In the announcements of many state grid suppliers, we can often see such a notice from cable companies: The thickness of the insulation layer of the produced power cable is unqualified. What impact does the thickness of the specific insulation layer have on the cable?
To This problem is easy to understand. After long-term operation, especially directly buried, immersed in water, exposed to the open air or easily corroded, due to long-term corrosion by external media, the insulation level of the thinnest point of the sheath and the mechanical The level will drop. Coupled with routine sheath test detection or line ground fault, the thinnest point may be broken down. In this way, the protective effect of the cable sheath will be lost. In addition, the internal consumption cannot be ignored. Long-term energization of wires and cables will generate a lot of heat. Here is a little common sense: the allowable working temperature of the conductor is 70°C, and the long-term use temperature of PVC should not exceed 65°C. Therefore, wires and cables are actually in a situation of "internal and external troubles".
With the development of global industry, more and more environments require high-voltage cable products to have a small outer diameter, and gaps should be considered during the laying process, so as to dissipate the heat generated by the wires and cables after they are energized, and the thickness of the sheath Too thick will increase the difficulty of laying, so the thickness of the sheath must strictly meet the relevant standards, otherwise it will not be able to protect the wires and cables. Nor can it blindly pursue its thickness.
Based on the above two analysis, it is not difficult to see that the quality of the product is good or bad, and its first feature is reflected in the appearance quality of the product. No matter what kind of product or semi-finished product, the appearance must be paid attention to in production. The quality is strictly controlled and inspected. The sheath is the appearance of the cable. Its appearance requirements are smooth and round, uniform gloss, not eccentric (not exceeding the specified deviation), no mechanical damage, flattening, no visible sundries, bubbles, sand holes, obvious particles, and bamboo Nodal shape, twist shape, etc. In addition to meeting the above quality requirements, the thickness of the sheath also has a certain impact on the quality of the cable.
It is well known that the function of the cable sheath is to protect the insulated core of the cable from damage under the action of external force, and play a protective role. If the thinnest point of the cable sheath fails to meet the requirements, the cable sheath will be damaged before reaching the normal maximum external destructive force.
If the sheath thickness of the cable in production is lower than the standard requirement, it is unqualified, and the thickness exceeds the standard requirement. For example: the cable model is DLD-KYJV22 3*1.5mm2, and the measured thickness of the sheath has an average thickness of 1.7mm. For this model, the thickness should be 1.2mm according to the GB9330-88 standard.
1) Calculate the thickness of the control sheath according to the standard, the calculation formula: D (outer diameter before extrusion) × 0.035+1;
2) Online measurement of sheath thickness: sheath thickness = (periphery after squeezing the sheath-perimeter before squeezing the sheath) / 2π or sheath thickness = (perimeter after squeezing the sheath-before squeezing the sheath Circumference)×0.1592;
3) The thinnest point of single core sheath: nominal value×85%-0.1;
4) The thinnest point of multi-core sheath: nominal value×80%-0.2;
5) Increase the temperature of the low-density polyethylene sheathed extruder to increase the stress crack resistance strength, because the extrusion temperature is too high, it is easy to scorch the plastic or "slip"; in addition, the shape stability of the extruded layer is poor , The shrinkage rate increases, and even causes the extruded plastic layer to become discolored and bubbles appear;
6) Do a good job of extruder body and screw cooling system to eliminate frictional overheating, so as to maintain the heat balance in the extrusion process, stabilize the extrusion pressure, promote uniform mixing of the plastic, and improve the quality of plasticization.