electric wire 1 having a single core wire 10. In particular, the present inventors have focused on the manufacturing process of the foamed insulation layer 11 and identified the reason why it is difficult to ensure adequate adhesion between the core wire 10 and the foamed insulation layer 11 in a foam-insulated electric wire 1 having a single core wire 10. This point will be explained below.

[0038] <Foam insulation layer manufacturing system> First, a foam insulation layer manufacturing system for forming the foam insulation layer 11 that covers the outer periphery of the core wire 10 will be described with reference to the drawings.

[0039] FIG. 2 is a diagram showing an example of a foam insulation layer manufacturing system.

[0040] 2, foamed insulation layer manufacturing system 100 includes a feeder 101, a core wire preheater 102, an extruder 103, a water tank 104, an outer diameter measuring machine 105, a take-up machine 106, and a winder 107. In foamed insulation layer manufacturing system 100 shown in FIG. 2, core wire 10 is first fed out from feeder 101. Then, core wire 10 fed out from feeder 101 is heated in core wire preheater 102. Then, the heated core wire 10 is fed into extruder 103 at a predetermined linear speed. Then, in extruder 103, a foamed insulation material containing a foaming agent (such as carbon dioxide gas) supplied from gas cylinder 103a is kneaded and extruded onto the outer periphery of core wire 10, which is moving at a predetermined linear speed. As a result, foamed insulation layer 11 is formed to cover the outer periphery of core wire 10. Thereafter, the core wire 10 covered with the foamed insulation layer 11 is carried into a water tank 104, and the capacitance is measured by a capacitance measuring machine 104a. Next, the outer diameter of the core wire 10 covered with the foamed insulation layer 11 is measured by an outer diameter measuring machine 105. Thereafter, the core wire 10 covered with the foamed insulation layer 11 is taken up by a take-up machine 106 and finally taken up by a winder 107. In this manner, the foamed insulation layer 11 is formed around the outer periphery of the core wire 10.

[0041] Next, details of how the foamed insulating layer 11 is formed around the outer periphery of the core wire 10 in the extruder 103 shown in FIG. 2 will be described with reference to the drawings.

[0042] FIG. 3 is a schematic diagram showing how a foamed insulating layer 11 is formed around the outer periphery of a core wire 10 by an extruder.

[0043] In extruder 103, an inert gas such as carbon dioxide gas, nitrogen gas, or a mixture of these gases is injected as a foaming agent through the barrel of extruder 103 and then kneaded with the fluororesin by the screw. That is, in extruder 103, foam insulation material 20 composed of fluororesin containing a foaming agent is kneaded. Then, as shown in FIG. 3 , core wire 10 is moved at a predetermined linear speed in the y direction, and the kneaded foam insulation material 20 is extruded from die 103b of extruder 103 onto the outer periphery of core wire 10. At this time, the extrusion speed of foam insulation material 20 from die 103b is equal to the linear speed of core wire 10. Then, when foam insulation material 20 is extruded from die 103b onto the outer periphery of core wire 10, the foaming agent contained in foam insulation material 20 becomes supersaturated due to the pressure difference between the inside of extruder 103, which is in a high-pressure state, and the outside of extruder 103, which is in an atmospheric (low-pressure) state. As a result, bubbles 30 are generated in the foamed insulation material 20 discharged from the nozzle 103b, and as these generated bubbles 30 grow, a foamed insulation layer 11 is formed around the core wire 10. At this time, as shown in Figure 3, the bubbles 30 grow so that their outer diameter increases toward the outside. As a result, a gap is generated between the core wire 10 and the foamed insulation layer 11, and this gap grows as a void portion 40.

[0044] For this reason, when the extruder 103 is used to form the foamed insulation layer 11 that covers the outer periphery of the core wire 10, a void 40 is inevitably formed between the core wire 10 and the foamed insulation layer 11. This means that the presence of the void 40 reduces the adhesion between the core wire 10 and the foamed insulation layer 11. In other words, when the extruder 103 is used to form the foamed insulation layer 11 that covers the outer periphery of the core wire 10, a void 40 is inevitably formed between the core wire 10 and the foamed insulation layer 11, resulting in a problem of reduced adhesion between the core wire 10 and the foamed insulation layer 11.

[0045] <When the core wire 10 is composed of a twisted wire> In this regard, for example, when the core wire 10 is composed of a twisted wire, even if a foamed insulation layer 11 covering the outer periphery of the core wire 10 is formed using the extruder 103 as described above, the decrease in adhesion between the core wire 10 and the foamed insulation layer 11 rarely becomes apparent as an area for improvement.

