Craftsmanship
The main insulation and sheath materials for plastic electrical wires and cables are plastics. Thermoplastic plastics have excellent properties and good processing performance, particularly making the production of insulation and sheathing layers for wires and cables straightforward. The basic method for producing plastic insulation and sheathing for wires and cables is continuous extrusion using a single-screw extruder. Due to the continuous nature of the extruder, the production process for plastic insulation and sheathing is also continuous. In wire and cable production, differences in product specifications and extrusion components often determine certain changes in equipment and process parameters. Generally speaking, the extrusion process for various products and components is similar. Below, the basics, processes, and mould types of extrusion are introduced mainly in general terms with some specifics. First Section Plastic Extrusion 1. Basic Principles of Plastic Extrusion The working principle of an extruder is: using a screw of a specific shape that rotates within a heated barrel to push plastic forward from the hopper, ensuring the plastic is uniformly plasticised (i.e., melted). Through the die head and different shaped moulds, the plastic is extruded into continuous layers and applied around cores and cables. 1. Plastic Extrusion Process Plastic insulation and sheathing for wires and cables are generally produced through continuous extrusion, commonly using a single-screw extruder. Before extrusion, it is important to check the plastic for moisture or other impurities, then preheat the screw before adding the material into the hopper. During extrusion, the plastic in the hopper is fed into the barrel by gravity or a feed screw, and pushed forward by the rotating screw, gradually moving from the preheating section to the homogenising section. At the same time, the plastic is mixed and pushed by the screw and, under external heating and shear friction between the plastic and the equipment, turns into a viscous flow, forming a continuous and uniform material flow in the screw grooves. At the specified process temperature, the plastic transforms from solid to a molten, plasticised state, and is then pushed into the die head by the screw. The material enters the annular gap between the die core and die sleeve, extruded through the die sleeve opening, enveloping the conductor or core to form a continuous and compact insulation or sheath layer, which is then cooled and solidified to produce the wire or cable. 2. Three Stages of Extrusion The main principle of plastic extrusion is the plastic state of the material. Formation in an extruder is a complex physical process, including mixing, breaking down, melting, plasticising, venting, compressing, and final shaping. It is important to note that this process occurs continuously. However, it is customary to divide the continuous process into stages based on the material's reactions: plasticising stage (mixing, melting, and homogenising); forming stage (extrusion shaping); and setting stage (cooling and solidifying the plastic layer). The first stage is the plasticising stage, also called the compression stage. It is carried out in the extruder barrel, where the screw rotation transforms the plastic from solid pellets to a viscous, plastic state. Heat for plasticising comes from two sources: external electric heating of the barrel and frictional heat generated as the screw rotates. Initially, heat comes from the external electric heaters, but during normal operation, heat is generated by friction between the compressed, sheared, and mixed material and the barrel wall, as well as internal friction among the material molecules.
The second stage is the forming stage. It takes place inside the machine head. Due to the rotation of the screw and the pressure, the viscous fluid is pushed towards the machine head, passes through the mould inside the head, and is formed into extruded material of various required sizes and shapes, coating the core or the conductor. The third stage is the setting stage. This occurs in a cooling water tank or cooling pipeline. After the plastic extrusion layer is cooled, it changes from an amorphous plastic state to a solid fixed state. 3. Changes in plastic flow during the plasticising stage During the plasticising stage, plastic is pushed along the screw axis towards the machine head and experiences changes in temperature, pressure, viscosity, and even chemical structure, which vary in different sections of the screw. The plasticising stage is artificially divided into three stages based on changes in the state of plastic flow, namely the feeding stage, melting stage, and homogenising stage. This is also the conventional method of segmenting an extrusion screw, with each section affecting the plastic extrusion differently and showing the plastic in various forms, thereby expressing the extrusion characteristics of the plastic. In the feeding stage, the first task is to provide softening temperature for the granular solid plastic, and the second is to use the shear stress generated between the rotating screw and the fixed barrel on the plastic particles to crush the softened plastic. The most important task is to generate enough continuous and stable thrust and reverse friction from the screw rotation to form a continuous and stable extrusion pressure, which stirs and mixes the crushed plastic and initially performs heat exchange, laying the foundation for continuous and stable extrusion. The continuity and stability of the thrust, the shear strain rate, and uniformity of crushing and mixing directly affect extrusion quality and output. In the melting stage, the pre-crushed, softened, and initially mixed plastic moves along the screw groove towards the machine head due to screw thrust, entering from the feeding stage into the melting stage. In this stage, plastic encounters high-temperature heat, with heat sources from both the external heating of the barrel and frictional heat from screw rotation. The thrust from the feeding stage and the reaction force from the homogenising stage create backflow in the screw groove and the gap between the screw and barrel. This backflow further mixes the material and enhances heat exchange, achieving surface thermal balance. As the temperature in this stage exceeds the plastic's flow temperature and the duration is significant, the plastic changes state. The material contacting the heated barrel starts to melt, forming a polymer film on the barrel's inner surface. When this film exceeds the gap between the screw thread tip and the barrel, it is scraped off by the rotating thread and accumulates in front of the advancing thread, forming a melt pool. The relative movement between the barrel and the screw root creates a circulating flow of material in the melt pool. Behind the screw flight is a solid bed of plastic. During its forward movement along the screw groove, as the melt stage groove depth gradually decreases towards the homogenising stage, the solid bed is pushed against the barrel wall, accelerating heat transfer from the barrel to the solid bed. The screw rotation also shears the melt film on the barrel wall, melting the material at the interface between the film and solid bed, gradually reducing the solid bed width until it completely disappears, transforming from solid to viscous flow. At this point, the plastic's molecular structure fundamentally changes, intermolecular tension is greatly relaxed, and for crystalline high polymers, crystalline regions decrease while amorphous regions increase. Apart from the very large molecules, the main body completes plasticising, known as 'initial plasticising.' Under pressure, air previously in the solid material is expelled, achieving initial densification.
