Knowledge of wire and cable production process

The main insulation material and protective layer material of industrial plastic wire and cable is plastic. Thermoplastic has superior performance and good processing performance, especially for wire and cable extrusion insulation and protective layer production process is simple. The basic way of the production of plastic insulation and protective layer of wire and cable is the continuous extrusion of a single screw extruder. Because the extruder has the characteristics of continuous extrusion, the production process of plastic insulation and sheathing is also continuous. As far as wire and cable production is concerned, differences in product specifications and different extruded parts often determine certain changes in extrusion equipment and process parameters. But in general, all kinds of products, each part of the extrusion coating process is much the same, the following is based on the general, individual as a supplement to the extrusion principle, process and mold type are introduced. Section 1 Extrusion of plastics 1, the basic principle of plastic extrusion The working principle of the extruder is: The use of a specific shape of the screw, rotating in the heated barrel, the plastic sent by the hopper is extruded forward, so that the plastic is uniformly plasticized (that is, melted), through the head and different shapes of the mold, so that the plastic is extruded into a continuous plastic layer of various shapes required, extruded on the core and cable. 1. Plastic extrusion process The plastic insulation and sheath of wire and cable are carried out by continuous extrusion, and the extrusion equipment is generally a single screw extruder. Before the plastic is extruded, it is necessary to check whether the plastic is wet or there is other debris in advance, and then add the screw to the hopper after preheating. In the extrusion process, the plastic loaded into the hopper enters the barrel with the help of gravity or the feeding screw. Under the thrust action of the rotating screw, it continuously pushes forward, and gradually moves towards the homogenizing section from the preheating section. At the same time, the plastic is stirred and squeezed by the screw, and under the action of the external heat of the barrel and the shear friction between the plastic and the equipment, the viscous flow is formed in the screw groove. Under the action of the temperature specified in the process, the plastic is transformed from a solid state to a plastic object in a molten state, and then pushed or stirred by the screw, the completely plasticized plastic is pushed into the head; The material flow reaching the head is extruded from the mold sleeve through the annular gap between the mold core and the mold suite, and is extruded around the conductor or wire core to form a continuous dense insulating layer or sheath layer, and then cooled and cured to make wire and cable products. 2. The three stages of the extrusion process The most important basis for plastic extrusion is the plastic has a plastic state. Plastic in the extruder to complete the plastic process molding is a complex physical process, that is, including mixing, crushing, melting, plasticizing, exhaust, compaction and finally shaping. It is important to note that this process is implemented continuously. However, traditionally, people often according to the different reactions of plastics extrusion process of this continuous process, artificial into different stages, namely: plasticizing stage (plastic mixing, melting and homogenization); Molding stage (extrusion molding of plastics); The setting stage (cooling and curing of the plastic layer). The first stage is plasticizing. Also known as the compression phase. It is completed in the barrel of the extruder, and through the rotation of the screw, the plastic is changed from a granular solid into a plastic viscous fluid. There are two sources of heat obtained by plastics in the plasticizing stage: one is the electric heating outside the barrel; The second is the friction heat generated when the screw rotates. The initial heat is generated by the electric heating outside the barrel, and when the normal driving, the heat is obtained by the friction between the screw selection material and the inner wall of the barrel and the internal friction between the material molecules in the process of compression, shearing and stirring. The second stage is the molding stage. It is carried out in the head, due to the rotation of the screw and pressure, the viscous fluid is pushed to the head, through the die in the head, the viscous fluid is formed into the extruded material of various sizes and shapes required, and is covered with the online core or the conductor. The third stage is the setting stage. It is carried out in a cooling tank or cooling pipe, and the plastic extruded cladding is cooled to change from an amorphous plastic state to a solid state. 