JP2012104333A - Foam insulated cable and method for manufacturing the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide means for making small both of a delay time (Td) and a delay time difference (Skew), forming a foam insulation having high formability and high stability in high productivity, and efficiently producing a cable having a small skew.SOLUTION: A form insulated cable 1 has a structure in which at least two foam insulated wires 10 each of which is constituted by forming a foam insulation 23 around a central conductor 21 are arranged in parallel or in stranded. A delay time (Td) of the foam insulated wires 10 is 3.9 ns/m or more and 4.2 ns/m or less, and the zero shear viscosity of the foam insulation 23 at 170°C is 3000 Pa s or more and 18000 Pa s or less.

Description

  The present invention relates to a foamed cable used for high-speed signal transmission and a method for manufacturing the same.

  In recent years, along with miniaturization and high performance of electronic devices, there is a strong demand for higher performance, that is, higher transmission speed and increased communication capacity in signal transmission cables used for them.

  The transmission speed is often represented by a delay time (Td), and in most cases, it is represented by a time (ns / m) required for a signal to pass through a 1-m wire. The smaller the Td, the higher the transmission speed.

  It is well known that the transmission speed is greatly affected by the dielectric constant of the insulator used for the electric wire, and the smaller the dielectric constant, the higher the transmission speed. Polyethylene (relative dielectric constant: about 2.3) and fluorine resin (ratio) A low dielectric constant insulator having a dielectric constant of 2.1 to 2.2) is used. Furthermore, since the relative permittivity of air is 1, a technology for suppressing the relative permittivity of the entire insulator by including bubbles in these insulators has been put into practical use, and is generally used as a foam insulated wire. It has become. Some types with a high degree of foaming have a Td of 4.0 ns / m or less.

  On the other hand, a differential transmission method is becoming mainstream as a method for dealing with an increase in communication capacity. In the differential transmission method, communication is performed using a high frequency signal in the GHz band, but the communication capacity is large. However, even if a slight shift occurs in the signal arrival time, communication failure occurs. Therefore, it is necessary to reduce the delay time difference (Skew) of the electric wires used in pairs.

  In order to improve the communication speed and capacity, a cable having both Td and Skew reduced is required for such applications.

  In addition, the manufacturing method of a foam insulated wire is divided roughly and there exist the following two types of manufacturing methods.

(Physical foaming method)
One is a method of injecting a high-pressure gas into a resin melted in an extruder, which is called a physical foaming method. The general procedure is as follows.
1) A resin is put into an extruder and melted by heating and kneading.
2) A high-pressure gas is injected from the middle of the resin flow path and dissolved.
3) A gas-dissolved resin is coated on the conductor.
4) As the conductor moves, the coated resin is moved out of the extruder.
5) Gas released from the pressure inside the extruder and dissolved in the resin becomes bubbles.
6) Cooling and solidifying the resin before the bubbles grow excessively and the insulator becomes non-uniform.

(Chemical foaming method)
The other is a method of adding a chemical foaming agent together with a resin, which is called a chemical foaming method. The general procedure is as follows.
1) Put resin and foaming agent in the extruder. The blowing agent may be used alone or kneaded in the resin.
2) Heat above the decomposition temperature of the blowing agent in the extruder. At that time, the pressure of the resin is kept high so as not to foam in the extruder, and the generated gas is dissolved in the resin.
3) A gas-dissolved resin is coated on the conductor.
4) As the conductor moves, the coated resin is moved out of the extruder.
5) Gas released from the pressure inside the extruder and dissolved in the resin becomes bubbles.
6) Cooling and solidifying the resin before the bubbles grow excessively and the insulator becomes non-uniform.

The physical foaming method has the following advantages over the chemical foaming method.
(1) It is easy to obtain a high foaming degree.
(2) Since no chemical foaming agent is used, there is little decrease in the electrical characteristics (dielectric constant ε and dielectric loss tangent tan δ) of the insulator due to the foaming agent and the residue of the foaming agent.

  For these reasons, the physical foaming method is frequently used in the production of high-performance foam insulated wires.

JP 2006-339099 A

  However, the foam insulated wire by the foaming method has a problem that bubble growth becomes unstable. It is known that the bubble growth is slow when the surrounding material viscosity is high and fast when the zero shear viscosity is low because the gas pressure in the bubble is the driving force. If the bubble growth is too fast, the bubble growth will vary, or abnormal growth will likely occur, and the Td of individual foam insulated wires will vary due to fluctuations in outer diameter, eccentricity (thickness deviation), and fluctuation in foaming degree. Occurs and Skew increases.

