The main insulating material and sheath material for plastic wire and cable is plastic.Thermoplastic is superior in performance and has good processing performance, especially in the production of insulating layer and sheath for wire and cable.The basic mode of the plastic insulation layer and the protective layer is the continuous extrusion of the single screw extruder.Since the extruder has the characteristics of continuous extrusion, the production process of plastic insulation and sheath is continuous.In terms of the production of wire and cable, the differences in product specifications and extruded parts often determine some changes in extruded equipment and process parameters.But in general, a variety of products, parts of extrusion cladding technology is more or less the same, the following for you with general is given priority to, the individual is complementary to extrusion principle, process and a brief introduction to the mold types.

Knowledge of production process of plastic wire and cable.

Section 1 extrusion of plastics.

The basic principle of plastic extrusion.

Extrusion machine working principle is: the use of specific shape screw, rotating in the heating barrel, will be sent by the hopper of plastic extrusion, plastic plasticizing evenly (melting), through the nose and the different shape of the mould, make plastic extrusion into the continuity needed for various shapes of plastic layer, extruded online core and the cable.

1. Plastic extrusion process.

The plastic insulation and sheath of wire and cable are made by continuous extrusion, and the extruder is usually single screw extruder.Before the plastic is extruded, check whether the plastic is wet or not, and then preheat the screw to join the hopper.In the process of extrusion, into the hopper of the plastic with the aid of gravity or spiral feeding into the machine barrel, under the effect of rotating screw thrust, constantly moving forward, starting from the preheating section gradually to the homogenization of motion;At the same time, plastic mixing and extrusion effect by the screw, and the barrel of the external heat and the plastic and equipment under the action of shear friction between into a viscous flow, forming continuous homogeneous material flow in the spiral groove.Under the temperature of the process, the plastic can be transformed from the solid state to the plastic material of molten state, and then the plastic can be pushed into the head with the push or stirring of the screw.Arrived in the nose of the material flow, the mold core and mold suites of annular space, from mouth extrusion die, extruded around a conductor or wire core, the formation of continuous dense layer insulation or sheath, and then by the cooling and solidification, made of wire and cable products.

Knowledge of production process of plastic wire and cable.

2. Three stages of extrusion process.

Plastic extruding is the most important basis of plastic.Plastic forming process in extruder is a complicated physical process, which includes mixing, crushing, melting, plasticizing, exhaust, compaction and final shaping.What you notice is that this process is continuous.However, it is customary for people to push the process of extrusion process according to the different reactions of plastic, which is divided into different stages, namely: plasticizing stage (mixing, melting and homogenization of plastics);Molding stage (plastic extrusion molding);Shaping stage (the cooling and curing of the plastic layer).

The first stage is the plasticization stage.Also known as the compression phase.It is accomplished in the extruder cylinder, through the rotation of the screw, the plastic from the granular solid to plastic fluid.There are two sources of heat source for plastic during plasticization: the first is the electric heating outside the cylinder.The second is the friction heat generated by the rotation of the screw.At first, the heat is generated by the barrel external electric heating, when the normal driving, heat gain by screw optional material in the process of compression, shear, mixing with barrel inner surface friction and of the internal friction of the material.

The second stage is the molding stage.It is conducted in the nose, because screw rotation and pressure, the sticky fluid into the nose, the nose of the mould, make the sticky fluid forming for the needs of various size shape extruded materials, the core and cladding online or outside the conductor.

The third stage is the formative stage.It is carried out in a cooling water tank or a cooling pipe, and the plastic extruded layer is cooled and then becomes a solid state with an undefined plastic state.

3. Changes in plastic flow during plasticization.

 

In plasticizing phase, plastic along the screw axial is in the process of the screw to the movement of the head, experiencing the temperature, pressure, viscosity, and the change of chemical structure, the changes it is different in different sections of the screw.Plasticizing stage according to the plastic flow of state change process is artificially divided into three stages, namely, charging, melting section, homogenization, this also is people's habit of segmentation method of extrusion screw, paragraphs have different effects on plastic extruder, plastic in paragraphs present different forms, so as to show the plastic extrusion characteristics.