In the homogenising section, there are several prominent process characteristics: this section has the shallowest screw thread depth, meaning the screw groove volume is smallest, so this is the working section where the pressure between the screw and the barrel is greatest; additionally, the thrust from the screw and the reaction forces from the screen plate are where the plastic is in direct contact; this section also has the highest extrusion process temperature, so the plastic here experiences the greatest radial and axial pressure, sufficient to remove all gases contained in the plastic, compacting and densifying the melt. This is why it is called the 'pressure equalisation section'. Due to the high temperature, polymers that were not fully plasticised in the melting section are completed here, eliminating 'particles' and ensuring the plastic is fully and evenly plasticised, before being uniformly extruded from the die in a controlled amount and pressure.
Flow state of plastic during extrusion: During extrusion, the rotation of the screw moves the plastic while the barrel remains stationary, creating relative motion between the barrel and screw. This relative motion causes friction, dragging the plastic forward. Additionally, the resistance from the die, multi-hole screen plate and filter in the die causes reaction forces as the plastic moves forward, complicating the flow within the screw and barrel. The plastic flow is generally considered to consist of four types: 1) Forward flow - referring to the movement of plastic along the screw grooves toward the die. It is generated by the pushing force of the rotating screw and is the most important of the four flow types. The amount of forward flow directly determines the extrusion output. 2) Backflow - also called reverse flow, it moves in the opposite direction to forward flow, caused by the resistance of the die, screen plates, and filters in the die, generating pressure in the die area (reaction opposing the plastic's forward movement). This forms 'pressure-driven backflow' from the die to the feed port, also called 'backpressure flow', which can cause production losses. 3) Transverse flow - flow of plastic along the axis, perpendicular to the screw grooves, also caused by the pushing of the rotating screw. Its movement is resisted by the groove side walls, and due to the interaction of opposite threads, the rotating screw causes the plastic to turn over within the groove, forming circular flow. Thus, transverse flow is essentially circular flow, which is essential for mixing and plasticising the plastic in the barrel, aiding heat exchange and improving extrusion quality, though it has little effect on extrusion rate. 4) Leakage flow - also caused by resistance from the die, screen plates, and filter. However, it occurs not in the screw groove but in the gap between the screw and barrel, forming reverse flow. It can also reduce production output. Since the gap between screw and barrel is usually very small, under normal conditions leakage flow is much smaller than forward or backflow. During extrusion, leakage flow affects the output, and an increase in leakage flow reduces the extrusion amount. These four flow states do not appear separately; for any given plastic particle, there is no true backflow or enclosed circular flow. In practice, the melt flow in the screw groove is a combination of these four states, moving forward in a spiral trajectory.
5. Extrusion Quality Extrusion quality mainly refers to whether the plastic is properly plasticised and whether the geometric dimensions are consistent, that is, whether the radial thickness is uniform and the axial outer diameter is even. Factors influencing plasticisation, aside from the plastic itself, mainly include temperature, shear strain rate, and duration of action. Excessively high extrusion temperatures not only cause fluctuations in extrusion pressure but also lead to plastic decomposition and may even result in equipment accidents. Reducing the groove depth and increasing the screw length-to-diameter ratio can promote heat exchange and extend heating time, meeting uniform plasticisation requirements, but this will affect the extrusion rate and make screw manufacturing and assembly more difficult. Therefore, the key to ensuring plasticisation is to increase the shear strain rate generated by the screw rotation on the plastic, achieving uniform mechanical mixing and balanced heat exchange, thus ensuring uniform plasticisation. The magnitude of this strain rate is determined by the shear strain force between the screw and barrel, with the shear strain rate calculated as: Where: Δ ―― shear strain rate (1/min) D ―― screw diameter (cm) N ―― screw rotation speed (r/min) ―― groove depth (cm) It can be seen that under the requirement of ensuring the extrusion rate, the groove depth can be increased while increasing the rotation speed. Additionally, the gap between the screw and barrel also affects extrusion quality. If the gap is too large, plastic backflow and leakage increase, which not only causes extrusion pressure fluctuations and affects the extrusion rate but also, due to increased recirculation, overheats the plastic, leading to scorching or poor moulding. 2. Operating Procedures of Plastic Extruders A plastic extruder unit consists of the extruder (main machine) and multiple auxiliary devices. During production, operators must coordinate closely. Operators must be familiar with the growth process and operating procedures. 1. Extrusion Process of Plastic Extruders Plastic extruders are hot extrusion equipment. Cable coils or cores are placed on the pay-off device, ensuring adequate tension. After passing through the tensioning and straightening device, the material enters the extruder head to extrude the insulation or sheath layer. Plastic pellets are fed into the extruder barrel via the hopper. Due to the screw rotation, the material enters the chamber, heats up, and is stirred by the screw, promoting plasticisation and pushing it to the die, extruding tightly and continuously onto the wire or cable cores. To control the thickness of the plastic layer and extrusion pressure, adjust the annular gap between the die core and die sleeve for uniform plastic coverage. Each machine in the unit has separate drive systems, and the operating speed between units can be adjusted individually. The screw and traction speed should be coordinated to ensure consistent outer diameter and plastic layer thickness on cables, meeting process specifications. The pay-off and take-up speeds should match the production speed of wires and cables to prevent other quality issues. Temperatures should be controlled according to process specifications, suitable moulds should be selected, and changes in the heating system, outer diameter, and speed should be frequently monitored to prevent eccentric layers, scorching, and poor plasticisation.
