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With China Plastic Machinery
With China Plastic Machinery
The twin-screw extruder is developed on the basis of the single-screw extruder. Due to its good feeding performance, mixing and plasticizing performance, exhaust performance, extrusion stability and other characteristics, it has been widely used in the molding processing of extruded products.
The twin-screw extruder consists of several parts such as the transmission device, feeding device, barrel and screw. The functions of each component are similar to those of the single-screw extruder. Its structure is shown in Figure 1. The difference from the single-screw extruder is that there are two parallel screws in the twin-screw extruder placed in the "¡Þ"-shaped cross-section material cylinder.
The twin-screw extruder used for profile extrusion is usually tightly meshed and rotates in opposite directions. Although a few also use co-rotating twin-screw extruders, they are generally operated at a relatively low screw speed of about 10 r/min. High-speed meshing co-rotating twin-screw extruders are used for compounding, exhaust or as continuous chemical reactors. The maximum screw speed range of this type of extruder is 300-600r/min. Non-intermeshing extruders are used for mixing, venting and chemical reactions. Their conveying mechanism is quite different from that of intermeshing extruders and is closer to that of single-screw extruders, although there are essential differences between the two.
From the perspective of the principle of motion, twin-screw extruders are different in terms of co-rotating meshing, counter-rotating meshing and non-intermeshing.
1. Co-rotating Meshing Twin-Screw Extruder
This type of extruder has two types: low-speed and high-speed. The former is mainly used for profile extrusion, while the latter is used for special polymer processing operations.
(1) Closely intermeshing extruder. Low-speed extruders have a closely intermeshing screw geometry, in which the flight shape of one screw closely matches the flight shape of the other screw, i.e., a conjugate screw shape.
(2) Self-cleaning extruder. High-speed co-rotating extruders have closely matched flight shapes. This screw can be designed to have a relatively small screw gap, so that the screw has a closed self-cleaning effect. This twin-screw extruder is called a close self-cleaning co-rotating twin-screw extruder.
2. Counter-rotating twin-screw extruder
The gap between the two screw grooves of the close-meshing counter-rotating twin-screw extruder is very small (much smaller than the gap in the co-rotating twin-screw extruder), so it can achieve positive conveying characteristics.
3. Non-meshing twin-screw extruder"}]}]},
The center distance between the two screws of the non-meshing twin-screw extruder is greater than the sum of the two screw radii.
Since it is easy to open, the degree of wear of the threaded components and the barrel liner can be found at any time, so that effective repair or replacement can be carried out. It is not necessary to find out when there is a problem with the extruded product, causing unnecessary waste.
When manufacturing masterbatch, it is often necessary to change the color. If it is necessary to change the product, the open processing area can be opened within a few minutes. In addition, the melt profile on the entire screw can be observed. Analyze the mixing process. At present, ordinary twin-screw extruders need to use a large amount of cleaning materials to clean the machine when changing colors, which is time-consuming, power-consuming, and wasteful of raw materials. The split twin-screw extruder can solve this problem. When changing colors, it only takes a few minutes to quickly open the barrel and perform manual cleaning. This way, no or less cleaning materials are needed, saving costs.
When repairing equipment, ordinary twin-screw extruders often have to remove the heating and cooling systems first, and then pull out the screw as a whole. The split twin-screw does not need this. Just loosen a few bolts, turn the worm gear box handle device to lift the upper half of the barrel, and then you can open the entire barrel for repair. This not only shortens the maintenance time, but also Reduced labor intensity.
At present, the development trend of twin-screw extruders in the world is towards high torque, high speed and low energy consumption. The effect of high speed is high productivity. The split twin-screw extruder belongs to this category, and its speed can reach 500 rpm. Therefore, it has unique advantages in processing high-viscosity and heat-sensitive materials.
In the core technology of high speed and high torque, only German and Japanese manufacturers currently master the core technology of asymmetric and symmetrical high-torque gearboxes, and the maximum speed can reach more than 1,800 rpm. In China, only Sichuan Zhongzhuang Technology has mastered this core technology, and it is currently one of the main choices for domestic high-end material processing manufacturers. 1. It is a nationally encouraged project for domestic independent innovation.
Wide application range, applicable to the processing of a variety of materials
It has other advantages of ordinary twin-screw extruders, and can achieve high output, high quality and high efficiency.
