Rail transit flame-retardant direct-current cable double-layer serial extrusion production line and extrusion molding process thereof

Abstract

The invention relates to the technical field of power transmission cable production, and discloses a rail transit flame-retardant direct-current cable double-layer serial extrusion production line and an extrusion process thereof, the production line comprises a pay-off rack, a front traction part, an extrusion machine, a production line cooling device, a rear traction part and a take-up rack, and the extrusion machine comprises a first extrusion machine and a second extrusion machine; the production line cooling device comprises a pre-cooling cavity, a water cooling tank and an air cooling box; the pre-cooling cavity annularly surrounds the cable and is arranged between the first plastic extruding machine and the second plastic extruding machine, and a temperature and humidity monitoring system is arranged at an inlet of the pre-cooling cavity; a fan is arranged at the top in the pre-cooling cavity, and a water mist nozzle and a water collecting tank are arranged at the bottom of the pre-cooling cavity; transparent observation windows are arranged on the two sides of the pre-cooling cavity. The invention aims to solve the problems of difficult heat dissipation after extrusion, low production efficiency and easy damage to the structural integrity of a sheath due to high heat capacity of a material in the existing production technology.

Description

Technical Field

[0001] This invention relates to the field of power transmission cable manufacturing technology, specifically to a double-layer series extrusion production line for flame-retardant DC cables for rail transit and its extrusion process. Background Technology

[0002] The double-layer co-extrusion process for the oxygen barrier and sheath in cable production is an advanced and innovative technology. It breaks with traditional production methods, allowing two layers of functional material to be extruded simultaneously in a single process: an inner oxygen barrier and an outer sheath. This double-layer co-extrusion method enables complete bonding of the two sheath layers, forming an organic whole and laying a solid foundation for improved cable performance.

[0003] Cables manufactured using this process possess numerous superior properties. The outer sheath is specially designed to exhibit excellent flame retardancy, effectively preventing the spread of flames and improving the cable's safety in extreme environments such as fires; the inner oxygen barrier layer has excellent mechanical properties and stability, enhancing the cable's resistance to pressure and tension, ensuring stable operation under complex working conditions.

[0004] However, this process also faces some challenges in actual production. The thickness of the oxygen barrier layer needs to be precisely controlled. But when the sheath is extruded onto the oxygen barrier layer, due to the simultaneous extrusion of the two layers of materials and the influence of factors such as the cooling system, it is very easy for insufficient cooling to occur, resulting in uneven thickness. Summary of the Invention

[0005] To address the existing problems, this invention aims to provide a double-layer tandem extrusion production line for flame-retardant DC cables used in rail transit and its extrusion process. This solves the problems in existing production technologies where the material has a large heat capacity, making it difficult to dissipate heat after extrusion, resulting in low production efficiency and easy damage to the integrity of the sheath structure. This invention aims to increase the flame-retardant and mechanical properties of the cable.

[0006] To achieve the above objectives, the present invention provides the following technical solution.

[0007] This invention provides a double-layer tandem extrusion production line for flame-retardant DC cables used in rail transit, comprising a wire feeding frame, a front traction machine, an extruder, a production line cooling device, a rear traction machine, and a take-up frame. The extruder comprises a first extruder and a second extruder; the production line cooling device comprises a pre-cooling chamber, a water-cooling tank, and an air-cooling box; the pre-cooling chamber surrounds the cable in a ring and is located between the first and second extruders, with a temperature and humidity monitoring system installed at the inlet; a fan is installed at the top of the pre-cooling chamber, and water mist nozzles and a water collection tank are installed at the bottom; transparent observation windows are provided on both sides of the pre-cooling chamber.

[0008] As a further improvement of the present invention, the water cooling tank includes a rapid cooling section, a slow cooling section and a shaping section, and the rapid cooling section and the slow cooling section are connected by a connecting pipe at their respective bottoms; the rapid cooling section is provided with an aeration device, which is connected to an air compressor; the rapid cooling section is also provided with a low-temperature water supply port, which is connected to a chiller unit.