In the feeding section, the first is to provide granular solid plastic softening temperature, second is the rotation of the screw and the effects of the shear stress between the fixed and barrel on plastic particles, realize to soften plastic broken.And the most main is produced by the rotation of the screw continuous and stable enough thrust and reverse friction, to form a continuous and stable extrusion pressure, thus realize the broken plastic stirring and mixing, and preliminary heat exchange, which provide basis for a stable and continuous extrusion.Whether the thrust produced at this stage is continuous and uniform, the shear strain rate is high and low, and whether the crushing and stirring are uniform directly affects the extrusion quality and output.

In the melting section, the old plastic, which is broken, softened and initially stirred, is moved from the feeding section into the melting section due to the extruding function of the screw, moving along the spiral groove to the machine head.In this section, the plastic meets the thermal effect of higher temperature, which is the heat source. Besides the point heating outside the cylinder, the friction heat of the screw rotation also plays a role.And thrust from the feed section and from homogenization period of reaction, make plastic formed in the course of moving back, this flow in the spiral groove, and the clearance between the barrel and screw, the backflow produced not only make the material mixing further, and make the plastic heat transfer effect more, up to the surface thermal equilibrium.Due to the action at this stage temperature exceeds the rheological temperature of plastic, and function for a long time, the plastic in the transformation of state and contact with the heater tube material began to melt, forming a layer of polymer melt in the barrel surface membrane, when the thickness of melt film more than between the threaded cap and barrel clearance, will be rotating screw down, gathered in front of the propulsion thread, form a molten pool.Because of the relative motion of the cylinder and the root of the thread, the molten pool produces the circulation of the material.Screw arris is a solid bed (solid plastic), behind the material along the spiral groove in the process of moving forward, as a result of the melting period of screw groove depth to the homogenization section becomes shallow gradually, solid bed continuously be pushed to the barrel wall, accelerated the barrel to the heat transfer process of solid bed, at the same time, the rotation of the screw barrel inner surface melt film shear action, so that the melt film and on the surface of the solid bed boundary material melt, gradually reduce the width of solid bed, know disappear completely, namely by the solid into a viscous flow.At this time, fundamental changes have taken place in the molecular structure of plastic, intermolecular tension relaxation, for crystalline polymer, its crystal area began to decline, besides the large molecules, the increase in the number of amorphous subject completed the plasticizing, known as the "preliminary plasticizing", and under the action of pressure, ruled out the solid material contained in the gas, realize the initial compaction.

In homogenization, have such a few outstanding technological characteristic: in this paragraph the most shallow depth of the screw thread, the spiral trough minimum volume, so here is the largest stress between the screw and barrel of work;In addition, the reaction force from the thrust of the screw and the sieve plate is the direct zone of the plastic "short arms".This paragraph is the highest temperature of extrusion process, so the plastic at this stage of the radial pressure and axial pressure, the biggest such high pressure effect, enough to contain all the gases within the plastic, compaction, and makes the melt density.This section is known as the "average pressure section".And due to the effect of high temperature, make after melting period failed to plasticizing of polymer in the period of complete plasticizing, thus finally eliminate the "particles", make plastic plasticizing uniform fully, and then completely plasticizing molten plastic quantitative and constant pressure by the nose out evenly.

 

4. The flow state of plastics during extrusion.

In extrusion process, due to the rotation of the screw plastic, and barrel is motionless, the relative movement between barrel and screw, the relative movement of plastic friction effect, make plastic dragged forward.In addition, due to the resistance of the mould, the porous sieve plate and the filter screen in the machine head, the reaction force of the plastic in the advance is made, which complicates the flow of the plastic in the screw and the barrel.The flow state of plastics is generally considered to be composed of the following four flow forms:

1) positive flow -- refers to the flow of plastic along the direction of the head of the screw.It is produced by the pushing force of screw rotation and is the most important of the four flow forms.The amount of positive flow determines the amount of extrusion directly.