[0046] The reason for this will be explained below.

[0047] FIG. 4 is a diagram schematically showing a foam-insulated electric wire having a core wire made of twisted wires.

[0048] In Fig. 4, the core wire 10 is formed by twisting together, for example, seven single wires 10a. At this time, a void 40 is formed between the core wires 10 that make up the stranded wire and the foamed insulation layer 11, but the foamed insulation layer 11 fills the spaces between the single wires 10a that make up the stranded wire. That is, in a foam-insulated electric wire 2 in which the core wire 10 is made up of a twisted wire, even if a void 40 is formed, there are still some portions where the core wire 10 and the foamed insulation layer 11 come into contact with each other. This prevents a decrease in adhesion between the core wire 10 made up of a twisted wire and the foamed insulation layer 11.

[0049] Furthermore, Fig. 5 is a cross-sectional view schematically showing the longitudinal direction (y direction) of a foam-insulated electric wire whose core wire is composed of a twisted wire. In Fig. 5, the core wire 10 of the foam-insulated electric wire 2 has a twisted structure in which a plurality of single wires 10a are twisted together, resulting in an uneven structure formed in the longitudinal direction (y direction) due to the twisted structure. The foam insulation layer 11 is formed to match the twisted structure of the core wires 10, so that an uneven structure is also formed on the inner circumferential surface of the foam insulation layer 11. As a result, for example, as shown in region AR in Fig. 5, the uneven structure resulting from the twisted structure of the core wires 10 and the uneven structure of the foam insulation layer 11 formed to match the twisted structure of the core wires 10 intermesh. Therefore, in the foam-insulated electric wire 2 whose core wire 10 is composed of a twisted wire, there is a region (region AR) where the twisted core wire 10 and the foam insulation layer 11 intermesh with each other, making it difficult to pull the core wire 10 out of the foam insulation layer 11. That is, the foam insulated wire 2 in which the core wire 10 is made up of twisted wires has a large bond strength.

[0050] In this way, in the foam-insulated electric wire 2 in which the core wire 10 is made of a twisted wire, even if the foam insulation layer 11 covering the outer periphery of the core wire 10 is formed using the extruder 103, the decrease in adhesion between the core wire 10 and the foam insulation layer 11 due to the structure unique to the twisted wire does not become apparent as an area for improvement.

[0051] <When the core wire 10 is composed of a single wire> In contrast, for example, when the core wire 10 is composed of a single wire, if the extruder 103 is used as described above to form the foamed insulation layer 11 covering the outer periphery of the core wire 10, the decrease in adhesion between the core wire 10 and the foamed insulation layer 11 becomes apparent as an area for improvement.

[0052] The reason for this will be explained below.

[0053] FIG. 6 is a diagram showing a foam-insulated electric wire having a core made of a single wire.

[0054] 6, the foam-insulated electric wire 3 has a core wire 10 made of a single wire 10b and a foamed insulation layer 11 provided to cover the outer periphery of the core wire 10. As a result of forming the foamed insulation layer 11 using an extruder 103, a void 40 is formed between the core wire 10 and the foamed insulation layer 11. Here, unlike the foam-insulated electric wire 2 in which the core wire 10 is made of a stranded wire, the foam-insulated electric wire 3 in which the core wire 10 is made of a single wire does not have the unique problem of the foamed insulation layer 11 getting between the single wires 10a that make up the stranded wire. For this reason, as shown in FIG. 6, in the foam-insulated electric wire 3 in which the core wire 10 is made of a single wire, there are no parts where the core wire 10 and the foamed insulation layer 11 are in direct contact with each other, and voids 40 are present in all parts between the core wire 10 and the foamed insulation layer 11. In this way, in a foam-insulated electric wire 3 in which the core wire 10 is made of a single wire, there is no part where the core wire 10 and the foam insulation layer 11 are in direct contact, making it difficult to ensure adhesion between the core wire 10 and the foam insulation layer 11.

