The extrusion process includes insulation and sheath production processes. Insulation production methods include coating, wrapping, extrusion, and their combinations. Currently, the main production methods for insulation are coating (winding wires, which are no longer within the scope of production license management) and extrusion (wires and cables).
　
1、 Plastic extrusion equipment and molds
　
1. Plastic extrusion equipment: extruder
1.1 Working principle of extrusion machine: Using a specific shaped screw to rotate in the heated barrel, the plastic sent from the hopper is pressed forward to evenly plasticize. Through the machine head and molds of different shapes, the plastic is extruded into various shapes of products.
1.2 Basic structure of the extruder: The extruder is composed of an extrusion system, a transmission system, and a heating system (cooling).
1.2.1 Extrusion system (including five parts: screw, barrel, hopper, head, and mold)
① Screw: The screw is an important part of an extrusion machine, which is made of high-strength, heat-resistant, and corrosion-resistant alloy steel. Its function is to push the plastic forward, generate pressure, stir, and generate friction heat with the plastic during rotation, causing the plastic to melt and continuously feed the melt into the machine for extrusion. It directly affects the application range and productivity of the extrusion machine.
② The barrel is a metal cylinder, usually made of high-strength, sturdy, wear-resistant, and corrosion-resistant alloy steel or composite steel lined with alloy steel. It forms the basic structure of the extrusion system for plastic plasticization and transportation with screws. The length of the barrel is generally 15-30 times its straight diameter, in order to ensure that the material is fully heated and plasticized. The barrel should have sufficient thickness, stiffness, and smooth inner wall. A resistance or induction heater measuring device and a cooling system are installed outside the barrel.
③ The hopper is usually a conical container with a capacity of at least 1 hour for material consumption. The bottom of the hopper is equipped with a cut-off device for material flow, and the side of the hopper is equipped with a sight hole for calibration and measurement.
④ The head is the forming component of an extrusion machine, which mainly consists of a filtering device (porous plate and screen), connecting pipes, splitters, mold core seats, molds, etc.
1.2.2 Transmission system
Its function is to ensure that the screw rotates uniformly with the required torque and speed. Generally, the Weidong system includes three necessary links: primary power - transmission - reducer. The rotational speed of the screw is required to be stable and not change with its load to ensure uniform and consistent product quality. But in different situations, it is also required that the screw can be variable speed to meet the requirement that a device can extrude products of different specifications. For this reason, drive motors generally use rectifier motors, DC motors, etc.
1.2.3 Heating and cooling system
Function: By heating or cooling the barrel, it ensures that the plastic is always extruded within its process temperature range.
① Heating method of the extruder: When the screw speed of the extruder is kept constant, the main factor affecting the stability of the glue output is the temperature of the extruder body.
There are usually two heating methods for cross-linked extrusion machines, namely carrier heating (such as adding hot water) and resistor heating. The characteristic of using carrier heating is uniform heating, which is not prone to local overheating. However, the carrier heating temperature has high requirements for the sealing performance of the system, and the cost is high. Therefore, it is used on extrusion machines with high temperature control requirements.
② Cooling of the plastic machine: During the extrusion process, there is often a phenomenon of excessive plastic temperature and heat inside the barrel. If the excessive heat is not discharged in a timely manner, it is easy to cause the material to "pre crosslink"; The extrusion machine generally cools in two parts: barrel cooling and head cooling.
a. Barrel cooling (there are two methods for barrel cooling: air cooling and water cooling)
From the perspective of cooling effect, air cooling is relatively gentle, with a slower cooling speed, while water cooling is faster, but more intense, which can easily cause strong "thermal vibration". The design of a water cooling system is relatively complex, and it should not only have good cooling effect, but also have good sealing to prevent leakage, so that when cooling is not needed, it is best to allow all water in the cooling system to escape, To avoid clogging of the cooling system due to scale generated by water storage, the structure should also be conducive to maintenance. Currently, most water cooling systems are spiral grooves drilled on the outer surface of the machine barrel and then coiled with cooling pipes.
b. Screw cooling
The main purpose of cooling the screw is to prevent plastic from overheating, and the feeding section of the cooling screw is also conducive to material transportation to the machine head. The cooling medium that enters the screw is usually water, and the water temperature can be maintained at a certain value according to requirements. Even the cooling length of the screw can be adjusted, sometimes for full length cooling, and sometimes only for a portion. The latest design of the extruder screw temperature can be adjusted in zones.
1.2.4 Extruder screw parameters
The main parameters of a screw include diameter to diameter ratio, compression ratio, pitch, groove width, helix angle, and the clearance between the screw and the barrel.
① Screw diameter D: The size specifications of an extruder are usually represented by the screw diameter. The larger the diameter, the greater the amount of glue produced, and the extrusion amount is approximately proportional to its square. Therefore, a slight increase in screw diameter will cause a significant increase in extrusion amount.
② Length to diameter ratio L/D: The ratio of the length L of the working part of the screw to the diameter D. When the diameter of the screw is fixed, increasing the aspect ratio means increasing the length of the screw. The larger the aspect ratio, the more favorable it is for the material to fully plasticize, and at the same time, it can generate greater pressure to ensure that the product is more dense and improve quality. However, if the aspect ratio is too large, it is not enough to cause excessive plasticization of the cross-linked material, causing early cross-linking of the material. When used in the extruder of the cross-linking unit, the aspect ratio is generally between 20-25 times.
