In this paper, the physical and mechanical properties of the extruded semi-conductive heat shrinkable tube shield are studied. We know that surface cracking, defects, insoluble impurities and sharp convex of semi-conductive shielding layer have great influence on the service life of insulated heat shrinkable tubes. Secondly, the selection of thermal expansion coefficient of semi-conductive shielding layer is also very important. It should be close to the thermal expansion coefficient of insulating material, so as to ensure the strong adhesion ability and prevent the interface from cracking or breaking off due to the cold and hot cycles of operation. For example: thick-walled flame-retardant double-walled heat-shrinkable tubes, military standard soft flame-retardant double-walled heat-shrinkable tubes and other products.
From the point of view of arbor friction, the semi-conductive shielding layer will be harmful to the shielding layer because of the scar of extrusion time, impurities on the interface, air gap and voids on the interface caused by the four traps of semi-conductivity. The degree of damage is judged by experience as follows: knot scar > impurity > void.
For the thickness of semi-conductive shielding layer, only a very thin layer can meet the shielding requirements from the electrical performance analysis. However, considering the uniform electric field, the semi-conductive shielding layer is required to have a certain thickness. Otherwise, it can not cover the twisted pattern of the guide wire, and can not form a smooth circular surface, thus reducing the function of improving the electric field. Considering the mechanical action, the shielding layer is too thin. When the cable bends, the bending stress will cause the shielding layer to crack. At the same time, a certain thickness can also play the role of thermal shielding. Generally, the shielding layer thickness of medium and low voltage cables is 0.8-1.0 mm, and the high voltage cables need to be slightly thicker.
If the factors such as low temperature laying and low temperature impact are considered at the same time, the elongation is generally required to be more than 30%. In fact, the larger manufacturers at home and abroad can exceed this requirement at present. Only the semi-conductive shielding layer of some township cable factories or individual manufacturers can not meet this requirement.