[0055] Furthermore, FIG. 7 is a cross-sectional view schematically illustrating the longitudinal direction (y direction) of a foam-insulated electric wire whose core wire is composed of a single wire. In FIG. 7, the core wire 10 of the foam-insulated electric wire 3 is composed of a single wire 10b. As a result, the core wire 10 does not have an uneven structure due to a stranded wire structure in the longitudinal direction (y direction), and the surface of the core wire 10 is flat. Furthermore, since the foamed insulation layer 11 is formed to match the surface shape of the core wire 10, the inner surface of the foamed insulation layer 11 is also flat. As a result, in a foam-insulated electric wire 3 whose core wire 10 is composed of a single wire 10b, the core wire 10 is easily pulled out of the foamed insulation layer 11. In other words, a foam-insulated electric wire 3 whose core wire 10 is composed of a single wire 10b has a low bond strength. Therefore, in a foam-insulated electric wire 3 whose core wire 10 is composed of a single wire 10b, a decrease in adhesion between the core wire 10 and the foamed insulation layer 11 becomes apparent as an area for improvement.

[0056] For example, if the adhesion between the core wire 10 and the foam insulation layer 11 is reduced, not only is the core wire 10 more likely to protrude during terminal processing to form a connection structure at the end of the foam-insulated electric wire 3, as shown in FIG. 8, but also the core wire 10 is more likely to protrude when the foam-insulated electric wire 3 is bent. Furthermore, if the adhesion between the core wire 10 and the foam insulation layer 11 is reduced, the core wire 10 is more likely to deviate from the center of the foam-insulated electric wire 3, as shown in FIG. 9. If the core wire 10 is deviated from the center of the foam-insulated electric wire 3, the capacitance of the foam-insulated electric wire 3 will vary in the longitudinal direction, making it difficult to keep the capacitance of the foam-insulated electric wire within a specified range. Thus, assuming that the core wire 10 is composed of a solid wire 10b, when the extruder 103 is used to form the foam insulation layer 11 that covers the outer periphery of the core wire 10, it becomes difficult to ensure adhesion between the core wire 10 and the foam insulation layer 11, which will result in a deterioration in the performance of the foam-insulated electric wire 3. That is, in the foam-insulated electric wire 3 in which the core wire 10 is composed of a single wire 10b, there is room for improvement specific to the foam-insulated electric wire 3 that is not apparent in the foam-insulated electric wire 2 in which the core wire 10 is composed of a twisted wire.

[0057] Therefore, in this embodiment, a measure has been taken to address the room for improvement inherent in the foam-insulated electric wire 3 in which the core wire 10 is composed of a single wire 10b. The technical concept of this embodiment in which this measure has been taken will be described below.

[0058] <Configuration of foam-insulated electric wire according to the embodiment> FIG. 10 is a cross-sectional view showing a schematic configuration of a foam insulated electric wire according to the present embodiment.

[0059] In FIG. 10, the y direction indicates the longitudinal direction in which the foam insulated wire 4 extends.

[0060] As shown in FIG. 10 , the foam-insulated electric wire 4 includes a core wire 10 formed of a solid wire and a foamed insulation layer 11 provided around the core wire 10. The foamed insulation layer 11 contains air bubbles 30. The foamed insulation layer 11 is in partial contact with the core wire 10. For example, as shown in FIG. 10 , the foamed insulation layer 11 and the core wire 10 are in contact in a region BR. On the other hand, in FIG. 10 , a void 40 is present between the foamed insulation layer 11 and the core wire 10 in a region other than the region BR. Thus, the foam-insulated electric wire 4 in this embodiment has a region where the foamed insulation layer 11 and the core wire 10 are in contact (region BR) and a region where a void 40 is present between the foamed insulation layer 11 and the core wire 10.

[0061] <Structural Features of the Embodiment> Next, the features of this embodiment will be described.

[0062] A feature of the present embodiment is that, assuming that the core wire 10 of the foam-insulated electric wire 4 is formed from a solid wire, there are portions where the foamed insulation layer 11 and the core wire 10 are in contact with each other and portions where a gap 40 is present between the foamed insulation layer 11 and the core wire 10, as shown in FIG. 10 . As a result, the foam-insulated electric wire 4 of the present embodiment can improve the adhesion between the foamed insulation layer 11 and the core wire 10. This is because, in the foam-insulated electric wire 4 of the present embodiment, there is a portion where the foamed insulation layer 11 and the core wire 10 are in contact with each other, as shown in region BR in FIG. 10 , and the contact force at this portion improves the adhesion between the foamed insulation layer 11 and the core wire 10. In other words, according to the present embodiment, the contact portion between the foamed insulation layer 11 and the core wire 10 functions as an appropriate contact resistance (friction resistance), and the bond strength can be kept within a specified range. Specifically, according to the foam insulated wire 4 of this embodiment, the bond strength, which indicates the force required to pull out the core wire 10 from the foam insulation layer 11, can be kept within a range of 0.1 N or more when the length of the foam insulated wire 4 is 50 mm, and 1.0 N or less when the length of the foam insulated wire 4 is 10 mm.