[0009] As a further improvement of the present invention, the slow cooling section is provided with a spiral guide plate and a water flow disturbance pump, which work together to form an axial spiral flow; the slow cooling section is provided with an electric heating rod to stabilize the water temperature.

[0010] As a further improvement of the present invention, the shaping section is connected to the filter water tank and the circulation pump, an overflow trough is provided at the top of the shaping section, and a rubber wiper is provided at the outlet of the shaping section.

[0011] As a further improvement of the present invention, the air-cooled box is a tunnel-type air box, and an axial flow fan is installed inside the air-cooled box.

[0012] As a further improvement of the present invention, the air outlet of the axial flow fan is provided with a guide shroud for blowing airflow radially along the cable.

[0013] As a further improvement of the present invention, it also includes a PLC controller; the PLC controller is electrically connected to the temperature and humidity monitoring system, the fan, the water mist nozzle, the chiller unit, and the electric heating rod.

[0014] This invention also provides an extrusion process based on a double-layer series extrusion production line for flame-retardant DC cables used in rail transit, characterized by comprising the following steps: Place the cable core to be processed on the pay-off frame and adjust the tension of the pay-off frame by front traction; After the cable core enters the first extruder, the first extruder evenly extrudes and wraps the corresponding insulating material on the cable core to form an oxygen barrier layer. The cable cores, wrapped with an oxygen-barrier layer, are pre-cooled in the production line's cooling device. The cable core wrapped with an oxygen barrier layer enters the second extruder, which evenly extrudes and wraps the corresponding sheath material. The cable cores, which are wrapped with an oxygen barrier and a sheath, enter the production line cooling device for cooling.

[0015] As a further improvement of the present invention, when the cable core wrapped with an oxygen barrier layer and a sheath enters the precooling chamber, the temperature at the inlet of the precooling chamber is greater than 140°C, and the flow rate of the water mist nozzle is increased; if the temperature at the inlet of the precooling chamber is still greater than 140°C after increasing the flow rate of the water mist nozzle, the air volume of the fan is increased.

[0016] The present invention also provides a cable structure based on a double-layer tandem extrusion production line for flame-retardant DC cables for rail transit, characterized in that, from the inside out, it includes a conductor, an insulation layer, an alkali-free glass fiber tape isolation layer, an oxygen barrier layer, and an oxygen barrier flame-retardant sheath.

[0017] The present invention has the following beneficial effects: This production line achieves double-layer tandem extrusion by setting up a first extruder and a second extruder, enabling the extrusion of both the oxygen barrier layer and the sheath layer in one pass, thus improving production efficiency. The cooling system of the production line is sequentially equipped with a pre-cooling chamber, a water-cooling tank, and an air-cooling box, which provides multi-stage cooling for the extruded cable to ensure cooling effect. The pre-cooling chamber surrounds the cable in a ring and is equipped with a temperature and humidity monitoring system, a fan, water mist nozzles, and a water collection tank, which can accurately control the temperature and humidity of the pre-cooling environment to prepare for subsequent cooling. Transparent observation windows on both sides allow operators to observe the cable status in real time, promptly identify problems, and adjust process parameters to ensure product quality.

[0018] Preferably, the water cooling tank is divided into a rapid cooling section, a slow cooling section, and a shaping section, each with a clearly defined function. The rapid cooling section uses an aeration device and a low-temperature water supply port to rapidly cool the cable by utilizing bubble disturbance and rapid heat exchange of low-temperature water, achieving rapid cooling and shaping. The rapid cooling section and the slow cooling section are connected by a bottom connecting pipe to ensure smooth water flow and improve the overall cooling effect.

[0019] Preferably, the slow cooling section is equipped with a spiral guide plate and a water flow disturbance pump to form an axial spiral flow, which can make the cable heat (or cool) more evenly during the slow cooling process, avoid excessive local temperature differences that may cause internal stress in the cable and affect the cable quality; the electric heating rod can stabilize the water temperature and prevent excessive water temperature fluctuations, further ensuring the stability of the slow cooling process and improving the cable performance.

