Cable Process - Extrusion

The extrusion process includes the production of insulation and sheathing. Insulation production methods are: coating, wrapping, extrusion, and their combinations. At present, the main insulation production is coating (winding wire, which is no longer within the scope of production license management) and extrusion (wire and cable).

1. Plastic extrusion process

(1) Using continuous extrusion

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 whether there are other debris, and then preheat the screw and add it into the hopper. During the extrusion process, the plastic loaded into the hopper enters the barrel by means of gravity or the feeding screw, and under the thrust of the rotating screw, it moves forward continuously, and gradually moves from the preheating section to the homogenization section; at the same time, the plastic It is stirred and squeezed by the screw, and is transformed into a viscous flow state under the action of the external heat of the barrel and the shear friction between the plastic and the equipment, forming a continuous and uniform material flow in the screw groove. Under the action of the temperature specified by the process, the plastic changes from a solid state to a plastic object in a molten state, and then pushes or stirs the fully plasticized plastic into the machine head through the pushing or stirring of the screw; the material flow reaching the machine head passes through the mold core. The annular gap between the die sleeve and the die sleeve is extruded from the die sleeve mouth, extruded around the conductor or core to form a continuous and dense insulating layer or sheath layer, and then cooled and solidified to make wire and cable products.

Two, the three stages of the extrusion process

The main basis for plastic extrusion is the plastic state of plastic. The plastic forming process in the extruder is a complex physical process, which includes mixing, crushing, melting, plasticizing, exhausting, compacting and finally forming. People often divide the continuous process of extrusion into different stages artificially according to the different reactions of plastics, namely:

(1) Plasticization stage (mixing, melting and homogenization of plastics)

It is completed in the barrel of the extruder, and through the rotation of the screw, the plastic is changed from a granular solid to a plastic viscous fluid. There are two sources of heat for plastics in the plasticizing stage: one is the electric heating outside the barrel; the other is the frictional heat generated when the screw rotates.

(2) Forming stage (extrusion of plastic)

It is carried out in the head. Due to the rotation of the screw and the pressure, the viscous fluid is pushed to the head, and the viscous fluid is formed into the required extrusion materials of various sizes and shapes through the die in the head. Overlaid on the core or conductor.

(3) Setting stage (cooling and curing of plastic layer)

It is carried out in a cooling water tank or a cooling pipe. After the plastic extrusion layer is cooled, it changes from an amorphous plastic state to a stereotyped solid state.

3. Changes in plastic flow during plasticization

In the plasticizing stage, the plastic undergoes changes in temperature, pressure, viscosity, and even chemical structure during the process of being pushed by the screw to the head along the screw axis. These changes are different in different sections of the screw. The plasticizing stage is artificially divided into the following three stages according to the physical state change process when the plastic flows.

(1) In the feeding section

The first is to provide the softening temperature for the granular solid plastic, and the second is to act on the plastic particles by the shear stress generated between the rotation of the screw and the fixed barrel to achieve the crushing of the softened plastic. The most important thing is to generate a sufficiently large continuous and stable thrust and reverse friction force by the rotation of the screw to form a continuous and stable extrusion pressure, so as to realize the stirring and uniform mixing of the broken plastic, and initially carry out heat exchange. , thus providing the basis for continuous and stable extrusion. The thrust generated at this stage directly affects extrusion quality and output.

(2) In the melting section

This section of plastic encounters the thermal effect of higher temperature, which is the heat source. In addition to the point heating outside the barrel, the frictional heat of the screw rotation also plays a role. The thrust from the feeding section and the reaction force from the homogenization section make the plastic form a backflow during the advance. The backflow not only further uniformly mixes the material, but also increases the heat exchange effect of the plastic, achieving a thermal balance on the surface. Since the action temperature at this stage has exceeded the rheological temperature of the plastic, and the action time is long, the material state of the plastic has changed, and the material in contact with the heating barrel begins to melt, forming a layer of polymerization on the inner surface of the barrel. When the thickness of the melt film exceeds the gap between the thread top and the barrel, it will be scraped off by the rotating thread and gather in front of the advancing thread to form a molten pool. Due to the relative movement of the barrel and the root of the thread, the molten pool produces a circulating flow of materials. There is a solid bed (solid plastic) behind the screw. During the process of moving the material forward along the screw groove, since the depth of the screw groove in the melting section gradually becomes shallower towards the homogenization section, the solid bed is continuously squeezed to the inner wall of the barrel, which accelerates the machine. The heat transfer process from the barrel to the solid bed, and the rotation of the screw produces a shearing effect on the molten film on the inner wall of the barrel, so that the material at the interface between the molten film and the solid bed melts, and the width of the solid bed gradually decreases until it disappears completely. That is, from a solid state to a viscous fluid state. At this time, the molecular structure of the plastic has fundamentally changed, and the intermolecular tension is extremely relaxed. If it is a crystalline polymer, its crystal region begins to decrease, and the amorphous shape increases. Except for the super-large molecules, the main body has completed plasticization, that is, The so-called "preliminary plasticization", and under the action of pressure, the gas contained in the solid material is excluded to achieve preliminary compaction.