[0063] In particular, in this embodiment, it is important that the foamed insulation layer 11 and the core wire 10 are in partial contact. This is because, for example, if the foamed insulation layer 11 and the core wire 10 are not in contact at all, as in Figure 7, there will be no contact resistance (frictional resistance) between the foamed insulation layer 11 and the core wire 10, resulting in an extremely low bond strength. On the other hand, if the foamed insulation layer 11 and the core wire 10 are in complete contact, the contact resistance (frictional resistance) between the foamed insulation layer 11 and the core wire 10 will be extremely high, resulting in an excessively high bond strength. In other words, it is important that the foamed insulation layer 11 and the core wire 10 are in partial contact from the perspective of obtaining a bond strength within the specified range.

[0064] Furthermore, the feature of this embodiment that there are portions where the foamed insulation layer 11 and the core wire 10 are in contact and portions where a gap 40 is present between the foamed insulation layer 11 and the core wire 10 has important technical significance from the viewpoint of keeping the capacitance of the foam core of the foamed insulated wire 4 within the specified range (50 pF / m or more and 70 pF / m or less) while attempting to make the foamed insulated wire 4 thinner.

[0065] For example, consider a configuration in which the foamed insulation layer 11 and the core wire 10 are in complete contact with each other, rather than a configuration in which the foamed insulation layer 11 and the core wire 10 are in partial contact. In this configuration, to achieve a capacitance of the foam core of the foamed insulation wire 4 of 50 pF / m or more and 70 pF / m or less, the foaming degree of the foamed insulation layer 11 must be 45% or more and 60% or less. However, to thin the foamed insulation wire 4 so that the outer diameter φ of the foamed insulation wire 4 is φ≦0.37 mm, the thickness of the foamed insulation layer 11 must be reduced to, for example, approximately 70 μm. It is very difficult to achieve a foaming degree of 45% or more and 60% or less in a foamed insulation layer 11 of such a thickness. This is because, if an attempt is made to achieve a foaming degree of 45% or more and 60% when the thickness of the foamed insulation layer 11 is 70 μm or less, the size of the bubbles 30 contained in the foamed insulation layer 11 will be 60 μm or more. As a result, there is a risk of the foamed insulation layer 11 suffering from coating tearing or other problems.

[0066] In contrast, in a configuration in which the foamed insulation layer 11 and the core wire 10 are in partial contact with each other, there are also portions in which voids 40 are present between the foamed insulation layer 11 and the core wire 10. In this case, the dielectric constant of the voids 40 is low, just like the dielectric constant of the bubbles 30 contained in the foamed insulation layer 11. Therefore, in this embodiment, not only the bubbles 30 formed inside the foamed insulation layer 11 but also the voids 40 present between the foamed insulation layer 11 and the core wire 10 contribute to reducing the capacitance. This means that it is no longer necessary to set the foaming degree of the foamed insulation layer 11 to 45% or more and 60% or less in order to keep the capacitance of the foam core of the foamed insulated electric wire 4, for example, between 50 pF / m and 70 pF / m or less. In other words, because the voids 40 contribute to reducing the capacitance, even if the foaming degree of the foamed insulation layer 11 is lower than 45%, the capacitance of the foam core of the foamed insulated electric wire 4 can be kept, for example, between 50 pF / m and 70 pF / m. Therefore, according to this embodiment, even if the thickness of the foamed insulation layer 11 is thinned to, for example, about 70 μm in order to make the outer diameter φ of the foamed insulated electric wire 4 φ≦0.37 mm, it is possible to suppress breakage of the coating of the foamed insulation layer 11, which occurs when the degree of foaming is increased.

[0067] As described above, according to the features of the present embodiment, even when the outer diameter of the foam-insulated electric wire 4, in which the core wire 10 is formed of a solid wire, is thinned to 0.37 mm or less, it is possible to improve the adhesion between the foam insulation layer 11 and the core wire 10 and reduce the capacitance of the foam-insulated electric wire 4. Specifically, according to the features of the present embodiment, the bond strength, which indicates the force required to pull the core wire 10 out of the foam insulation layer 11, can be set within a range of 0.1 N or more when the length of the foam-insulated electric wire 4 is 50 mm and 1.0 N or less when the length of the foam-insulated electric wire 4 is 10 mm, and the capacitance of the foam core of the foam-insulated electric wire 4 can be set within a range of 50 pF / m or more and 70 pF / m or less. Therefore, according to the present embodiment, it is possible to thin the foam-insulated electric wire 4, in which the core wire 10 is formed of a solid wire, and to improve its performance at the same time.

