An audio and video cable with a physical foaming core wire diameter less than 1.8 mm and a preparation process thereof

Abstract

The present application relates to the field of audio and video cables, and discloses an audio and video cable with a physical foaming core wire diameter less than 1.8 mm and a preparation process thereof, wherein the preparation process is as follows: S1, conductor preheating; S2, insulation extrusion: HDPE, LDPE and nucleating agent are mixed uniformly in a mass ratio of (1.0-1.5):(3-4):0.08, heated to melt and introduced with 2.5-3.5 MPa nitrogen; the conductor core head is passed through an extrusion die with an outer die outlet aperture 0.15-0.35 mm smaller than the actual core wire diameter, an inner die outlet aperture 0.1-0.3 mm larger than the conductor diameter, and an outer die gallery length of 1.0-1.8 mm; the insulating layer rubber is extruded and foamed through the outer die outlet to obtain a semi-finished core wire; S3, the semi-finished core wire is cooled; S4, the cooled core wire is pulled and shaped to obtain an audio and video cable with a core wire diameter less than 1.8 mm. The present application improves the process so that physical foaming can also produce core wires with thinner insulation layers.

Description

Technical Field

[0001] The present application relates to the field of audio and video cable manufacturing, and in particular to an audio and video cable with a physical foaming core wire diameter less than 1.8 mm and a preparation process thereof. Background Art

[0002] Audio and video cable manufacturing usually adopts physical foaming or chemical foaming methods. Physical foaming is to directly inject gas or liquid during the plastic extrusion process, and then expand or vaporize it to achieve the effect of foaming insulation. Nitrogen is a widely used physical foaming gas. Nitrogen foaming refers to pressure control through a steady flow valve, and then continuous and stable pressure mist nitrogen is injected into the extruder screw through the gas injection needle. A certain amount of nitrogen is dissolved and diffused into the molten polyethylene polymer mixed with a nucleating agent in a molecular state. Under the action of the screw, it is evenly mixed. The molten polymer is coated on the surface of the conductor and pushed to the die outlet for forming. At this time, the nitrogen gas pressure is quickly released, and the nitrogen molecules dissolved in the molten polymer form bubbles due to supersaturation. The polymer expands rapidly at this time to form a lightweight and porous foamed polyethylene core wire. Traditional chemical foaming is to add an appropriate proportion of thermal effect foaming agent during the production of polyethylene insulation materials. When the core wire is extruded, the temperature is used to cause the foaming agent to produce a chemical decomposition reaction, so that bubbles are formed inside the polyethylene insulation material.

[0003] Compared with chemical foaming, physical foaming has controllable nitrogen flow and foaming degree, and the tensile strength of the insulating layer is 176% of that of chemical foaming. The foaming agent of chemical foaming contains impurities and there are still residues after thermal decomposition, while physical foaming uses high-purity nitrogen for foaming, with almost no residues. The most commonly used chemical foaming agents are calcium carbonate, magnesium carbonate, sodium bicarbonate, etc. Among them, sodium bicarbonate releases some carbon dioxide and nitrogen dioxide gas when decomposing, which are corrosive to metal conductors and will cause intermittent blackening of metal conductors. Therefore, physical foaming is usually used to manufacture audio and video cables.

[0004] In the existing extrusion manufacturing process technology, the physical foaming method is generally used in the preparation process of audio and video cables with an average insulation thickness greater than 0.6mm and a diameter greater than 2mm. If the physical foaming method is used to prepare audio and video cables with thinner insulation thickness and a core wire diameter of less than 2mm, the surface of the audio and video cables produced will be rough and uneven, and cannot be put into production and use. Although the existing technology can prepare audio and video cables with a core wire diameter of less than 2mm by using the physical foaming method and three-layer co-extrusion technology, the three-layer co-extrusion equipment is expensive and the investment cost is too high, which is not conducive to promotion and popularization.

[0005] In view of the above-mentioned related technologies and technical problems, the inventors believe that there are still many technical obstacles in using physical foaming to produce audio and video cables with thinner insulation layers and a diameter of less than 2 mm, and there is still much room for improvement. Summary of the invention

[0006] In order to improve the process of preparing a thinner insulating layer and a core wire with a diameter less than 2 mm by physical foaming, and thereby enable the physical foaming method to produce a core wire with a diameter less than 1.8 mm, the present application provides a physically foamed core wire with a diameter less than 1.8 mm and a preparation process thereof.

