Characteristics of co-directional twin screw extruders and basic principles of screw combination

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Features of twin screw extruders with meshing

There are many types of twin-screw extruders. Among them, twin-screw extruders are a type of production and processing equipment widely used in the plastics industry. This type of extruder is a combination of two “building block” screws, barrel, power unit, temperature control device, etc., and can have multiple feeding ports and vacuum/non-vacuum degaussing ports on the fuselage.

The main characteristics of meshing twin screw extruders are as follows:

  1. The two screws rotate in parallel and in the same direction, producing a uniform shearing effect between the contact part and the barrel. Moreover, the strength of this shearing effect can be adjusted by screw combination, spacing design, etc.
  2. The geometry and co-rotation of the screw block make the screw have good material distribution and mixing ability, which is suitable for mixing operation. After the material enters the barrel is softened, because the twin screw is in the opposite direction at the meshing place, one screw should pull the material into the meshing gap, and the other screw pushes it out of the gap, so the material is transferred from one screw to the other screw here, with a “∞” movement, this movement has a large relative speed at the meshing point, which is very conducive to the mixing and equalization of the material, and the meshing area The gap is small, and the thread and groove at the kneading place The speed is the opposite, and has a high shear effect, so as to achieve uniform plasticization.
    3.The screw and barrel are combined. There are many types of threaded elements, including conveying elements, kneading elements, shearing elements, reverse threaded elements and booster threaded elements, etc., which each play different roles. According to the needs of material processing, various components are combined by building blocks, and through optimization design, it can be adapted to the processing of various process formula materials.
  3. The co-directional twin screw extruder has the ability to react. It is a dynamic reactor. After the material is melted in the barrel, it can undergo a series of chemical reactions, such as polymerization and grafting. Reactive extrusion is mainly used for: polymerization of monomers or oligomers (free radical polymerization, polymerization, condensation and copolymerization); controllable cross-linking and degradation of polyolefins; grafting modification of polymers (functionalization or polarity grouting of polymers to achieve the purpose of material modification and preparation of solubilizers); forced blending modification of various materials. It also includes physical modification of materials, such as filling, mixing, toughening and strengthening.
PP Talc Powder Environmental Protection Sheet Extrusion Line
PP Talc Powder Environmental Protection Sheet Extrusion Line

Basic principles of screw combination

For a twin screw extruder, the screw is mainly divided into feeding section, melting section, mixing section, exhaust section and homogenizing section. Threaded elements mainly include conveying, melting, shearing, material mixing, residence time control, etc. The threaded components of a twin-screw extruder are combined in a “building block” manner and can be adjusted according to different production needs in practice, so screw combination is the key to customizing the twin screw extrusion process.

The screw combination should take into account the performance and shape of the main and auxiliary materials, the order and position of the feeding, the position of the exhaust port, the temperature setting of the barrel, etc. At the same time, the objects of mixing are very diverse, and a reasonable screw combination is required for each specific mixing process. Despite this, there are basic rules to follow in the screw combination of meshing to twin-screw extruders.

Here are a few basic principles of screw combinations.

(1) A large lead thread should be used at the feeding port to ensure smooth unloading.
(2) In the melting section, a small lead thread should be used to establish pressure, so that the material is compressed and melted, the kneading block with a misalignment angle of 90° can be set to balance the pressure, or the kneading block with a misalignment angle of 30° can be used to initially distribute and mix the material.
(3) In the mixing section, the main purpose is to cut, refine and disperse material particles. The setting of threaded elements in this segment is very complex and requires rich practical experience of designers. In this section, kneading blocks with misalignment angles of 45° and 60° are mainly used to strengthen the shear, and special elements such as toothed elements or “S” shaped elements are assisted.
However, it should be noted that the kneading and shearing elements should not be set too much, nor should they be arranged too tightly, so as not to cut too strong. In addition, in order to enhance the conveying capacity of this section of material, the threaded conveying element should be arranged at intervals, that is, the kneading block and the thread conveying element are staggered.
(4) A reverse thread element or reverse kneading block should be set before the exhaust port or vacuum port, a large lead thread element should be set at the exhaust port or vacuum port, and a small lead thread element should be set up after the exhaust port or vacuum port.
(5) In the homogenization section, the thread lead gradient should be small to achieve supercharging and reduce the length of the back pressure section. At the same time, pay attention to the use of single-head thread and wide thread thread to improve discharge capacity and avoid charging.

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