Plastic extruder refers to the machinery used for plastic processing and forming. The main machine of plastic extruder is the extruder, which consists of extrusion system, transmission system and heating and cooling system.

1. Extrusion system The extrusion system consists of a screw, a barrel, a hopper, a head, and a mold. The plastic is plasticized into a uniform melt by an extrusion system and is continuously compressed by the screw under the pressure established during this process. Extruder head.

(1) Screw: It is the most important part of the extruder. It is directly related to the application range and productivity of the extruder. It is made of high-strength, corrosion-resistant alloy steel.

(2) Barrel: It is a metal cylinder, generally made of composite steel pipe with high heat resistance, high compressive strength, strong wear-resisting, corrosion-resistant alloy steel or inner alloy steel. The barrel and the screw cooperate to realize the pulverization, softening, melting, plasticizing, exhausting and compacting of the plastic, and continuously and uniformly conveying the rubber material to the forming system. The length of the barrel is generally 15 to 30 times of the diameter. Plastics are fully heated and fully plasticized.

(3) Hopper: The bottom of the hopper is equipped with a cut-off device to adjust and cut off the material flow. The side of the hopper is equipped with a sight hole and a calibration metering device.

(4) Machine head and die: The machine head is composed of an inner casing made of alloy steel and a carbon steel jacket. The machine head is equipped with a molding die. The function of the machine head is to convert the rotating plastic melt into parallel linear motion, which is even and stable. Into the mold set, and to give the plastic with the necessary molding pressure, the plastic in the barrel plasticized and compacted, through the porous filter plate along the head through the head neck into the head molding die, the mold core mold fit, An annular gap with decreasing cross-section is formed so that the plastic melt forms a continuous dense tubular coating around the core wire. In order to ensure that the plastic flow path in the head is reasonable and the dead corner of the accumulated plastic is eliminated, a diverter sleeve is often installed. In order to eliminate pressure fluctuations when the plastic is extruded, a pressure equalizing ring is also provided. The head is also equipped with mold correction and adjustment device to facilitate adjustment and correction of the concentricity of the mold core and mold sleeve.

The extruder is divided into an angle head (angle 120o) and a right angle head according to the angle between the head stream direction and the screw center line. The shell of the machine head is fixed on the fuselage by bolts. The mould in the machine head has a core seat and is fixed on the inlet port of the machine head with a nut. The front of the core seat is equipped with a mould core, a mould core and a mould core seat. There is a hole in the center for passing through the core wire; a pressure equalizing ring is arranged in the front of the machine head to equalize the pressure; the extrusion molding part is composed of a mold housing and a mold sleeve; the position of the mold sleeve can be adjusted by the bolt through the support In order to adjust the relative position of the die set to the core, it is convenient to adjust the thickness of the extruded layer. The outside of the nose is equipped with a heating device and a temperature measuring device.

2. Drive system The role of the drive system is to drive the screw. The torque and speed required by the feed screw in the extrusion process are usually composed of a motor, a reducer, and a bearing.

3. Heating and cooling device Heating and cooling are necessary conditions for the plastic extrusion process.

(1) Nowadays, electric heating is usually used in the extruder, which is divided into resistance heating and induction heating. The heating plate is mounted on the fuselage, neck, and nose. The heating device heats the plastic in the cylinder from the outside to raise the temperature so as to reach the temperature required for the process operation.

(2) The cooling device is set to ensure that the plastic is in the temperature range required by the process. Specifically, it is to eliminate the excess heat generated by the shear friction of the screw rotation so as to prevent the plastic from decomposing, scorching, or setting difficulties due to excessive temperature. The barrel cooling is divided into water cooling and air cooling. Generally, small and medium-sized extruders are suitable for air cooling. Larger ones are mostly water-cooled or combined with two types of cooling. The screw cooling is mainly center-cooled to increase the material solids transfer rate. , Stabilize the amount of plastic, while improving product quality; but in the hopper at the cooling, first, in order to strengthen the solid material transport role, to prevent the plastic pellets due to temperature rise stick to obstruct the material, and second, to ensure the normal operation of the transmission part.