[0020] Preferably, the shaping section connects the filter water tank and the circulating pump, which can filter and recycle the cooling water, saving water resources and ensuring water quality; the overflow trough at the top can discharge excess water in time, maintain a stable water level, and ensure the cooling effect; the rubber wiper at the outlet can effectively remove residual moisture on the cable surface and prevent moisture from adversely affecting the subsequent performance of the cable.

[0021] Preferably, the air-cooled box adopts a tunnel design, which can provide a longer cooling path for the cable and ensure that the cable is fully cooled; the internal axial flow fan can generate a stable airflow, accelerate the airflow on the cable surface, remove heat, and further improve the cooling efficiency.

[0022] Preferably, the air outlet of the axial flow fan is equipped with a guide shroud, which can sweep the airflow along the radial direction of the cable, making the airflow more concentrated and effective on the cable surface, enhancing the cooling effect, ensuring uniform cooling of all parts of the cable, and improving product quality.

[0023] Prioritizing the connection between the PLC controller and the temperature and humidity monitoring system, fans, water mist nozzles, chiller units, and electric heating rods, the entire production line is automated. Operators can centrally set and adjust the parameters of each device through the PLC controller, monitor the system's operating status in real time, and automatically adjust parameters such as temperature, humidity, water flow, and air volume according to actual conditions. This improves the stability and controllability of the production process, reduces human error, and enhances production efficiency and product quality.

[0024] The extrusion process of this invention clarifies the specific steps for producing flame-retardant DC cables for rail transit. Starting from placing the cable core on the wire feeding rack, the tension is adjusted by front traction to ensure that the cable core smoothly enters the first extruder to form an oxygen barrier layer, then enters the second extruder to wrap the flame-retardant sheath material, and finally enters the production line cooling device for cooling. The entire process is clear and reasonable, and can efficiently and stably produce cable products that meet the requirements.

[0025] This step outlines detailed countermeasures for situations where the cable core, encased in an oxygen-barrier layer and flame-retardant sheath, reaches excessively high temperatures upon entering the pre-cooling chamber. When the temperature at the pre-cooling chamber inlet exceeds 140°C, the flow rate of the water mist nozzles is first increased to utilize the heat absorption through water mist evaporation and lower the temperature. If this is ineffective, the fan volume is then increased to enhance airflow and accelerate heat dissipation. This tiered control method allows for more precise and effective temperature control within the pre-cooling chamber, ensuring the cable reaches a suitable temperature range during the pre-cooling stage, preparing it for subsequent cooling processes and guaranteeing cable quality. The cable structure of this invention comprises, from the inside out, a conductor, an insulation layer, an alkali-free fiberglass tape insulating layer, an oxygen barrier layer, and an oxygen barrier flame-retardant sheath. Each layer has a clearly defined function and works in concert with the others. The insulation layer ensures the electrical insulation performance of the cable; the alkali-free fiberglass tape insulating layer provides isolation and reinforcement, improving the mechanical strength of the cable; the oxygen barrier layer effectively prevents oxygen from entering, slowing down the combustion rate; and the oxygen barrier flame-retardant sheath further enhances the flame-retardant performance of the cable, improving its safety in harsh environments such as fires, and meeting the high-performance and high-safety requirements of rail transit cables. Attached Figure Description

[0026] The accompanying drawings described herein are for illustrative purposes only and are not intended to limit the scope of the invention in any way. Furthermore, the shapes and proportions of the components in the drawings are merely schematic to aid in understanding the invention and are not intended to specifically limit the shapes and proportions of the components. In the drawings: Figure 1 This is a schematic cross-sectional view of a cable product from a double-layer tandem extrusion production line for flame-retardant DC cables used in rail transit, as shown in Example 1. Figure 2 This is a schematic diagram of a double-layer tandem extrusion production line for flame-retardant DC cables for rail transit in Example 1; Figure 3This is a schematic diagram of the production line cooling device for a double-layer tandem extrusion production line of flame-retardant DC cables for rail transit in Example 1. Figure 4 This is a schematic diagram of the spiral guide plate of the production line cooling device in Example 1; Figure 5 This is a schematic diagram of the production line cooling device for a double-layer tandem extrusion production line of flame-retardant DC cables for rail transit in Example 2.