  In particular, in the case of thin-diameter, thin-walled foam insulated wires and cables, a slight difference in bubble growth leads to fluctuations in the foam insulation, so the bubble growth is often moderated using a relatively high viscosity material. . However, when forming a highly foamed insulator, even if a high-viscosity material is used, bubbles are likely to grow abnormally because it is foamed with a higher pressure gas, and such wires and cables are less productive. End up.

As one of countermeasures against this problem, a method (Patent Document 1) that uses an extremely fine foam nucleating agent has been proposed. This is aimed at preventing the abnormal growth of bubbles by generating a large amount of bubbles by using a fine particle nucleating agent that becomes the starting point of foaming and reducing the gas flowing into each bubble. However, this method also has the following problems.
(1) Since the ultrafine particle nucleating agent is difficult to uniformly disperse in the resin, problems such as secondary aggregation and poor dispersion are likely to occur.
(2) When the nucleating agent becomes ultrafine particles, the work environment is likely to be contaminated, so that handling is troublesome and workability is lowered.
(3) Even if a fine particle nucleating agent is used, if the amount added is increased in order to improve the degree of foaming, the dielectric constant ε and dielectric loss tangent tanδ as a foamed insulator are likely to be adversely affected.

  As described above, when Td is reduced, Td varies, and it is difficult to reduce Skew.

  The present invention has been made with respect to the point of application, and provides a foamed cable having a small Td and Skew and a manufacturing method thereof.

  In order to solve the above-mentioned problems, the present invention relates to a foam cable having a structure in which at least two foam insulated wires formed by forming a foam insulator on the outer periphery of a central conductor are arranged in parallel or at least two pairs. The foam insulated wire has a delay time (Td) of 3.9 ns / m to 4.2 ns / m and the foamed insulator has a zero shear viscosity at 170 ° C. of 3000 Pa · s to 18000 Pa · s. It is a cable.

  It is preferable that the foamed insulator has an outer diameter of 2 mm or less and the center conductor has a diameter of 1 mm or less.

  The delay time difference (Skew) of the foamed cable is preferably 8 ps / m or less.

  In the foamed cable, the foamed insulator coated on the outer periphery of the center conductor may be made of a material mainly composed of linear low density polyethylene.

  In addition, the present invention extrudes a resin composition containing a polyolefin resin and a foaming agent or a foaming nucleating agent with an extruder and chemically foams or physically foams it, and forms a foamed insulated wire by covering the outer periphery of the center conductor with a foamed insulator. In the method of manufacturing a foamed cable in which at least two foamed insulated wires are arranged in parallel or formed by at least two pairs, the foamed insulator includes a polyolefin resin and a foaming agent or a foaming nucleating agent. And a zero shear viscosity of the foamed insulator at 170 ° C. of 3000 Pa · s to 18000 Pa · s.

  The foaming degree of the foamed insulator is preferably 50% or more.

  According to the present invention, both Td and Skew of the foamed cable can be reduced.

It is sectional drawing of the foamed cable which concerns on one embodiment of this invention. It is sectional drawing of the foam insulated wire used for the foam cable of FIG. It is a figure which shows the measurement result of the extensional viscosity of polyethylene.

  Hereinafter, an embodiment of the present invention will be described with reference to the accompanying drawings.

  In the foamed cable according to the present invention, at least two foam insulated wires formed by forming a foam insulator made of a resin composition containing a polyolefin resin and a foaming agent or a foam nucleating agent are arranged in parallel on the outer periphery of the central conductor, or The structure has at least two pairs.

  As an example of the foamed cable according to the present invention, the twinax cable shown in FIG. 1 having a structure in which two foam insulated wires are arranged in parallel will be described. However, the foamed cable is not limited to the structure of FIG. Alternatively, a twisted wire structure in which two foam insulated wires are twisted together may be used. Moreover, it is good also as a structure which twists together 4 or 6 foam insulation electric wires, and uses 2 facing each other as 1 set, and a structure which uses a spacer and an intermediary material. Further, a plurality of these can be combined into one cable.

  As shown in FIG. 1, the foamed cable 1 according to the present embodiment has two foam insulated wires 10 arranged in parallel and a drain wire 11 arranged along the foam insulated wires 10. A shield tape 12 is coated on the outer periphery, and a resin tape 13 is wound around the outer periphery.

  The shield tape 12 is for shielding external noise, and the external noise shielded by the shield tape 12 is dropped to the external ground through the drain wire 11.