[0007] In the first aspect, the present application provides a preparation process for an audio and video cable with a physical foaming core wire diameter less than 1.8 mm, which adopts the following technical solution:

[0008] A preparation process for an audio and video cable with a physical foaming core wire diameter less than 1.8 mm, comprising the following steps;

[0009] S1, conductor preheating;

[0010] S2, insulation extrusion:

[0011] S2-1, mixing high-density polyethylene (HDPE), low-density polyethylene (LDPE) and a nucleating agent in a mass ratio of (1-1.5):(3-4):0.08 to form an insulating layer rubber material, and heating and melting the mixture;

[0012] S2-2, passing the conductor core head through an extrusion die having an outer die outlet aperture 0.15-0.35 mm smaller than the preset core wire diameter, an inner die outlet aperture 0.1-0.3 mm larger than the conductor diameter, and an outer die gallery length of 1.0-1.8 mm;

[0013] S2-3, nitrogen is introduced into the molten insulating layer rubber at a pressure of 2.5-3.5 MPa; the molten insulating layer rubber mixed with nitrogen passes through the gap between the outer mold and the inner mold, covers the surface of the conductor passing through the inner mold at the outer mold corridor, and is extruded and foamed through the outer mold outlet to obtain a semi-finished core wire;

[0014] S3, cooling the semi-finished core wire;

[0015] S4, pulling and shaping the cooled core wire to obtain an audio and video cable with a core wire diameter less than 1.8 mm.

[0016] At present, physical foaming is usually only used to manufacture core wires with an average insulation thickness greater than 0.6mm and a diameter greater than 2mm. If physical foaming is used to prepare core wires with thinner insulation layers and less than 2mm in diameter, the produced core wires will have problems such as rough and uneven surfaces of the insulation layer due to excessive and uneven foaming. To solve the above problems, the inventors believe that the preparation process of physical foaming core wires with thinner insulation layers can be improved from two aspects: controlling the size of foaming pores and reducing the time that the insulation layer rubber stays in the extruder head and is subjected to pressure.

[0017] In order to meet the requirements of the insulation layer performance with an average insulation thickness greater than 0.6mm, the existing head mold usually uses a mold with an outer mold corridor length of 4-6mm and a larger outlet aperture; the longer the outer mold corridor is, the longer the insulation layer rubber stays in the head, the greater the degree of decomposition of the foaming agent, and the higher the degree of melt cross-linking. The insulation layer rubber can have sufficient plasticization reaction time in the outer mold corridor, and the probability of the gas in the melt diffusing to the external surface is increased, thereby ensuring the mechanical properties of the thicker insulation layer after molding.

[0018] The present application shortens the outer mold gallery length to 1.0-1.8mm by adopting the above-mentioned technical scheme, which reduces the time that the insulating layer rubber stays in the extruder head to a certain extent, and further reduces the time that the insulating layer rubber stays in the extruder head and is subjected to pressure. However, the insulating layer rubber stays in the extruder head for a short time, the decomposition process of the foaming agent is short, and insufficient gas is generated, which seriously affects the foaming degree of the insulating layer rubber. Furthermore, the short residence time of the rubber, the short cross-linking time, the low cross-linking degree, and the melt is easy to stick to the wall. As the amount of gas decomposed by the foaming agent increases, the low cross-linking melt cannot withstand the gas pressure, which will cause the melt to rupture after extrusion.

[0019] Therefore, in this application, the foaming pore size is controlled by setting the nitrogen introduction pressure to 2.5-3.5MPa, and the outer mold outlet pore size is limited to 0.15-0.35mm smaller than the preset core wire diameter, and the inner mold outlet pore size is 0.1-0.3mm larger than the conductor diameter, so as to regulate the pressure applied by the mold to the insulating layer rubber material; the preset core wire diameter in this application is the theoretical core wire diameter obtained in production. For example, when a core wire with a diameter of 1.27mm needs to be produced, the preset core wire diameter is 1.27mm. In the present application, the corridor length, nitrogen pressure and the inner and outer mold outlet apertures cooperate with each other, so that the pressure on the insulating layer rubber in the machine head is appropriate, the foaming is uniform and dense, and the problems caused by the shorter corridor length are overcome. Combined with the appropriate ratio between high-density, low-density polyethylene and nucleating agent and other appropriate process parameters, the various parameters involved in the process reach a delicate balance with each other, thereby enhancing the adhesion between the obtained core wire insulation layer and the conductor, making the surface of the core wire insulation layer smooth, the tensile strength performance of the core wire stable, and the overall performance of the core wire further improved.