③ The compression ratio is the ratio of the volume of the first slot in the feeding section to the volume of the last screw slot in the homogenization section. The selection of the compression ratio should be based on the raw material, plastic properties, and compression ratio, with larger particles and smaller compression ratios; The particle size is small and the compression ratio is large, with a compression ratio of 2-3 times for low-density polyethylene.
④ Screw groove depth H: refers to the difference between the outer radius of the thread and the root radius. According to the compression requirements, the groove depth of the feeding section is greater than the melting section, and the melting section is also greater than the homogenizing section. The large depth of the screw groove in the feeding section is conducive to improving its conveying capacity. The shallow groove in the melting section and homogenizing section allows the screw to produce a high shear speed for the material, which is conducive to the heat transfer from the cylinder wall to the material and the mixing and plasticization of the material.
　
2. Extruder mold
2.1 The molds of the extruder are divided into extrusion type, semi extrusion tube type, and extrusion tube type.
2.2 Characteristics of various molds:
2.2.1. Advantages of extrusion type:
1) The extruded plastic layer structure is tight and sturdy.
2) The insulation and conductor are tightly bonded without gaps, and the insulation strength of the extruded layer is reliable.
3) The outer surface is flat and smooth.
2.2.2. Disadvantages of extrusion type:
1) It is not easy to adjust the deviation of the core.
2) The accuracy requirements for mold matching are high, and product quality depends heavily on the mold.
3) The bending of the extrusion line is not good.
2.2.3 Advantages of Squeezing Tube
1) Fast extrusion speed.
2) Easy to operate, easy to adjust the deviation core.
3) Large clearance, low wear, and long service life.
4) Convenient mold matching.
5) Plastic directional stretching increases the extrusion strength of plastic.
6) The thickness of the sheath is easy to control.
2.2.4 Disadvantages of extruded tube type:
1) Poor density.
2) Poor adhesion between plastic and wire core.
2.2.5 Semi extruded tube type: absorbs the advantages of both extruded and extruded tube types.
　
2、 The crosslinking methods of cross-linked polyethylene materials are divided into two methods: chemical and physical
　
1. Chemical crosslinking: peroxide crosslinking (1. Steam crosslinking (SCP); 2. Infrared crosslinking method (RCP) and dry crosslinking; 3. MDCV cross-linking; 4. Pressure lava salt crosslinking (PLCV) process; 5. Silicone oil crosslinking (FZCV) process Graft crosslinking with silane (one-step method, two-step method, copolymerization method).
2. Physical crosslinking: High energy radiation crosslinking.
　
3、 Crosslinked polyethylene material
　
Crosslinked polyethylene material is a mixture of low-density polyethylene, peroxide crosslinking agents, antioxidants, etc. When heated, the peroxide decomposes into free radicals with high chemical activity. These free radicals seize the hydrogen atoms in the polyethylene molecule, making some carbon atoms in the main chain of polyethylene active free radicals and combining with each other, that is, C-C cross-linking bond is generated, forming a network macromolecular structure. The insulator uses diisopropylbenzene peroxide (DCP) as the cross-linking agent. Taking DCP as an example, the crosslinking reaction of polyethylene is as follows:
　
1. DCP decomposes into two free radicals
2. Activation of Polyethylene, Free Radical Transfer, and Generation of Cumyl Alcohol
3. Crosslinking between polyethylene molecules
4. Kuryl alcohol is an unstable compound that needs to decompose at high temperatures. Due to the high temperature required for the decomposition of DCP, the entire cross-linking process must be carried out under high temperature and pressure to increase the reaction speed and compress the residual by-product gas in the insulation.
　
4、 Basic requirements for insulation process
　
1. No microporous insulation, with the lowest water content
Adopting a fully dry cross-linking and cooling system. Minimize the moisture content and micropores in the insulation. The cross-linked tube heating system should ensure that the temperature of the heating tube is uniform and there are no "hot spots". The system should be easy to operate and have a short reaction time.
The air-cooled blower of the fully dry cross-linking production line allows nitrogen to quickly circulate in the cooling pipe and achieve heat exchange, resulting in high cooling efficiency. Air cooling is mostly used in the production process of high-voltage and ultra-high voltage cables. The fully dry cross-linking production line can also be equipped with water cooling, which adopts the principle of closed circuit circulation and heat exchange, and is mostly used in the production of medium voltage cables. This fully dry crosslinking process can be applied to both suspension chain and vertical crosslinking production lines.
2. Smooth interlayer interface
The smoothness of the interface between the cable insulation layer and the semi conductive layer is one of the important factors affecting the service life of the cable. Especially for the protrusion and embedding of the semi conductive shielding layer at the interface of high-voltage and ultra-high voltage cables, it can lead to excessive local electric field strength, accelerate insulation aging, and increase the possibility of water tree phenomenon. In order to achieve smooth layer interfaces, ultra smooth semi conductive materials are used in the production of high-voltage and ultra-high voltage cables.
3. Cable insulation concentricity
Cable insulation concentricity refers to the position of a conductor on each insulation layer, with good coaxial symmetry. Due to the fact that the distribution of electric fields in insulation is a key factor in the long-term operation of cables, the circular shape minimizes the electric field strength of the conductor shielding layer, which is ideal and also the required shape. The requirements for cable concentricity are also becoming increasingly strict. In addition, both vertical and catenary production lines can produce thick insulated cables that meet the requirements of contract centrality. The roundness of the insulated cable depends on the various processes of the entire cross-linking production line, namely extrusion, cross-linking, and cooling, but it is impossible to require absolute symmetry, especially in the suspension chain cross-linking production line.