- Jul 04, 2020-
Since the extrusion temperature, extrusion speed and crosslinking speed used by different manufacturers of silane crosslinked cable materials are not the same, it has a greater impact on the surface quality of the low-voltage cable insulation core extrusion. In the process of producing silane cross-linked polyethylene insulated core, the common phenomena and their causes are probably as follows:
1. The phenomenon of raw rubber
The cross-linked insulated wire core sometimes has a raw rubber phenomenon during the production process, and it also becomes an insulating block or hard block. The main reasons for this phenomenon are:
(1) The temperature of the machine neck or machine head is not enough, causing local cold glue inside;
(2) Poor local plasticization due to insufficient temperature or shear force of the machine body;
(3) When the liner of the filter is not in place, it will eventually affect the extrusion pressure of the cross-linked insulating material, and will also produce raw rubber, making the surface of the insulated wire core appear uneven and hard.
2. Old glue phenomenon
The cross-linked insulated wire core sometimes has the phenomenon of old rubber during the production process, and it also becomes pre-cross-linked. It is caused by the high temperature of the die when the cross-linked material is extruded or the dead corner of the cross-linked material staying in the runner for a long time. Old glue is amber, the longer the residence time, the darker its color, and the higher the temperature, the darker its color. The general causes of old glue are:
(1) Extrusion speed is too fast, the faster the screw speed, the stronger the shearing force of the cross-linked material in the barrel, which will increase the local temperature of the machine body and cause the phenomenon of old glue;
(2) The residence time of the cross-linked insulating material in the barrel is too long, and a part of the insulating material is cross-linked prematurely, so that the surface of the core is uneven when it is ejected from the mold;
(3) The displacement of the filter gasket causes uneven pressure distribution of the rubber at the splitter plate, forming a dead corner of the flow channel. In this case, old rubber will also be produced, and it may also lose part of the filtering function due to the filter. After a long time After production, the old rubber accumulated on the screw head will be taken out during extrusion, and the old rubber particles will be mixed into the insulating layer, causing quality problems in the insulated core.
3. Material stability
Ordinary two-step silane cross-linked polyethylene, firstly use silane and PE to produce a graft reaction to produce cross-linkable PE (abbreviated as material A), in order to accelerate the cross-linking reaction, a masterbatch containing catalyst (abbreviated as material B) ), and then mix A and B materials evenly according to a certain ratio, and then crosslink in warm water after extrusion molding. The main reasons for the quality of the insulated core caused by the stability of the material are:
(1) The components of the cross-linked polyethylene insulating material are not uniformly mixed. Because material A contains a cross-linking agent and a small amount of antioxidants, if the antioxidants are not evenly mixed during the production process, the insulating material will pass through the extruder at high temperature. After extrusion, the more concentrated antioxidant is heated and vaporized, forming a bulge in the insulating layer;
(2) Improper storage of cross-linked polyethylene insulation material. Even if the silane cross-linked polyethylene material is not added with a catalyst, it will slowly cross-link at room temperature. This is because the material contains a small amount of water (about 50×10- 4%). If stored at high temperature and humidity, the surface of the extruded insulation layer will not be smooth.
4. Cooling process
In addition to the factors of extrusion temperature, extrusion speed, cross-linking speed and material stability, the cooling process after the cross-linked polyethylene is extruded from the die also has factors that may cause the surface quality of the insulated wire core;
(1) Air bubbles are attached to the surface of the cable. When the cable enters the water, if there are air bubbles attached to the surface, there will be "small pits" after the position is cooled. This is because the air bubbles cover the air on the insulating surface, and the part covered by the air bubbles The cooling rate is slower than the part in contact with water. The reason for the formation of bubbles attached to the surface of the cable is generally that the circulating water used for cooling enters the water tank with a fast flow rate and rapid water flow. The high-speed water flow brings air into the water, and a part of the air adheres to the surface of the cable to form bubbles;
(2) Before entering the cooling water, water droplets splash on the surface of the cable. The part in contact with the water cools and shrinks first, while the part in contact with the air is still in a high temperature state. Due to the difference in cooling speed, the surface of the insulated core is uneven.