In the announcements of many suppliers of China Netcom, we can often see such a notice from Cable Enterprises: the thickness of insulation layer of power cables produced is unqualified. What is the impact of unqualified thickness of specific insulating layer on the cable? What kind of sheath is qualified? How do we make qualified cables in production?
Hazards of unqualified sheath thickness
1. Reduce the service life of wire and cable products
It is easy to understand that after long-term operation, especially directly buried, immersed in water, exposed to the open air or easily corroded environment, the insulation level and mechanical level of the thinnest point of the sheath will decrease due to the corrosion of external media for a long time.
In addition to routine sheath test detection or line grounding 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 can not be ignored. Long term power on of wires and cables will produce a lot of heat.
Here is a little common sense: the allowable working temperature of conductor is 70 ℃, and the long-term service temperature of PVC should not exceed 65 ℃. Therefore, wires and cables are actually in a situation of “internal and external troubles”.
2. It increases the difficulty in the laying process
With the development of global industry, more and more environmental requirements require that the outer diameter of high-voltage cable products must be small. It is necessary to consider leaving a gap in the laying process, so as to dissipate the heat generated by the power on of wires and cables. Too thick sheath will increase the difficulty of laying, so the thickness of sheath shall strictly comply with relevant standards, Otherwise, it cannot protect wires and cables. Nor can we blindly pursue its thickness.
What kind of sheath is qualified?
According to the above two analyses, it is not difficult to see that the first feature of product quality is reflected from the appearance quality of products. No matter what kind of products or semi-finished products, we must pay attention to the appearance quality in production and strictly control and inspect them.
The sheath is the appearance of the cable. Its appearance requirements are smooth and round, uniform luster, no deviation from the core (not exceeding the specified deviation), no mechanical damage, flattening, no visible sundries, bubbles, sand holes, obvious particles, bamboo knots, twist, etc. In addition to meeting the above quality requirements, the sheath thickness also has a certain impact on the cable quality.
As we all know, the function of cable sheath is to protect the insulated core of cable from damage under external force. If the thinnest point of the cable sheath fails to meet the requirements, the cable sheath will be damaged under the action of the normal maximum external destructive force.
If the sheath thickness of the cable in production is lower than the standard requirements, it is unqualified, and if the thickness exceeds the standard requirements, it is also unqualified. For example, the cable model is dld-kyjv22 3 * 1.5mm2, and the measured average thickness of sheath is 1.7mm. If this model refers to the requirements of gb9330-88 standard, the thickness should be 1.2mm.
Several points needing attention in sheath production
1. Calculate the thickness of control sheath according to the standard. Calculation formula: D (outer diameter before extrusion) × 0.035+1
2. Online measurement of sheath thickness
Sheath thickness = (perimeter after sheath extrusion – Perimeter before sheath extrusion) / 2 π
Sheath thickness = (perimeter after sheath extrusion – Perimeter before sheath extrusion) × zero point one five nine two
3. Thinnest point of single core sheath: nominal value × 85%-0.1
4. Thinnest point of multi-core sheath: nominal value × 85%-0.1
5. Increase the temperature of the LDPE sheath extruder and improve the stress cracking 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 increases, and even causes discoloration and bubbles of the extruded plastic layer
6. Do a good job in the extrusion body and screw cooling system to eliminate friction overheating, so as to maintain the heat balance in the extrusion process, stabilize the extrusion pressure, promote uniform mixing of plastics and improve the plasticization quality.
To sum up, in the production process, we only operate the equipment carefully and strictly control the sheath thickness according to the standard requirements, which can not only save resources, reduce material consumption and increase profits for the enterprise, but also ensure the cable quality and create high-quality and low-cost products.