Ultra-low attenuation radio frequency coaxial cable and leaky coaxial cable and preparation method thereof

Abstract

The present application belongs to the field of wireless communication technology, and more specifically, it relates to an ultra-low attenuation radio frequency coaxial cable and a leaky coaxial cable and a method for preparing the same. The present application adopts a three-layer co-extrusion technology to prepare the foam layer, and adjusts the injection pressure of the foaming gas in each layer of the foaming material and the extrusion pressure of each layer of the foaming material through the process, so as to finally achieve a foaming degree of 85% or more in the foam layer of the radio frequency coaxial cable and the leaky coaxial cable, and a uniform distribution of pore diameters in the foam layer, thereby effectively reducing the dielectric constant and signal transmission attenuation while maintaining the compressive strength of the cable required by the standard. The attenuation value of the radio frequency coaxial cable and the leaky coaxial cable prepared by the method of the present application in the 100MHz-3700MHz frequency band is optimized by an average of about 13.3% compared to the attenuation value of the existing conventional process.

Description

Technical Field

[0001] The present application belongs to the field of wireless communication technology, and more specifically, relates to an ultra-low attenuation radio frequency coaxial cable and a leaky coaxial cable and a preparation method thereof. Background Art

[0002] Communication cable is a type of cable used to transmit electromagnetic energy within the radio frequency range and is widely used in various radio communication systems and electronic equipment. It plays an important role in wireless communication, broadcasting, television, radar, navigation and other fields. Among them, coaxial cable is the most widely used type, mainly used as antenna feeder for radio transmitting or receiving equipment, as well as internal or interconnecting wires for various communication and electronic equipment.

[0003] The insulation layer of traditional RF coaxial cables and leaky coaxial cables usually uses physical foamed polyethylene materials. By increasing the foaming degree to 82% or above, the dielectric constant and signal transmission attenuation can be effectively reduced. However, when faced with a thick insulation layer, if the foaming degree is further increased, although in theory it can further reduce the dielectric constant, reduce signal loss and increase characteristic impedance, it will cause a series of problems in the actual preparation process. As the foaming degree increases, the foaming cooling problem will cause excessive foaming of the inner insulation, and the thickness of the inner pore wall will be significantly thinned, resulting in insufficient mechanical strength and uneven distribution of pore size, which will easily cause the insulation layer to collapse or create an excessive gap with the outer conductor. This gap will destroy the geometric symmetry of the cable, causing characteristic impedance fluctuations, and then generate signal reflections, seriously affecting the quality of high-frequency signal transmission, and the attenuation increase can reach 15%. Summary of the invention

[0004] In view of the defects of the prior art, the present application provides an ultra-low attenuation RF coaxial cable and a leaky coaxial cable and a preparation method thereof, which aims to solve the technical problems of decreased cable structural stability and electrical performance under high foaming degree.

[0005] To achieve the above-mentioned objectives, in a first aspect, the present application provides an ultra-low attenuation radio frequency coaxial cable, comprising an inner conductor, a composite insulation layer, an outer conductor and an outer sheath concentrically nested from the inside to the outside, the composite insulation layer comprising a foam layer, the foam layer being prepared by three-layer co-extrusion technology, the extruded foaming material of each layer being the same, the extrusion pressure of each layer increasing in steps, and the injection pressure of the foaming gas in the extruded foaming material of each layer increasing in steps; the foaming degree of the foam layer is not less than 85%; the pore size distribution of all pores in the foam layer is uniform.

[0006] In the second aspect, the present application provides an ultra-low attenuation leakage coaxial cable, comprising an inner conductor, a composite insulation layer, an outer conductor and an outer sheath concentrically nested from the inside to the outside, the composite insulation layer comprising a foam layer, the foam layer being prepared by three-layer co-extrusion technology, each layer of extruded foaming material being the same, the extrusion pressure of each layer increasing in steps, and the injection pressure of the foaming gas in each layer of extruded foaming material increasing in steps; the foaming degree of the foam layer is not less than 85%; the pore size distribution of all pores in the foam layer is uniform.

[0007] Preferably, the thickness of the foam layer is 8 mm-15 mm; the thickness of each layer in the foam layer does not exceed 40% of the thickness of the foam layer and is not less than 25% of the thickness of the foam layer.

[0008] Preferably, the coefficient of variation of the pore size is ≤5%.