The difference between twin-screw extruder and single-screw extruder is mainly reflected in the following two aspects.
In a single-screw extruder, friction drag is used in the solid conveying section, and viscous drag is used in the melt conveying section. The friction properties of solid materials and the viscosity of molten materials determine the conveying behavior. If some materials have poor friction properties, it is difficult to feed the materials into a single-screw extruder if the feeding problem is not solved. In twin-screw extruders, especially intermeshing twin-screw extruders, the material is conveyed to some extent by positive displacement, and the degree of positive displacement depends on the proximity of the flight of one screw to the relative groove of the other screw. The screw geometry of the tightly intermeshing counter-rotating extruder can obtain a high degree of positive displacement conveying characteristics.
The flow velocity distribution of materials in single-screw extruders has been described quite clearly, while the flow velocity distribution of materials in twin-screw extruders is quite complex and difficult to describe. Many researchers simply analyze the flow velocity field of materials without considering the material flow in the meshing zone, but these analysis results are very different from the actual situation. Because the mixing characteristics and overall behavior of twin-screw extruders mainly depend on the leakage flow occurring in the meshing zone, and the flow situation in the meshing zone is quite complex. The complex flow spectrum of materials in twin-screw extruders shows advantages that single-screw extruders cannot match on a macro scale, such as sufficient mixing, good heat transfer, large melting capacity, strong exhaust capacity and good material temperature control.
1. Granulation of glass fiber reinforced and flame retardant materials (such as PA6, PA66, PET, PBT, PP. PC reinforced flame retardant, etc.)
2. Granulation of high filler materials (such as PE, PP filled with 75% CaCO. )
3. Granulation of heat-sensitive materials (such as PVC, XLPE cable materials)
4. Concentrated masterbatch (such as: filled with 50% color powder)
5. Anti-static masterbatch, alloy, coloring, low-filling blending granulation
6. Granulation of cable materials (such as: sheath materials, insulation materials)
7. Granulation of XLPE pipe materials (such as: masterbatch for hot water cross-linking)
8. Mixing and extrusion of thermosetting plastics (such as: phenolic resin, epoxy resin, powder coating)
9. Hot melt adhesive, PU reaction extrusion granulation (such as: EVA hot melt adhesive, polyurethane)
10. K resin, SBS devolatilization granulation
The most common type of plastic extrusion waste is eccentricity, and various types of bending of the core are one of the important reasons for insulation eccentricity. In the sheath extrusion, the scratches on the sheath surface are often caused by the bending of the cable core. Therefore, the straightening device in various extruders is indispensable. The main types of straightening devices are: roller type (divided into horizontal and vertical types); pulley type (divided into single pulley and pulley group); winch type, which plays multiple roles such as dragging, straightening, and stabilizing tension; pressure wheel type (divided into horizontal and vertical types), etc.
Cable core preheating is necessary for both insulation extrusion and sheath extrusion. For the insulation layer, especially the thin layer insulation, the existence of pores cannot be allowed. The wire core can be completely cleaned of surface moisture and oil by high temperature preheating before extrusion. For sheath extrusion, its main function is to dry the cable core to prevent the possibility of pores in the sheath due to moisture (or moisture in the wrapping cushion layer). Preheating can also prevent the residual internal pressure of the plastic due to sudden cooling during extrusion. In the process of extruding plastic, preheating can eliminate the large temperature difference formed when the cold wire enters the high-temperature die and contacts the plastic at the die mouth, avoid the fluctuation of plastic temperature and cause the fluctuation of extrusion pressure, so as to stabilize the extrusion volume and ensure the extrusion quality. Electric heating wire core preheating device is used in the extrusion unit, which requires sufficient capacity and rapid temperature rise to make the wire core preheating and cable core drying efficient. The preheating temperature is restricted by the wire release speed and is generally similar to the die temperature.
The formed plastic extrusion layer should be cooled and shaped immediately after leaving the die, otherwise it will deform under the action of gravity. The cooling method usually adopts water cooling, and it is divided into rapid cooling and slow cooling according to the water temperature. Rapid cooling is direct cooling with cold water. Rapid cooling is beneficial to the shaping of the plastic extrusion layer, but for crystalline polymers, due to sudden heating and cooling, it is easy to leave internal stress inside the extrusion layer, resulting in cracking during use. Generally, PVC plastic layer adopts rapid cooling. Slow cooling is to reduce the internal stress of the product. Water of different temperatures is placed in the cooling water tank in sections to gradually cool down the product and set the shape. Slow cooling is used for the extrusion of PE and PP, that is, hot water, warm water and cold water are used for cooling.