[0020] Preferably, in S2-2, the outer die outlet aperture is 0.25 mm smaller than the actual core wire diameter, and the inner die outlet aperture is 0.1 mm larger than the conductor diameter.

[0021] By adopting the above technical solution, when the outer mold outlet aperture is 0.25mm smaller than the actual core wire diameter and the inner mold outlet aperture is 0.1mm larger than the conductor diameter, it is beneficial to adjust the pressure of the mold on the insulating layer rubber during extrusion, so that the pressure applied by the head to the insulating layer rubber is within a suitable range, which is beneficial to improve the adhesion between the insulating layer rubber and the conductor, so that the obtained core wire diameter is closer to the preset core wire diameter.

[0022] Preferably, in S2-3, the core wire extrusion speed is set to 75-80 m / min.

[0023] The core wire extrusion speed has an important influence on the adhesion between the insulating layer rubber and the conductor. Excessive extrusion speed will lead to loose adhesion of the core wire, and the core wire is prone to deformation. Excessive extrusion speed will lead to large extrusion thickness and the occurrence of rubber accumulation or empty tube. By adopting the above technical solution, the core wire extrusion speed is controlled at 75-80m / min, and the extrusion speed is uniform and stable, which can make up for the shortening of the outer mold corridor length. The effect of shortening the time that the insulating layer rubber stays in the die head and is subjected to the die head pressure can be compensated, thereby reducing the problem of low adhesion between the insulating layer rubber and the conductor, and further stabilizing the tensile strength and other properties of the core wire.

[0024] Preferably, in S2-1, the heating and melting of the insulating layer rubber material is divided into a melting section and a homogenizing section, and the temperature settings of each section are respectively: 170-175°C for the first melting section, 180-185°C for the second melting section, 190-195°C for the first homogenizing section, and 205-210°C for the second homogenizing section; in S2-3, the extrusion of the molten insulating layer rubber material is divided into three sections, and the temperature settings of each section are respectively: 200-205°C for the first extrusion section, 195-200°C for the second extrusion section, and 190-195°C for the third extrusion section.

[0025] During the heating, melting and extrusion process of the insulation layer, the transformation of the material aggregation state and the viscosity that determines the flow of the material are all dependent on the temperature; the heating and melting temperature of the insulation layer is wide, and melt fracture is easily formed during low-temperature extrusion, resulting in a rough surface and poor brightness, and residual internal stress will also be generated, leading to cracking of the insulation layer in the later stage; if the extrusion temperature is too high, the insulation layer is easy to be scorched, or "slip" will occur, resulting in poor shape stability of the extruded layer, increased shrinkage, and even causing discoloration and bubbles in the extruded insulation layer. Therefore, the regulation of temperature during the heating, melting and extrusion of the insulation layer is an important link in the insulation layer processing technology. By adopting the above technical solution, during the melting, homogenization and extrusion of the insulation layer, when the temperature of each section is within the above temperature range, the temperature difference between the temperature sections is relatively appropriate, so that the insulation layer rubber can still achieve relatively uniform plasticization under the condition of shortening the length of the outer mold gallery, which helps to improve the tensile strength of the insulation layer, and thus improve the mechanical properties of the obtained core wire.

[0026] Preferably, in S2-1, the mass ratio of high-density polyethylene HDPE to low-density polyethylene LDPE and nucleating agent is 1:(3-4):0.08.