3. Changes in plastic flow In the plasticizing stage, during the movement of the plastic along the screw axis by the screw to the head, it experiences changes in temperature, pressure, viscosity, and even chemical structure, and these changes are different in different sections of the screw. The plasticizing stage is divided into three stages according to the physical state change process of the plastic flow, that is, the feeding section, the melting section, and the homogenization section, which is also the segmentation method that people are accustomed to on the extrusion screw, each segment has different effects on the plastic extrusion, and the plastic presents different forms in each segment, thus showing the extrusion characteristics of the plastic. In the feeding section, first of all, the softening temperature is provided for the granular solid plastic, and then the shear stress generated between the rotation of the screw and the fixed barrel acts on the plastic particles to achieve the crushing of the softened plastic. The most important is to rotate the screw to generate enough continuous and stable thrust and reverse friction to form a continuous and stable extrusion pressure, so as to achieve the mixing and uniform mixing of broken plastics, and initially implement heat exchange, so as to provide the basis for continuous and stable extrusion. At this stage, whether the thrust generated is continuous, uniform and stable, the shear strain rate is high or low, and whether the crushing and stirring are uniform directly affect the extrusion quality and yield. In the melting section, the plastic, which has been broken, softened and initially mixed, moves along the screw groove to the head due to the pushing action of the screw, and enters the melting section from the feeding section. In this section of the plastic encountered a higher temperature of heat, which is a heat source, in addition to the point heating outside the barrel, the friction heat of the screw rotation is also playing a role. The thrust from the feeding section and the reaction force from the homogenization section make the plastic form a reflux in the advance, which is generated in the screw groove and the gap between the screw and the barrel, the reflux generation not only makes the material further evenly mixed, but also increases the heat exchange effect of the plastic, to achieve the surface heat balance. Since the action temperature at this stage has exceeded the rheological temperature of the plastic, coupled with the longer action time, the plastic has a change of state, and the material in contact with the heating cylinder begins to melt, forming a layer of polymer melt film on the inner surface of the cylinder. When the thickness of the melt film exceeds the gap between the top of the thread and the cylinder, it will be scraped off by the rotating thread and gather in front of the push thread. Forming a molten pool. Due to the relative movement of the barrel and the root of the thread, the circulating flow of materials is generated in the molten pool. Behind the screw edge is a solid bed (solid plastic). In the process of material moving forward along the screw groove, the depth of the melt groove in the melt section gradually becomes shallower toward the homogenization section, and the solid bed is constantly squeezed toward the inner wall of the barrel, accelerating the heat transfer process from the barrel to the solid bed. At the same time, the rotation of the screw has a shearing effect on the melt film on the inner wall of the barrel, thus melting the material at the interface between the melt film and the solid bed. The width of the solid bed gradually decreases until it completely disappears, that is, from a solid state to a viscous flow state. At this time, the molecular structure of the plastic has undergone a fundamental change, the intermolecular tension is extremely relaxed, if it is a crystalline polymer, the crystal zone begins to reduce, amorphous increase, in addition to the large molecules, the main body completed plasticization, the so-called "preliminary plasticization", and under the action of pressure, the gas contained in the solid material is excluded to achieve preliminary compaction. In the homogenization section, there are several outstanding process characteristics: this section of the screw thread depth is the shallowest, that is, the screw groove volume is the smallest, so this is the working section where the pressure between the screw and the barrel is the largest; In addition, the thrust force from the screw and the reaction force at the screen plate and other places are the direct zones of plastic "hand-to-hand contact"; This section is the highest temperature of the extrusion process, so the radial pressure and axial pressure of the plastic at this stage are the largest, and this high pressure is enough to exclude all the gases contained in the plastic, and make the melt compacted and dense. This section has the "pressure equalization section" is thus called. Due to the effect of high temperature, the polymer that has not been plasticized through the melting section is plasticized in this section, so that the "particles" are finally eliminated, so that the plastic plasticization is fully uniform, and then the plastic that is completely plasticized and melted is quantitatively and at constant pressure uniformly extruded by the head. 4. The flow state of the plastic during the extrusion process, due to the rotation of the screw, the plastic is pushed, and the barrel is not moving, which produces a relative motion between the barrel and the screw, and this relative motion has a friction effect on the plastic, so that the plastic is dragged forward. In addition, due to the resistance of the mold, porous screen and filter in the head, the plastic produces a reaction force in the advance, which complicates the flow of plastic in the screw and barrel. The flow state of plastic is usually regarded as consisting of the following four flow forms: 1) positive flow - refers to the flow of plastic along the screw groove to the head direction. It is produced by the pushing force of the screw rotation and is the most important of the four flow forms. The size of the positive flow directly determines the extrusion amount. 2) Backflow - also known as countercurrent, its direction is the opposite of the flow direction of the positive flow. It is caused by the pressure generated in the head area (the reaction force of the plastic forward) caused by the mold, screen plate, and filter in the head that hinder the positive movement of the plastic. From the head to the feed port, a "return under pressure" is formed, also known as "reverse pressure flow". It can cause a loss of productive capacity. 