(3) In the homogenization section

This section of the screw thread has the shallowest depth, that is, the volume of the screw groove 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 from the screw and the reaction force from the sieve plate are the direct result of the "short-handed connection" of plastics. This section is also the section with the highest temperature in the extrusion process, so the radial pressure and axial pressure on the plastic at this stage are the largest. This high pressure effect is enough to remove all the gas contained in the plastic and make the melt Body compacted and dense. This section has the name of the "equalizing section" that is derived from this.

Fourth, the flow state of the plastic during the extrusion process

 During the extrusion process, due to the rotation of the screw, the plastic is moved, and the barrel is not moving, which produces a relative motion between the barrel and the screw, which produces friction on the plastic, causing the plastic to be dragged go ahead. In addition, due to the resistance of the mold, porous sieve plate and filter screen in the machine head, the plastic produces a reaction force in the advancement, which complicates the flow of the 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 direction of 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 size of the positive flow directly determines the amount of extrusion.

(2) Backflow (countercurrent)

Its direction is exactly opposite to that of the positive flow. It is caused by the pressure (reaction force of the plastic moving forward) in the area of the machine head due to the mold, sieve plate, and screen in the machine head hindering the forward movement of the plastic. From the head to the feeding port, a "reflow under pressure" is formed, also known as "back pressure flow". It can cause a loss of productivity.

  (3) Cross flow

It is the plastic flow along the axis, which is perpendicular to the thread groove. It is also formed by the pushing of the screw when it rotates. Its flow is resisted by the sidewall of the thread groove. Due to the mutual resistance of the threads on both sides, and the screw is rotating, the plastic is turned over in the thread groove to form a circular flow, so the cross flow is essentially a circular flow. The effect of circulation on the mixing and plasticization of plastics in the barrel into a molten state is inseparable from the effect of circulation. The circulating flow makes the material agitate and mix in the barrel, and is conducive to the heat exchange between the barrel and the material. It is of great significance to improve the extrusion quality, but has little effect on the extrusion flow rate.

(4) Leakage

It is also produced by the resistance of the die, sieve plate and screen 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 productivity. Because the gap between the screw and the barrel is usually very small, under normal circumstances, the leakage flow rate is much smaller than the forward flow and reverse flow. During the extrusion process, the leakage flow will affect the extrusion volume. The leakage flow will increase and the extrusion volume will decrease. The four flow states of the plastic will not appear in separate forms. For a certain plastic particle, neither There will be true reverse flow and no closed circulation. The actual flow of the molten plastic in the thread groove is the synthesis of the above four flow states, and the flow is forward in a spiral trajectory.

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. In addition to the plastic itself, the factors that determine the plasticization are mainly factors such as temperature, shear strain rate and action time. Excessive extrusion temperature not only causes fluctuations in extrusion pressure, but also leads to the decomposition of plastics, and may even lead to equipment accidents. Reducing the depth of the screw groove and increasing the length-diameter ratio of the screw is beneficial to the heat exchange of the plastic, prolonging the heating time, and meeting the requirements of uniform plasticization, but it will affect the extrusion volume and cause difficulties in screw manufacturing and assembly. Therefore, an important factor to ensure plasticization should be to increase the shear strain rate generated by the rotation of the screw to the plastic, so as to achieve uniform mechanical mixing and balanced extrusion heat exchange, and thus provide a guarantee for uniform plasticization. The size of this strain rate is determined by the shear strain force between the screw and the barrel. It can be seen that under the requirement of ensuring the extrusion volume, the depth of the screw groove can be increased under the condition of increasing the rotational speed. In addition, the gap between the screw and the barrel also affects the extrusion quality. When the gap is too large, the backflow and leakage of the plastic will increase, which will not only cause fluctuations in the extrusion pressure and affect the extrusion volume; Overheating can cause the plastic to scorch or be difficult to form.