  As shown in FIG. 2, the foam insulated wire 10 is formed by extrusion coating a foam insulator 23 having countless bubbles 22 around the outer periphery of the center conductor 21. The central conductor 21 may be a single wire or a stranded wire. In the case of a stranded wire, the conductor cross-sectional area is smaller than that of a single wire.

  The outer diameter of the foam insulator 23 of the foam insulated wire 10 is 2 mm or less, and the diameter of the central conductor 21 is 1 mm or less. By doing so, the thin diameter thin-walled foamed cable 1 is formed.

  Here, in the foam insulated wire, it is desired to reduce the delay time (Td) from the viewpoint of increasing the transmission speed. For this purpose, it is necessary to increase the degree of foaming in the foam insulation of the foam insulated wire.

  However, even if Td is reduced by increasing the foaming degree of the foamed insulating material, it is difficult to make the foam uniform over the longitudinal direction of the foamed insulated wire, resulting in variations in Td. Thus, when there is variation in Td, it is difficult to reduce the delay time difference (Skew) in a foamed cable formed by bundling a plurality of foam insulated wires, such as a twinax cable configured using the Td.

  Therefore, the present inventors have defined the conditions that can prevent the variation of Td in the foam insulated wire and reduce the skew when the foamed cable is used.

  That is, according to the present invention, the foam insulated wire has a Td of 3.9 ns / m or more and 4.2 ns / m or less, and the foam insulation of the foam insulated wire has a zero shear viscosity at 170 ° C. of 3000 Pa · s or more and 18000 Pa · s. The following guarantees that the skew can be reduced to 8 ps / m or less when a foamed cable is used.

  Hereafter, each item is demonstrated with the manufacturing method of the foamed cable 1. FIG.

  First, the foamed insulated wire 10 is formed by extrusion-coating the foamed insulator 23 having countless bubbles 22 on the outer periphery of the center conductor 21. The conditions at this time are as follows.

(Foaming method, conditions)
The foaming method is described. There are two foaming methods, physical foaming and chemical foaming. The physical foaming method is preferred as the method applied to the present invention, but the chemical foaming method can be selected according to the purpose and required performance of the product. .

  The Td of the foam insulated wire 10 is adjusted by the foaming degree of the foam insulator 23. For example, when the main component of the resin composition is LLDPE (linear low density polyethylene), Td can be 4.2 ns / m or less if the foaming degree is 50% or more.

(Zero shear viscosity)
The zero shear viscosity of the resin composition is in the range of 3000 Pa · s to 18000 Pa · s, more preferably in the range of about 8000 Pa · s to 12000 Pa · s. When the viscosity is further increased from this range, foaming is inevitably performed using a high-pressure gas, and nest formation and abnormal bubble growth are likely to occur. On the other hand, if the viscosity is low, eccentricity (thickness) due to liquid dripping tends to occur when the outer periphery of the center conductor 21 is covered. For this reason, the viscosity of the material is desirably 3000 Pa · s or more and 18000 Pa · s or less centering on the optimum range.

  For the measurement of the zero shear viscosity, for example, a dynamic viscosity measuring device ARES manufactured by TA Instruments is used, and the frequency is measured at 170 ° C. using a φ20 mm parallel plate as a parameter. The zero shear viscosity is obtained by extending this measurement result to the low frequency side and extrapolating the shear rate to zero.

  The zero shear viscosity at 170 ° C. of the foamed insulation formed and coated on the center conductor is the same as the zero shear viscosity at 170 ° C. of the resin composition before coating formation.

(Formulation of resin composition)
The present invention is characterized by foaming a resin composition containing a polyolefin resin having a zero shear viscosity at 170 ° C. of 3000 Pa · s or more and 18000 Pa · s or less and a foaming agent or a foaming nucleating agent, and particularly defines a material composition. However, a more preferable example of the composition of the resin composition is shown in Table 1.

  That is, HDPE (high density polyethylene) having an MFR (190 ° C., load 2.16 kg) measured in accordance with JIS-K7210 of 0.3-2 and LLDPE having an MFR of 6-10, a foaming agent as a third component or The ratio (parts by mass) of the master batch (MB) containing the foam nucleating agent is 15 to 35, 55 to 75, 20 or less. The MB will be described later.

