Wire and cable production process knowledge

Technology
The main insulation and sheath material of plastic wire and cable is plastic. Thermoplastics are superior in performance and have good processing properties, especially when used in the extrusion of insulating layers and sheaths for wires and cables. The basic method for the production of plastic insulation and sheathing of wire and cable is the continuous extrusion of a single screw extruder. Due to the continuous extrusion characteristics of the extruder, the plastic insulation and jacket production process is also continuous. In the case of wire and cable production, differences in product specifications and differences in extruded components often determine certain changes in the extrusion equipment and process parameters. However, in general, the extrusion coating processes for various products and components are similar, and the following are general, and individual supplements are for extrusion principles, processes, and mold types.
Section 1 Plastic Extrusion
First, the basic principle of plastic extrusion
The working principle of the extruder is: using a specially shaped screw, rotating in a heated barrel, extruding the plastic sent from the hopper forward, plasticizing the plastic uniformly (ie, melting), passing the machine head and different shapes The molds, which squeezing the plastic into a continuous layer of the various shapes required for the plastic layer, are extruded over the core and the cable.
1. Plastic extrusion process
The plastic insulation and sheathing of the wire and cable are made by continuous extrusion. The extrusion equipment is generally a single screw extruder. Before plastics are extruded, it is necessary to check in advance whether the plastics are damp or have other impurities. Then the screws are preheated and added to the hoppers. During the extrusion process, the plastic loaded into the hopper enters into the barrel by means of gravity or feed screw. Under the action of the thrust of the rotating screw, it continuously advances, gradually moving from the preheating section to the homogenization section; at the same time, the plastic By the stirring and squeezing action of the screw, and under the action of the external heat of the barrel and the shear friction between the plastic and the equipment, it becomes a viscous flow state, and a continuous and uniform stream is formed in the screw groove. Under the action of the temperature specified by the process, the plastic changes from a solid state to a meltable plastic object, and then pushes or stirs the screw to push the completely plasticized plastic into the machine head; the stream that reaches the machine head passes through the core. The annular gap between the die case and the die case is extruded from the die bushing and extruded around the conductor or wire core to form a continuous dense insulating layer or jacket layer, which is then cooled and solidified to produce a wire and cable product.
2. Three stages of the extrusion process
The most important basis for plastic extrusion is the plastic state of plastics. Plastic molding in the extruder is a complex physical process that involves mixing, crushing, melting, plasticizing, venting, compaction, and final shaping. It is worth noting that this process is implemented continuously. However, customarily, people often use the different reactions of plastics to separate the continuous process of the extrusion process into different stages, namely: the plasticization stage (mixing, melting, and homogenizing of the plastic) and the forming stage (plastic extrusion Forming); setting stage (cooling and curing of the plastic layer).
The first stage is the plasticization stage. Also known as the compression phase. It is completed in the barrel of the extruder, through the rotation of the screw, the plastic from a granular solid into a plastic viscous fluid. There are two main sources of heat for the plastic during the plasticization stage: one is the electrical heating outside the barrel; the other is the frictional heat generated when the screw rotates. The initial heat is generated by the external heating of the barrel. When the vehicle is normally driven, the heat is obtained from the friction between the screw's optional material and the inner wall of the barrel during compression, shearing, and stirring. Caused by friction.
The second stage is the molding stage. It is carried out in the head. Due to the rotation of the screw and the pressure, the viscous fluid is pushed toward the head and the mold in the head is used to form the viscous fluid into the required extruded materials of various sizes and shapes. Cover the core or conductor outside the wire.
The third stage is the finalization stage. It is carried out in a cooling water tank or a cooling pipe. After the plastic extruded layer is cooled, it changes from an amorphous plastic state to a fixed solid state.
3. Changes in plastic flow during plasticization
In the plasticization stage, the movement of the plastic along the screw axis by the screw towards the head undergoes temperature, pressure, viscosity, and even changes in the chemical structure. These changes are different in different sections of the screw. The plasticization stage is divided into three stages according to the physical state of plastics flow, ie, the feed section, the melting section, and the homogenization section. This is also the customary method of segmenting the extrusion screw. Out of play a different role, the plastic presents different forms in each segment, thus showing the plastic extrusion characteristics.
