Twin screw extruderThe twin-screw extruder is developed based on 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.
Structure and type
The twin-screw extruder consists of several parts, such as a transmission device, a feeding device, a barrel and a screw. The functions of each part are similar to those of a single-screw extruder. Its structure is shown in the “Structural Schematic Diagram of a Twin-Screw Extruder”. The difference from a single-screw extruder is that a twin-screw extruder has two parallel screws placed in a barrel with an “∞” shaped cross section.
Twin-screw extruders used for profile extrusion are usually tightly meshed and counter-rotating, although a few use co-rotating twin screws. They are generally operated at relatively low screw speeds, around 10 r/min. High-speed intermeshing co-rotating twin screw extruders are used for compounding, venting, or as continuous chemical reactors. The maximum screw speed of these extruders ranges from 300-600 r/min. Non-meshing extruders are used for mixing, venting, and chemical reactions. Their conveying mechanism is very different from that of intermeshing extruders and is closer to that of single-screw extruders, although there are essential differences between the two.
Working principle
From the perspective of movement principle, the co-rotating meshing, counter-rotating meshing and non-meshing types in twin-screw extruders are different.
1. Co-rotating intermeshing twin screw extruder
These extruders are available in both low-speed and high-speed versions, the former primarily used for profile extrusion and the latter for specialty polymer processing operations.
(1) Closely meshing extruder. Low-speed extruders have a closely meshing 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 screw flight shapes. This type of screw can be designed to have a relatively small screw gap, so that the screw has a closed self-cleaning effect. This type of twin-screw extruder is called a tightly self-cleaning co-rotating twin-screw extruder.
2. Counter-rotating intermeshing twin screw extruder
The gap between the two screw grooves of the tightly meshed counter-rotating twin-screw extruder is very small (much smaller than the gap in the same-direction meshing twin-screw extruder), so positive conveying characteristics can be achieved.
3. Non-intermeshing twin-screw extruder
The center distance between the two screws of a non-intermeshing twin-screw extruder is greater than the sum of the two screw radii.
Advantages
Wear
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, so as not to find out the problem when the extruded product has a problem, causing unnecessary waste.
Reduce manufacturing cost
When manufacturing masterbatches, it is often necessary to change colors. If it is necessary to change products, the open processing area can be opened within a few minutes. In addition, the mixing process can be analyzed by observing the melt profile on the entire screw. When changing colors, ordinary twin-screw extruders need to use a large amount of cleaning materials to clean the machine, 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, so that no or less cleaning materials are used, saving costs.
Improve labor efficiency
When repairing the equipment, ordinary twin-screw extruders often need to remove the heating and cooling systems first, and then pull out the screw as a whole. However, this is not necessary for split twin-screw extruders. You only need to loosen a few bolts, turn the worm gear box handle device to lift the upper part of the barrel, and then open the entire barrel for repair. This not only shortens the repair time, but also reduces labor intensity.
High torque, high speed
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. Split twin-screw extruders belong to this category, and their speed can reach 500 rpm. Therefore, they have unique advantages in processing high-viscosity and heat-sensitive materials.
In terms of the core technology of high speed and high torque, only German and Japanese manufacturers have mastered the core technology of asymmetric and symmetric high torque gearboxes, and the maximum speed can reach more than 1,800 revolutions. In China, only Sichuan Zhongzhuang Technology has mastered this core technology, which is also one of the main choices for domestic high-end material processing manufacturers and is a national encouragement project for domestic independent innovation.
Wide range of applications
Wide range of applications, suitable for processing a variety of materials
High yield, high quality
It has other advantages of ordinary twin-screw extruders and can achieve high output, high quality and high efficiency.
The difference
The difference between twin-screw extruder and single-screw extruder is mainly reflected in the following two aspects.
Material delivery method
In a single-screw extruder, the solids conveying section is frictional drag, and the melt conveying section is viscous drag. The frictional properties of the solid material and the viscosity of the molten material determine the conveying behavior. For example, some materials have poor frictional properties and are difficult to feed into a single-screw extruder if the feeding problem is not solved. In twin-screw extruders, especially intermeshing twin -screw extruders, the material conveying is to some extent positive displacement conveying, 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 a tightly intermeshing counter-rotating extruder can achieve a high degree of positive displacement conveying characteristics. [2]
Material flow velocity field
At present, 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 conditions in the meshing zone, but these analysis results are very different from the actual situation. This is because the mixing characteristics and overall behavior of a twin-screw extruder mainly depend on the leakage flow occurring in the meshing zone, and the flow conditions in the meshing zone are quite complex. The complex flow spectrum of materials in a twin-screw extruder exhibits advantages that a single-screw extruder cannot match on a macro scale, such as sufficient mixing, good heat transfer, large melting capacity, strong exhaust capacity, and good material temperature control.