Second, auxiliary equipment "target = _blank> auxiliary equipment:

The auxiliary machines of the plastic extruder [1] group mainly include pay-off devices, straightening devices, preheating devices, cooling devices, traction devices, meter counters, spark testers, and take-up devices. Extruder units are used for different purposes and the auxiliary equipment used for their selection is not the same, such as disconnectors, blowers, and printing devices.

Straightening device: The most common type of plastic extrusion waste type is eccentricity, and bending of various types of wire core is one of the important reasons for insulation eccentricity. In sheath extrusion, scratches on the jacket surface are also often caused by the bending of the cable core. Therefore, the straighteners in various extrusion units are indispensable. The main types of straightening devices are: drum type (divided into horizontal type and vertical type); pulley type (divided into single pulleys and pulley sets); twisted wheel type, which also functions as dragging, straightening, and stabilizing tension; Pneumatic wheel type (horizontal and vertical type) etc.

Preheating device: Core preheating is necessary for insulation extrusion and sheath extrusion. For the insulation layer, especially the thin layer insulation, the presence of pores cannot be allowed. The preheating of the core before high temperature can completely remove the moisture and oil on the surface. For sheath extrusion, its main role is to dry the core, preventing the possibility of air pockets in the sheath due to moisture (or moisture around the mat). Preheating can also prevent the residual pressure of the plastic from escaping during extrusion. In the process of squeezing plastic, preheating can eliminate the temperature difference between the cold line entering the high-temperature machine head and contact with the plastic at the die mouth, avoiding fluctuation of the plastic temperature and fluctuating the extrusion pressure, thereby stabilizing the extrusion volume and ensuring Extrusion quality. Extruder unit uses electric heating core preheating device, which requires sufficient capacity and guarantees rapid temperature rise, so that core preheating and core drying efficiency are high. The preheating temperature is restricted by the line speed, and it is generally similar to the head temperature.

Cooling device: After the molded plastic extruded layer leaves the machine head, it should be immediately cooled and set, otherwise it will deform under the action of gravity. The cooling method is usually water-cooled and divided into quench and slow cooling depending on the water temperature. The quenching is the direct cooling of the cold water. The quenching is beneficial to the shaping of the extruded plastic layer. However, for the crystalline polymer, due to sudden cooling, internal stress is easily left in the extruded layer structure, resulting in cracks during use. Generally, PVC is used. The plastic layer is quenched. Slow cooling is to reduce the internal stress of the product. In the cooling water tank, water is placed at different temperatures in sections, so that the product gradually cools and stereotypes. The extrusion of PE and PP is performed by slow cooling, that is, through hot water, warm water, and cold water. Three stages of cooling.

Third, the control system:

The control system of the plastic extruder includes a heating system, a cooling system, and a process parameter measurement system, and is mainly composed of electrical appliances, meters, and actuators (ie, control screens and consoles). Its main functions are: controlling and adjusting the drive motor of main and auxiliary engines, outputting the speed and power that meet the technological requirements, and enabling the main and auxiliary machines to work in coordination; detecting and regulating the temperature, pressure, and flow rate of the plastic in the extruder; Control or automatic control of the entire unit.

The electrical control of the extrusion unit can be broadly divided into two major parts, drive control and temperature control, to achieve the extrusion process including temperature, pressure, screw rotation, screw cooling, barrel cooling, product cooling and outside diameter control, and traction speed , neatly arranges the line and guarantees the constant tension take-up control from the empty plate to the full plate on the take-up reel.

1. Temperature control of the extruder host

Plastic extrusion of wire and cable insulation and jackets is based on the deformation characteristics of thermoplastics, making them in a viscous fluid state. In addition to external heating of the screw and barrel, it is transmitted to the plastic to melt it out, but also to consider the screw's own heat when extruding the plastic. Therefore, the temperature of the host should be considered from the whole. It is necessary to consider not only the heating of the heater but also the Off, it is necessary to take into account the factors of the extrusion heat of the screw to be cooled, and there must be effective cooling facilities. It is required to determine the location and installation method of the thermocouple of the measuring element correctly and reasonably, and to accurately reflect the actual temperature of each section of the host from the temperature control meter reading. As well as requiring that the accuracy of the temperature-controlled instrument match well with the system, the fluctuation stability of the entire host temperature control system can meet the requirements of various plastic extrusion temperatures.

