The pace of my country's urban rail transit construction is accelerating, and the increase in railway operating mileage has driven the expansion of the demand for AC and DC power cables for urban rail transit vehicles, and the demand for AC and DC power cables for urban rail transit vehicles has also increased. Due to the particularity of urban rail transit construction, higher requirements are put forward for the safety and environmental protection of AC and DC power cable products for urban rail transit vehicles. AC and DC power cables for urban rail transit vehicles have excellent In addition to electrical properties and insulation properties, it must also have good flame retardancy, water resistance and anti-rodent and ant properties. Therefore, compared with traditional power cables, this product has new updates in material selection and process routes. The following is an introduction to the problems that are prone to occur in the production process of this product and the solutions.
The main problems in the manufacturing process of rail transit power cables are insulation eccentricity, uneven comprehensive sheathing and high spark breakdown rate; halogen-free, low-smoke, flame-retardant polyolefin materials are not smooth and have pores. After repeated analysis and experiments, it is believed that the reasons for these problems are mainly as follows:
1. Insulation eccentricity during co-extrusion.
Because the insulation thickness is relatively thick and the contact surface between the insulation and the conductor is small, under the conditions of high temperature and high pressure in the cross-linked tube, the plastic is in a viscous flow state, and there will be a certain degree of sag, which is prone to unqualified eccentricity.
2. The comprehensive protective layer is uneven and the electric spark breakdown rate is high.
The main reasons for this problem are:
⑴The longitudinally wrapped aluminum-plastic tape is not uniformly stressed during forming, causing the appearance of the longitudinally wrapped wrinkle;
⑵When the aluminum-plastic composite tape is longitudinally wrapped, the joints are not glued and cause warping;
⑶Flanging of aluminum-plastic tape;
⑷Halogen-free low-smoke flame-retardant polyolefin material foaming, perforation, etc.;
3. The surface of the halogen-free, low-smoke and flame-retardant polyolefin material is not smooth and has pores.
The main reasons for this problem are:
⑴Improper storage of raw materials, resulting in moisture;
⑵Because the processing temperature of the material is relatively low, excessively high extrusion temperature will cause the material to foam;
⑶ Due to the high flame retardancy, more fillers are added to the raw materials, which may cause the appearance of unevenness and cracks during extrusion.
4. Solution measures.
In response to the above-mentioned main problems in the trial production process, the technicians of the Guangdong South Cable Technology Department organized tackling problems one process at a time, and tracked the implementation. After analysis and research, the following solutions were adopted, which achieved the expected results and satisfactorily meet customer requirements.
⑴The measures to solve the problem of insulation eccentricity in three-layer co-extrusion are:
The cross-linked polyethylene plastic is in a viscous flow state in the cross-linked pipe. We use the American Dow Chemical 35KV low sag cross-linked insulating material to increase the viscosity of the plastic itself, thereby reducing the sag of the material in the high-temperature molten state. The layer co-extrusion equipment is equipped with an online eccentricity meter to ensure real-time monitoring. After the insulated core is cooled, the roundness is observed in time. In this way, the insulation performance and eccentricity of the product are effectively controlled, which fully meets the requirements of Changsha Metro for eccentricity of 5 Within %.
⑵The measures to solve the problems of uneven comprehensive protective layer and high electric spark breakdown rate are:
This phenomenon is mainly due to the large outer diameter of the cable core, and the uneven deformation of the aluminum tape during the longitudinal wrapping process causes unevenness after the aluminum tape is longitudinally wrapped. The first is the aluminum-plastic composite tape longitudinal wrapping mold. There are two molds from large to small. Group, the size of the mold must be strictly checked before production to prevent the aluminum-plastic tape from being difficult to form due to the large mold; if the aluminum-plastic tape is not on the same horizontal line from deformation to molding during production, wrinkles and inequality will occur. The position of the front and rear molds must be adjusted in time to ensure that the aluminum-plastic tape is flat; the aluminum-plastic tape flanging is often because the pay-off rack of the aluminum-plastic tape is not aligned with the forming mold, and the pay-off rack must be strictly fixed on the floor; for aluminum The plastic tape does not adhere to the overlap, and timely adjustment of the position and temperature of the heat gun can solve this problem better;
(3) The solutions to the problems of non-halogen, low-smoke, flame-retardant polyolefin materials such as uneven surface, foaming, pores, and perforations are:
The first is to strictly prevent the mixing of impurities in the rubber material; enter the factory to check whether the material packaging is damaged, and if it is found to be damaged, it should be cleaned in time; the second is to prevent moisture from the plastic. If the weather is wet, the plastic should be dried for at least 4 hours before squeezing the package Treatment; 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 with ordinary die is not smooth. After analysis, it is considered that the working surface of the die sleeve is longer, and the plastic stretching causes the internal The stress is relatively large, so we designed a special mold to reduce the working surface of the mold sleeve, and adopted a water tank from high temperature to low temperature to remove the internal stress; fourth is to use a low compression ratio screw to reduce the shear heat of the screw; 5. Control the stretch ratio and keep it below 2.0 to reduce the risk of degumming of the sheath. After trial production, the appearance problem was effectively solved; after taking the above-mentioned measures, the problems of uneven surface, foaming, pores, and breakdown were effectively solved.