Applications
- Glass fiber reinforcement, flame retardant material granulation (such as: PA6, PA66, PET, PBT, PP. PC reinforced flame retardant, etc.)
- Granulation of high-filling materials (such as PE and PP filled with 75% CaCO.)
- Granulation of heat-sensitive materials (such as PVC, XLPE cable materials)
- Concentrated masterbatch (e.g. filled with 50% color powder)
- Antistatic masterbatch, alloy, coloring, low filling blending granulation
- Granulation of cable materials (such as sheath materials, insulation materials)
- Granulation of XLPE pipe materials (e.g. masterbatch for hot water cross-linking)
- Compounding and extrusion of thermosetting plastics (such as phenolic resin, epoxy resin, powder coating )
- Hot melt adhesive, PU reaction extrusion granulation (such as: EVA hot melt adhesive, polyurethane)
- K resin, SBS devolatilization granulation
Auxiliary equipment
Straightening device
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 sheath extrusion, scratches on the sheath surface are often caused by the bending of the cable core. Therefore, straightening devices in various extrusion units are indispensable. The main types of straightening devices are: drum type (divided into horizontal and vertical types); pulley type (divided into single pulley and pulley group); winch type, which has multiple functions such as dragging, straightening, and stabilizing tension; pressure wheel type (divided into horizontal and vertical types), etc.
Preheating device
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 is not allowed. The surface moisture and oil can be completely removed by high temperature preheating of the core 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 the extrusion pressure caused by the fluctuation of the plastic temperature, thereby stabilizing the extrusion volume and ensuring the extrusion quality. Electric heating core preheating devices are used in extrusion units, which are required to have sufficient capacity and ensure rapid heating, so that the core preheating and cable core drying efficiency are high. The preheating temperature is restricted by the pay-off speed, and is generally similar to the die temperature.
Cooling device
The formed plastic extruded layer should be cooled and shaped immediately after leaving the die, otherwise it will deform under the action of gravity. Water cooling is usually used for 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 extruded layer, but for crystalline polymers, due to sudden cooling, internal stress is easily left inside the extruded 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 shape it. Slow cooling is used for the extrusion of PE and PP, that is, it is cooled in three stages: hot water, warm water, and cold water.
Daily maintenance
1. After 500 hours of use, there will be iron filings or other impurities in the reduction box caused by the gear grinding. Therefore, the gears should be cleaned and the reduction box lubricating oil should be replaced.
2. After using it for a period of time, you should conduct a comprehensive inspection of the extruder and check the tightness of all screws.
3. If there is a sudden power outage during production, the main drive and heating will stop. When the power is restored, each section of the barrel must be reheated to the specified temperature and kept warm for a period of time before the extruder can be started.
4. If the instrument or pointer is found to be turning full, check whether the contact of the thermocouple and other side lines is good.
Principles
1. Structural principles
The basic mechanism of the extrusion process is simply that 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 law, if an object is stationary in a certain direction, then the force on the object in this direction is balanced. 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 equilibrium. 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 same direction to another object. Obviously, the thrust it applies is acting on the thrust bearing behind the feed port. Most single screws are right-handed threads. If you look at it 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 law.
2. Temperature principle
Extrudable plastics are thermoplastics, 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 molten plastic come from? First of all, the preheating of the feed and the barrel/ die heater may play a role and is very important at startup. In addition, the motor input energy, that is, the friction heat generated in the barrel when the motor turns the screw against the resistance of the viscous melt, is also the most important heat source for all plastics, of course, small systems, slow screw speeds, high melt temperature plastics and extrusion coating applications are the exceptions. 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 meshing or solids conveying speed in the feed. In general, except for some specific purposes (such as glazing, fluid distribution or pressure control), the die and mold temperature should reach the required melt temperature or close to this temperature.
3. Deceleration principle
In most extruders, the screw speed is varied by adjusting the motor speed. The drive motor usually runs at a full speed of about 1750 rpm, which is too fast for an extruder screw. If it runs at such a fast speed, too much friction heat will be generated, and the residence time of the plastic will be too short to produce a uniform, well-mixed melt. The typical reduction ratio should be between 10:1 and 20:1. The first stage can be either gear or pulley, but it is better to use gears for the second stage and position the screw in 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 30 rpm or less (ratio up to 60:1). On the other hand, some very long twin screws used for mixing can run at 600 rpm or faster, 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 set between the motor and the first reduction stage to change the maximum speed, which either increases the screw speed even beyond the previous limit, or reduces the maximum speed. This increases the available energy, reduces the current value and avoids motor failure. In both cases, the output may increase due to the material and its cooling needs.
