Process
The main insulating materials and sheathing materials for plastic wires and cables are plastics. Thermoplastic plastics have excellent performance and good processing characteristics, especially when used for extruding insulation and sheathing layers on wires and cables, the process is simple. The basic method for producing plastic insulation and sheathing layers on wires and cables is continuous extrusion using a single-screw extruder. Because the extruder has the characteristic of continuous extrusion, the production process of plastic insulation and sheathing is also carried out continuously. For wire and cable production, differences in product specifications and extruded components often determine certain changes in extrusion equipment and process parameters. But generally speaking, the extrusion coating processes for various products and components are similar. The following introduces extrusion principles, processes, and mold types, focusing on general principles and supplementing with specific details.
Section 1: Plastic Extrusion
1. Basic Principle of Plastic Extrusion
The working principle of an extruder is: using a screw of a specific shape rotating inside a heated barrel, the plastic fed from the hopper is pushed forward, uniformly plasticized (i.e., melted), and then forced through the head and different shaped dies to form a continuous plastic layer of various required shapes, which is extruded onto the core or cable.
1.1 Plastic Extrusion Process
The plastic insulation and sheathing of wires and cables are produced by continuous extrusion, generally using a single-screw extruder. Before extrusion, the plastic should be checked for moisture or other debris, then the screw is preheated and the plastic is added to the hopper. During extrusion, the plastic in the hopper enters the barrel by gravity or a feed screw. Under the thrust of the rotating screw, it continuously moves forward, gradually progressing from the preheating zone to the metering zone. At the same time, the plastic is stirred and compressed by the screw, and under the external heat of the barrel and the shear friction between the plastic and the equipment, it becomes a viscous fluid, forming a continuous and uniform melt flow in the screw channel. At the process-specified temperature, the plastic transforms from a solid state to a molten plastic state. The fully plasticized material is pushed into the head by the screw's rotation or stirring. The material reaching the head passes through the annular gap between the core die and the sleeve die, exits from the die orifice, and is extruded around the conductor or core, forming a continuous and compact insulation or sheath layer. It is then cooled and solidified to produce the finished wire or cable product.
1.2 Three Stages of the Extrusion Process
The main basis for plastic extrusion is the plastic's thermoplastic state. The process of plasticizing and shaping plastic in the extruder is a complex physical process, including mixing, crushing, melting, plasticizing, degassing, compacting, and finally shaping and setting. It is important to note that this process is achieved continuously. However, by convention, this continuous extrusion process is artificially divided into different stages based on the plastic's different behaviors: the plasticizing stage (mixing, melting, and homogenization of the plastic); the shaping stage (extrusion shaping of the plastic); and the setting stage (cooling and solidification of the plastic layer).
Stage one is the plasticizing stage, also called the compression stage. It takes place inside the extruder barrel. Through the rotation of the screw, the plastic changes from granular solid to a plastic viscous fluid. The plastic gains heat in this stage from two sources: external electric heating of the barrel, and frictional heat generated by the rotating screw. Initially, heat comes from the external electric heating. After normal operation, heat comes from the friction between the material and the barrel inner wall and between material molecules as the screw rotates and the material is compressed, sheared, and stirred.
Stage two is the shaping stage. It takes place inside the head. Due to screw rotation and pressure, the viscous fluid is pushed toward the head, passes through the die in the head, and is shaped into the required dimensions and forms of extrusion material, which is then coated around the core or conductor.
Stage three is the setting stage. It takes place in a cooling water tank or cooling pipe. After cooling, the extruded plastic layer changes from an amorphous plastic state to a set solid state.
1.3 Changes in Plastic Flow During the Plasticizing Stage
During the plasticizing stage, as the plastic moves axially along the screw toward the head, it undergoes changes in temperature, pressure, viscosity, and even chemical structure. These changes vary in different zones of the screw. The plasticizing stage is artificially divided into three zones based on the physical state changes of the plastic during flow: the feed zone, the melting zone, and the metering zone. This is also the conventional way to segment an extrusion screw. Each zone has a different effect on the extrusion, and the plastic exhibits different forms in each zone, thus showing different extrusion characteristics.
