The extrusion system includes screw, barrel, hopper, die and die head. Plastic is plasticized into a uniform melt by an extrusion system, and is continuously extruded by a screw under the pressure established in the process. Screw (1): It is the most important part of the extruder, which is directly related to the application scope and productivity of the extruder. It is made of high-strength corrosion-resistant alloy steel. (2) Machine barrel: it is a metal cylinder, generally made of alloy steel with high heat resistance, high compressive strength, strong wear resistance and corrosion resistance or composite steel pipe lined with alloy steel. The barrel cooperates with the screw to crush, soften, melt, plasticize, exhaust and compact the plastic, and continuously and evenly convey the rubber to the molding system. Generally, the length of the barrel is 15 ~ 30 times of its diameter, so that the plastic can be fully heated and filled into the wire extruder.
The principle of plasticization is divided. (3) Hopper: The bottom of the hopper is equipped with a cutting device to adjust and cut off the material flow, and the side of the hopper is equipped with an observation hole and a calibration metering device. (4) Mold: The mold consists of an alloy steel inner sleeve and a carbon steel outer sleeve, and a molding mold is installed in the mold. The function of the mold is to transform the rotating plastic melt into parallel linear motion, introduce it into the mold sleeve evenly and stably, and give the plastic the necessary molding pressure. Plastic is plasticized and compacted in the barrel, and flows into the die from the neck of the die through the porous filter plate through a certain flow channel. The die sleeve is properly matched to form an annular gap with gradually decreasing cross section, so that the plastic melt can be in the core. In order to ensure the reasonable plastic flow channel in the mold and eliminate the dead angle of plastic accumulation, a shunt sleeve and a pressure equalizing ring are often installed to eliminate the pressure fluctuation during plastic extrusion. The die head is also equipped with a device for correcting and adjusting the die, which is convenient for adjusting and correcting the concentricity of the die core and the die sleeve. According to the angle between the material flow direction of the die and the center line of the screw, the extruder can divide the die into oblique die (angle 120o) and right-angle die. The shell of the machine head is fixed on the machine body through bolts, and the die in the machine head has a die core seat, which is fixed on the wire inlet of the machine head through nuts. The mold core is installed in front of the mold core seat, and the centers of the mold core and the mold core seat are provided with holes for the core wire to pass through; An equalizing ring is installed in front of the machine head to equalize the pressure; The extrusion molding part consists of a die sleeve seat and a die sleeve, and the position of the die sleeve can be adjusted through bolt support, so that the relative position of the die sleeve and the die core can be adjusted, and the thickness uniformity of the extrusion layer can be adjusted conveniently. A heating device and a temperature measuring device are installed outside the machine head.
2. Transmission system
The function of the transmission system is to drive the screw and supply the torque and rotation speed required by the screw in the extrusion process, which is usually composed of a motor, a reducer and a bearing.
3. Heating and cooling devices
Heating and cooling are necessary conditions for plastic extrusion process. (1) At present, extruders usually adopt electric heating, which can be divided into resistance heating and induction heating. The heating plate is installed in the extruder of the fuselage mold.
, neck, nose. The heating device heats the plastic in the cylinder from the outside to raise the temperature to the temperature required by the process operation. (2) Set a cooling device to ensure that the plastic is within the temperature range required by the process. Specifically, it is to eliminate the excess heat generated by the rotating shear friction of the screw, and to avoid the decomposition, scorching or difficult setting of plastics caused by too high temperature. Barrel cooling can be divided into water cooling and air cooling. Generally speaking, air cooling is more suitable for small and medium-sized extruders, and water cooling or a combination of the two forms is used for large extruders. Screw cooling mainly adopts central water cooling, aiming at increasing the solid conveying rate of materials, stabilizing the glue output and improving the product quality; However, the cooling at the hopper is to strengthen the conveying function of solid materials, prevent plastic particles from sticking and blocking the material opening due to temperature rise, and ensure the normal operation of the transmission part.
Edit this paragraph II. ancillary equipment
Auxiliary machines of plastic extrusion unit mainly include pay-off device, straightening device, preheating device, cooling device, traction device, meter, spark testing machine and take-up device. The use of extrusion unit is different, and the auxiliary equipment selected is also different, such as cutter, dryer, printing device, etc.
straightening machine
One of the most common plastic extrusion wastes is eccentric, and various types of wire cores are bent to produce insulated eccentric extruders.
One of the important causes of the heart. In the process of sheath extrusion, the scratch on the surface of sheath is often caused by the bending of cable core. Therefore, straightening devices in each extrusion unit are essential. The main types of straightening devices are: drum type (divided into horizontal and vertical); Pulley type (divided into single pulley and pulley block); The twisted wheel has many functions such as dragging, straightening and stabilizing tension. Press wheel (divided into horizontal and vertical), etc.
Preheating device
The preheating of cable core is necessary for insulation extrusion and sheath extrusion. For the insulation layer, especially the thin insulation layer, no air holes are allowed, and the water and oil stains on the surface of the wire core can be completely removed by preheating at high temperature before extrusion. For sheath extrusion, its main function is to dry the cable core and prevent the possibility of air holes in the sheath due to moisture (or moisture around the cladding). Preheating can also prevent the residual internal pressure caused by quenching in the extrusion process of plastics. In the process of plastic extrusion, preheating can eliminate the wide temperature difference formed when the cold wire enters the high-temperature die and contacts with the plastic at the die mouth, and avoid the extrusion pressure fluctuation caused by the plastic temperature fluctuation, thus stabilizing the extrusion quantity and ensuring the extrusion quality. The extrusion unit adopts electric heating wire core preheating device, which requires sufficient capacity, fast temperature rise, high wire core preheating and cable core drying efficiency. The preheating temperature is limited by the linear velocity, which is generally similar to the head temperature.
cooling device
The molded plastic extrusion coating should be cooled immediately after leaving the die, otherwise it will be extruded under the action of gravity.