[0027] The components include: 1. Oxygen barrier layer; 2. Flame-retardant sheath; 3. Cable feeder; 4. Front traction; 5. First extruder; 6. Second extruder; 7. Production line cooling device; 70. Pre-cooling chamber; 701. Water mist nozzle; 702. Water collection tank; 703. Fan; 704. Transparent observation window; 705. Temperature and humidity monitoring system; 71. Rapid cooling section; 711. Aeration device; 712. Low-temperature water supply port; 713. Connecting pipe; 72. Slow cooling section; 721. Spiral guide. 722. Water flow disturbance pump; 723. Electric heating rod; 73. Shaping section; 731. First circulation pump; 732. First filter water tank; 733. Second circulation pump; 734. Second filter water tank; 735. Overflow trough; 736. Rubber wiper; 74. Air-cooled box; 8. Rear traction; 9. Cable take-up frame; 10. Chiller unit; 11. PLC controller; 12. Wire; 13. Insulation layer; 14. Alkali-free fiberglass tape isolation layer; 15. Cable support. Detailed Implementation

[0028] To enable those skilled in the art to better understand the technical solutions of this invention, the technical solutions of the embodiments of this invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this invention, and not all embodiments. Based on the embodiments of this invention, all other embodiments obtained by those skilled in the art without creative effort should fall within the scope of protection of this invention.

[0029] It should be noted that when an element is referred to as being "set on" another element, it can be directly on the other element or there may be an intervening element. When an element is referred to as being "connected to" another element, it can be directly connected to the other element or there may be an intervening element. The terms "vertical," "horizontal," "left," "right," and similar expressions used herein are for illustrative purposes only and do not represent the only embodiments.

[0030] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. The terminology used herein in the specification of this invention is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. The term "and / or" as used herein includes any and all combinations of one or more of the associated listed items.

[0031] Example 1 like Figure 1 As shown, this invention provides a cable structure based on a double-layer tandem extrusion production line for flame-retardant DC cables used in rail transit. From the inside out, it includes a conductor 12, an insulation layer 13, an alkali-free glass fiber tape insulating layer 14, an oxygen barrier layer 1, and an oxygen barrier flame-retardant sheath 2. The insulation layer 13 can be selected as cross-linked polyethylene insulation or ethylene propylene rubber insulation.

[0032] like Figure 2 As shown, this embodiment also provides a double-layer tandem extrusion production line for flame-retardant DC cables used in rail transit, including a wire feeding frame 3, a front traction frame 4, an extruder, a production line cooling device 7, a rear traction frame 8, a take-up frame 9, and a PLC controller 11. Optionally, wire support devices 15 can be installed between the wire feeding frame 3 and the front traction frame 4, and between the rear traction frame 8 and the take-up frame 9.

[0033] The extruder includes a first extruder 5 and a second extruder 6, which can be selected as a combination of Φ150+150 or Φ120+150; for example Figure 3 As shown, the production line cooling device 7 includes a precooling chamber 70, a water-cooled tank, and an air-cooled box 74; the precooling chamber 70 is arranged in a ring around the cable and between the first extruder 5 and the second extruder 6; a temperature and humidity monitoring system 705 is installed at the inlet of the precooling chamber 70; a fan 703 is installed at the top of the precooling chamber 70, and a water mist nozzle 701 and a water collection tank 702 are installed at the bottom of the precooling chamber 70; transparent observation windows 704 are provided on both sides of the precooling chamber 70.