2) reverse flow -- also known as reverse flow, its direction is opposite to the positive flow direction.It is caused by the pressure in the head area (the reaction force of the plastic) due to the positive motion of the mould, sieve plate and filter screen in the head.The "backflow", also known as "backpressure flow", is formed from the head to the feeding port.It can cause loss of productive capacity.

3) transverse flow -- it is a plastic flow along the direction of the axis, which is perpendicular to the thread groove.It is also formed by the extrusion of the screw.The flow resistance by thread slot wall, due to the mutual resistance on both sides of the screw, the screw is in the rotation, make plastic to produce reverse movement in the spiral groove, a circular flow, so the cross flow essence is circulation.The circulation of plastic in the cylinder of mixing, plastic melt state, is and the circulation of the function inseparable.The circulation causes the material to be stirred and mixed in the cylinder, which is beneficial to the heat exchange between the cylinder and the material. It is of great significance to improve the extrusion quality, but it has little effect on the extrusion flow rate.

4) leakage flow -- it is also caused by the resistance of mould, sieve plate and screen in the machine head.However, it is not the flow in the spiral groove, but the backflow formed in the gap between the screw and the cylinder.It can also cause loss of productivity.Since the gap between the screw and the cylinder is usually very small, the leakage flow in normal circumstances is much smaller than the positive and backward flow.In the extrusion process, the leakage flow will affect the extrusion volume, the leakage flow will increase, the extrusion quantity will be reduced.

The four flow states of plastic do not appear in a single form, and there is no real reflux or closed circulation in a plastic particle.The actual flow of melt plastic in the thread groove is the synthesis of the above four flow states, and a flow with the spiral trajectory forward.

shear strain rate is as follows:

Among them: Δ - for the shear strain rate (1 / min)

D -- for screw diameter (cm)

N - for screw speed (r/min)

-- for spiral groove depth (cm)

It can be seen that, under the requirement of ensuring the extrusion quantity, the depth of the spiral groove can be increased under the condition of increasing the speed.In addition, the gap between the screw and the cylinder also has an effect on the extrusion quality. When the gap is too large, the backflow and leakage flow of the plastic are increased, which not only causes the pressure fluctuation, but also affects the extrusion quantity.And because of the increase of these reflux, the plastic overheats and leads to plastic coke or molding difficulties.

2. Operation procedures of plastic extruder.

The plastic extruder is composed of extruding machine (host) and multiple auxiliary equipment, and the production crew should cooperate closely. The operator must be familiar with the growth process and operating procedures.

1. Extrusion process of plastic extruder.

Plastic extruder is a hot extrusion equipment.The cable or cable in the disk is placed on the line device, and it is guaranteed to have certain tension. After tensioning the straightening device, it enters the extruder insulation layer or sheath layer.

Plastic particles through the hopper to join the extrusion machine machine barrel, due to the rotation of the screw, the chamber into the machine, on the one hand, on the one hand by the screw rotation mixing, heating plastic plasticizing, and into the nose, from die extrusion, complete close continuous extrusion packets on the wire or cable core cable core.

In order to control the thickness and extrusion pressure of the plastic layer, the annular spacing between the mould core and the mould sleeve should be adjusted so that the plastic layer is even.

Each unit in the unit adopts single transmission, and the working speed of each unit can be adjusted separately.The speed of the screw and the traction should be coordinated with each other to ensure the uniform thickness of the outer diameter and plastic layer of the wire and cable, and meet the requirements of the process size.Line and line speed should be combined with the production speed of wire and cable to prevent other quality problems.

As stipulated in the process of controlling temperature, selecting suitable mould, often observe the change of heating system, the change of outside diameter, the change of speed, prevent the eccentricity of the plastic layer, coke burning and plasticizing the bad phenomena appear.

The production cable can be divided into more than 50 varieties and subdivided into 1000 specifications.All products have passed national certification, such as China compulsory certification, bv certification, Nigerian SONCAP certificate, China national industrial certification, etc. China national industrial production license, etc.It also has the ability to produce products that meet international standards, such as iec, ce, rohs, etc.
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