[0027] By adopting the above technical scheme, the mixing ratio range of LDPE, HDPE and nucleating agent can be more accurately controlled, which is conducive to making the melt flow rate of LDPE and HDPE more uniform after melting, thereby reducing the internal stress caused by the large difference in the mixed melt flow rate of the two. The internal stress can cause the melt fracture problem caused by stress recovery when the molten colloid is extruded. Therefore, accurate control of the ratio of LDPE and HDPE can reduce the internal stress of the insulation layer; controlling the nucleating agent within a suitable ratio range is conducive to the nucleating agent to better play the role of promoting foaming, so that the insulation layer foams evenly and densely, thereby ensuring the quality of the core wire.

[0028] Preferably, in S3, the semi-finished core wire is first placed in water at 75-85°C to cool, and then placed in water at 25-30°C to cool.

[0029] The foaming characteristics of the insulation layer are greatly affected by the cooling conditions. The bubbles begin to expand after the outer eye exits. Insufficient cooling will cause excessive heat shrinkage and core eccentricity. For the insulation layer that has just solidified from a molten state to a semi-solid state, it is best to cool it slowly. Therefore, when the first cooling water temperature is set at 75-85℃ and the second cooling water temperature is set at 25-30℃, it can avoid internal stress in the insulation layer due to sudden cooling due to too low water temperature, and it can also avoid excessive water temperature, which will form bubbles and bulges on the surface of the insulation layer, thereby helping to ensure the quality of the product.

[0030] Preferably, during cooling in S3, the electrostatic capacitance in the water is controlled to be 90-100 PF / m.

[0031] Water capacitance is an important indicator reflecting the foaming degree of the insulation layer. The larger the water capacitance, the lower the foaming degree. Since the foaming degree affects the strength performance of the insulation layer, the greater the foaming degree, the greater the strength decrease. Therefore, it is necessary to control the water capacitance of the core wire. By adopting the above technical solution, when the water capacitance of the core wire is controlled at 90-100PF / m, the foaming degree of the insulation layer is about 30%, and the dielectric constant is about 1.84, which can well meet the strength performance requirements of the audio and video cables with a core wire diameter less than 1.8mm in this application.

[0032] In the second aspect, the present application provides an audio and video cable with a physical foaming core wire diameter less than 1.8 mm, which adopts the following technical solution:

[0033] An audio and video cable with a physical foaming core wire diameter less than 1.8 mm is prepared by adopting the preparation process of the audio and video cable with a physical foaming core wire diameter less than 1.8 mm.

[0034] By adopting the above technical scheme, the conductor is first preheated, which helps to improve the adhesion between the insulating layer rubber and the conductor; then the parameters of the temperature of each section of melting, homogenization and extrusion of the insulating layer rubber, the extrusion line speed, the extrusion die and the nitrogen pressure are adjusted to a suitable range to extrude and foam the insulating layer rubber, and under the coordination and cooperation between the corridor length, the nitrogen pressure and the inner and outer die outlet apertures, as well as the coordination and cooperation of other process parameters, the adhesion between the obtained core wire insulation layer and the conductor is enhanced, the surface of the core wire insulation layer is smooth, and it has relatively excellent tensile strength.

[0035] In summary, the present application includes at least one of the following beneficial technical effects:

[0036] 1. In this application, by setting the outer mold corridor length to 1.0-1.8mm, the outer mold outlet aperture to be 0.15-0.35mm smaller than the actual core wire diameter, the inner mold outlet aperture to be 0.1-0.3mm larger than the conductor diameter, and limiting the nitrogen gas introduction pressure to 2.5-3.5MPa, the insulating layer rubber is coordinated with each other so that the pressure in the die is appropriate and the foaming is uniform and dense. By further adjusting other parameters, the entire process parameters are in a delicate balance, which not only overcomes the technical problems caused by the shorter corridor length, but also enhances the adhesion between the obtained core wire insulation layer and the conductor, makes the surface of the core wire insulation layer smooth, and further improves the tensile strength performance of the core wire;

[0037] 2. In this application, the outer mold outlet aperture is set to be 0.25mm smaller than the actual wire diameter, and the inner mold outlet aperture is set to be 0.1mm larger than the conductor diameter, which is conducive to adjusting the pressure of the mold on the insulating layer rubber during extrusion, so that the pressure applied by the die head to the insulating layer rubber is in a suitable range, so that the mold outlet aperture and the nitrogen pressure and the shorter corridor length can be better matched, which can not only better overcome the insulating layer rubber quality problem caused by the shorter corridor length, but also help to improve the adhesion between the insulating layer rubber and the conductor, thereby improving the mechanical properties of the obtained audio and video cable;