In the feed zone, first, the granular solid plastic is provided with the softening temperature. Second, the shear stress generated between the rotating screw and the stationary barrel acts on the plastic particles to crush the softened plastic. Most importantly, the screw rotation generates a sufficiently large, continuous, and stable thrust and opposing friction to form a continuous and stable extrusion pressure, thereby achieving stirring and uniform mixing of the crushed plastic and preliminary heat exchange, providing the basis for continuous and stable extrusion. The continuity, uniformity, and stability of the thrust generated in this stage, the shear strain rate, and the uniformity of crushing and mixing directly affect extrusion quality and output.
In the melting zone, the solid plastic that has been crushed, softened, and preliminarily mixed is pushed by the screw along the screw channel toward the head, moving from the feed zone into the melting zone. In this zone, the plastic encounters higher temperatures. The heat sources, besides the external electric heating of the barrel, also include the frictional heat from the screw rotation. The thrust from the feed zone and the reaction force from the metering zone cause the plastic to form backflow as it moves forward. This backflow occurs in the screw channel and in the gap between the screw and the barrel. The backflow not only further uniformly mixes the material but also increases heat exchange, achieving surface thermal equilibrium. Because the temperature in this zone exceeds the plastic's rheological temperature and the action time is relatively long, the plastic undergoes a physical state change. The material in contact with the heated barrel begins to melt, forming a polymer melt film on the inner surface of the barrel. When the melt film thickness exceeds the gap between the screw flight tip and the barrel, it is scraped off by the rotating flight and accumulates in front of the advancing flight, forming a melt pool. Due to the relative motion between the barrel and the screw root, the melt pool generates circulating flow of the material. Behind the flight edge is a solid bed (solid plastic). As the material moves forward along the screw channel, because the channel depth in the melting zone gradually decreases toward the metering zone, the solid bed is continuously squeezed toward the barrel inner wall, accelerating heat transfer from the barrel to the solid bed. Simultaneously, the screw rotation shears the melt film on the barrel inner wall, causing the material at the interface between the melt film and the solid bed to melt. The width of the solid bed gradually decreases until it disappears completely, i.e., transitions from solid to viscous flow state. At this point, the molecular structure of the plastic has fundamentally changed, inter-molecular tension is greatly relaxed. For crystalline polymers, the crystalline regions begin to decrease and amorphous regions increase. Except for the largest molecules, the main body has been plasticized – so-called "preliminary plasticization." Under pressure, gases contained in the solid material are expelled, achieving preliminary compaction.
In the metering zone, several prominent process characteristics exist: the screw channel depth here is the shallowest, meaning the channel volume is smallest, so this zone generates the highest pressure between the screw and the barrel. Also, the thrust from the screw and the reaction force from the screen pack and breaker plate create a direct "close-quarters" force on the plastic. This zone also has the highest extrusion process temperature. Therefore, the plastic experiences the greatest radial and axial pressure in this stage. This high pressure is sufficient to expel all gases contained in the plastic and compact the melt. This zone is therefore also called the "pressure metering zone." Due to the high temperature, any high polymers that were not plasticized in the melting zone complete plasticization here, finally eliminating "granules," making the plastic fully and uniformly plasticized. The completely plasticized and molten plastic is then uniformly extruded through the head at a constant rate and pressure.
1.4 Flow States of Plastic During Extrusion
During extrusion, the screw rotates and moves the plastic, while the barrel is stationary. This relative motion between the barrel and the screw creates friction that drags the plastic forward. Additionally, the resistance from the die, breaker plate, and screen pack in the head creates a reaction force against the forward movement of the plastic, complicating the flow of plastic in the screw and barrel. The flow state of the plastic is generally considered to consist of four types of flow:
Drag flow – refers to the flow of plastic along the screw channel toward the head. It is generated by the pushing force of the rotating screw and is the most important of the four flow types. The magnitude of the drag flow directly determines the extrusion output.
Back flow – also called reverse flow, its direction is exactly opposite to the drag flow. It is caused by the pressure generated in the head region (the reaction force against forward movement) due to the obstruction of the forward movement by the die, screen pack, and breaker plate in the head. From the head to the feed opening, a "pressure-induced backflow" is formed, also called "reverse pressure flow." It causes a loss of production capacity.