[0034] A pre-cooling section is set up after the oxygen barrier layer 1 is extruded and before the flame-retardant sheath 2 is applied. This section uses a combination of air cooling and water mist evaporation to rapidly reduce the surface temperature of the oxygen barrier layer 1 from its post-extrusion temperature. The core function of this stage is to dissipate the surface heat of the oxygen barrier layer 1 in advance, reducing the total heat load of the subsequent double-layer structure and preventing heat accumulation due to excessively high initial temperature after the flame-retardant sheath 2 is applied. The pre-cooling process requires controlled cooling intensity to ensure that the oxygen barrier layer 1 maintains a certain viscosity to guarantee interfacial adhesion with the flame-retardant sheath 2.

[0035] The water cooling tank includes a rapid cooling section 71, a slow cooling section 72, and a shaping section 73. The rapid cooling section 71 and the slow cooling section 72 are connected by a connecting pipe 713 at their respective bottoms. The length of the rapid cooling section 71 can be selected from 0 to 2 m, the length of the slow cooling section 72 can be selected from 2 to 5 m, and the length of the shaping section 73 can be selected from 5 to 8 m.

[0036] Specifically, the rapid cooling section 71 is equipped with an aeration device 711, which is connected to an air compressor; the rapid cooling section 71 also has a low-temperature water supply port 712, which is connected to the chiller unit 10. The rapid cooling section 71 uses low-temperature water (15-20℃) for immersion cooling, and in conjunction with the bottom aeration device 711 to generate microbubbles, it breaks the vapor film on the cable surface, causing the temperature of the outer layer of the flame-retardant sheath 2 to drop from 160-180℃ to 80-100℃ in a short time, quickly solidifying the surface and preventing deformation. The rapid cooling section 71 is designed as an arc-shaped water tank, and it is a liftable water tank. The lifting design is close to the arc surface of the cable, which allows the cooling water to quickly contact the sheath surface when starting the vehicle, reducing the length of cable that cannot be cooled in time and cable loss.

[0037] like Figure 3 and 4 As shown, the slow cooling section 72 is equipped with a spiral guide plate 721 ( Figure 4 (A top view of the cross-section) and the water flow disturbance pump 722 work together to form an axial spiral flow; the slow cooling section 72 is equipped with an electric heating rod 723 to stabilize the water temperature. The water temperature gradient increases to 25-30℃, reducing the internal and external temperature difference stress. At the same time, the water flow disturbance pump 722 and the spiral guide plate 721 enhance the heat exchange at the interface between the oxygen barrier layer 1 and the flame-retardant sheath 2, so that the heat inside the oxygen barrier layer 1 is continuously transferred to the surface.

[0038] The shaping section 73 connects the first filter water tank 731 and the first circulating pump 732, the second filter water tank 733 and the second circulating pump 734. An overflow trough 735 is provided at the top of the shaping section 73, and a rubber wiper 736 is provided at the outlet of the shaping section 73. Circulating filtered water is used to maintain a stable water temperature of 30-35℃, achieving uniform cooling.

[0039] The air-cooled box 74 is a tunnel-type air box, and an axial flow fan 703 is installed inside the air-cooled box 74. High-speed airflow blows away residual moisture on the cable surface, while simultaneously lowering the overall temperature to ambient temperature, preventing surface oxidation or secondary heating caused by residual moisture. During this stage, an infrared thermometer can be used to monitor the temperature in real time to ensure stable temperature at the end of the cooling process.

[0040] Optionally, the air outlet of the axial flow fan 703 is provided with a guide shroud to blow airflow radially along the cable.