[0038] 3. In the present application, the mass ratio of high-density polyethylene HDPE to low-density polyethylene LDPE and nucleating agent is controlled to be 1:(3-4):0.08, which is beneficial to make the melt flow rate of LDPE and HDPE more uniform after melting, thereby reducing the problem of melt fracture caused by internal stress caused by the large difference in the mixed melt flow rate of the two, and is beneficial for the nucleating agent to better play the role of promoting foaming, thereby making the foaming of the obtained core wire insulation layer uniform and dense, thereby ensuring the quality of the obtained core wire. BRIEF DESCRIPTION OF THE DRAWINGS

[0039] Figure 1 It is a cross-sectional view of the overall structure of the head mold of the embodiment of the present application, and is particularly used to show the length of the outer mold gallery in the present application.

[0040] Description of reference numerals:

[0041] 1. External mold; 2. Internal mold; 3. External mold corridor. DETAILED DESCRIPTION

[0042] The following is combined with Figure 1 And embodiments further illustrate the application in detail.

[0043] The sources of raw materials for the examples and comparative examples of the present application are detailed in Table 1.

[0044] Table 1

[0045] raw material model Source Information High Density Polyethylene HDPE H411 Guangzhou Jingxin Polymer Technology Co., Ltd. Low Density Polyethylene LDPE L416 Guangzhou Jingxin Polymer Technology Co., Ltd. Nucleating Agent 01FG-DOW-14533 American Pottery

[0046] Example

[0047] Example 1

[0048] Reference Figure 1 The extruder head mold includes an outer mold 1 and an inner mold 2. An outer mold gallery ring 3 is opened near the outlet of the outer mold 1. The gallery length of the outer mold gallery ring 3 in the mold of the prior art is usually 4-6 mm, while the gallery length of the outer mold gallery ring 3 in the embodiment of the present application is 1.0-1.8 mm.

[0049] This embodiment provides a preparation process for an audio and video cable with a physical foaming core wire diameter less than 1.8 mm, comprising the following steps:

[0050] S1, conductor preheating: Place a flexible oxygen-free copper conductor with a diameter of 0.45 mm in a preheating machine, set the preheating machine voltage to 35 V, current to 2 A, heating temperature to 170 ° C, and preheat for 0.75 seconds:

[0051] S2, insulation extrusion:

[0052] S2-1, 10.0 kg high-density polyethylene HDPE, 30.0 kg low-density polyethylene LDPE and 0.8 kg nucleating agent are mixed evenly (i.e., the mass ratio of HDPE, LDPE and nucleating agent is 1:3:0.08), to obtain an insulating layer rubber material with an initial mass of 40.8 kg, and add it to the extruder hopper; the temperature of each section of the extruder during the heating and melting process of the insulating layer rubber material is set: the first melting section is 177°C, the second melting section is 187°C, the first homogenizing section is 197°C, the second homogenizing section is 215°C, the first extrusion section (neck) is 207°C, the second extrusion section (die head) is 203°C, and the third extrusion section (eye mold) is 197°C, and the insulating layer rubber material passes through each section of the extruder in turn, and is then heated and melted in the extruder;

[0053] S2-2, pass the preheated conductor core through the inner die 2 of the extruder die; set the preset core wire diameter to 1.25 mm, and select the extruder die with an outer die outlet aperture of 1.1 mm, an inner die outlet aperture of 0.55 mm, and an outer die 1 corridor length of 1.8 mm; the aspect ratio of the extruder screw is 30:1;

[0054] S2-3, set the temperature of the extruder for extruding the insulating layer rubber material: 207°C for the first extrusion section (neck), 203°C for the second extrusion section (die head), and 197°C for the third extrusion section (eye die); after the temperatures of the extruder heating and melting and each extrusion section have reached the set values, start the extruder and the traction device, and introduce nitrogen into the molten insulating layer rubber material evenly and smoothly at a pressure of 2.5MPa; the molten insulating layer rubber material mixed with nitrogen passes through the gap between the outer die 1 and the inner die 2, and is coated on the surface of the conductor passing through the inner die 2 at the outer die gallery 3, and is extruded and foamed through the outer die outlet at an extrusion line speed of 70m / min to obtain a semi-finished core wire;