Deformation. The cooling method is usually water cooling, which is divided into quick cooling and slow cooling according to the different water temperatures. Quenching is the direct cooling of cold water, which is beneficial to the forming of plastic extrusion coating. However, for crystalline polymers, due to sudden thermal cooling, internal stress is easy to remain in the extruded coating, which leads to cracks in the use process. Usually, the PVC plastic layer is quenched. Slow cooling is to reduce the internal stress of the product. Put the moisture at different temperatures into the cooling tank, so that the product can be gradually cooled and shaped. The extrusion of PE and PP adopts slow cooling mode, that is, after three stages of cooling: hot water, warm water and cold water.
Edit the third paragraph. control system
The control system of plastic extruder includes heating system, cooling system and process parameter measurement system, which is mainly composed of electrical appliances, instruments and actuators (namely control panel and operation console). Its main functions are: to control and adjust the traction motors of the main and auxiliary machines, output the rotating speed and power that meet the technological requirements, and make the main and auxiliary machines work in harmony; Detect and adjust the temperature, pressure and flow of plastic in the extruder; Realize the control or automatic control of the whole unit. The electrical control of extrusion unit can be roughly divided into two parts: transmission control and temperature control. The extrusion process includes temperature, pressure, screw revolution, screw cooling, barrel cooling, product cooling and outer diameter control, as well as traction speed, orderly arrangement of wires and constant tension take-up control from empty disk to full disk.
1. Temperature control of extruder main engine
The plastic extrusion of wire and cable insulation and sheath is carried out according to the deformation characteristics of thermoplastic, which makes it in a viscous state. In addition to the external heating of the screw and the barrel, it is required to melt and extrude the plastic, and the heat generated when the screw extrudes the plastic should also be considered. Therefore, it is required to consider the temperature of the main engine as a whole, not only to turn on and off the heating of the heater, but also to consider the factors of heat overflow from screw extrusion for cooling, and to provide effective cooling facilities. It is also required to correctly and reasonably determine the position and installation method of the measuring element thermocouple, which can accurately reflect the actual temperature of each section of the main engine from the reading of the temperature control instrument. It is also required that the precision of the temperature control instrument should be well matched with the system, so that the fluctuation stability of the whole host temperature control system can meet the requirements of various plastic extrusion temperatures.
2. Pressure control of extruder
In order to reflect the extrusion situation of the die, it is necessary to detect the die pressure during extrusion. Because there is no die pressure sensor in domestic extruders, the measurement of die pressure is usually replaced by the measurement of screw thrust after extrusion, and the screw load table (ammeter or voltmeter) can correctly reflect the extrusion pressure. The fluctuation of extrusion pressure is also one of the important factors leading to the instability of extrusion quality. The fluctuation of extrusion pressure is closely related to extrusion temperature, the use of cooling device and the length of continuous operation. When abnormal phenomena occur, they can be quickly eliminated. If it is necessary to reorganize production, it should be stopped decisively, which can not only avoid the increase of waste products, but also prevent accidents. The pressure state of plastic in the extrusion process can be known by detecting the reading of pressure gauge, and the back thrust limit value is generally taken for alarm control.
3. Control of screw speed
The adjustment and stability of screw speed is one of the important technical requirements of main engine transmission. The screw speed directly determines the glue output and extrusion speed. Normal production always wants to achieve the highest speed and high yield as much as possible. When the screw speed of the extruder needs to be started to the required working speed, the available speed regulation range is larger. Moreover, the stability of rotation speed is required, because the fluctuation of rotation speed will lead to the fluctuation of extrusion quantity and affect the extrusion quality, so the outer diameter of the cable will change under the condition of constant traction linear speed. Similarly, if the linear speed of the traction device fluctuates greatly, the outer diameter of the cable will also change. The linear speed of screw and traction can be reflected by the corresponding instruments on the operating table, and should be closely observed during extrusion to ensure high quality and high yield.
4. Outside diameter control
As mentioned above, in order to ensure the cable outer diameter size of the product, in addition to controlling the dimensional tolerance of the cable core, the extrusion temperature, screw speed and linear speed of the traction device should also be controlled and ensured. The measurement and control of the outer diameter comprehensively reflect the accuracy and level of the above control. In the equipment of the extrusion unit, especially in the high-speed extrusion production line, an on-line outer diameter detector should be equipped to detect the outer diameter of the cable at any time, feed back the out-of-tolerance signal, adjust the traction or screw speed, and correct the out-of-tolerance of the outer diameter.
5. Tension control required for winding.
In order to ensure the constant tension requirements of winding at different linear speeds, it is hoped that the winding device will have a wire storage tension adjustment mechanism from empty drum to full drum, or the winding of constant linear speed system and constant tension system will be considered electrically.
6. Electrical automation control of the whole machine
This is the process control requirement of realizing high-speed extrusion production line, which mainly includes: starting temperature interlock; Working pressure protection and interlocking; Proportional synchronous control of extrusion and traction drive: synchronous control of take-up and traction; On-line detection and feedback control of outer diameter; According to different needs, the control of single machine and whole machine tracking consists of components.