China’s rail transit construction pace accelerated, the increase in the number of miles of railway operations, led to the expansion of rail transit cable demand, the demand for wire and cable has 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 having excellent electrical properties and insulation performance, but also must have good flame retardant properties, water resistance and anti-rodent performance, so the product and the traditional power cable compared to the material selection and process routes have been updated.
Power cable is used for the transmission and distribution of electrical energy cables, power cables are commonly used in urban underground power grids, power stations lead lines, industrial and mining enterprises internal power supply and over the river and sea underwater transmission lines. In the power lines, the proportion of cables is gradually increasing. Power cables are used to transmit and distribute high-power electrical energy in the main lines of the power system, including 1-500kV and above, various voltage levels, various insulation of power cables.
The main problems that arise in the manufacturing process of power cables for rail transportation are insulation eccentricity, uneven comprehensive sheath and high spark breakdown rate; halogen-free low smoke flame retardant polyolefin material surface is not smooth, air holes, etc.. After repeated analyses and tests, it is believed that the causes of these problems are mainly in the following areas:
(A) Insulation eccentricity when three layers are co-extruded.
As the insulation thickness is thicker, the insulation and conductor contact surface is smaller, so under the high temperature and high pressure conditions in the cross-linked tube, the plastic is viscous flow state, there will be a certain degree of sagging, so that the eccentricity is prone to the phenomenon of unqualified.
The solution to the problem of insulation eccentricity when co-extruding three layers is
Cross-linked polyethylene plastic in the cross-linked tube is viscous flow state, can use the United States Dow Chemical low sag 35KV level cross-linked insulation material, in order to increase the viscosity of the plastic itself, so as to reduce the high temperature melt state of the material sag, in addition to the three-layer co-extrusion equipment with online eccentricity meter, to ensure real-time monitoring, and finally, after the insulation core cooling in time to slice the observation of roundness, so that the product insulation performance and eccentricity are In this way, the insulation performance and eccentricity of the product are both effectively controlled and fully meet the Changsha Metro’s requirement of an eccentricity of 5% or less.
(B) Unevenness of the joint sheathing and high rate of EDM breakdown.
The main causes of the problem are:
(1) Uneven force on the longitudinal package of aluminium-plastic tape during forming, resulting in wrinkling of the appearance of the longitudinal package;
(2) When the aluminium-plastic composite tape is wrapped longitudinally, the lap interface is not bonded, resulting in warping;
(3) Flanging of the aluminium-plastic tape;
(4) Halogen-free, low-smoke flame retardant polyolefin material foaming, perforation, etc.;
The solution to the problem of uneven sheathing and high EDM penetration rate is
The phenomenon is mainly due to the cable cable core outside diameter is larger, aluminum belt longitudinal package molding due to uneven deformation of the force caused by aluminum belt longitudinal package uneven, first of all, aluminum-plastic composite belt longitudinal package mold, from large to small before and after a total of two groups, before production must be strictly checked mold size, to prevent the mold is large resulting in aluminum-plastic belt molding difficulties; if the production of aluminum-plastic belt from deformation to molding, not in the same level, there will be wrinkles, inequality If the strip is not on the same level from deformation to forming, wrinkles and inequalities will occur, then the position of the front and rear moulds should be adjusted in time to ensure the flatness of the strip; the flanging of the strip is often due to the fact that the discharge frame of the strip and the forming moulds are not aligned, so the discharge frame needs to be fixed strictly on the floor; for the non-bonding of the strip, timely adjustment of the position and temperature of the hot air gun can better solve the problem;
(C) The surface of halogen-free, low-smoke flame retardant polyolefin material is not smooth and has air holes.
The main causes of this problem are:
(1) Improper storage of the raw material, resulting in moisture;
(2) Due to the low processing temperature of the material, high extrusion temperatures can lead to foaming of the material;
(3) Due to the high flame retardancy, more fillers are added to the raw material, resulting in an extrusion that is prone to an unsmooth and cracked appearance.
For halogen-free low-smoke flame retardant polyolefin material surface is not smooth, foaming, porous, perforated and other problems to solve measures are:
1 – is to strictly prevent impurities in the rubber material mixed in, into the factory to check whether the material packaging is broken, found broken in time to clean treatment.
If the weather is wet, the plastic should be dried for at least 4 hours before extrusion.
3- is to improve the extrusion die, found in the production, low smoke halogen-free flame retardant polyolefin materials with ordinary die extrusion surface is not smooth, after analysis that the working surface of the die sleeve is longer, plastic stretching resulting in greater internal stress, so can reduce the die sleeve working surface of the special die, and the use of water tank from high to low temperature cooling method to eliminate internal stress.
4 – is the use of low compression ratio screw, reduce the screw shear heat.
5- is to control the stretch ratio, keeping the ratio below 2.0 to reduce the risk of sheath extrusion debonding.