Problems and Solutions in production of power cables for rail transit
China's rail transit construction has accelerated and the increase in railway operating mileage has driven the expansion of demand for rail transit cables, and the demand for wires and cables has also risen. Due to the particularity of rail transit construction, higher requirements are put forward for the safety, environmental protection and other technical levels of wire and cable products, in addition to excellent electrical performance and insulation performance, wires and cables must also have good flame retardant performance, water blocking performance and rat and Ant prevention performance, therefore, compared with the traditional power cable, this product has a new update in material selection and process route. The following is an introduction to the problems that are easy to occur in the production process of this product and the solutions. The main problems in the manufacturing process of power cables for rail transit include insulation eccentricity, uneven comprehensive sheath and high electric spark breakdown rate; Halogen Free low smoke flame retardant polyolefin material surface is not smooth, there are pores, etc. After repeated analysis and tests, it is believed that the main causes of these problems are as follows: first, insulation eccentricity during three-layer coextrusion. Because the insulation thickness is relatively thick and the contact surface between the insulation and the conductor is relatively small, under the condition of high temperature and high pressure in the cross-linking tube, the plastic is viscous and will sag to a certain extent, in this way, it is easy to have unqualified eccentricity. Second, the comprehensive sheath is uneven and the electric spark breakdown rate is high. The main causes of this problem are: (1) the stress of the longitudinal aluminum-plastic belt is uneven during molding, resulting in the appearance of the longitudinal package; (2) when the aluminum-plastic composite belt is wrapped vertically, the joint is not bonded, resulting in warping; (Iii) aluminum-flanging; (4) halogen-free low smoke flame retardant polyolefin material foaming, perforation, etc; Three, halogen-free low smoke flame retardant polyolefin material surface is not smooth and has pores. The main causes of this problem are: (1) improper storage of raw materials, resulting in moisture; (2) due to the low processing temperature of the material, too high extrusion temperature will lead to material foaming; (3) due to high flame retardancy, more fillers are added to the raw materials, resulting in unsmooth appearance and cracks during extrusion. Fourth, the solution measures. In view of the main problems in the above-mentioned trial production process, the technicians of Guangdong South Cable Technology Department, one process, one process, organized and tracked the implementation. After analysis and research, the following solutions have been adopted to achieve the expected results and better meet the customer's requirements. (1) the solution to the problem of insulation eccentricity during three-layer co-extrusion is: cross-linked polyethylene plastic is viscous in the cross-linked pipe. We use American Dow Chemical low vertical 35KV cross-linked insulating material to increase the viscosity of the plastic itself, so as to reduce the sagging of the material in the state of high temperature melting. In addition, an on-line deflector is equipped on the three-layer co-extrusion equipment to ensure real-time monitoring. Finally, after the insulated wire core is cooled down, slice and observe the roundness in time, in this way, the insulation performance and eccentricity of the product are effectively controlled, fully meeting the requirement of Changsha subway that the eccentricity is within 5%. (2) the solution to the problem of uneven comprehensive sheath and high electric spark breakdown rate is: This phenomenon is mainly due to the large outer diameter of cable core, when the aluminum strip is formed, the deformation force is uneven, causing the aluminum strip to be uneven after the longitudinal package. The first is the aluminum-plastic composite strip longitudinal package mold, from the big to the small front and back, there are two groups, before production, the mold size must be strictly checked to prevent the forming difficulty of aluminum-plastic belt caused by the large mold; If the aluminum-plastic belt is not on the same horizontal line from deformation to molding during production, there will be wrinkles and inequality. At this time, the position of the front and back molds should be adjusted in time to ensure the aluminum-plastic belt is smooth; The flanging of the aluminum-plastic belt is often that the drawing frame of the aluminum-plastic belt and the molding mold are not aligned, and the drawing frame needs to be strictly fixed on the floor; In view of the non-adhesion of the aluminum-plastic belt, the timely adjustment of the position and temperature of the hot air gun can better solve this problem; (3) the solutions to the problems of non-smooth surface, foaming, porosity and perforation of halogen-free low smoke flame retardant polyolefin materials are as follows: first, prevent impurities from mixing in the rubber; Enter the factory to check whether the material packaging is damaged, and clean it in time if it is found damaged; The second is to do a good job of plastic moisture-proof, if the weather is wet, the plastic should be dried for at least 4 hours before squeezing the bag; The third is to improve the extrusion die. In the production, we found that the extrusion surface of low-smoke halogen-free flame retardant polyolefin materials is not smooth with ordinary dies. After analysis, it is considered that the working surface of the die sleeve is longer, plastic stretching leads to large internal stress, so we designed a special mold to reduce the working face of the mold sleeve, and adopted the cooling method from high temperature to low temperature in the water tank to eliminate internal stress; Fourth, low compression ratio screw is adopted to reduce the shear heat of the screw; V. Control the Stretch ratio and keep it at 2. Below 0, reduce the risk of sheath extrusion and degumming. After trial production, the appearance problem has been effectively solved; Finally, after taking the above measures, the problems of surface not only, foaming, porosity, breakdown and so on are effectively solved.