[0068] <Manufacturing process of foam insulation layer 11> As described above, in the foam insulated electric wire 4 of this embodiment, on the premise that the core wire 10 of the foam insulated electric wire 4 is configured from a single wire, there are coexisting portions where the foamed insulation layer 11 and the core wire 10 are in contact with each other and portions where a gap 40 is interposed between the foamed insulation layer 11 and the core wire 10, as shown in Fig. 10, for example. In the following, a manufacturing process of the foamed insulation layer 11, which forms the foamed insulation layer 11 around the outer periphery of the core wire 10 to achieve this configuration, will be described.

[0069] FIG. 11 is a schematic diagram showing how a foamed insulating layer is formed around the outer periphery of a core wire by an extruder.

[0070] In the extruder 103, an inert gas, typically carbon dioxide gas, nitrogen gas, or a mixture of these, serving as a foaming agent is injected through the barrel of the extruder 103 and then kneaded with the fluororesin by the screw. That is, in the extruder 103, a foaming insulation material 20 composed of a fluororesin containing a foaming agent is kneaded. Then, as shown in FIG. 11 , while the core wire 10 is moved in the y direction at a predetermined linear speed, the kneaded foaming insulation material 20 is extruded from the die 103b of the extruder 103 onto the outer periphery of the core wire 10. As the foaming insulation material 20 is extruded from the die 103b onto the outer periphery of the core wire 10, the pressure difference between the inside of the extruder 103, which is in a high-pressure state, and the outside of the extruder 103, which is in an atmospheric (low-pressure) state, causes the foaming agent contained in the foaming insulation material 20 to become supersaturated. As a result, bubbles 30 are generated in the foaming insulation material 20 extruded from the die 103b. As the generated bubbles 30 grow, a foamed insulation layer 11 is formed around the outer periphery of the core wire 10.

[0071] <Features of the manufacturing method in the embodiment> Here, a feature of this embodiment is that, for example, as shown in Fig. 11 , in the process of covering the outer periphery of core wire 10 with foamed insulation layer 11, the discharge flow rate of foamed insulation material 20 is made faster than the linear speed (drawing speed) of core wire 10. This results in, for example, foamed insulation material 20 being supplied in excess to core wire 10. As a result, as shown in Fig. 11 , foamed insulation material 20 discharged from mouthpiece 103b is extruded onto the outer periphery of core wire 10 in a wavy pattern. Therefore, according to this embodiment, foamed insulation layer 11 covering the outer periphery of core wire 10 can be formed so that there are both areas where foamed insulation layer 11 and core wire 10 are in contact and areas where voids 40 are present between foamed insulation layer 11 and core wire 10. That is, the manufacturing method characteristic of this embodiment is that, for example, instead of making the linear speed (pulling speed) of the core wire 10 and the discharge flow speed of the foamed insulation material 20 equal as shown in Figure 3, the discharge flow speed of the foamed insulation material 20 is made faster than the linear speed (pulling speed) of the core wire 10, as shown in Figure 11.

[0072] 11 , for example, core wire 10 consisting of a single wire is moved at a predetermined linear speed, while extruding foamable insulation material 20 from extruder 103 at a predetermined discharge flow rate so as to cover the outer periphery of core wire 10, thereby forming foamed insulation layer 11 made of foamed insulation material 20 that covers the outer periphery of core wire 10. Specifically, in this embodiment, the ratio of the discharge flow rate of foamed insulation material 20 to the linear speed of core wire 10 (discharge flow rate / linear speed) is 1.1 or more and 1.3 or less. As a result, according to the characteristic feature of the manufacturing method of this embodiment, foamed insulation layer 11 can be brought into partial contact with core wire 10.

[0073] <Photo of foam-insulated wire> Figures 12 and 13 are photographs showing the cross-sectional structure of the foamed insulation layer 11 in this embodiment. Figure 14 is a photograph showing the cross-sectional structure of the core wire 10 covered with the foamed insulation layer 11. Furthermore, Figure 15(a) is a cross-sectional photograph taken along line AA in Figure 14, Figure 15(b) is a cross-sectional photograph taken along line BB in Figure 14, and Figure 15(c) is a cross-sectional photograph taken along line CC in Figure 14.