[0041] This invention also provides an extrusion process based on a double-layer series extrusion production line for flame-retardant DC cables used in rail transit, characterized by comprising the following steps: Place the cable core to be processed on the pay-off frame 3, and adjust the tension of the pay-off frame 3 by the front traction 4; After the cable core enters the first extruder 5, the first extruder 5 evenly extrudes and wraps the corresponding insulating material on the cable core to form an oxygen barrier layer 1. The cable core wrapped with oxygen barrier layer 1 enters the production line cooling device 7 for pre-cooling; The cable core wrapped with the oxygen barrier layer 1 enters the second extruder 6, and the second extruder 6 evenly extrudes and wraps the corresponding flame-retardant sheath 2 material. The cable core, which is wrapped with an oxygen barrier layer 1 and a flame-retardant sheath 2, enters the production line cooling device 7 for cooling.

[0042] In traditional processes, when the flame-retardant sheath 2 is extruded onto the surface of the oxygen barrier layer 1, the residual heat from the oxygen barrier layer 1 will act in the opposite direction to the inner layer of the sheath through heat conduction, creating a temperature gradient of "cold outside, hot inside". During conventional cooling, the outer layer of the sheath reaches the curing temperature first, but the inner layer remains in a molten state due to the continuous heating from the oxygen barrier layer. This causes the cooling process of the sheath to be "repeated". The material that has already cured on the outer layer may generate stress due to the shrinkage of the inner layer, and may even develop microcracks, affecting the overall structural stability. In this embodiment, the cable core wrapped with the oxygen barrier layer 1 is pre-cooled before entering the cooling device 7 on the production line, specifically avoiding the heat dissipation difficulties of "cold outside, hot inside" in traditional processes.

[0043] Example 2 The difference between this embodiment and Embodiment 1 is that: 1) The PLC controller 11 is electrically connected to the temperature and humidity monitoring system 705, the fan 703, the water mist nozzle 701, the chiller unit 10, and the electric heating rod 723.

[0044] 2) The extrusion process also includes the following steps: when the cable core wrapped with oxygen barrier layer 1 and sheath enters the precooling chamber 70, the temperature at the inlet of the precooling chamber 70 is greater than 140°C, the flow rate of the water mist nozzle 701 is increased; if the temperature at the inlet of the precooling chamber 70 is still greater than 140°C after increasing the flow rate of the water mist nozzle 701, the air volume of the fan 703 is increased.

[0045] like Figure 5 As shown, the PLC controller 11 is electrically connected to the temperature and humidity monitoring system 705, the fan 703, the water mist nozzle 701, the chiller unit 10, and the electric heating rod 723.

[0046] Example 3 The difference between this embodiment and Embodiment 1 is that: 1) The PLC controller 11 is electrically connected to the temperature and humidity monitoring system 705, the fan 703, the water mist nozzle 701, the chiller unit 10, and the electric heating rod 723.

[0047] 2) Temperature sensors are installed at both the inlet and outlet of the quench section 71, and the temperature sensors of the quench section 71 are electrically connected to the PLC controller 11.

[0048] 3) The shaping section 73 is equipped with a temperature sensor, and the temperature sensor of the shaping section 73 is electrically connected to the PLC controller 11.

[0049] The extrusion process in this embodiment also includes the following steps: If the cooling rate of the rapid cooling section 71 of the water tank is less than 8℃ / s, increase the amount of low-temperature water supplied. If the temperature fluctuation in the shaping section 73 exceeds ±3℃, adjust the electric heating rod 723 to maintain a stable water temperature.

[0050] In summary, the combination of water tank cooling with segmented cooling and pre-cooling processes in this invention, by constructing a three-stage cooling chain of "pre-cooling and load reduction - deep cooling in the water tank - air cooling and shaping", achieves efficient heat dissipation in a stepwise manner. This not only solves the problem of inadequate cooling, but also ensures the mechanical performance of the cable and production efficiency, making it a practical and economical solution.

[0051] The above embodiments are merely one of the implementation methods for achieving the technical solution of the present invention. The scope of protection claimed by the present invention is not limited to this embodiment, but also includes any variations, substitutions and other implementation methods that can be easily conceived by those skilled in the art within the scope of the technology disclosed in the present invention.

