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Twin Screw Extruder
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Twin Screw Extruder

There is a wide variety of extruders, produced today in the world; in general, they can be classified into two main families, that is the single screw extruder and twin screw extruder.

The corotating twin screw extruders manufactured today very little heat through mechanical work, far below the amount needed for the material to reach the temperature required for gas mixing; additional heat is applied to reach it. After the mixing, no more heat is applied, and due to the low shear developed, the temperature of the melt will decrease; because of the relatively slow movement down the barrel, with a reasonable little cooling, the temperature will easily reach the one required for the extrusion.

The principle of operation that characterizes corotating intermeshing twin-screws extruders can be summarized as follows:
-The screws can be starve-fed by an independent mechanism so that screws speed setting does not affect output.
-Positive forwarding action eliminates problems like the bridging or slipping of solid material in the feed section, allowing the feed of pellets, beads, or powder.
-Shear rate is very low, and additional external heat, much easier to control, is added; the shear stress distribution can be adjusted by changing screw speed and/or output.
-Stock temperature can be raised to the desired level without exceeding it, and low-temperature extrusion is easily obtainable.
-Positive conveying of the melt eliminates surging and promotes better dimensional control of the extrudate.
-Mixing is also positive and accomplished without overheating the material.
-Better dispersing and homogenization.
-Self-cleaning is fast and complete and accomplished without overheating the material.
-Lower HP required per rated output, resulting in a lesser stress on the machine.
-Energy-saving, as the actual energy consumption is very close to the theoretical value needed.
 
Therefore, twin screw extruders are especially suitable for the extrusion of heat-sensitive polymers, for products that have to be extruded at low temperature, for the extrusion of uncompounded dry blends and materials that present difficulty in feeding or have to be thoroughly degassed.

Finally, with the proper screw configuration, foamed products can be produced by mixing additives, melting at relatively high temperature, injecting the blowing agent, and then cooling the melt to the lower temperature required for foaming; all these operations are performed in one extruder.
 
Working principle of twin screw extruder
Although the structure of the twin screw extruder is very similar to that of a single screw extruder, there are great differences between them in operating principle. In the single screw extruder, the power of material moving forward mainly depends on the difference of friction coefficient between material, barrel, and screw. If, the friction coefficient of material and machine barrel inner wall is too small, when screw rolling, the material will hold screw rolling together, so that thread can not play its role in promoting, the material can not be transported forward.

Because the twin-screw extruder has made the material in the twin-screw retention time is short and uniform, not easy to produce local degradation transformation characteristics, this processing material retention time will be long thermal decomposition, curing or agglutination of heat-sensitive plastic, such as PVC material coloring, mixing, product production has a common advantage.

Because of the excellent mixing and plasticizing characteristics of the twin-screw extruder, it is mainly used for polymer modification, such as blending, filler, enhanced extrusion, and other operations, but also can directly extrude plate, pipe, profile and other products.

The twin screw extruder has a wide range of applications, can be applied to the processing of a variety of materials, can complete high yield, high quality, high efficiency, and other characteristics, although its structure is more complex, it is still widely attention and rapid development.

In recent years, with the development of new fields, new products, and new technologies such as synthetic data and fine chemical industry, more and more special or special conditions of polymers to be processed are needed, which makes the application of twin screw extruder more and more extensive. At present, the world twin screw extruder is the trend to high torque, high speed, low energy consumption direction.
 

Twin screw extruder, the extrusion system consists of two meshing or mesh, synthetic rotary or differential rotary screw and cylinder, less heat due to friction, the material of shear is evener, screw large conveying capacity, under the same extrusion amount, the material short residence time in the screw, so is suitable for those who stay a long time will be cured or condensed material shading and blending.
Twin screw extruder extrusion method generally refers to the high temperature of about 200 degrees to melt the plastic, melting plastic through the mold to form the required shape. The function of the twin-screw extruder is to heat, pressurize and shear solid plastics into a uniform and uniform melt and send the melt to the next process. The production of melt involves the process of mixing additives such as color masterbatch, mixing resins, and re-grinding. Compared with other molding methods, it has the advantages of high efficiency and low unit cost.

The extrusion method is mainly used in the formation of thermoplastic plastics, also can be used in some thermosetting plastics. Extruded products are continuous profiles, such as pipe, rod, wire, plate, film, wire, and cable cladding. Besides, can also be used for plastic mixing, plasticizing granulation, coloring, mixing, and so on. Extruded products can be called "profile" because the cross-section shape is mostly irregular, so it is also called "profile".
 

Advantages of twin screw extruder:
1. The realization of large-scale twin-screw extruders can reduce production costs, which has more obvious advantages in large twin-screw plastic granulator sets, film blowing sets, pipe extruders, and other aspects.
2. Twin screw extruder modular production can adapt to the special requirements of different users, shorten the development cycle of new products, strive for a larger market share.
3. The twin screw extruder to the extrusion melting process of friction heatless.
4. The plasticizing in the barrel of the twin-screw extruder is stable and even under the action of double screw meshing and shearing. The plasticized raw materials are better in quality.
5. The molten plasticizing time of raw materials in the barrel is shorter (i.e. the residence time of raw materials in the barrel is shorter than that of single screw extruder), and the production efficiency is higher than that of a single screw extruder.
6. It can directly use the powder in the twin screw extruder extrusion, mixing plastic resin, product quality is more stable, save the use of single screw extruder molding products must be mixed and granulating process.
7. Double screw extruder meshing rotation work, the barrel residual material can be automatically cleaned.

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