[0074] As shown in Figures 12 and 13, it can be seen that air bubbles 30 have formed inside the foamed insulation layer 11 and that the inner periphery of the foamed insulation layer 11 is wavy. Furthermore, the photograph shown in Figure 14 reveals that the structures shown in Figures 10 and 11 have been realized. Furthermore, in Figures 15(a) and 15(c), there are areas where the foamed insulation layer 11 is in partial contact with the core wire 10, and the core wire 10 is fixed to the foamed insulation layer 11. Meanwhile, in Figure 15(b), it can be seen that the foamed insulation layer 11 is not in complete contact with the core wire 10, and the core wire 10 is not fixed to the foamed insulation layer 11. Here, the state in which the core wire 10 is not fixed to the foamed insulation layer 11 corresponds to a state in which the bond strength is zero in the bond strength evaluation method described below.

[0075] From the above, it can be seen from the actual photographs shown in Figures 12 to 15 that the characteristic feature of this embodiment, that there are areas where the foamed insulation layer 11 and the core wire 10 are in contact and areas where a gap 40 is present between the foamed insulation layer 11 and the core wire 10, has been realized.

[0076] <Example> In the following, assuming that the core wire of the foam-insulated electric wire is composed of a single wire, the results of verification of the useful effect that the foam-insulated electric wire of this embodiment can achieve both improved adhesion between the core wire and the foam insulation layer and reduced capacitance of the foam-insulated electric wire will be described based on examples.

[0077] <<Exam Contents>> The bond strength evaluation method is used to evaluate the adhesion between the core wire and the foam insulation layer. The bond strength evaluation method is explained below.

[0078] FIG. 16 is a diagram for explaining a method for evaluating bond strength.

[0079] First, as shown in Figure 16(a), a sample (foamed core) is prepared in which a portion of a core wire 10 made of a conductor with an overall length of 100 mm to 120 mm (a 50 mm coated portion (also called a 50 mm sample length) or a 10 mm coated portion (also called a 10 mm sample length)) is covered with a foamed insulation layer 11.

[0080] 16(b), the core wire of the sample is tied to the pull gauge 200, and the covering portion (50 mm or 10 mm) covered with the foamed insulation layer 11 is sandwiched with the tape 210. When sandwiching the covering portion with the tape 210, the foamed insulation layer 11 portion is not pressed down, and the covering portion and the tape 210 are bonded by the weight of the tape 210 itself.

[0081] For example, DS2-50N manufactured by IMADA is used as the pull gauge 200. For example, masking tape N-300 12×30 manufactured by NITTO TAPE is used as the tape 210. The width of the tape 210 is 12 mm.

[0082] 16(c), the foamed insulation layer 11 is pulled out from the core wire 10 while holding down only the tape 210 and not the foamed insulation layer 11. The maximum value applied to the pull gauge 200 during this pulling out is recorded as the bond strength. The tape 210 is pulled out at a speed of approximately 1 m / min.

[0083] Next, the method for measuring capacitance will be explained.

[0084] As shown in Figure 17, the capacitance was measured by placing a 1 m foam core 300a, or a foam core 300a with a skin layer made of skin tape formed on the outer periphery (member 300b) in a metal pipe 301 filled with water 302, connecting one end to the metal pipe 301 and the other end to an LCR meter 303 manufactured by Yokogawa Hewlett-Packard, and measuring the capacitance at 1 MHz.

[0085] <<Evaluation Results>> The evaluation results are shown in Table 2. Each test was performed on 10 samples. The minimum bond strength value for the 50 mm sample is listed, and the maximum bond strength value for the 10 mm sample is listed. The maximum and minimum values ​​for the capacitance of the foam core are also listed.

[0086] [Table 2]

[0087] The foam insulation material used was a full compound made by blending PFA (MFR 70g / 10min) or FEP (MFR 36g / 10min) with 0.5 parts by mass of boron nitride as a foam nucleating agent. The extruder used to manufacture the foam insulation layer was equipped with a nozzle in the cylinder that could inject the foaming agent, carbon dioxide, nitrogen, or a mixture of these gases. The foaming agent was used by reducing the primary pressure on the cylinder side with a secondary pressure reducing valve. Here, "PFA" refers to a fluororesin, a tetrafluoroethylene-perfluoroalkoxyethylene copolymer, and "FEP" refers to a tetrafluoroethylene-hexafluoropropylene copolymer. "MFR" stands for melt flow rate, an index of resin fluidity. The higher the "MFR," the greater the fluidity when melted.