[0009] Preferably, in the three-layer co-extrusion technology, the extrusion pressure of the inner layer is 90 bar-100 bar, the extrusion pressure of the middle layer is 105 bar-115 bar, and the extrusion pressure of the outer layer is 120 bar-130 bar.

[0010] Preferably, in the three-layer co-extrusion technology, the injection pressure of the foaming gas in the inner layer foaming material is 105bar-110bar, the injection pressure of the foaming gas in the middle layer foaming material is 120bar-125bar, and the injection pressure of the foaming gas in the outer layer foaming material is 135bar-140bar.

[0011] In a third aspect, the present application provides a method for preparing a coaxial cable, wherein the method adopts a three-layer co-extrusion technology to prepare a foam layer in a composite insulating layer;

[0012] Among them, the extrusion pressure of the inner flow channel is 90bar-100bar, the extrusion pressure of the middle flow channel is 105bar-115bar, and the extrusion pressure of the outer flow channel is 120bar-130bar;

[0013] The injection pressure of the foaming gas in the foaming material extruded by the inner runner is 105bar-110bar, the injection pressure of the foaming gas in the foaming material extruded by the middle runner is 120bar-125bar, and the injection pressure of the foaming gas in the foaming material extruded by the outer runner is 135bar-140bar.

[0014] Preferably, the thickness of the foam layer is 8 mm-15 mm; the thickness of each layer in the foam layer does not exceed 40% of the thickness of the foam layer and is not less than 25% of the thickness of the foam layer.

[0015] Preferably, the foaming materials of the inner layer flow channel, the middle layer flow channel and the outer layer flow channel are extruded simultaneously.

[0016] Preferably, three extruders are used to inject three foaming materials into three flow channels in a die head respectively, and then three-layer co-extrusion technology is used to extrude the three foaming materials simultaneously through the inner flow channel, the middle flow channel and the outer flow channel respectively.

[0017] In general, the above technical solutions conceived by this application have the following technical advantages compared with the prior art:

[0018] (1) The present application proposes a radio frequency coaxial cable and a leaky coaxial cable, wherein the foam layer is prepared by a three-layer co-extrusion technology, wherein the extrusion material of each layer is the same, the extrusion pressure from the inner layer to the outer layer increases in a step-by-step manner, and the injection pressure of the foaming gas in the foaming material from the inner layer to the outer layer increases in a step-by-step manner, thereby comprehensively unifying the foaming degree of each layer and the pore diameter of each layer; a foam layer with a high foaming degree and good pore diameter uniformity is prepared, which effectively reduces the dielectric constant and signal transmission attenuation of the cable, and at the same time avoids the decrease in the compressive performance of the foam layer under high foaming degree.

[0019] (2) The preparation method of the radio frequency coaxial cable and the leaky coaxial cable of the present application is simple and easy, and is suitable for large-scale production. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 It is a schematic diagram of the end face structure of the radio frequency coaxial cable and the leaky coaxial cable of Examples 1-6 and Comparative Examples 1-3 of the present application.

[0021] Figure 2 This is a schematic diagram of the end face structure of the radio frequency coaxial cable and the leaky coaxial cable in Example 7 of the present application and Comparative Example 4.

[0022] In all the drawings, the same reference numerals are used to denote the same elements or structures, wherein: 1 - inner conductor; 2 - composite insulation layer; 3 - outer conductor; 4 - outer sheath; 21 - adhesive layer; 22 - foam layer; 23 - outer skin layer. DETAILED DESCRIPTION

[0023] In order to make the purpose, technical scheme and advantages of the present application clearer, the technical scheme in the embodiment of the present application will be clearly and completely described below in conjunction with the embodiments of the present application. It should be understood by those skilled in the art that the embodiments are only to help understand the present application and should not be regarded as specific limitations of the present application. Based on the embodiments in the present application, all other embodiments obtained by those of ordinary skill in the art without making creative work are within the scope of protection of the present application.

[0024] The embodiments of the present application are implemented on the premise of the technical solution of the present application, and detailed implementation methods and processes are given, but the protection scope of the present application is not limited to the following embodiments. The process parameters for which specific conditions are not specified in the following embodiments are usually based on conventional conditions.

