How to solve the cracking problem of polyethylene outer sheath with steel tape armor Polyethylene is one of the most used raw materials in modern plastics.
Because of its good mechanical strength, toughness and excellent heat resistance, insulation and chemical stability, it has been widely used in the insulation and sheath of electric wire and cable, telephone cable.
Another advantage of polyethylene is that it has better waterproof performance. Therefore, polyethylene is commonly used as the sheath of cable when designing the structure of water-resistant cable.
According to the cable laying environment, some customers require the cable must have good waterproof performance, not only require polyethylene as the cable inner cushion, but also require polyethylene as the outer sheath material.
But a more important disadvantage of polyethylene is its poor resistance to environmental stress cracking.
Due to the structure of the material itself, it is very sensitive to external stress. Under special circumstances, cracking occurs in a short time under very small stress, which affects the service life of the material.
Our company has twice received this kind of contract (YJY23 8.7/10kV 3*400), after extruded polyethylene outer sheath rewind coil, sheath cracking phenomenon occurred at the edge of the outer steel belt, and we negotiated with the other party to use PVC as the outer sheath of the cable.
Then how does the cracking of polyethylene outer sheath outside steel tape armor come into being?
1. Extrusion temperature and cooling speed
Extrusion temperature has a great impact on the performance of polyethylene sheath. Too high temperature is not easy to set, too low temperature will cause uneven plasticization, directly affect the mechanical properties of products and surface finish, so the setting of extrusion temperature is particularly important.
Polyethylene is easy to crystallized under high temperature tension, making the material embrittlement, in the production process, in a very short time, the polyethylene from more than 200 degrees into dozens of degrees of cold water, (the winter may be only a few degrees) after the outer cooling volume fixed, after the inner cooling shrinkage will produce microholes.
Some polyethylene chain segments are frozen without adjustment, which makes the vitrification temperature of the sheath very high and increases the probability of cracking under the action of a large external force.
So the brittleness caused by the cooling temperature can be reduced by using low temperature extrusion and cooling water tank section by section.
The flatness of armored surface and the thickness of sheathing
Due to the large armored outer diameter and heavy cable, extruded tube or semi-extruded tube can only be used in the production of outer sheath.
Plus armoring layer is composed of double steel band gap around the package, and like YJY23 8.7/10 kv 3 * 400 the thickness of the steel strip used for 0.8 mm, surface roughness, after drawing the coated on the surface of the armor, sheath thickness is not consistent, stretching process visible left: sheathed in the stretching process, from (1) through (2), (3), (4), (5) (a loop) thickness of sheath will happen very big change, (2), (4) two dots (i.e., the outer edges of the strip) the thinnest thickness of sheath, the focus of internal stress is the main location of future cracking.
3. Thermal shrinkage performance of polyethylene itself
The polyethylene material itself also has a thermal shrinkage performance, the shrinkage rate from hot state to cold state is generally around 8%, so the hot polyethylene sheath coated on the armored surface will slowly cool and finally turn into cold state after entering the tank. The cold polyethylene sheath will tightly wrap the double-layer steel belt, making it impossible for the steel belt to slip from side to side.
In addition, the sheathing thickness at the edge of the outer steel belt mentioned above is the thinnest, and the edge of the sheathing layer becomes warped when the cable top coil is bent, so there is a tension effect on the outer sheathing, which acts on the weakest part of the sheathing and finally causes the sheathing to crack.
In November 2004, our company received the contract of Suzhou Suyuan Power Supply Bureau YJY23 8.7/15kV 3*400 for about 30km, with a total of more than 60 cable. The technical specification of Suyuan Power Supply Bureau also mentioned that polyethylene is required to be used as the inner cushion and outer protective layer of the cable.
In order to solve this long-standing problem, we finally solved the cracking problem of polyethylene outer sheath outside steel tape armor through multiple verification.
Specific measures are as follows:
1, reduce the extrusion temperature, using warm water cooling segment by segment.
2. The most important point is that, in addition to the polyethylene sheath, a layer of non-woven cloth with an overlap of 10% should be longituinally covered on the steel strip armored surface.
Because there is a certain gap between the non-woven cloth and steel tape armor layer of the longitudinal bale, the surface flatness before the sheath is alleviated, making the thickness of polyethylene sheath uniform and consistent.
By utilizing the thermal shrinkage performance of PE itself, the uniform and uniform loose sheath is made to shrink gradually after entering warm water, so that the polyethylene sheath wrapped in longitudinal non-woven cloth will neither loosen nor wrap too tightly, thus reducing the internal stress.
3. A small drawing ratio (1.5~2.0) is adopted when selecting molds.
Finally through the above three measures, complete the delivery task as scheduled, polyethylene cracking rate of 0%, to ensure the quality of products.