Problems and Solutions in the Production of Power Cables for Rail Transit

With the acceleration of the construction of rail transit in China, the increase in the mileage of railway operations has led to the expansion of rail transit cable demand, and the demand for wire and cable has also increased. Due to the special characteristics of rail transit construction, higher requirements are placed on the technical level of safety and environmental protection of wire and cable products. In addition to excellent electrical and insulation properties, wire and cable must also have good flame retardant properties. , Water resistance and rodent resistance, so the product has a new update on material selection and process route compared with traditional power cables. The following is a brief introduction to the problems that may occur during the production process and the solutions.

The main problems that arise in the manufacturing process of rail transit power cables are the insulation eccentricity, the unevenness of the integrated sheath and the high electric spark breakdown rate; the surface of the halogen-free and low-smoke flame-retardant polyolefin material is not smooth and has pores. After repeated analysis and testing, it is believed that the causes of these problems are mainly the following:

One or three layers co-extrusion insulation eccentric.

Because the insulation thickness is relatively thick, the contact surface between the insulation and the conductor is small, so under the conditions of high temperature and high pressure in the cross-linked pipe, the plastic is in a viscous flow state, there will be a certain degree of sagging, so the phenomenon of unacceptable eccentricity is easy to occur.

Second, the overall sheath is not flat and the breakdown rate of EDM is high.

The main causes of this problem are:

(1) The longitudinally wrapped aluminum-plastic belt has uneven force during molding, resulting in wrinkling of the longitudinal package;
(2) When the aluminum-plastic composite tape is longitudinally wrapped, the joint is not bonded to cause the tilt;
(3) aluminum plastic belt flange;
(4) Foaming and perforation of halogen-free low-smoke flame retardant polyolefin materials;

Third, halogen-free low-smoke flame-retardant polyolefin material surface is not smooth and there are pores.

The main causes of this problem are:
(1) Improperly preserved raw materials, resulting in moisture;
(2) Due to the low processing temperature of the material, excessive extrusion temperatures can lead to foaming of the material;
(3) Due to high flame retardancy, a large amount of fillers are added to the raw materials, resulting in the appearance of smoothness, cracks, and the like during extrusion.

Fourth, solutions.

In response to the above-mentioned major problems in the trial production process, the technicians of the Guangdong Southern Cable and Cable Technology Department organized a process and a process to tackle the problem and follow up. After analysis and research, the following solutions were adopted to achieve the desired results and better meet the customer's requirements.

(1) The measures to solve the problem of insulation eccentricity in the three-layer co-extrusion are:

The cross-linked polyethylene plastic in the cross-linked pipe is in a viscous flow state. We use Dow Chemical's low-slip 35KV cross-linked insulation to increase the viscosity of the plastic itself, thereby reducing the sagging of the material under high-temperature melting conditions. The layer co-extruded equipment is equipped with an on-line polarizer to ensure real-time monitoring. Finally, the roundness is sliced ​​and observed after the insulated core is cooled. In this way, the product insulation performance and eccentricity are effectively controlled, and the eccentricity of the Changsha subway is fully satisfied. Within 5%.

(2) The solution to the problem of unevenness in the integrated sheath and high spark breakdown rate is:

This phenomenon is mainly due to the large diameter of the cable core, and the aluminum strip is not even after the longitudinal deformation of the aluminum strip. The first is the aluminum-plastic composite strip longitudinal mold. Before the production, the size of the mold must be strictly inspected to prevent the molding of the aluminum-plastic belt caused by the overlarge mold; if the aluminum-plastic belt is deformed and formed during production, it is not on the same level, and wrinkling and inequality will occur. It is necessary to adjust the position of the front and rear molds in time to ensure that the aluminum-plastic belt is flat; the aluminum-plastic belt flanges are often the alignment frames of the aluminum-plastic belts and the molding molds are not aligned, and the pay-off rack needs to be firmly fixed on the floor; Plastic straps do not bond together, and timely adjustment of the position and temperature of the hot air gun can solve this problem.

(3) The measures for solving the problems of non-smooth, low-smoke flame-retardant polyolefin materials such as non-smooth surface, foaming, porosity, and perforation are:

Firstly, prevent the impurities in the rubber from mixing in. Check whether the packaging of the material is damaged or not. If it is found damaged, it must be cleaned in time. Second, make the plastic moisture-proof. If the weather is wet, the plastic should be dried at least 4 hours before being squeezed. Processing; third is to improve the extrusion die, we found in the production, low-smoke halogen-free flame-retardant polyolefin materials with ordinary die extrusion surface is not smooth, the analysis is considered to be a long working face of the mold sleeve, plastic stretching caused by the inner The stress is relatively large. Therefore, we designed a special mold for reducing the work surface of the die set, and used the sink cooling method from high temperature to low temperature to eliminate the internal stress. Fourth, the low compression ratio screw was used to reduce the shear heat of the screw. Fifth, control the stretching ratio, keep it below 2.0, reduce the risk of jacket extrusion and degumming. After trial production, the appearance problem was effectively solved. Finally, after taking the above measures, the problems such as surface blemishes, foaming, porosity, and breakdown were effectively solved.

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