[0055] S3, cooling the semi-finished core wire: after being extruded from the extruder, the semi-finished core wire enters the mobile water tank after 1m of air cooling, and performs the first stage of cooling at a water temperature of 65°C in the mobile water tank, and then enters the fixed water tank for the second stage of cooling. The water temperature of the fixed water tank is set to 27°C. The water capacitance tester in the fixed water tank measures the water capacitance value in real time. The water capacitance value is controlled within the range of 90-100PF / m. If it exceeds this range, the distance between the mobile water tank and the die head is adjusted in time;

[0056] S4, the traction machine pulls the cooled core wire at a traction speed of 77m / min. The cooled core wire is tested for pinhole impurities through a 1500V spark voltage test. After inspection, it is pulled to a double-reel automatic wire take-up machine, which winds the core wire into a roll to obtain an audio and video wire roll with a core wire diameter of 1.30mm.

[0057] Embodiment 2-7

[0058] The preparation process steps of Example 2-7 are the same as those of Example 1, but the specific process parameters are different. The main process parameters are detailed in Table 2 (the process parameters not recorded in the table are assumed to be the same as those of Example 1, and the initial mass of the insulating layer rubber in Examples 2-7 is 40.8 kg).

[0059] Table 2

[0060]

[0061]

[0062] Comparative Examples 1-12

[0063] The preparation process steps of Comparative Examples 1-11 are the same as those of Example 1, but the specific process parameters are different. The specific process parameters are detailed in Tables 3 and 4.

[0064] Table 3

[0065]

[0066] Table 4

[0067]

[0068]

[0069] Performance test experiment

[0070] 1. The core wires prepared in Examples 1-7 and Comparative Examples 1-12 of the present application were tested according to the performance testing standards and methods in our company's corporate standard specification "Physical Foaming Core Wire Inspection Specification". The specific contents of our company's corporate standard specification "Physical Foaming Core Wire Inspection Specification" are shown in Table 5.

[0071] Table 5

[0072]

[0073]

[0074] 2. Tensile strength test: According to the test method in ISO9001-2015 "Wire and Cable Insulation Tensile Strength Test Specification", the core wires prepared in Examples 1-7 and Comparative Examples 1-12 were subjected to tensile strength tests, and the tensile strengths before and after aging were recorded. The tensile strength standard is shown in Table 6: (U.S. wire standard is used in the examples of this application)

[0075] Table 6

[0076]

[0077] The test data of Examples 1-7 are detailed in Table 7.

[0078] Table 7

[0079]

[0080] The test data of Comparative Examples 1-12 are detailed in Tables 8 and 9.

[0081] Table 8

[0082]

[0083]

[0084] Table 9

[0085]

[0086] According to the test data of Examples 1-7 in Table 7, Examples 1-7 shorten the outer mold corridor length to 1.0-1.8 mm, limit the outer mold outlet aperture to 0.15-0.35 mm smaller than the preset core wire diameter (1.25 mm), the inner mold outlet aperture is 0.1-0.3 mm larger than the conductor diameter (0.45 mm), the nitrogen injection pressure is reduced to 2.5-3.5 MPa, and the mass ratio of HDPE to LDPE and nucleating agent in the insulating layer rubber material is controlled to be (1-1.5): (3-4): 0.08 for physical foaming to prepare a core wire with a thinner insulating layer. The diameters of the core wires obtained in Examples 1-7 are all less than 1.8 mm, and meet various standards in the specification. In particular, in Examples 5-7, the core wires were prepared after lowering the temperatures of the various sections of the extruder, increasing the extrusion line speed and the pulling speed accordingly, and increasing the temperature of the cooling water tank. The adhesion and tensile strength of the core wires prepared in Examples 5-7 were significantly improved compared to the adhesion and tensile strength of the core wires prepared in Examples 1-4, indicating that process parameters such as the temperatures of the various sections of the extruder, the extrusion line speed and the pulling speed, and the cooling water tank temperature have a certain influence on the adhesion between the insulation layer and the conductor and the tensile strength performance of the core wire. Based on the process of preparing core wires with a diameter of about 1.25 mm in Examples 1-4, further precise adjustment of other conventional parameters can obtain core wires with better mechanical properties.