In the feeding section, the first is to provide the 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 breakage of the softened plastic. The most important is that the screw rotation generates a large enough continuous and stable thrust and reverse friction force to form a continuous and stable extrusion pressure, thereby achieving agitation and uniform mixing of the crushing plastic, and initial implementation of heat exchange This provides the basis for continuous and stable extrusion. The continuous and stable thrust, the shear strain rate, and the uniformity of crushing and stirring all directly affect the extrusion quality and production at this stage.
In the melting section, the plastic in the state of being broken, softened, and preliminarily mixed by stirring, moves along the groove to the nose due to the pushing action of the screw, and enters the melting section from the feeding section. During this period, the plastic encountered a higher temperature thermal effect. This is a 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 feed section and the reaction force from the homogenization section cause the plastic to form a backflow during the advancement. This backflow is generated in the screw groove and in the gap between the screw and the barrel, and the backflow not only makes the material evenly mixed. Moreover, the heat exchange effect of the plastic is increased to reach the thermal balance of the surface. Since the action temperature at this stage has exceeded the rheological temperature of the plastic and the longer action time, a change in the state of the plastic occurs, and the material in contact with the heating cylinder starts 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 top of the screw thread and the barrel, the material film will be scraped off by the rotating thread and accumulated in front of the thread to form the molten pool. Due to the relative movement of the barrel and the root of the thread, the melt pool produces a circulating flow of material. Behind the screw edge is a solid bed (solid plastic). During the process in which the material moves forward along the screw groove, the depth of the screw groove of the melting section gradually becomes shallower toward the homogenization section, and the solid bed is continuously pushed toward the inner wall of the barrel, accelerating the machine. The heat transfer process from the drum to the solid bed, while the rotation of the screw has a shearing effect on the melt film on the inner wall of the barrel, so that the material at the interface between the melt film and the solid bed melts, and the width of the solid bed gradually decreases, and the disappearance is known. That is, the transition from a solid state to a viscous state. At this point, the molecular structure of plastics has undergone a fundamental change. The inter-molecular tension is extremely relaxed. If it is a crystalline polymer, the crystal area begins to decrease, and the amorphous area increases. Except for the extra large molecules, the main body is plasticized. The so-called "preliminary plasticization", and under the effect of pressure, eliminates the gas contained in the solid material to achieve initial compaction.
In the homogenization section, there are several outstanding process characteristics: The depth of the screw thread is the shallowest in this section, ie the screw groove volume is the smallest, so here is the working section where the pressure between the screw and the barrel is the largest; in addition the thrust and the screen from the screw The reaction force at the board and the like is the direct area where the plastics “join the soldiers”; this section is the highest temperature in the extrusion process, so the radial pressure and the axial pressure that the plastic receives at this stage are the greatest. It is sufficient to eliminate all the gas contained in the plastic and make the melt compact and compact. The term "equalizing section" in this paragraph is derived from this. Due to the effect of high temperature, the polymer that has not been plasticized in the melting zone is completely plasticized in this stage, so that the “particles” are finally eliminated, the plastic is fully plasticized, and then the plasticized molten plastic is quantified and fixed. The uniform extrusion from the nose.
4. Flow of plastic during extrusion
During the extrusion process, the plastic is pushed by the rotation of the screw, and the barrel is immobile, which creates a relative movement between the barrel and the screw. This relative movement creates a friction effect on the plastic and drags the plastic. go ahead. In addition, due to the resistance of the molds, perforated screens, and strainers in the head, the plastics react in advance, which complicates the flow of plastics in the screw and barrel. The flow state of plastics is usually seen as consisting of the following four flow forms:
1) Positive flow - refers to the flow of plastic along the screw groove toward the nose. It is generated by the pushing force of the screw rotation and is the most important one of the four flow forms. The amount of positive flow directly determines the amount of extrusion.
2) Backflow - Also known as reverse flow, its direction is the opposite of the flow direction of positive flow. It is caused by the impediment to the positive movement of the plastic, such as the molds, screens, and strainers in the head, caused by the pressure generated in the nose region (the reaction force of plastic advance). From the nose to the feed port, a "return under pressure" is formed, also called "back pressure flow." It can cause loss of productivity.
