Polyethylene (PE) is a non-polar material with good electrical properties, but its molecular structure is linear and easily cracked when extruded into an insulating layer. Cross-linking technology is one of the main means of PE modification. The cross-linked PE can significantly improve its environmental stress resistance, service temperature and mechanical properties at high temperature. Cross-linking of C-Si-O-Si-C between PE molecules as a cross-linking of silanes by one of PE cross-linking methods is currently being cabled in the production of low-voltage cables for its simple, economical and superior performance Merchants respected. As different manufacturers of silane cross-linked cable materials used in the extrusion temperature, extrusion speed and cross-linking speed and other processing technology is not the same, low voltage cable insulation core extrusion surface quality is also greater. In this paper, the production of silane cross-linked polyethylene core insulation in the process of surface roughness, bulging problems are analyzed, and propose solutions and control measures.
1 causes
1.1 raw rubber phenomenon
Cross-linked insulated core in the production process sometimes appear unborn plastic phenomenon, but also become insulating block or lump. The main reasons for this phenomenon are: (1) neck or nose temperature is not enough, resulting in the internal local cold gel produce;
(2) Local plasticization is not good due to insufficient temperature or shearing force of the fuselage;
(3) When the filter liner is not in place, the final impact of cross-linked insulation material extrusion pressure, will produce raw rubber, so that the surface of the insulating core rugged lumps.
1.2 old glue phenomenon
Cross-linked insulated core in the production process sometimes appear old plastic phenomenon, but also become pre-cross-linked, is due to cross-linked material extrusion head temperature is too high or long-term stay in the cross-linked material in the dead zone within the flow path caused by the Old plastic was amber, stay longer, the darker the color, the higher the temperature the darker the color. The reasons for the old plastic generally have:
(1) Extrusion speed is too fast, the faster the screw speed, the stronger the shear force of the cross-linked material in the screw cylinder, so that local body temperature increases, leading to the phenomenon of the old glue;
(2) cross-linked insulation material in the barrel residence time is too long, there is a part of premature cross-linking insulation material, so that when the core in the mold caused by the uneven surface.
(3) The displacement of the filter liner causes the pressure distribution of the rubber material in the manifold to be uneven, forming the dead zone of the flow channel. In this case, the old glue will also be generated. At the same time, it may also lose some filtration due to the filter. After a long time After production, the old glue accumulated on the screw head will also be taken out during extrusion. The old glue particles are mixed in the insulation layer, causing the quality problems of the insulated wire core.
1.3 material stability
General two-step silane cross-linked polyethylene, silane and PE first generated graft reaction to produce cross-linkable PE (referred to as A material), in order to speed up its cross-linking reaction, made of the catalyst containing the masterbatch ), And then A, B material according to a certain percentage of uniform mixing, then cross-linked in warm water after extrusion molding. Mainly caused by the material stability of the quality problems of the insulated core are:
(1) cross-linked polyethylene insulation materials mixed with the various components, because the A material contains a cross-linking agent and a small amount of antioxidants, if the antioxidant is not stirred in the production process, will result in insulating material through the extruder high temperature After extrusion, the more concentrated anti-oxidant vaporized by heat and formed a bulge in the insulating layer.
(2) Improper storage of cross-linked polyethylene insulation material, the silane cross-linked polyethylene material will slowly cross-link at room temperature even without addition of a catalyst because the material contains traces of moisture (about 50 × 10- 4%) caused, if stored at high temperature and humidity, will result in extruded insulation surface is not smooth.
1.4 cooling process
In addition to the extrusion temperature, extrusion speed, crosslinking speed and material stability factors, the cross-linked polyethylene cooling process from the head extrusion there are also factors that may lead to the surface quality of the insulated core:
(1) Air bubbles are attached to the surface of the cable. If air bubbles adhere to the surface of the cable after it enters the water, there will be a "pocket bag" after it is cooled. This is because air bubbles cover the insulating surface and the air bubble covered part The cooling rate is slower than the contact with water. Formation of air bubbles attached to the surface of the cable is generally cooling the circulating water into the sink when the fast flow velocity, water flow, high-speed flow of air into the water, part of the air attached to the cable surface, the formation of bubbles;
(2) Water droplets splash on the surface of the cable before entering the cooling water, and then cool and contract with the water first, while the contact part with the air is still in a high temperature state, resulting in the uneven surface of the insulated core due to the difference in cooling rate.
2 solution
2.1 Insulation surface of the "raw rubber" and "old plastic" phenomenon, almost the same core appearance, easily lead to confusion, so pay attention to distinguish. Solution:
(1) From the boot time to judge, a short time to produce "raw plastic" is likely to produce a long time, "old plastic" is likely;
(2) judging from the change of process conditions and combining with the reasons of "raw rubber" and "old rubber";
(3) Extrusion line speed should be strictly in accordance with the requirements of the implementation of the process specifications, the extrusion speed changes, we must pay close attention to changes in the temperature of the extruder;
(4) Check the nose of the temperature control of various parts is accurate enough to find out which part is caused, and then adjusted according to the actual situation;
(5) If necessary, adjust the temperature, adjust the extrusion speed, clean the filter and the nose and other reasons.
2.2 Select the production process is stable, high technology and management, continuous monitoring of impurity content, extrusion resin melt index constant silane cross-linked polyethylene insulation material manufacturers to ensure the stability of the material properties. Silane cross-linked material produced by the two-step process has a short shelf-life, generally less than 6 months. It should avoid high temperature and humidity when stored to prevent the cross-linking of the material and result in a non-smooth extruded surface.
2.3 insulated cable core extruded from the extruder head, the overall should be smooth into the water, to avoid water splashes on the surface. Cooling water circulation process, water to the sink to pay special attention, should adopt a large diameter, low flow rate of water, it is because the nozzle diameter is bound to improve the water velocity, high velocity water will bring a lot of air into the water, the other in the insulated wire About half a meter after the core into the water to increase the fine cotton yarn and other devices in addition to bubbles, brush the attached bubbles, keep the surface of the insulated wire core clean.
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