  LLDPE is characterized by a high strain hardening property of extensional viscosity compared to other HDPE and LDPE (low density polyethylene) having the same level of zero shear viscosity (see FIG. 3). Therefore, when bubbles grow large, strain hardening develops and the viscous resistance of the resin increases, thereby reducing problems such as bubble breakage and nest generation due to abnormal growth, and fluctuations in the outer diameter. By using this LLDPE as a main material, it is possible to manufacture a foam insulated wire 10 having a stable performance by preventing abnormal growth of bubbles while having a low viscosity. Examples of LLDPE having both low viscosity and strain-hardening properties include Ultexex (registered trademark) 15150J manufactured by Prime Polymer Co., Ltd. and Sumikasen (registered trademark) L-5721 manufactured by Sumitomo Chemical Co., Ltd.

  Of course, the above shows the idea of the present invention, and the material composition of the resin composition can be selected according to the performance required for the foamed cable 1.

(Foaming nucleating agent MB)
When the physical foaming method is employed, a foam nucleating agent can be used as a starting point for generating gas dissolved in the resin as bubbles. In most cases, the foam nucleating agent is in the form of fine powder, and when these are put into an extruder, poor dispersion tends to occur in the resin. For this reason, the method of adding the compound which mix | blended the foam nucleating agent with the high concentration previously called a masterbatch (MB) is common.

  Since the foam nucleating agent MB is intended to disperse a high concentration foaming nucleating agent, its nature and form are not particularly limited. Further, in order to further improve the dispersibility in the extruder, it is possible to carry out dilution kneading with a part or all of HDPE, LDPE or LLDPE used in the present invention in advance.

  Various types of foam nucleating agents can be considered depending on the organic substance, the inorganic substance, or the size and shape, but are not particularly limited, and can be selected depending on the purpose and effect.

  Examples of organic substances include azo compounds represented by ADCA (azodicarbonamide), nitroso compounds represented by NN′-dinitrosopentamethylenetetramine, OBSH (4,4′-oxybis (benzenesulfonylhydrazide)), Examples include hydrazine derivatives represented by HDCA (hydrazodicarbonamide). Although these also have an effect | action as a below-mentioned foaming agent, using it as a foaming nucleating agent is not restrict | limited. Further, polyester, polyimide, fluororesin, polymethylpentene, cyclic olefin copolymer, polystyrene, styrene copolymer, polylactic acid, polyimide, polyamideimide, polyetherimide, polyetherketone, and other various resin powders can be selected.

  Moreover, the method of exhibiting the effect as a foam nucleating agent by adding resin different from polyolefin resin as a foam nucleating agent, kneading | mixing and stirring in an extruder can also be selected.

  Silica, talc, and other metal compounds can be selected as the inorganic substance.

  Of course, addition of a foam nucleating agent is common, but depending on the use and purpose of the foam insulated wire 10, a method in which the foam nucleating agent is not added can also be selected.

(Foaming agent MB)
On the other hand, when chemical foaming is performed, it is necessary to knead a foaming agent as a generation source of foaming gas in the resin. In the case of a foaming agent, it is common to pre-form MB like the foaming nucleating agent MB, but it can be put into the extruder alone or diluted and kneaded in advance.

  As the foaming agent, an azo compound typified by ADCA (azodicarbonamide), a nitroso compound typified by NN′-dinitrosopentamethylenetetramine, OBSH (4,4′-oxybis (benzenesulfonylhydrazide)), A hydrazine derivative typified by HDCA (hydrazodicarbonamide), sodium hydrogen carbonate and the like can be used according to the purpose and purpose.

(Other additives)
From the original purpose of the foam insulated wire 10 used in the present embodiment, it is preferable that the resin composition is purely composed of a polyolefin resin and a foaming agent or a foaming nucleating agent as far as possible in terms of its electrical characteristics. It is possible to use additives that are unavoidable for maintenance and production of the resin composition.

  In the former example, there are an antioxidant, a dispersion aid for dispersing the foam nucleating agent, a colorant for identifying a large number of foam insulated wires 10, etc. In the latter example, the molecular weight control during the synthesis of the resin composition ( Such as a deactivator for preventing excessive polymerization) and a catalyst residue. These can be used according to the purpose and effect.

  Further, a method of separately preparing a resin composition containing a colorant (pigment, dye, etc.) and coating the outer layer of the foamed insulator 23 is also possible.

(Structural variation)
Structural variations are also conceivable. It is possible not only to coat the outer layer with a resin composition containing a colorant, but also to provide a non-foamed layer directly above (outer periphery) of the central conductor 21 and provide a foamed layer on the outer periphery thereof. This is effective in preventing the phenomenon of gas escape along the eyes of the strands, particularly when stranded wires are used. As a modification of this, there is a method of slightly foaming the inner layer in order to improve the degree of foaming even a little.

