In this paper, the physical and mechanical properties of the shielding layer of the heat shrinkable casing are studied by using the extrusion type semi-conductive heat shrinkable tube shielding layer as the object. We know that the surface cracking, defects, insoluble impurities, sharp bumps and so on of the semi-conductive shielding layer have great influence on the service life of the insulated heat shrinkable tube. Secondly, the selection of thermal expansion coefficient of semi-conductive shielding layer is also very important, it should be similar to the thermal expansion coefficient of insulating materials, so as to ensure strong adhesion ability, to prevent the two interface due to the operation of hot and cold cycle and cracking or detachment. For example: Thick wall flame retardant double-walled heat shrinkable tube, military standard soft flame retardant double-walled heat shrinkable pipe and other products.

From the point of view of the initiation of branch angle, semi-conductive shielding layer due to extrusion season scar, impurities on the boundary surface, semi-conductive four subsidence caused by air gap and interface hole and other reasons will be harmful to the shielding layer, its degree of harm is judged by experience: scar of the point of the world of impurities in the light of the hollow.

For the thickness of semi-conductive shielding layer, from the electrical performance analysis, only a very thin layer can meet the shielding requirements. However, from the aspect of uniform electric field, it is required that the semi-conductive shielding layer has a certain thickness, otherwise it can not cover the twisting pattern of the guide wire, and can not form a smooth round surface, thus improving the function of the electric field. Consider in terms of mechanical action. The shielding layer is too thin, and when the cable bends and deforms, the bending stress causes the shielding layer to crack. At the same time has a certain thickness can also play a thermal shielding effect, the general common medium and low voltage cable, its shielding layer thickness of 0.8-1.0mm, high voltage cable requirements slightly thicker.

If at the same time consider low-temperature laying and low-temperature impact and other factors, the general requirements of elongation in more than 30%, in fact, at present, the larger manufacturers at home and abroad can exceed this requirement, only some township cable factory or individual manufacturers of semi-conductive shielding layer can not meet this requirement.

热缩管屏蔽层物理、机械性能

在此主要以挤出型半导电热缩管屏蔽层为对象研究热缩套管屏蔽层的物理、机械性能。我们己知半导电屏蔽层表面开裂、缺陷、不溶性杂质、尖凸物等对绝缘热缩管的使用寿命影响极大。其次，半导电屏蔽层的热膨胀系数选择也至关重要，它应与绝缘材料的热膨胀系数相近，这样才能保证坚强的粘附能力，防止两者界面因运行的冷热循环作用而开裂或脱离。例如：厚壁阻燃双壁型热缩管、军标柔软阻燃双壁型热缩管等产品。

从引发树枝角摩来看，半导电屏蔽层由于挤出时节疤、分界面上杂质、半导电四陷造成气隙和界面上空洞等原因将对屏蔽层产生危害，其危害程度据经验判断为:节疤>杂质>空洞。

对于半导电屏蔽层的厚度，从电性能上来分析，只需有很薄一层即能达到屏蔽要求。但从均匀电场方面考虑，则要求半导电屏蔽层具有一定厚度，否则无法覆盖导丝扭纹纹路，不能形成光滑圆面，从而使改善电场功能下降。从机械作用方面考虑.屏蔽层太薄，当电缆弯曲变形时，弯曲应力会使屏蔽层开裂。同时有了一定的厚度也才能起热屏蔽作用，一般常见的中低压电缆，其屏蔽层厚度为0 .8-1 .0mm，高压电缆要求略厚一些。

若同时考虑低温敷设和低温冲击等因素，一般要求伸长率在30%以上，实际上目前国内外较大的生产厂商都能超过这一要求，只有某些乡镇电缆厂或个别厂家的半导电屏蔽层不能满足这一要求。