The main insulation material and protective layer material of plastic wires and cables is plastic. Thermoplastics have superior performance and excellent processing performance, especially in the production of extruded insulation and protective layers for wires and cables, where the process is simple. The basic method of producing plastic insulation and protective layers for wires and cables is to use a single screw extruder for continuous extrusion. Due to the continuous extrusion characteristic of the extruder, the production process of plastic insulation and sheath is also continuous. In terms of wire and cable production, differences in product specifications and extrusion components often determine certain changes in extrusion equipment and process parameters. However, overall, the extrusion coating process for various products and components is similar and slightly different. Below, the extrusion principle, process, and mold type will be introduced with general principles as the main focus and individual examples as auxiliary.

Section 1 Basic Principles of Plastic Extrusion

The working principle of an extrusion machine is to use a specific shape of screw to rotate in the heated barrel, squeeze the plastic sent from the hopper forward, and make the plastic uniformly plasticized (i.e. melted). Through the machine head and different shapes of molds, the plastic is extruded into various shapes of plastic layers required for continuity, and extruded onto the wire core and cable.

1. Plastic extrusion process
The plastic insulation and sheath of wires and cables are made using a continuous extrusion method, and the extrusion equipment is usually a single screw extruder. Before extrusion, plastic should be checked for moisture or other debris, and then the screw should be preheated and added to the hopper. During the extrusion process, the plastic loaded into the hopper enters the barrel with the help of gravity or feeding screws. Under the thrust of the rotating screw, it continuously advances and gradually moves towards the homogenization section starting from the preheating section; At the same time, the plastic is stirred and squeezed by the screw, and under the external heat of the machine barrel and the shear friction between the plastic and the equipment, it transforms into a viscous flow state, forming a continuous and uniform material flow in the screw groove. Under the temperature specified by the process, plastic is a plastic object that transforms from a solid state to a molten state, and then is pushed or stirred by a screw to push the fully plasticized plastic into the machine head; The material flow that reaches the machine head is extruded through the annular gap between the mold core and the mold sleeve, and wrapped around the conductor or wire core to form a continuous and dense insulation layer or sheath layer. Then, it is cooled and solidified to produce wire and cable products.

1- Payoff wheel 2- Tension wheel 3- Preheater 4- Φ 150 type extrusion machine host 5- cooling water tank 6- water meter 7- double traction wheel 8- cable rack 9- take-up wheel 10- gearbox 11- feeding port 12- belt pulley 13- DC motor

2. Three stages of extrusion process
The main basis for plastic extrusion is the plasticity of plastic. Plastic forming in an extruder is a complex physical process that involves mixing, crushing, melting, plasticizing, venting, compacting, and ultimately shaping. It should be noted that this process is implemented continuously. However, in practice, people often divide the extrusion process into different stages based on the different reactions of plastics, namely the plasticization stage (mixing, melting, and homogenization of plastics); Forming stage (extrusion molding of plastic); The shaping stage (cooling and curing of the plastic layer).
The first stage is the plasticization stage. Also known as the compression stage. It is completed inside the barrel of the extruder, and through the rotation of the screw, the plastic changes from a granular solid to a plastic viscous fluid. There are two sources of heat obtained by plastics during the plasticizing stage: firstly, electric heating outside the barrel; The second is the frictional heat generated when the screw rotates. The initial heat is generated by electric heating outside the barrel. After normal driving, the heat is obtained by the friction between the selected materials of the screw and the inner wall of the barrel during compression, shearing, and stirring, as well as the internal friction between the material molecules.     
The second stage is the forming stage. It is carried out inside the machine head, and due to the rotation and pressure of the screw, the viscous fluid is pushed towards the machine head. Through the mold inside the machine head, the viscous fluid is formed into various sizes and shapes of extrusion materials required, and wrapped around the wire core or conductor.     
The third stage is the finalization stage. It is carried out in a cooling water tank or cooling pipeline, and after cooling, the plastic extrusion layer changes from an amorphous plastic state to a fixed solid state.