[0088] Examples 1 to 3 (required capacitance: 60 pF / m or more and 70 pF / m or less) are examples in which the ratio of the discharge flow rate of the foamed insulating material to the linear speed of the core wire was 1.1 to 1.3 times. Here, this ratio was adjusted by keeping the linear speed constant and increasing the screw rotation speed of the extruder to increase the discharge flow rate. In Examples 1 to 3, the bond strength values ​​for sample lengths of 50 mm and 10 mm were within the specified range. Furthermore, in Examples 1 to 3, the capacitance of the foamed core was also within the specified range at 63 pF / m to 69 pF / m, and the capacitance of the foamed core after the skin layer process was also within the specified range at 63 pF / m to 68 pF / m.

[0089] Example 4 (required capacitance: 60 pF / m or more and 70 pF / m or less) is an example in which the diameter of the core wire is 46 AWG (0.04 mm). In Example 4, the ratio of the discharge flow rate of the foamed insulation material to the line speed of the core wire was also 1.1, and both the bond strength and capacitance were within the specified range.

[0090] Examples 5 and 6 (required capacitance: 60 pF / m or more and 70 pF / m or less) show examples in which nitrogen gas or a 1:1 mixture of nitrogen gas and carbon dioxide gas is used as the blowing agent. In this case, nitrogen gas has a lower solubility in fluororesin than carbon dioxide gas, so the capacitance of the foamed core tends to be higher, but in both Examples 5 and 6, the bond strength and the capacitance of the foamed core were both within the specified range.

[0091] Example 7 (required capacitance: 60 pF / m or more and 70 pF / m or less) is an example in which FEP (MFR 36 g / 10 min) was used as the foam insulation material. Because the MFR of PFA is about twice that of FEP, the viscosity of FEP during melting is higher than that of PFA. This slows down bubble growth, which tends to slightly increase the capacitance of the foam core, but both the bond strength and the capacitance of the foam core were within the specified range. Here, "FEP" refers to a fluororesin, a copolymer of tetrafluoroethylene and hexafluoropropylene.

[0092] Example 8 (required capacitance: 50 pF / m or more and 60 pF / m or less) had a smaller capacitance of the foamed core compared to Examples 1 to 7. The capacitance can be reduced by increasing the thickness of the foamed insulation layer (increasing the outer diameter of the foamed core) and / or increasing the foaming degree of the foamed insulation layer. Example 8 shows the results for 42 AWG. To increase the foaming degree, the carbon dioxide gas injection pressure was increased to 1 MPa, and the diameter of the foamed insulation layer was also increased to φ = 0.267 mm. By setting the foamed insulation material discharge flow rate ratio to the specified 1.1 times, the bond strength and capacitance were within the specified range. Similarly, Example 9 (required capacitance: 50 pF / m or more and 60 pF / m or less) was for 46 AWG, but the bond strength and capacitance were within the specified range.

[0093] In this way, in Examples 1 to 9 having the manufacturing method characteristics of the present embodiment, both the bond strength and the capacitance of the foamed core were within the specified range, and therefore the overall evaluation of each of Examples 1 to 9 was Pass. Therefore, it can be seen that, based on the premise that the core wire of the foam-insulated electric wire is composed of a single wire, Examples 1 to 9 can achieve both improved adhesion between the core wire and the foamed insulation layer and reduced capacitance of the foam-insulated electric wire.

[0094] On the other hand, Comparative Example 1 is an example in which the ratio of the discharge flow rate of the foam insulation material to the linear velocity of the core wire was 1.0. In Comparative Example 1, the inner surface of the foam insulation layer was flat and did not come into contact with the core wire, resulting in a bond strength of zero. In addition, the position of the core wire was not fixed, and as a result, the capacitance of the foam core was outside the specified range, resulting in failure.

[0095] Comparative Example 2 is an example in which the ratio of the discharge flow rate of the foamed insulation material to the wire speed of the core wire was 1.4. In Comparative Example 2, the discharge rate of the foamed insulation material was too fast, pulling the core wire, causing the core wire to break and making it impossible to manufacture.