[0025] The endpoints and any values ​​of the ranges disclosed in this application are not limited to the precise ranges or values, and these ranges or values ​​should be understood to include values ​​close to these ranges or values. For numerical ranges, the endpoint values ​​of each range, the endpoint values ​​of each range and the individual point values, and the individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be regarded as specifically disclosed in this application.

[0026] The term "and / or" in this application is a description of the association relationship of associated objects, indicating that there can be three relationships. For example, A and / or B can represent: A exists alone, A and B exist at the same time, and B exists alone. The symbol " / " in this article indicates that the associated objects are in an or relationship, for example, A / B means A or B.

[0027] In this application, unless otherwise specified and / or described, all numerical values ​​involving the amounts of components are "parts by weight" from beginning to end. The process parameters in the following examples where specific conditions are not specified are generally based on conventional conditions.

[0028] The present application first further illustrates the preparation method of the ultra-low attenuation RF coaxial cable and the leaky coaxial cable of the present application through some embodiments, which specifically includes the following steps:

[0029] (1) Pre-treat the inner conductor;

[0030] (2) Low-density polyethylene and glue (EVA resin) are mixed in a ratio of 4:1 and extruded at a temperature of 140°C-220°C, and evenly coated on the surface of the inner conductor of a 17.6mm diameter corrugated copper tube to form a solid inner skin layer with a thickness of 0.15mm;

[0031] (3) High-density polyethylene, low-density polyethylene and nucleating agent are mixed in a mass ratio of 68.5:30:1.5, and three extruders are used respectively. After high-temperature plasticization at 130°C-195°C, high-pressure CO2 is injected in a molten state, and the mixture is fully mixed in the screw to form a foaming aerosol. Then, the three extruders respectively inject the foaming aerosol into three flow channels in a die head, and then three layers of foaming aerosol are simultaneously extruded from the three flow channels using a three-layer co-extrusion technology to attach to the solid inner skin layer, and the foaming is performed and cooled and shaped using cooling water at a temperature of 20°C-25°C, thereby finally forming a foam layer in the composite insulation layer;

[0032] The foaming aerosols extruded from the above three extruders are respectively injected into the inner flow channel, middle flow channel and outer flow channel in the die head; the foaming aerosols extruded from the inner flow channel, middle flow channel and outer flow channel form the inner layer, middle layer and outer layer of the foam layer respectively.

[0033] In the inner flow channel, the injection pressure of CO2 in the foaming aerosol is 105 bar-110 bar, and the pre-extrusion pressure of the inner flow channel is 90 bar-100 bar.

[0034] In the middle-layer flow channel, the injection pressure of CO2 in the foaming aerosol is 120 bar-125 bar, and the pre-extrusion pressure of the middle-layer flow channel is 105 bar-115 bar.

[0035] In the outer flow channel, the injection pressure of CO2 in the foaming aerosol is 135 bar-140 bar, and the pre-extrusion pressure of the outer flow channel is 120 bar-130 bar.

[0036] When the foaming material is extruded, the greater the pressure difference between the material inside the die and the outside, the greater the expansion after exiting the die. Conversely, if the pressure of the material inside and outside the die is smaller during extrusion, the expansion of the material after exiting the die is smaller. After the foam enters the cooling water tank, due to heat conduction, the outer layer cools the fastest and the inner layer cools the slowest. Therefore, the foam layer with a small extrusion pressure in the inner layer has a long foaming time, the foam layer with a large extrusion pressure in the outer layer has a short foaming time, and the middle layer is between the two. The three-layer co-extrusion technology is used to adjust the extrusion pressure of the three layers and the gas content in the extruded material, so as to balance the foaming degree and pore size of the three layers of foam. Finally, under the combined effect of different extrusion pressures and different CO2 injection pressures in the foaming aerosol, the foam layer achieves uniform pore size distribution from the inside to the outside and consistency of foam foaming degree. The foaming degree of the inner layer / middle layer / outer layer is 85%-86%.

[0037] (4) A solid outer skin layer of low-density polyethylene with a thickness of 0.1 mm is extruded and evenly coated on the periphery of the foam layer, so that the composite insulation layer including the foam layer has better sealing properties and prevents water vapor from penetrating. After the outer skin layer is extruded, an insulating foam cable core with a diameter of 42.6 mm is finally obtained.

