China's rail transit construction to speed up the pace of railroad operation mileage increase, driven by the expansion of demand for rail transit cable, wire and cable demand also climbed. Due to the special nature of rail transit construction, the safety of wire and cable products, environmental protection and other technical levels put forward higher requirements, wire and cable in addition to excellent electrical performance and insulation properties, but also must have good flame retardant properties, water resistance and anti-rodent and ant performance, so the product and the traditional power cables compared to the selection of materials and process routes have been a new update. The following is an introduction to the problems and solutions that are likely to occur in the production process of this product.

　　Rail transit power cables in the manufacturing process, the main problems are insulation eccentricity, integrated sheath uneven and spark breakdown rate is high; halogen-free, low smoke flame retardant polyolefin material surface is not smooth, there are air holes and so on. After repeated analysis and testing that cause these problems are mainly due to the following aspects:

　　First, three-layer co-extrusion insulation eccentricity.

　　Due to the insulation thickness is thicker, insulation and conductor contact surface is smaller, so in the cross-linked tube under high temperature and high pressure conditions, the plastic is viscous flow state, there will be a certain degree of sagging, so that it is easy to appear eccentricity failure phenomenon.

　　Second, the comprehensive protective layer is uneven and high rate of electric spark breakdown.

　　The main reasons for this problem are:

　　(1) longitudinal package of aluminum-plastic belt in the molding force is not uniform, resulting in the appearance of the longitudinal package wrinkled;

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

　　(3) Aluminum-plastic tape flap;

　　(4) Halogen-free low-smoke flame retardant polyolefin material foaming, perforation, etc.;

　　Third, the surface of halogen-free low-smoke flame retardant polyolefin materials are not smooth and have air holes.

　　The main reasons for the problem are:

　　(1) Improper preservation of raw materials, resulting in moisture;

　　(2) Due to the low processing temperature of the material, the high extrusion temperature will lead to material foaming;

　　(3) Due to the high flame retardancy, more fillers are added to the raw material, resulting in the extrusion of the appearance of non-smooth, cracks and so on easily.

　　Fourth, the solution measures.

　　For the above main problems in the trial production process, Guangdong Cable Technology Department of technical staff, a process by process organization and tracking the implementation of the attack. After analyzing and researching the following measures to solve the problem, to achieve the expected results, better meet customer requirements.

　　(1) for three-layer co-extrusion when the insulation eccentricity problem solution measures are:

　　Crosslinked polyethylene plastic in the crosslinking tube was viscous flow state, we use the U.S. Dow Chemical low-sag 35KV grade crosslinked insulation material, in order to increase the viscosity of the plastic itself, so as to reduce the sagging of the material in the high temperature molten state, in addition to three-layer co-extruded equipment with an on-line bias meter, to ensure that the real-time monitoring and control, and finally, in the insulation of the core cooled down in a timely manner to observe the degree of rounding of the slices, in this way, the product insulation and eccentricity have been effectively controlled, fully meet the Changsha Metropolitan Government's requirements. In this way, the insulating performance and eccentricity of the product are effectively controlled, fully meeting the requirement of Changsha Metro that the eccentricity is within 5%.

(2) The solution measures for the problem of unevenness of the integrated sheath and high rate of spark breakdown are:

　　The phenomenon is mainly due to the cable core outer diameter is large, aluminum belt longitudinal package forming due to the deformation of the uneven force caused by the longitudinal package of aluminum belt uneven, first of all, aluminum-plastic composite belt longitudinal package molds, from large to small before and after a total of two groups, the production of the mold must be strictly check the mold size, to prevent molds large aluminum-plastic belt caused by molding difficulties; if the aluminum-plastic belt from the deformation of the production to the molding, not at the same level, there will be wrinkles, inequality, etc., then we must promptly adjust the front and back of the molds, and the molds should be adjusted in time. If the production of aluminum composite belt from deformation to forming is not on the same level, there will be wrinkles, inequality and so on, then we should adjust the position of the front and rear molds in a timely manner to ensure that the aluminum composite belt is flat; Aluminum composite belt flanging is often the aluminum composite belt payoff bracket and molding molds are not aligned with the need for the payoff bracket is strictly fixed to the floor; aluminum composite belt for the overlap of the mouth is not bonded to the timely adjustment of the location of the hot air gun and the temperature, to be able to better solve the problem;

(3) for halogen-free low-smoke flame-retardant polyolefin material surface is not smooth, foam, air holes, perforation and other issues to solve the problem is:

　　First, strictly prevent impurities in the rubber material mixed; into the factory to check whether the material packaging is broken, found broken to be cleaned in a timely manner to deal with; the second is to do a good job of plastic moisture, if the weather is humid, the plastic in the extrusion of the package should be at least 4 hours before the drying process; third is to improve the extrusion molds, in the production of low-smoke halogen-free flame-retardant polyolefin materials extrusion surface is not smooth with an ordinary mold, after analysis of the die sleeve is considered to be the working surface of the long After analysis, it is thought that the working surface of the die set is longer, and the plastic stretching leads to greater internal stress, so we designed a special mold to reduce the working surface of the die set, and use the water tank from high temperature to low temperature cooling method to eliminate the internal stress; Fourth, the use of low compression ratio screws to reduce the shear heat of the screw; Fifth, control the stretch ratio, to keep below 2.0, to reduce the risk of extruded sheath deglossing. After trial production, the appearance problem is effectively solved; finally, after taking the