3) Cross flow - it is the plastic flow along the direction of the axis, that is, perpendicular to the thread groove. It is also formed by the pushing of the screw when it rotates. Its flow is subject to the resistance of the side wall of the threaded groove, due to the mutual resistance of the threads on both sides, and the screw is in rotation, so that the plastic in the spiral groove produces a turnover movement, forming a circular flow, so the cross-flow is essentially a circulation. Circulation is inseparable from the effect of circulation on the mixing and plasticizing of plastics in the barrel. Circulation makes the material stir and mix in the barrel, and is conducive to the heat exchange between the barrel and the material. It has important significance for improving the extrusion quality, but has little effect on the extrusion flow rate. 4) Leakage flow - it is also caused by the resistance of the mold, screen plate and filter in the head. However, it is not the flow in the screw groove, but the reverse flow formed in the gap between the screw and the barrel. It can also cause a loss of productive capacity. Because the gap between the screw and the barrel is usually very small, under normal circumstances, the leakage flow is much smaller than the positive flow and the reverse flow. In the extrusion process, the leakage flow will affect the extrusion amount, the leakage flow will increase, the extrusion amount will decrease. The four flow states of plastic do not appear in a separate form, and there is neither a true backflow nor a closed circulation for a particular plastic particle. The actual flow of melt plastics in the threaded groove is a synthesis of the above four flow states, and a flow forward in a spiral trajectory. 5. Extrusion quality Extrusion quality mainly refers to whether the plasticization of plastics is good, whether the geometric size is uniform, that is, whether the radial thickness is consistent, and whether the axial outer diameter is uniform. In addition to the plastic itself, the main factors that determine the plasticization are temperature, shear strain rate and action time. High extrusion temperature not only causes the fluctuation of extrusion pressure, but also leads to the decomposition of plastics, and may even lead to equipment accidents. While reducing the depth of the screw groove and increasing the length-diameter ratio of the screw, although it is conducive to the heat exchange of the plastic and the extension of the heating time to meet the uniform requirements of plasticization, it will affect the extrusion amount and cause difficulties for the manufacture and assembly of the screw. Therefore, an important factor to ensure plasticization should be to increase the shear strain rate generated by the screw rotation on the plastic, so as to achieve uniform mechanical mixing and balanced extrusion heat exchange, and thus provide protection for uniform plasticization. The size of this strain rate is determined by the shear stress between the screw and the barrel, and the shear strain rate value is: Δ -- shear strain rate (1/min) D -- screw diameter (cm) N -- screw speed (r/min) -- screw groove depth (cm) It can be seen that under the requirements of ensuring the extrusion volume, the depth of screw groove can be increased under the condition of increasing the speed. In addition, the gap between the screw and the barrel also has an impact on the extrusion quality. When the gap is too large, the backflow and leakage of the plastic increase, which not only causes the extrusion pressure fluctuation, but also affects the extrusion quantity. And due to the increase of these backflows, the plastic overheats and causes the plastic to scorch or form difficult. The plastic extruder unit is composed of an extruder (host) and a number of auxiliary equipment, and the crew should closely cooperate with the operation in the production. Operators must be familiar with the growing process and operating procedures. 1. Extrusion process of plastic extruder Plastic extruder is a hot extrusion equipment. A disk of cable or cable core is placed on the pay-off device, and ensure that there is a certain tension, after the tensioning straightening device into the extruder head extruded insulation layer or sheath layer. Plastic particles are added to the extruder barrel through the hopper, due to the rotation of the screw, into the chamber, on the one hand, heating, on the other hand, by the rotation and stirring of the screw, to promote plastic plasticization, and pushed to the head, extruded from the die, complete and tight continuous extrusion wrapped in the wire and cable core or cable core. In order to control the thickness and extrusion pressure of the plastic layer, the annular spacing between the mold core and the mold suite should be adjusted to make the plastic layer uniform. Each unit in the unit adopts a separate drive, and the working speed between the units can be adjusted separately. The speed of the screw and traction should be matched with each other to ensure that the outer diameter of the wire and cable extrusion and the thickness of the plastic layer are uniform, and meet the requirements of the process size. The speed of pay-off and take-up should be matched with the production speed of wire and cable to prevent other quality problems. According to the control temperature stipulated by the process, the appropriate mold is selected, and the change of the heating system, the change of the outer diameter and the change of the speed are often observed to prevent the phenomenon of eccentricity, coke burning and poor plasticization of the plastic layer.