[0038] (5) Use a 0.31mm thick smooth copper tape (the copper tape surface of the leaky coaxial cable has periodically distributed grooves, and the copper tape surface of the RF coaxial cable has no grooves) to weld into a copper tube by forming argon arc welding, and then wrap the insulating foam cable core for corrugation. The corrugation trough sinks into the insulating foam cable core, so that the composite insulation layer in the outer conductor and the insulating foam cable core fits tightly into a whole. The outer diameter of the corrugation peak is 46.50mm.

[0039] (6) A layer of polyethylene sheath material is extruded on the surface of the outer conductor, and the sheath thickness is 1.4 mm.

[0040] In some embodiments, the thickness of the prepared foam layer is 8 mm, wherein the thickness of the inner layer, the middle layer, and the outer layer are 25%×8 mm, 35%×8 mm, and 40%×8 mm, respectively.

[0041] In some embodiments, the thickness of the prepared foam layer is 15 mm, wherein the thickness of the inner layer, the middle layer, and the outer layer are 40%×15 mm, 35%×15 mm, and 25%×15 mm, respectively.

[0042] In some embodiments, the thickness of the prepared foam layer is 12 mm, wherein the thickness of the inner layer, the middle layer, and the outer layer are 40%×12 mm, 25%×12 mm, and 35%×12 mm, respectively.

[0043] In some embodiments, the thickness of the prepared foam layer is 10 mm, wherein the thickness of the inner layer, the middle layer, and the outer layer are 30%×10 mm, 40%×10 mm, and 30%×10 mm, respectively.

[0044] In some embodiments, in step (3), high-pressure nitrogen is injected to form a foaming aerosol.

[0045] In some embodiments, in step (3), the three-layer co-extrusion technology controls the extrusion pressure of each layer by adjusting the speed of the pump in the flow channel of each layer.

[0046] In some embodiments, in step (3), the three-layer co-extrusion technology controls the extrusion pressure of each layer by the die size of the flow channel of each layer.

[0047] It is understandable that the present application does not limit the material of each layer structure in the above-mentioned radio frequency coaxial cable, and all materials disclosed in the prior art are applicable to the present application.

[0048] In some embodiments, the inner conductor includes but is not limited to copper-clad aluminum wire, spiral corrugated copper tube, smooth copper tube, etc. In other embodiments, high-purity silver material may also be used. The surface of the inner conductor is roughened to form a microscopic rough structure.

[0049] In some embodiments, the outer conductor is one of a spiral corrugated metal outer conductor, an annular corrugated metal outer conductor or a metal wire braided outer conductor. In some embodiments, the metal inner conductor and outer conductor are both copper conductors.

[0050] In some embodiments, the foam layer may be foamed polyolefin, including but not limited to foamed polyethylene, and the outer layer may be low-density polyethylene LDPE or high-density polyethylene HDPE.

[0051] In some embodiments, the sheath is a polyolefin sheath, including but not limited to polyethylene resin, polypropylene resin, polystyrene resin, etc.

[0052] The radio frequency coaxial cable and the leaky coaxial cable of the present application are now further described through some embodiments.

[0053] A plurality of embodiments of radio frequency coaxial cables and leaky coaxial cables are prepared by the above preparation method. The structure of the cable is as follows: Figure 1As shown, an inner conductor 1, a composite insulating layer 2, an outer conductor 3 and an outer sheath 4 are coaxially arranged in sequence from the inside to the outside, wherein the composite insulating layer 2 includes an adhesive layer 21, a foam layer 22 and an outer skin layer 23 in sequence from the inside to the outside.

[0054] The key parameters of the structures and preparation processes of the various embodiments are shown in Table 1:

[0055] Table 1

[0056]

[0057] Then, a plurality of radio frequency coaxial cables and leaky coaxial cables are prepared by conventional preparation methods. The structures of the comparative examples are as follows: Figure 1 As shown, an inner conductor 1, a composite insulating layer 2, an outer conductor 3 and an outer sheath 4 are coaxially arranged in sequence from the inside to the outside, wherein the composite insulating layer 2 includes an adhesive layer 21, a foam layer 22 and an outer skin layer 23 in sequence from the inside to the outside.