Claims

1. A double-layer tandem extrusion production line for flame-retardant DC cables used in rail transit, comprising a wire feeding frame, a front traction unit, an extruder, a production line cooling device, a rear traction unit, and a wire take-up frame, characterized in that, The extruder includes a first extruder and a second extruder; the production line cooling device includes a precooling chamber, a water-cooling tank, and an air-cooling box; the precooling chamber surrounds the cable in a ring and is located between the first extruder and the second extruder, and a temperature and humidity monitoring system is installed at the inlet of the precooling chamber; a fan is installed at the top of the precooling chamber, and a water mist nozzle and a water collection tank are installed at the bottom of the precooling chamber; transparent observation windows are provided on both sides of the precooling chamber.

2. The double-layer tandem extrusion production line for flame-retardant DC cables for rail transit according to claim 1, characterized in that, The water cooling tank includes a rapid cooling section, a slow cooling section, and a shaping section. The rapid cooling section and the slow cooling section are connected by connecting pipes at their respective bottoms. The rapid cooling section is equipped with an aeration device, which is connected to an air compressor. The rapid cooling section is also equipped with a low-temperature water supply port, which is connected to a chiller unit.

3. The double-layer tandem extrusion production line for flame-retardant DC cables for rail transit according to claim 2, characterized in that, The slow cooling section is equipped with a spiral guide plate and a water flow disturbance pump, which work together to form an axial spiral flow; the slow cooling section is also equipped with an electric heating rod to stabilize the water temperature.

4. The double-layer tandem extrusion production line for flame-retardant DC cables for rail transit according to claim 2, characterized in that, The shaping section is connected to the filter water tank and the circulation pump. An overflow trough is set at the top of the shaping section, and a rubber wiper is set at the outlet of the shaping section.

5. The double-layer tandem extrusion production line for flame-retardant DC cables for rail transit according to claim 1, characterized in that, The air-cooled box is a tunnel-type air box, and an axial flow fan is installed inside the air-cooled box.

6. The double-layer tandem extrusion production line for flame-retardant DC cables for rail transit according to claim 5, characterized in that, The air outlet of the axial flow fan is equipped with a guide shroud to blow airflow radially along the cable.

7. The double-layer tandem extrusion production line for flame-retardant DC cables for rail transit according to claim 1, characterized in that, It also includes a PLC controller; the PLC controller is electrically connected to the temperature and humidity monitoring system, fan, water mist nozzle, chiller unit, and electric heating rod.

8. The extrusion process based on a double-layer tandem extrusion production line for flame-retardant DC cables for rail transit as described in claims 1-7, characterized in that, The following steps are involved: Place the cable core to be processed on the pay-off frame and adjust the tension of the pay-off frame by front traction; After the cable core enters the first extruder, the first extruder evenly extrudes and wraps the corresponding insulating material on the cable core to form an oxygen barrier layer. The cable cores, wrapped with an oxygen-barrier layer, are pre-cooled in the production line's cooling device. The cable core wrapped with an oxygen barrier layer enters the second extruder, which evenly extrudes and wraps the corresponding sheath material. The cable cores, which are wrapped with an oxygen barrier and a sheath, enter the production line cooling device for cooling.

9. The extrusion process based on a double-layer tandem extrusion production line for flame-retardant DC cables for rail transit as described in any one of claims 8, characterized in that, Includes the following steps: When the cable core, which is wrapped with an oxygen barrier and a sheath, enters the precooling chamber, if the temperature at the inlet of the precooling chamber is greater than 140°C, increase the flow rate of the water mist nozzle; if the temperature at the inlet of the precooling chamber is still greater than 140°C after increasing the flow rate of the water mist nozzle, increase the air volume of the fan.

10. A cable structure based on a double-layer tandem extrusion production line for flame-retardant DC cables used in rail transit, characterized in that, From the inside out, it includes a conductor, an insulation layer, an alkali-free fiberglass tape isolation layer, an oxygen barrier layer, and an oxygen barrier flame-retardant sheath.

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