[0087] According to the test data of Comparative Examples 1-12 in Tables 8 and 9, Comparative Examples 1-3 only shortened the outer mold corridor length based on the existing process. The core wires prepared in Comparative Examples 1-3 had problems such as cracks, empty tubes and impurity particles. In addition, the adhesion between the insulation layer and the conductor of the core wire prepared in Comparative Examples 1-3 was poor, and the tensile strength of the insulation layer was low, indicating that the shortening of the outer mold corridor length had an important influence on the quality of the prepared core wire. In order to obtain a core wire with a core wire diameter less than 1.8 mm, it is necessary to shorten the time that the insulating layer rubber stays in the extruder head and is subjected to pressure. Comparative Examples 1-3 shorten the outer mold gallery length to 1.0-1.8 mm. Although the shortening of the outer mold gallery length reduces the time that the insulating layer rubber stays in the extruder head to a certain extent, and thus reduces the time that the insulating layer rubber stays in the extruder head and is subjected to pressure, the insulating layer rubber stays in the extruder head for a short time, the decomposition process of the foaming agent is short, and insufficient gas is generated, which seriously affects the foaming degree of the insulating layer rubber. Moreover, the short residence time of the rubber, the short cross-linking time, the low cross-linking degree, the melt is easy to stick to the wall, and as the amount of gas decomposed by the foaming agent increases, the low cross-linking melt cannot withstand the gas pressure, which will cause the melt to rupture after extrusion. Comparative Examples 1-3 prove that simply shortening the gallery length cannot produce qualified core wires.

[0088] In response to the problems of Comparative Examples 1-3, Comparative Examples 4-6 adjusted the outer die outlet aperture and the inner die outlet aperture based on the corridor length of Comparative Examples 1-3, that is, the pressure applied by the die aperture on the insulating layer rubber material was regulated. The core wire prepared in Comparative Examples 4-6 has a certain improvement in adhesion compared with the core wire prepared in Comparative Examples 1-3. The adjustment of the inner and outer die outlet apertures is beneficial to increase the pressure of the head die on the extruded rubber material during extrusion, which can enhance the adhesion between the insulating layer and the conductor to a certain extent; however, the core wire prepared in Comparative Examples 4-6 still has problems such as cracking and impurities, and the other indicators do not meet the standards, indicating that only adjusting the inner and outer die outlet apertures cannot overcome the technical defects caused by shortening the corridor length.

[0089] Comparative Examples 7-9 further reduce the nitrogen injection pressure on the basis of Comparative Examples 4-6. In Comparative Examples 7-9 in Table 9, the nitrogen pressure is gradually reduced, and the corridor length is shortened and the outlet apertures of the inner and outer molds are restricted. The core wire prepared in Comparative Example 8 has basically reached the specification standard except for the problem of flat wire. However, the core wires prepared in Comparative Examples 7 and 9 still have many defects, which shows that reducing the nitrogen injection pressure and adjusting the outlet apertures of the inner and outer molds can overcome the technical defects caused by the shorter corridor length to a certain extent, but there is great instability among the three.

[0090] In view of the instability existing in Comparative Examples 7-9, the ratio of LDPE, HDPE and nucleating agent in the insulating layer rubber material is adjusted in Comparative Examples 10-12. Although the core wire obtained in Comparative Examples 10-12 solves the problem of cracking of the insulating layer and improves the tensile strength of the core wire, the adhesion between the insulating layer of the core wire and the conductor decreases, and there are still impurity particles in the insulating layer. The uneven melt flow rate between LDPE and HDPE after melting will produce internal stress, and the internal stress will cause the molten insulating layer colloid to break due to stress recovery during extrusion. At the same time, the ratio between the two and the nucleating agent also affects the effect of the nucleating agent in promoting foaming to a certain extent. Therefore, the mass ratio of LDPE and HDPE mixed needs to be precisely controlled to enable the nucleating agent to better play the role of promoting foaming, so that the foaming of the insulating layer is uniform and dense, thereby ensuring the quality of the core wire.