  The conductor used for the central conductor 21 is not limited to a copper wire, and other metals, alloys, and ceramics or organic filaments with conductivity can be used as long as sufficient conductivity can be secured. .

  Further, regarding the presence or absence of plating and the type thereof, gold, silver, tin, or other plating can be selected according to the purpose and application. As a surface modification method other than plating, coating, sintering, use of a clad material, or the like can be selected.

  After forming the foam insulated wire 10 under the above conditions, the two foam insulated wires 10 are arranged in parallel, the drain wire 11 is placed along the foam insulated wire 10, the shield tape 12 is coated on the outer periphery thereof, and the resin is disposed on the outer periphery. The foam cable 1 is manufactured by winding the tape 13.

  In the foamed cable 1 obtained in this way, since the bubbles 22 of the foamed insulator 23 do not grow abnormally, the Td does not vary and the skew is as small as 8 ps / m or less.

  In short, according to the present invention, by defining the Td of the foam insulated wire and the zero shear viscosity at 170 ° C. of the foamed insulator, both the Td and Skew of the foamed cable can be reduced.

  In addition, according to the present invention, since foaming can be performed uniformly, a thin foamed cable having a thin diameter using a foam insulated wire in which the outer diameter of the foam insulator is 2 mm or less and the diameter of the center conductor is 1 mm or less is formed. It can cope with downsizing and high performance of electronic equipment.

  Furthermore, by using a resin composition containing LLDPE as a main component for the foamed insulation, it is possible to produce a foamed insulated wire with stable performance while preventing abnormal bubble growth while having a low viscosity.

  Next, examples of the present invention will be described together with comparative examples.

Table 2 shows material compositions of Examples and Comparative Examples. As the resin composition, density 0.951 g / cm ^3, HDPE of MFR0.8, density 0.937g / cm ^3, MFR8 of LLDPE, foam nucleating agent MB (the nucleating agent MB) were blended respectively, zero shear in the table A material with viscosity was obtained. The nucleating agent MB uses LDPE having a density of 0.918 g / cm ³ and MFR4 as a base resin, and contains 10% by mass of ADCA as a nucleating agent.

  Electric wire trial manufacture was performed using the resin composition which consists of these material composition. Table 3 shows the prototype conditions. The target of Td was set to 3.9 ns / m and 4.2 ns / m, a foamed insulator made of each resin composition was formed on the central conductor, and each Td electric wire was prototyped.

  For the trial manufacture, an extruder having a diameter of 45 mm and L / D25 (L: cylinder length of the extruder, D: diameter of the cylinder of the extruder) was used. In the prototype line, capacitance, outer diameter, and eccentricity measuring instruments are installed, and the degree of foaming and outer diameter obtained from the capacitance and outer diameter are matched to the respective targets. Adjusted. The extrusion temperature was 150 to 190 ° C., the linear velocity was 80 to 150 m / min, and the gas pressure was 20 to 60 MPa.

  The conductor used was a single wire of tin-plated copper wire and had an outer diameter of 0.81 mm (20 AWG). This conductor was subjected to foam extrusion to obtain a foam insulated wire having an outer diameter of 1.84 mm.

  The results of trial manufacture are shown in Tables 4 and 5. In order to make it easy to see the relationship between the prototype results and the material viscosity, Table 4 displays the prototype results targeting 4.2 ns / m in Table 4, and Table 5 displays the prototype results targeting 3.9 ns / m. Arranged in the order of. In Tables 4 and 5, the evaluation results are also shown, but specific criteria will be described later.

  The outline of the measuring method is described for each evaluation item in Tables 4 and 5.

  Each sample was prepared with a length of 5000 m or more, and the amount of flatness and the amount of fluctuation in foaming degree were evaluated from the data of sensors provided in the line of the extruder.

  Using these foam insulated wires, a cable was prepared from a twinax cable and a pair, and Skew was evaluated. In the evaluation, 10 samples of 3 m were collected at intervals of 100 m or more and measured, and the maximum values were described.

  About Td, it confirmed that each sample satisfy | filled target value (4.2 ns / m, 3.9 ns / m), and performed the following evaluation.

  Table 6 shows the judgment criteria of the prototype foam insulated wires and cables. A three-level evaluation including not only good and bad of the acceptance criteria but also excellent ◎ was made. Table 7 shows specific criteria for each item.

  A method for measuring the variation in foaming degree, Td and Skew shown in Table 7 will be described in detail.