A - Normal flow B - Countercurrent C - Cross flow (also known as circulation) D - Leakage flow 1- Φ 150 screw 2- Head 3- Body 4- Barrel 5- Resistance wire heating 6- Mold core seat 7- Mold sleeve seat 8- Matching screw 9- Filter device 10- Diverter 11- Feeding port 12- Cooling water pipe 13- Screw rotation direction

3. Changes in plastic flow during the plasticization stage
During the plasticizing stage, plastic is pushed towards the machine head along the axial direction of the screw, undergoing changes in temperature, pressure, viscosity, and even chemical structure. These changes vary in different sections of the screw. The plasticization stage is artificially divided into three stages based on the changes in the physical state of plastic during flow, namely the feeding stage, the melting stage, and the homogenizing stage. This is also a customary segmented method for extrusion screws, where each stage has different effects on plastic extrusion, and plastic exhibits different shapes in each stage, thus exhibiting the extrusion characteristics of plastic.
In the feeding section, the first step is to provide a softening temperature for the granular solid plastic, and secondly, 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 thing is to generate sufficient continuous and stable thrust and reverse friction force through the rotation of the screw, in order to form a continuous and stable extrusion pressure, thereby achieving stirring and uniform mixing of broken plastics, and initially implementing heat exchange, thereby providing a foundation for continuous and stable extrusion. The continuous, uniform and stable thrust generated during this stage, the level of shear strain rate, and the uniformity of crushing and stirring directly affect the extrusion quality and yield.
In the melting section, the old plastic, which has been crushed, softened, and initially mixed, moves along the screw groove towards the machine head due to the pushing effect of the screw, and enters the melting section from the feeding section. At this stage, the plastic encountered a higher temperature thermal effect, which is the heat source. In addition to the point heating outside the barrel, the frictional heat of screw rotation also plays a role. The thrust from the feeding section and the reaction force from the homogenization section form a reflux in the plastic as it moves forward. This reflux occurs in the screw groove and the gap between the screw and the barrel. The generation of reflux not only further mixes the material evenly, but also increases the heat exchange effect of the plastic, achieving surface heat balance. Due to the temperature exceeding the rheological temperature of the plastic during this stage, coupled with a longer action time, the plastic undergoes a phase transition. 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 screw pattern and the cylinder, it will be scraped off by the rotating thread and gathered in front of the advancing thread, forming a melt pool. Due to the relative movement between the barrel and the root of the thread, the molten pool generates a circulating flow of materials. Behind the screw edge is a solid bed (solid plastic). As the material moves forward along the screw groove, the depth of the screw groove in the melting section gradually becomes shallower towards the homogenization section, and the solid bed is continuously squeezed towards 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 produces a shear effect on the molten film on the inner wall of the barrel, causing the material at the interface between the molten film and the solid bed to melt. The width of the solid bed gradually decreases until it completely disappears, That is, the transition from a solid state to a viscous flow state. At this time, the molecular structure of the plastic has undergone a fundamental change, and the intermolecular tension is extremely relaxed. If it is a crystalline polymer, its crystal area begins to decrease and amorphous increases. In addition to the extra large molecules, the main body has completed the plasticization, which is called "preliminary plasticization". Under the effect of pressure, the gas contained in the solid material has been eliminated to achieve preliminary compaction.     
In the homogenization section, there are several prominent process characteristics: this section has the shallowest screw thread depth, which means the smallest screw groove volume, so this is the working section where the maximum pressure is generated between the screw and the barrel; In addition, the thrust from the screw and the reaction force from the sieve plate are the direct areas where plastic is "in close contact"; This section is also the section with the highest extrusion process temperature, so the plastic is subjected to the maximum radial and axial pressure during this stage. This high-pressure effect is sufficient to eliminate all gases contained in the plastic and make the melt compact and dense. The term "equalizing section" used in this section is derived from this. Due to the effect of high temperature, the polymer that cannot be plasticized after passing through the melting section completes plasticization in this section, ultimately eliminating "particles" and making the plastic plasticization fully uniform. Then, the fully plasticized and melted plastic is quantitatively and uniformly extruded from the machine head under constant pressure.