3) Cross-flow - it is along the axis, that is, the plastic flow perpendicular to the thread groove. It is also formed by pushing when the screw rotates. Its flow is resisted by the side wall of the thread groove. Due to the mutual resistance of the two threads, the screw is rotating, causing the plastic to make a turning motion in the groove to form an annular flow, so the cross flow is essentially a circulation. The circulation of the plastic in the barrel's mixing, plasticizing into a molten state is inseparable from the effect of the circulation. The circulation makes the material stir and mix in the barrel, and facilitates heat exchange between the barrel and the material. It has important significance for improving the extrusion quality, but it has little effect on the extrusion flow rate.
4) Leakage - It is also caused by the resistance of the die, screen, and strainer in the head. However, it is not the flow in the groove, but the reverse flow formed in the gap between the screw and the barrel. It can also cause loss of productivity. Since the gap between the screw and the barrel is usually small, under normal conditions, the leakage flow is much smaller than the forward flow and reverse flow. In 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 plastics do not occur in a single form. For a plastic material, there is neither true backflow nor closed circulation. The actual flow of melt plastic in the thread groove is a combination of the four flow conditions described above, with a forward flow of the spiral trajectory.
5. Extrusion quality
The extrusion quality mainly refers to whether the plasticization of plastics is good and whether the geometric dimensions are uniform, that is, whether the radial thickness is uniform and whether the axial outer diameter is uniform. The factors that determine the plasticization of the plastics, besides the plastic itself, are temperature, shear strain rate, and time of action. Excessively high extrusion temperatures not only cause fluctuations in the extrusion pressure, but also lead to the decomposition of plastics and may even cause equipment accidents. However, reducing the screw groove depth and increasing the length-to-diameter ratio of the screw will not only facilitate the heat exchange of the plastic and prolong the heating time, but also meet the requirements for uniform plasticization, but it will affect the extrusion volume and cause difficulties for screw manufacturing and assembly. Therefore, an important factor for ensuring plasticization should be to increase the shear strain rate of the screw rotation on the plastic, in order to achieve uniform mechanical mixing, balanced extrusion heat exchange, and thus provide uniform protection for plasticization. The magnitude of this strain rate is determined by the shear strain between the screw and the barrel. The shear strain rate is:
Where: Δ - is the shear strain rate (1/min)
D - screw diameter (cm)
N - screw speed (r/min)
——The groove depth (cm)
It can be seen that, under the requirement of ensuring the amount of extrusion, the depth of the screw groove can be increased at a higher speed. In addition, the gap between the screw and the barrel also affects the extrusion quality. When the gap is too large, the reverse flow and leakage flow of the plastic increase. This not only causes the extrusion pressure to fluctuate, but also affects the extrusion volume. Moreover, due to the increase of these reflows, the plastic Overheating causes difficulty in plastic scorching or molding.
Second, the plastic extruder operating procedures
The plastic extrusion unit is composed of an extruder (host) and a number of auxiliary equipment. The production crew should closely cooperate with the operator. The operators must be familiar with the growth process and operating procedures.
1. Extrusion process of plastic extruder
Plastic extruders are hot extrusion equipment. The plated cable or cable core is placed on the payout device, and a certain tension is ensured. After being passed through the tensioning and straightening device, the extruder head is extruded into the insulating layer or the jacket layer.
Plastic granules are fed into the barrel of the extruder through the hopper. As the screw rotates, it enters the machine casing. On the one hand, it is heated. On the other hand, the screw rotates to stir, which promotes the plasticization of the plastic and pushes it toward the machine head. It is extruded from the die opening. Continuous extrusion on the wire and cable core or cable core.
In order to control the thickness of the plastic layer and extrusion pressure, the annular spacing between the core and the mold sleeve should be adjusted so that the plastic layer is uniform.
Each single unit in the unit adopts a separate transmission, and the working speed between the units can be adjusted separately. The speed of the screw and the traction should be matched with each other to ensure that the outer diameter of the extruded wire and cable and the thickness of the plastic layer are uniform and meet the requirements of the process dimensions. The payoff and takeoff line speed should be matched with the wire and cable production speed to prevent other quality problems.
According to the process control temperature, matching the right mold, often observe changes in the heating system, changes in the outer diameter, speed changes, to prevent the plastic layer eccentricity, scorching, poor plasticity and other phenomena.

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