[0091] Comparing Example 1 with Comparative Examples 10-12, Example 1 maintains the determined matching relationship among the corridor length, nitrogen pressure, and inner and outer die outlet apertures on the basis of Comparative Examples 10-12, and further adjusts the mixing ratio of LDPE, HDPE and nucleating agent in the insulating layer rubber material, as well as the process parameters such as the temperature of each section of the extruder, the extrusion line speed and the pulling speed, and the cooling water tank temperature, so that the various parameters involved in the process reach a delicate balance with each other. The core wire prepared in Example 1 not only overcomes the technical problems caused by the shorter corridor length, but also solves the influence of changes in various process parameters, so that the adhesion between the insulation layer of the prepared core wire and the conductor is enhanced, the surface of the insulation layer of the core wire is smooth and round, the tensile strength performance of the core wire remains stable, and the overall performance of the core wire meets the standard specifications.

[0092] The above are all preferred embodiments of the present application, and the protection scope of the present application is not limited thereto. Therefore, any equivalent changes made according to the structure, shape, and principle of the present application should be included in the protection scope of the present application.

Claims

1. A process for preparing an audio and video cable with a physical foaming core wire diameter less than 1.8 mm, characterized in that: The steps include: S1, conductor preheating; S2, insulation extrusion: S2-1, mixing high-density polyethylene (HDPE), low-density polyethylene (LDPE) and a nucleating agent in a mass ratio of (1.0-1.5):(3-4):0.08 to form an insulating layer rubber material, and heating and melting the mixture; S2-2, passing the conductor core head through an extrusion die having an outer die outlet aperture 0.15-0.35 mm smaller than the preset core wire diameter, an inner die outlet aperture 0.1-0.3 mm larger than the conductor diameter, and an outer die (1) corridor length of 1.0-1.8 mm; S2-3, nitrogen is introduced into the molten insulating layer rubber material at a pressure of 2.5-3.5 MPa; the molten insulating layer rubber material mixed with nitrogen passes through the gap between the outer mold (1) and the inner mold (2), is coated on the surface of the conductor passing through the inner mold (2) at the outer mold gallery (3), and is extruded and foamed through the outer mold outlet to obtain a semi-finished core wire; S3, cooling the semi-finished core wire; S4, pulling and shaping the cooled core wire to obtain an audio and video cable with a core wire diameter less than 1.8 mm; In the S2-1, the heating and melting of the insulating layer rubber material is divided into a melting section and a homogenizing section, and the temperature settings of each section are respectively: 170-175°C for the first melting section, 180-185°C for the second melting section, 190-195°C for the first homogenizing section, and 205-210°C for the second homogenizing section; in the S2-3, the extrusion of the molten insulating layer rubber material is divided into three sections, and the temperature settings of each section are respectively: 200-205°C for the first extrusion section, 195-200°C for the second extrusion section, and 190-195°C for the third extrusion section; In S2-3, the core wire extrusion speed is set to 75-80 m / min; In S3, the semi-finished core wire is first placed in water at 75-85°C for cooling, and then placed in water at 25-30°C for cooling.

2. The process for preparing an audio and video cable with a physical foaming core wire diameter less than 1.8 mm according to claim 1, characterized in that: In S2-2, the outer die outlet aperture is 0.25 mm smaller than the actual core wire diameter, and the inner die outlet aperture is 0.1 mm larger than the conductor diameter.

3. The process for preparing an audio and video cable with a physical foaming core wire diameter less than 1.8 mm according to claim 1 or 2, characterized in that: In the S2-1, the mass ratio of high-density polyethylene HDPE to low-density polyethylene LDPE and nucleating agent is 1:(3-4):0.

08.

4. The process for preparing an audio and video cable with a physical foaming core wire diameter less than 1.8 mm according to claim 1, characterized in that: During cooling in S3, the electrostatic capacitance in the water is controlled to be 90-100PF / m.

5. An audio and video cable with a physical foaming core wire diameter less than 1.8 mm, characterized in that: The audio and video cable is prepared by using the preparation process of a physical foaming core wire with a diameter less than 1.8 mm as described in any one of claims 1 to 4.

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