[0096] Comparative Example 3 is an example of an unfoamed insulated wire. In Comparative Example 3, because the foamed insulating material was unfoamed, the inner diameter did not expand after extrusion. As a result, the bond strength exceeded the core strength, resulting in core wire breakage (conductor breakage). Furthermore, because the foamed core was unfoamed, the capacitance of the foamed core was outside the specified range and was therefore unacceptable.

[0097] For the reasons described above, Comparative Examples 1 to 3 do not embody the manufacturing method features of the present embodiment, and in these cases, they failed one of the various tests. This means that if the conditions for embodying the features of the present embodiment are not met, it is difficult to achieve both improved adhesion between the core wire and the foam insulation layer and reduced capacitance of the foam insulated electric wire, assuming that the core wire of the foam insulated electric wire is composed of a single wire. Therefore, a comparison of Examples 1 to 9 with Comparative Examples 1 to 3 supports the idea that, according to the features of the present embodiment, it is possible to achieve both improved adhesion between the core wire and the foam insulation layer and reduced capacitance of the foam insulated electric wire, assuming that the core wire of the foam insulated electric wire is composed of a single wire.

[0098] The invention made by the inventor has been specifically described above based on the embodiments thereof, but it goes without saying that the present invention is not limited to the above-described embodiments and can be modified in various ways without departing from the spirit of the invention.

[0099] The above embodiment includes the following aspects.

[0100] (Appendix 1) A method for manufacturing an electric wire, comprising the steps of: moving a core wire made of a single wire at a predetermined linear speed; and extruding a foaming insulating material from an extruder at a predetermined discharge flow rate so as to cover the outer periphery of the core wire; thereby forming a foaming insulating layer made of the foaming insulating material and covering the outer periphery of the core wire, The method for producing an electric wire, wherein the ratio of the discharge flow rate to the linear velocity (discharge flow rate / linear velocity) is 1.1 or more and 1.3 or less. [Explanation of symbols]

[0101] 1. Foam-insulated wire 2. Foam-insulated wire 3. Foam-insulated wire 4. Foam-insulated wire 10 core wire 10a single track 11 Foam insulation layer 12 Skin Tape 13 Outer conductor 14 Sheath 20 Foam insulation material 30 bubbles 40 Cavity 100 Foam insulation layer manufacturing system 101 Sending machine 102 Wire preheater 103 Extruder 103a Gas Cylinder 103b nozzle 104 Aquarium 104a Capacitance measuring device 105 Outer diameter measuring machine 106 Pick-up machine 107 Winder 200 pull gauge 210 Tape 300a foam core 300b material 301 Metal Pipe 302 Water 303 LCR meter AR area BR area

Claims

1. a core wire consisting of a single wire; a foam insulation layer provided on the outer periphery of the core wire; A foam-insulated electric wire comprising: The capacitance of the foam core including the core wire and the foam insulation layer is 50 pF / m or more and 70 pF / m or less, a bond strength, which indicates the force required to pull the core wire out of the foam insulation layer, of 0.1 N or more when the length of the foam insulated electric wire is 50 mm, and of 1.0 N or less when the length of the foam insulated electric wire is 10 mm; A foam insulated electric wire, wherein a gap is partially interposed between the foam insulation layer and the core wire. (However, foam-insulated wires with wavy cores are excluded.)

2. The foam insulated wire according to claim 1, The foam insulated electric wire has an outer diameter φ that satisfies the relationship φ≦0.37 mm.

3. The foam insulated electric wire according to claim 1 or 2, The foamed insulation layer comprises a fluororesin containing a foam nucleating agent.

4. The foam insulated wire according to claim 3, the fluororesin contains tetrafluoroethylene-perfluoroalkoxyethylene copolymer or tetrafluoroethylene-hexafluoropropylene copolymer, The foam insulated wire, wherein the foam nucleating agent is boron nitride.

5. 5. The method for producing a foamed insulated electric wire according to claim 1, wherein a core wire made of a single wire is moved at a predetermined linear speed, and a foamed insulating material is extruded from an extruder at a predetermined discharge flow rate so as to cover an outer periphery of the core wire, thereby forming a foamed insulating layer made of the foamed insulating material and covering the outer periphery of the core wire, The method for manufacturing a foam-insulated electric wire, wherein the discharge flow rate is greater than the wire speed.

6. 6. The method for producing a foam insulated electric wire according to claim 5, a ratio of the discharge flow rate to the wire speed (discharge flow rate / wire speed) of 1.1 to 1.3.

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