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Problems and Solutions in the Production of Power Cables for Rail Transit
May 09, 2018

With the development of modern science and technology and the improvement of people’s living standards, the pace of China’s rail transit construction has been accelerating. The increase in the number of railway operations has driven the expansion of rail transit equipment demand, and the demand for wire and cable has also increased. Due to the speciality of rail transit construction, higher requirements are placed on the technical level of safety and environmental protection of cable products. In addition to excellent electrical performance and insulation performance, cables must also possess good flame retardant performance and resistance. Water performance and rodent-resistant performance, so the product compared with the traditional power cable in the material selection and process route has a new update. The following is a brief introduction to the problems that may occur in the production process and the solutions.


The main problems in the manufacturing process of the product are the insulation eccentricity, the unevenness of the comprehensive sheath and the high breakdown rate of the electric spark; the surface of the halogen-free 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:


1, three-layer co-extrusion insulation eccentricity


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, and there will be a certain degree of sagging, so the phenomenon of unacceptable eccentricity tends to occur.


2. The unevenness of the integrated sheath and the high rate of electric spark breakdown


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;


3, 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.


4. Solution


In response to the above-mentioned major problems in the trial production process, our technical staff of the Joint Technical Department organized a process and a process to organize research and follow-up. After analysis and research, the following solutions were adopted to achieve the desired results and better meet the customer's requirements:

4.1 The solution to the problem of insulation eccentricity in the three-layer co-extrusion is:


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 insulation performance and eccentricity of the product are effectively controlled, and the eccentricity of the Changsha subway is fully satisfied. Within 5%.


4.2 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 mold being too large. If the aluminum-plastic belt is deformed and formed at the same level, the 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.


4.3 The solutions to the problem of non-smooth, low-smoke flame-retardant polyolefin materials with non-smooth surface, foaming, porosity, and perforation are:


One is to prevent impurities in the rubber compound from entering the factory. Check whether the material packaging is damaged or not. If any damage is found, it must be promptly cleaned.


    The second is to do a good job of moisture-proof plastics. If the weather is wet, the plastics should be dried for at least 4 hours before being squeezed. Third, the extrusion die is improved. In the production, we find that low-smoke halogen-free flame-retardant polyolefin materials are commonly used. The extrusion surface of the die is not smooth. After analysis, it is considered that the work surface of the die set is longer and the internal stress is larger due to the plastic tension. Therefore, we have designed a special die for reducing the work surface of the die set, and adopted the sink from high temperature to low temperature. Sectional cooling method to eliminate internal stress; Fourth, the use of low compression ratio screw, reduce the shear heat of the screw; Fifth, control the tensile ratio, keep it below 2.0, reduce the risk of jacket extrusion degumming. After trial production, the appearance problem was solved effectively; finally, after taking the above-mentioned measures, the problems such as surface dullness, foaming, porosity, breakdown, etc. were effectively solved.


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