[0058] The structures of several comparative examples and the key parameters of the preparation process are shown in Table 2:

[0059] Table 2

[0060]

[0061] By comparing Table 1 and Table 2, it can be seen that in the cable prepared by the method disclosed in the present application, the foaming degree of the foam layer is higher than that of the cable prepared by the conventional preparation method, the coefficient of variation of the pore size of the pores in the foam layer is smaller, and the best data of the attenuation value of the cable prepared by the method of the present application (taking the attenuation at 1800MHz frequency as a representative) is about 13.3% lower than that of the conventional method on average, and the attenuation average value is reduced by about 16.5%.

[0062] An increase in the degree of foaming means an increase in the proportion of bubbles in the composite insulation layer, which usually reduces the dielectric constant of the material, thereby possibly improving the signal transmission efficiency of the cable. The small coefficient of variation of the pore size indicates that the pores are evenly distributed and have good size consistency. The uniform pore structure can reduce local electric field concentration and avoid local discharge while maintaining stable dielectric properties. If the coefficient of variation is small, the overall electrical properties of the material will be more stable. It can be seen that the cable prepared by the method of the present application can effectively reduce the dielectric constant and signal transmission attenuation of RF coaxial cables and leaky coaxial cables.

[0063] The compression test is then conducted to see the effect of increasing the foaming degree and reducing the coefficient of variation of the pore size on the compression strength of the cable of the present application. The RF coaxial cables of Examples 1-3 and the leaky coaxial cables of Examples 4-6 are subjected to compression tests respectively.

[0064] Compression test 1:

[0065] (1) Place the RF coaxial cable of Example 1-3 flat on a bottom steel plate, apply a pressure of 2000N to a 100mm portion thereof, and the distance between the center of the compressed length and the nearest cable end is no more than 5m.

[0066] (2) The duration of pressure application is 2 minutes.

[0067] (3) After the pressure is removed, the RF coaxial cable embodiments 1-3 are allowed to recover for 2 minutes.

[0068] (4) Repeat the test of the voltage standing wave ratio of the RF coaxial cable embodiments 1-3. The voltage standing wave ratio data thereof are shown in Table 3.

[0069] Table 3

[0070]

[0071] As can be seen from the data in Table 3, the RF coaxial cable prepared by the preparation method of the present application meets the voltage standing wave ratio requirements after the compression test.

[0072] Compression test 2:

[0073] (1) Leaky coaxial cable of Example 4-6 was placed flat on a bottom steel plate, and a pressure of 8 N / mm was applied at a distance of 0.5 m from the test end.

[0074] (2) The pressure load is applied for 2 minutes.

[0075] (3) After the pressure load is removed, the leaky coaxial cable embodiments 4-6 are allowed to recover for 2 minutes.

[0076] (4) The voltage standing wave ratio of the leaky coaxial cable embodiments 4-6 was tested. The voltage standing wave ratio data thereof are shown in Table 4.

[0077] Table 4

[0078]

[0079] As can be seen from the data in Table 4, the leaky coaxial cable prepared by the preparation method of the present application meets the voltage standing wave ratio requirement after the compression test.

[0080] Based on the above experiments, it can be seen that although the foaming degree of the foam layer of the cable prepared by the preparation method of the present application increases, its compressive strength still meets the industry standards for RF coaxial cables and leaky coaxial cables.

[0081] The radio frequency coaxial cable of the present application is further described through some embodiments.

[0082] A radio frequency coaxial cable embodiment 7 having a two-layer composite insulation layer is prepared by the preparation method of the present application. The structure of the cable embodiment 7 is as follows: Figure 2 As shown, an inner conductor 1, a composite insulating layer 2, an outer conductor 3 and an outer sheath 4 are coaxially arranged in sequence from the inside to the outside, wherein the composite insulating layer 2 includes an adhesive layer 21 and a foam layer 22 in sequence from the inside to the outside.

[0083] The structure and key parameters of the preparation process of this embodiment are shown in Table 5:

[0084] Table 5

[0085]

[0086] A radio frequency coaxial cable comparative example having a two-layer composite insulation layer is prepared by a conventional preparation method. The structure of the comparative example is as follows: Figure 2 As shown, an inner conductor 1, a composite insulating layer 2, an outer conductor 3 and an outer sheath 4 are coaxially arranged in sequence from the inside to the outside, wherein the composite insulating layer 2 includes an adhesive layer 21 and a foam layer 22 in sequence from the inside to the outside.