2. Pressure control of the extruder

In order to reflect the extrusion of the head, it is necessary to detect the head pressure during extrusion. Since the domestic extruder does not have a head pressure sensor, the measurement of the thrust after the screw extrusion is generally used instead of the head pressure measurement. The screw load table (Ammeter or voltmeter) can correctly reflect the size of the extrusion pressure. The fluctuation of the extrusion pressure is also one of the important factors causing the instability of the extrusion quality. The fluctuation of the extrusion pressure is closely related to the extrusion temperature, the use of the cooling device, and the length of the continuous operation time. When an abnormal phenomenon occurs, the rapid elimination that can be ruled out and the reorganization of production must be decisively stopped, which not only avoids the increase of waste products, but also prevents the occurrence of accidents. Through the pressure gauge readings, the pressure state of the plastic during extrusion can be known. Generally, the back pressure limit value is used to control the alarm.

3. Screw speed control

The adjustment and stabilization of the screw speed is one of the important process requirements of the host drive. The screw speed directly determines the glue volume and the extrusion speed. The normal production always hopes to achieve the highest speed and achieve high output. When the screw speed of the extruder is required from the start to the required working speed, the available speed range is larger. . Moreover, the stability of the rotational speed is required to be high, because the fluctuation of the rotational speed will cause the fluctuation of the extrusion volume and affect the extrusion quality. Therefore, under the condition that the pull line speed does not change, the external diameter of the cable will change. Similarly, if the line speed of the traction device fluctuates greatly, the external diameter of the cable will also change. The speed of the screw and the traction line can be reflected by the corresponding instruments on the operation table, and the extrusion should be closely observed to ensure high quality and high output.

4. Outer diameter control

As mentioned above, in order to ensure the dimension of the outer diameter of the cable of the product, in addition to the dimensional tolerance of the control wire core (cable core), the extrusion temperature, the screw rotation speed, and the line speed of the traction device should be controlled and guaranteed. The measurement control comprehensively reflects the accuracy and level of the above control. In the extrusion unit equipment, especially on the high-speed extrusion production line, an on-line OD detector shall be used to detect the outside diameter of the cable at any time, and the over-difference signal shall be fed back to adjust the rotation speed of the traction or screw to correct the outside diameter. Very bad.

5. Tension control required for winding

In order to ensure the line take-up at different line speeds, from the constant tension requirement of the empty plate to the full plate, it is hoped that the line tension device will be provided for the line device, or that the constant line speed system and the constant tension system will be taken into consideration in electrical terms. and many more.

6. The complete electrical automation control

This is the process control requirements for realizing high-speed extrusion lines. The main requirements are: start-up temperature interlocking; work pressure protection and interlocking; proportional and synchronous control of extrusion and traction of two major components; synchronous control of take-up and traction; Outer diameter on-line detection and feedback control; according to a variety of different needs of the components of the single machine and machine tracking control.

Fourth, the extruder's classification:

Plastic extruders are divided into twin screw extruders and single screw extruders

The difference between the two extruders:

Single-screw machines and twin-screw machines: one is a screw and one is two screws. They are all driven by a single motor. The power varies depending on the screw. The power of 50 cones is about 20KW, about 65 For 37KW, the output is related to material and screw. The output of 50 cones is about 100-150KG/H, and the cone of 65 is about 200-280KG/H. The output of single screw is only half.

The extruder can be divided into single-screw, twin-screw and multi-screw extruders according to the number of its screws. At present, the single-screw extruder is the most widely used and suitable for extrusion processing of general materials. The twin-screw extruder has less heat due to friction, more uniform shearing of the material, greater conveying capacity of the screw, relatively stable extrusion volume, long stay of the material in the barrel, and even mixing.