Cross flow – refers to plastic flow perpendicular to the screw channel direction (i.e., across the channel). It is also generated by the pushing action of the rotating screw. Its flow is resisted by the side walls of the screw channel. Due to the mutual resistance of the two side walls, and because the screw is rotating, the plastic undergoes a tumbling motion within the channel, forming a circulating flow. Therefore, cross flow is essentially a circulation flow. The circulation flow is inseparable from the mixing and plasticizing of the plastic into a molten state in the barrel. It stirs and mixes the material in the barrel and facilitates heat exchange between the barrel and the material. It is significant for improving extrusion quality but has little effect on extrusion rate.
Leakage flow – it is also caused by the resistance of the die, screen pack, and breaker plate in the head. However, it is not flow within the screw channel but backflow in the gap between the screw and the barrel. It also causes a loss of production capacity. Because the gap between the screw and the barrel is usually very small, under normal conditions, the leakage flow rate is much smaller than the drag flow and back flow. During extrusion, leakage flow affects the extrusion output; as leakage flow increases, output decreases.
These four flow states of plastic do not appear in isolation. For a given plastic particle, there is neither true back flow nor a completely closed circulation. The actual flow of molten plastic in the screw channel is a combination of the above four flow states, moving forward in a spiral trajectory.
1.5 Extrusion Quality
Extrusion quality mainly refers to whether the plastic is adequately plasticized and whether the geometric dimensions are uniform – that is, whether the radial thickness is consistent and the axial outer diameter is uniform. The factors determining plasticization, 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 plastic decomposition and possibly equipment accidents. Reducing the screw channel depth or increasing the screw length-to-diameter ratio, while beneficial for heat exchange and extending heating time to meet uniform plasticization requirements, reduces output and creates difficulties in screw manufacturing and assembly. Therefore, the key factor for ensuring plasticization should be increasing the shear strain rate on the plastic generated by screw rotation, to achieve uniform mechanical mixing and balanced heat exchange, thereby providing assurance for uniform plasticization. The magnitude of this strain rate is determined by the shear stress between the screw and the barrel. The shear strain rate is:
Where: Δ – shear strain rate (1/min)
D – screw diameter (cm)
N – screw speed (r/min)
h – screw channel depth (cm)
Thus, while meeting output requirements, the screw channel depth can be increased with increased speed. Additionally, the gap between the screw and the barrel also affects extrusion quality. If the gap is too large, backflow and leakage flow increase, causing fluctuations in extrusion pressure and affecting output. Moreover, the increase in these backflows can overheat the plastic, leading to burning or difficulty in shaping.
2. Operating Procedure for Plastic Extruders
The plastic extrusion line consists of the extruder (main machine) and several auxiliary devices. During production, the line crew must cooperate closely. Operators must be familiar with the production process and operating procedures.
2.1 Extrusion Process of the Plastic Extruder
The plastic extruder is a hot extrusion machine. A reel of cable or core is placed on the pay-off stand with proper tension. It passes through a tensioner and straightener before entering the extruder head to extrude the insulation or sheath layer.
Plastic pellets are fed into the extruder barrel through the hopper. Due to the screw rotation, they enter the barrel, where they are heated and stirred by the screw rotation, promoting plasticization. The plastic is pushed to the head, exits through the die orifice, and is continuously extruded tightly and completely around the wire core or cable core.
To control the thickness of the plastic layer and the extrusion pressure, the annular gap between the core die and the sleeve die should be adjusted to ensure a uniform plastic layer.
Each machine in the line is driven separately, and the working speeds of each unit can be adjusted independently. The screw speed and the take-up speed must be matched to ensure uniform extruded outer diameter and plastic layer thickness, meeting the process dimensional requirements. The pay-off and take-up speeds must be coordinated with the production speed of the wire or cable to prevent other quality issues.
The appropriate die should be selected according to the process-specified control temperatures. Frequent observation of the heating system changes, outer diameter changes, and speed changes is necessary to prevent eccentricity, burning, poor plasticization, etc., of the plastic layer.
