The acceleration of China's rail transit construction and the increase in railway operating mileage have drivenRail Transit CableThe expansion of demand has led to an increase in the demand for wires and cables. Due to the special nature of rail transit construction, higher requirements have been put forward for the safety, environmental protection and other technical levels of wire and cable products. In addition to excellent electrical and insulation properties, wire and cable products must also have good flame retardancy, water resistance and rodent and ant resistance. Therefore, compared with traditional power cables, this product has new updates in material selection and process routes. Below is an introduction to the common problems and solutions that may arise during the production process of this product.

The main problems encountered in the manufacturing process of power cables for rail transit include insulation eccentricity, uneven comprehensive protective layer, and high electric spark breakdown rate; The surface of halogen-free low smoke flame-retardant polyolefin materials is not smooth and has pores. After repeated analysis and experimentation, it is believed that the main reasons for these problems are as follows:

1、 Insulation eccentricity during three-layer co extrusion.

Due to the thick insulation thickness and small contact surface between the insulation and the conductor, under high temperature and high pressure conditions inside the cross-linked tube, the plastic exhibits a viscous flow state and may sag to a certain extent, which can easily lead to unqualified eccentricity.

2、 The comprehensive protective layer is uneven and has a high rate of electrical discharge breakdown.

The main reasons for this problem are:

(1) The uneven force on the longitudinal aluminum-plastic strip during molding causes wrinkles on the appearance of the longitudinal package;

(2) When the aluminum-plastic composite tape is longitudinally wrapped, the overlapping interface is not bonded, resulting in warping;

(3) Aluminum plastic tape flanging;

(4) Foaming and perforation of halogen-free, low smoke, flame-retardant polyolefin materials;

3、 The surface of halogen-free low smoke flame-retardant polyolefin materials is not smooth and has pores.

The main reasons for this problem are:

(1) Improper storage of raw materials leads to moisture;

(2) Due to the low processing temperature of the material, excessive extrusion temperature can cause foaming of the material;

(3) Due to its high flame retardancy, a large amount of filler is added to the raw material, which can lead to uneven appearance and cracks during extrusion.

4、 Solution measures.

In response to the main problems encountered during the trial production process, the technical personnel of Guangdong South Cable Technology Department organized and tracked the implementation of each process step by step. After analysis and research, the following solutions have been adopted, achieving the expected results and meeting customer requirements well.

(1) The solution to the problem of insulation eccentricity during three-layer co extrusion is as follows: cross-linked polyethylene plastic is in a viscous flow state inside the cross-linked pipe. We use Dow Chemical's low viscosity 35KV cross-linked insulation material to increase the viscosity of the plastic itself, thereby reducing the sagging of the material under high temperature melting. In addition, an online analyzer is equipped on the three-layer co extrusion equipment to ensure real-time monitoring. After the insulation core cools down, the roundness is observed by slicing in a timely manner. In this way, the insulation performance and eccentricity of the product are effectively controlled. * Changsha Metro requires eccentricity within 5%.

(2) The solution to the problem of uneven comprehensive protective layer and high electrical discharge breakdown rate is as follows: this phenomenon is mainly caused by the large outer diameter of the cable core, and the uneven deformation and stress during the longitudinal wrapping of the aluminum strip, resulting in unevenness after the longitudinal wrapping of the aluminum strip. Firstly, there are two sets of longitudinal wrapping molds for aluminum-plastic composite strips, from large to small. Before production, the mold size must be strictly checked to prevent difficulties in forming aluminum-plastic strips caused by oversized molds; If the aluminum-plastic tape is not on the same horizontal line from deformation to molding during production, wrinkling and unevenness will occur. At this time, it is necessary to adjust the position of the front and rear molds in a timely manner to ensure the flatness of the aluminum-plastic tape; The flanging of aluminum-plastic tape is often due to the misalignment between the wire rack and the molding mold of the aluminum-plastic tape, and it is necessary to strictly fix the wire rack on the floor; Timely adjustment of the position and temperature of the hot air gun can effectively solve the problem of non bonding of aluminum-plastic tape joints;

(3) The solutions to the problems of uneven surface, foaming, porosity, and perforation of halogen-free low smoke flame-retardant polyolefin materials are: firstly, to strictly prevent impurities from entering the rubber material; Inspect the material packaging upon entering the factory for any damage, and promptly clean and dispose of any damages found; The second is to do a good job of moisture-proof plastic. If the weather is humid, the plastic should be treated with at least 4% moisture before squeezingHourly drying treatment; The third is to improve the extrusion mold. In production, we found that the surface of low smoke halogen-free flame-retardant polyolefin materials extruded with ordinary molds is not smooth. After analysis, it is believed that the working surface of the mold sleeve is longer, and the plastic stretching causes greater internal stress. Therefore, we designed a mold to reduce the working surface of the mold sleeve and used a segmented cooling method from high temperature to low temperature in a water tank to eliminate internal stress; The fourth is to use low compression ratio screws to reduce the shear heat of the screws; 5、 Control the stretching ratio and keep it below 2.0 to reduce the risk of sheath extrusion and delamination. After trial production, the appearance issue has been effectively resolved; After taking the above measures, the problems of uneven surface, foaming, porosity, and breakdown were effectively solved.