1. After 500 hours of use, there will be iron filings or other impurities in the gearbox. Therefore, the gears should be cleaned and the lubricating oil of the gearbox should be replaced.
2. After using it for a period of time, the extruder should be fully inspected to check the tightness of all screws.
3. If there is a sudden power outage during production, the main drive and heating stop. When the power is restored, the barrel sections must be reheated to the specified temperature and kept warm for a period of time before the extruder can be started.
4. If the full degree of the instrument and pointer is found, the contact of the thermocouple and other side lines should be checked to see if they are in good condition.
1. Structural principles
For the basic mechanism of the extrusion process, simply put, a screw rotates in the barrel and pushes the plastic forward. The screw structure is an inclined plane or slope wrapped around the center layer, the purpose of which is to increase pressure in order to overcome greater resistance. As far as the extruder is concerned, there are three kinds of resistance that need to be overcome during operation: one is friction, which includes the friction of solid particles (feed) on the barrel wall and the mutual friction between them during the first few turns of the screw (feed zone); the second is the adhesion of the melt on the barrel wall; the third is the internal logistics resistance of the melt when it is pushed forward.
According to Newton's theorem, if an object is stationary in a certain direction, then the object is in a state of force balance in this direction. For a screw that moves in a circumferential direction, it has no axial movement, that is, the axial force on the screw is in a state of balance. So if the screw applies a large forward thrust to the plastic melt, it also applies a thrust of the same magnitude but in the opposite direction to another object. Obviously, the thrust it applies is acting on the thrust bearing behind the feed port. Most single screws have right-handed threads. If you look at them from the back, they rotate in opposite directions. They rotate backward out of the barrel through rotational motion. In some twin-screw extruders, the two screws rotate in opposite directions in the two barrels and cross each other, so one must be right-handed and the other left-handed. For interlocking twin screws, the two screws rotate in the same direction and must have the same orientation. However, in either case, there is a thrust bearing that bears the backward force, which still conforms to Newton's theorem.
2. Temperature principle
Extrudable plastics are hot plastics. They melt when heated and solidify again when cooled. Therefore, heat is required during the extrusion process to ensure that the plastic can reach the melting temperature. So where does the heat for melting plastic come from? First of all, the feed preheating and barrel/die heater may play a role and are very important at startup. In addition, the motor input energy, that is, the friction heat generated in the barrel when the motor overcomes the resistance of the viscous melt to rotate the screw, is also the most important heat source for all plastics, except for small systems, low-speed screws, high melt temperature plastics and extrusion coating applications. In operation, it is important to realize that the barrel heater is not actually the main heat source, and its effect on extrusion may be smaller than we expect. The rear barrel temperature is more important because it affects the speed of solids conveying in the teeth or feed. In general, except for some specific purposes (such as glazing, fluid distribution or pressure control), the die head and mold temperature should reach the required melt temperature or close to this temperature.
3. Speed reduction principle
In most extruders, the change of screw speed is achieved by adjusting the motor speed. The drive motor usually rotates at a full speed of about 1750rpm, which is too fast for an extruder screw. If it were to run at such a high speed, too much frictional heat would be generated and a uniform, well-mixed melt would not be prepared because the residence time of the plastic would be too short. Typical reduction ratios should be between 10:1 and 20:1. The first stage can be either gears or pulleys, but it is better to use gears for the second stage and position the screw at the center of the last large gear. For some slow-running machines (such as twin screws for UPVC), there may be three reduction stages and the maximum speed may be as low as 30rpm or less (ratios of 60:1). On the other hand, some very long twin screws used for mixing can run at 600rpm or more, so a very low reduction ratio and more deep cooling are required. If the reduction ratio is not matched to the work, too much energy will be wasted. It may be necessary to add a pulley between the motor and the first reduction stage to change the maximum speed, which either increases the screw speed to even exceed the previous limit, or reduces the maximum speed. This can increase the available energy, reduce the current value and avoid motor failure. In both cases, the output may increase due to the material and its cooling needs.
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