4. The flow state of plastic during the extrusion process
During the extrusion process, the rotation of the screw causes the plastic to move, while the barrel remains stationary, resulting in relative motion between the barrel and the screw. This relative motion creates friction on the plastic, causing it to be dragged forward. In addition, due to the resistance of the molds, porous sieve plates, and filter screens in the machine head, a reaction force is generated during the plastic movement, which complicates the flow of plastic in the screw and barrel. The flow state of plastic is usually regarded as composed of the following four flow forms:
1) Positive flow - refers to the flow of plastic along the screw groove towards the machine head. It is generated by the pushing force of the screw rotation and is the most important of the four flow forms. The magnitude of the positive flow rate directly determines the extrusion amount.    
2) Reverse flow - also known as reverse flow, its direction is aligned opposite to the direction of normal flow. It is caused by the pressure (the reaction force of the plastic forward) generated in the head area due to the mold, sieve plate, and filter screen in the head hindering the forward movement of the plastic. A "backflow under pressure" is formed from the machine head to the feeding port, also known as "backpressure flow". It can cause a loss of production capacity.     
3) Cross flow - It is the plastic flow along the axis, perpendicular to the threaded groove. It is also formed by the pushing and squeezing of the screw during rotation. Its flow is subject to the resistance of the side walls of the threaded groove. Due to the mutual resistance of the threads on both sides, the screw is rotating, causing plastic to flip inside the groove, forming a circular flow. Therefore, cross flow is essentially a circular flow. The mixing and plasticization of plastic in the barrel into a molten state by circulation is inseparable from the effect of circulation. Circulation causes stirring and mixing of materials in the barrel, and facilitates heat exchange between the barrel and the material. It is of great significance for improving extrusion quality, but has little impact on extrusion flow rate.      
4) Leakage - It is also caused by the resistance of the mold, sieve plate, and filter in the machine 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 production capacity. Due to the usually small gap between the screw and the barrel, under normal circumstances, the leakage flow rate is much smaller than that of normal flow and backflow. During the extrusion process,

1. Homogenization section (shallower groove) 2. Plasticization section (shallower groove) 3. Feeding section (deeper groove)

5. Extrusion quality
The extrusion quality mainly refers to whether the plasticization of the plastic is good, whether the geometric dimensions are uniform, that is, whether the radial thickness is consistent, and whether the axial outer diameter is uniform. The factors that determine the plasticization situation, besides the plastic itself, are mainly temperature, shear strain rate, and action time. Excessive extrusion temperature not only causes fluctuations in extrusion pressure, but also leads to the decomposition of plastic and may even lead to equipment accidents. Reducing the depth of the screw groove and increasing the aspect ratio of the screw, although beneficial for the heat exchange of plastics and prolonging the heating time to meet the requirements of uniform plasticization, will affect the extrusion amount and create difficulties for screw manufacturing and assembly. So an important factor to ensure plasticization should be to increase the shear strain rate generated by screw rotation on the plastic, in order to achieve uniform mechanical mixing, balanced extrusion heat exchange, and thus provide guarantee for uniform plasticization. The magnitude of this strain rate is determined by the shear strain force between the screw and the barrel, indicating that, while ensuring the extrusion amount, the depth of the screw groove can be increased while increasing the rotational 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 plastic backflow and leakage increase, which not only causes fluctuations in extrusion pressure but also affects the extrusion quantity; Moreover, due to the increase in these backflows, the plastic overheats, resulting in difficulty in charring or forming.

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