SJSZ series conical twin-screw extruder has the characteristics of forced extrusion, high quality, wide adaptability, long life, low shear rate, material is not easy to decompose, good mixing and plasticization performance, direct molding of powder, etc. Vacuum exhaust and other devices. Applicable to the production of pipes, plates, special-shaped materials and other products.

Single-screw extruders occupies an important position as plastic granulation machinery or molding processing machinery. In recent years, the single-screw extruder has made great progress. At present, the large pelletizing single-screw extruders produced in Germany have a screw diameter of 700 mm and a production volume of 36 t/h.

The main sign of the development of single-screw extruders is the development of its key part, the screw. In recent years, a large number of theoretical and experimental researches on screw have been carried out. Up to now, there have been nearly 100 types of screws, and there are commonly used separation type, shear type, barrier type, shunt type, and wavy type.

From the standpoint of single-screw development, although single-screw extruders have been relatively complete in recent years, with the continuous development of polymer materials and plastic products, more innovative new types of screws and special single-screw extruders will emerge. In general, single-screw extruders are moving toward high speed, high efficiency, and specialization.

Twin-screw extruder has good feeding characteristics and is suitable for powder processing. It has better mixing, venting, reaction and self-cleaning functions than single-screw extruder. It is characterized by poor thermal stability of plastics and blends. When it is expected, it shows its superiority. In recent years, foreign twin-screw extruders have made great progress. Various types of twin-screw extruders have been serialized and commercialized, and there are also many manufacturers. They are roughly classified as follows:

(1) According to the relative positions of two axes, there are parallel and conical points;

(2) According to the two screw engagement procedures, there is a meshing type and a non-meshing type;

(3) According to the direction of rotation of the two screws, there are points in the same direction and in different directions. In the opposite direction, there are points inward and outward; â–¡

(4) According to the screw rotation speed, there are high speed and low speed;

(5) According to the structure of the screw and the barrel, there are integral and combination points.

On the basis of twin-screw extruders, some manufacturers have developed multi-screw extruders such as planetary extruders to make it easier to process poorly thermally stable blends.

Fifth, the mechanical principle of the extruder:

Water or suitable liquid is added to the raw material powder and stirring is continued. The agitated material is extruded from the perforated head or wire mesh with high extrusion pressure.

Usually the material is placed in a cylindrical container and the material is extruded with a screw. After the inverter technology is used, the pressure can be controlled so that the most suitable linear speed can be selected.

Single screw extruder principle

The single screw is generally divided into three sections in the effective length, and the effective length of the three sections is determined according to the screw diameter and pitch screw depth, generally divided by one third.

The last thread of the material mouth is called the conveying section: the material here must not be plasticized, but it must be preheated, pressed and crushed. In the past, the old extrusion theory thought that the material was loose, and later it was proved that the material was actually The solid plug, that is to say the material here after being squeezed, is a solid like a plug, so as long as the completion of the transfer task is its function.

The second section is called the compression section. At this time, the volume of the screw groove gradually becomes smaller, and the temperature must reach the plasticizing degree of the material. Here, the compression is generated by the conveying section three, where it is compressed to one, which is called the compression ratio of the screw. At 3:1, some machines also changed, and the plasticized materials entered the third segment.

The third section is the metering section, where the material maintains the plasticizing temperature, just as the metering pump can accurately and quantitatively convey the melted material to supply the head. At this time, the temperature cannot be lower than the plasticizing temperature, which is generally slightly higher.

Sixth, extruder use note:

1. The extruder must be operated in a forward direction to avoid reversal.

2, avoid fasting operation, must be hot machine feeding operation, this can avoid the occurrence of stick (shaft) phenomenon.

3. It is forbidden to enter the iron or other miscellaneous parts in the feed inlet and vent hole of the extruder to avoid accidents and affecting production.

4, safe electricity, grounding line.

5, when the machine is running, the feed port, discharge port, belt, gear and other rotating parts are forbidden to touch by hand.

6. Lubricate the machine before use to avoid damage to the machine.