(Flat amount)
In the measurement of the flatness, the major axis and the minor axis when the sample was flattened were read with a flatness measuring machine, and the difference was 10 μm or less as ◯, and 5 μm or less as ◎.

(Foaming degree variation)
The measurement of the foaming fluctuation amount was performed together with the measurement of the outer diameter fluctuation amount. For measurement of the outer diameter fluctuation amount, two outer diameter measuring machines (XY axes) installed on the electric wire production line were used. By measuring the outer diameter every 0.2 seconds and accumulating data in the PC via the data logger, the outer diameter variation over time was measured, and the capacitance C was also continuously measured. . Since the dielectric constant εs of the foamed insulator can be calculated from the conductor diameter a, the outer diameter b of the foam insulated wire, and the capacitance C, and further, the dielectric constant εp of the resin composition before foaming is known. The degree of foaming F can be calculated. Thereby, the amount of foaming fluctuation was measured. The detailed calculation method of a foaming degree is as the following Formula (1) (2). The foaming degree was 50% at Td 4.2 ns / m and 70% at Td 3.9 ns / m.

(Measurement method of Td and Skew)
In either case, measurement was performed in a TDT (Time Domain Transmission) mode or a TDR (Time Domain Reflection) mode of a high-function oscilloscope.

  The results of the examples and comparative examples will be compared.

  Table 4 comparing the trial results of Td 4.2 ns / m will be described. In Comparative Example 1 using the lowest-viscosity material, since the flattening amount was large, the skew was large even if the foaming degree fluctuation was within the standard, and the standard exceeded the standard. On the other hand, in Examples 1 to 4, Skew was 8 ps / m or less, and it was found that both the amount of flatness and the variation in foaming degree were small enough to withstand practical use. In Comparative Example 2 using a high-viscosity material, the amount of flatness was very small, but the skew was also large due to the large amount of variation in the degree of foaming.

  Table 5 that compares the trial results of Td 3.9 ns / m will be described. In Comparative Example 3 using a low-viscosity material, both the amount of flatness and the amount of fluctuation in foaming degree are large, and Skew clearly exceeds the standard. In Example 5, each item falls within the reference value and there is no tolerance, but there is no practical problem. In Examples 6 and 7, it can be seen that all items have a sufficient margin with respect to the reference. In Example 8, the variation in foaming degree is large and the tolerance is small, but it is within the standard of Skew. In Comparative Example 4, since the variation in foaming degree is large, it can be seen that Skew exceeds the reference value and is not suitable for practical use.

  As described above, as shown in the evaluation results of Tables 4 and 5, the foam insulated wire according to the present invention has a small amount of flatness and variation in the degree of foaming, and the skew when the cable is made is smaller than the conventional one. all right. Thereby, a high-performance foam cable for high-speed signal transmission can be produced efficiently.

DESCRIPTION OF SYMBOLS 1 Foam cable 10 Foam insulated wire 11 Drain wire 12 Shield tape 13 Resin tape 21 Center conductor 22 Air bubble 23 Foam insulator

Claims (5)

  In a foamed cable having a structure in which at least two foam insulated wires formed by forming a foam insulator on the outer periphery of the central conductor are arranged in parallel or at least two pairs, a delay time (Td) of the foam insulated wire A foamed cable, wherein the foamed insulator has a zero shear viscosity at 170 ° C. of 3000 Pa · s or more and 18000 Pa · s or less of 3.9 ns / m to 4.2 ns / m.   2. The foamed cable according to claim 1, wherein an outer diameter of the foamed insulator is 2 mm or less and a diameter of the central conductor is 1 mm or less.   The foamed cable according to claim 2, wherein a delay time difference (Skew) of the foamed cable is 8 ps / m or less.   The foamed cable according to claim 3, wherein the foamed insulator covered on the outer periphery of the center conductor is made of a material mainly composed of linear low-density polyethylene.   A resin composition containing a polyolefin resin and a foaming agent or a foam nucleating agent is extruded with an extruder and chemically foamed or physically foamed to form a foamed insulated wire by covering the outer periphery of the center conductor with a foamed insulation wire. In the method for producing a foamed cable in which at least two are arranged in parallel or at least two pairs are formed, the foamed insulator is made of a resin composition containing a polyolefin resin and a foaming agent or a foam nucleating agent, and the foamed cable A method for producing a foamed cable, wherein the insulator has a zero shear viscosity at 170 ° C. of from 3000 Pa · s to 18000 Pa · s.