[0087] The structure and key parameters of the preparation process of this comparative example are shown in Table 6:

[0088] Table 6

[0089]

[0090] By comparing Tables 5 and 6, it can be seen that both Example 7 and Comparative Example 4 are RF coaxial cables with two-layer structure composite insulation layers. Similarly, in cables of this structure, the foaming degree and pore size variation coefficient of Example 7 of the present application are better than those of the conventional cable of Comparative Example 4.

[0091] It can be seen that the radio frequency coaxial cable and the preparation process thereof of the present application are suitable for radio frequency coaxial cables having a two-layer structural composite insulation layer.

[0092] It will be easily understood by those skilled in the art that the above description is only a preferred embodiment of the present application and is not intended to limit the present application. Any modifications, equivalent substitutions and improvements made within the spirit and principles of the present application shall be included in the scope of protection of the present application.

Claims

1. An ultra-low attenuation radio frequency coaxial cable, comprising an inner conductor, a composite insulating layer, an outer conductor and an outer sheath which are concentrically nested from the inside to the outside, wherein the composite insulating layer includes a foam layer, and is characterized in that: The foam layer is prepared by three-layer co-extrusion technology, each layer has the same extruded foaming material, the extrusion pressure of each layer increases in steps, and the injection pressure of the foaming gas in each layer of extruded foaming material increases in steps; the foaming degree of the foam layer is not less than 85%; the pore sizes of all pores in the foam layer are evenly distributed.

2. An ultra-low attenuation leaky coaxial cable, comprising an inner conductor, a composite insulating layer, an outer conductor and an outer sheath concentrically nested from the inside to the outside, wherein the composite insulating layer includes a foam layer, characterized in that: The foam layer is prepared by three-layer co-extrusion technology, each layer has the same extruded foaming material, the extrusion pressure of each layer increases in steps, and the injection pressure of the foaming gas in each layer of extruded foaming material increases in steps; the foaming degree of the foam layer is not less than 85%; the pore sizes of all pores in the foam layer are evenly distributed.

3. The coaxial cable according to claim 1 or 2, characterized in that: The thickness of the foam layer is 8mm-15mm; the thickness of each layer in the foam layer does not exceed 40% of the thickness of the foam layer and is not less than 25% of the thickness of the foam layer.

4. The coaxial cable according to claim 1 or 2, characterized in that: The coefficient of variation of the pore size is ≤5%.

5. The coaxial cable according to claim 1 or 2, characterized in that: In the three-layer co-extrusion technology, the inner layer extrusion pressure is 90bar-100bar, the middle layer extrusion pressure is 105bar-115bar, and the outer layer extrusion pressure is 120bar-130bar.

6. The coaxial cable according to claim 1 or 2, characterized in that: In the three-layer co-extrusion technology, the injection pressure of the foaming gas in the inner layer foaming material is 105bar-110bar, the injection pressure of the foaming gas in the middle layer foaming material is 120bar-125bar, and the injection pressure of the foaming gas in the outer layer foaming material is 135bar-140bar.

7. A method for preparing a coaxial cable as claimed in claim 1 or 2, characterized in that: The preparation method adopts a three-layer co-extrusion technology to prepare the foam layer in the composite insulation layer; Among them, the extrusion pressure of the inner flow channel is 90bar-100bar, the extrusion pressure of the middle flow channel is 105bar-115bar, and the extrusion pressure of the outer flow channel is 120bar-130bar; The injection pressure of the foaming gas in the foaming material extruded by the inner runner is 105bar-110bar, the injection pressure of the foaming gas in the foaming material extruded by the middle runner is 120bar-125bar, and the injection pressure of the foaming gas in the foaming material extruded by the outer runner is 135bar-140bar.

8. The preparation method according to claim 7, characterized in that: The thickness of the foam layer is 8mm-15mm; the thickness of each layer in the foam layer does not exceed 40% of the thickness of the foam layer and is not less than 25% of the thickness of the foam layer.

9. The preparation method according to claim 7, characterized in that: The foaming materials of the inner layer flow channel, the middle layer flow channel and the outer layer flow channel are extruded simultaneously.

10. The preparation method according to claim 7, characterized in that: Three extruders are used to inject three foaming materials into three flow channels in a die head respectively, and then the three foaming materials are extruded simultaneously through the inner flow channel, the middle flow channel and the outer flow channel respectively using the three-layer co-extrusion technology.

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