Design of the Screw Configuration—Co-rotating Twin Screw Extruder

There are many types of twin screw extruders, of which the intermeshing co-rotating twin screw extruder is widely used in the plastics industry. This type of extruder consists of two intermeshing “building block” screws, barrels, power devices, temperature control devices and others, there may be multiple feed ports and vacuum/non-vacuum evacuation ports on the body.

Engaging the same direction twin-screw extruder
Engaging the same direction twin-screw extruder
Engaging the same direction twin-screw extruder
Engaging the same direction twin-screw extruder
  1. The two screws rotate in parallel and in the same direction, and shear uniformly in the contact area with each other and between the barrels, and the strength of this shearing effect can be adjusted by means of screw combinations, spacing design and other means.
  2. The geometry and co-rotation of the screw block make the screw have good material mixing ability, which is suitable for mixing. After the material is softened, due to the twin screws in the engagement of the opposite direction, so the material here by a screw transfer to another screw, was “∞” movement, this movement in the engagement of the relative speed is greater, conducive to the mixing of materials and homogenization of the engagement area of the gap is very small, pinch threads and grooves at the speed of the opposite, has a high shear The role of plasticization, so as to achieve uniformity of the role of plasticization.
  3. The screw and barrel are combined type. There are many kinds of thread elements, such as conveying element, kneading element, shearing element, reverse thread element and pressurized thread element, etc. According to the material, various elements will be optimized design, in order to adapt to the processing of various kinds of process formula materials.
  4. Coaxial twin-screw extruder has the ability to react, is a dynamic reactor, the material in the barrel of the melt can occur a series of chemical reactions, such as polymerization, grafting and so on. Reactive extrusion processing is mainly used for: polymerization reaction of monomers or oligomers (free radical polymerization, polymerization, polycondensation and copolymerization); controlled cross-linking and degradation of polyolefins; grafting modification of polymers (to make the polymer functional groups or polar groups, to achieve the purpose of the material modification and preparation of bulking agents); forced blending modification of a variety of materials. It also includes physical modification of materials, such as filling, blending, toughening and strengthening.

The basic principles of screw combination design

  1. A large lead thread should be used at the charging port to ensure smooth material discharging.
  2. In the melting section should be used in small lead thread to build up pressure, can also be used to stagger the angle of 30 ° pinch block of the material for the initial distribution of mixing, note that the pinch block should be spaced out.
  3. In the mixing section mainly to shear, refine and disperse the material particles for the purpose of this section of the thread element is very complex to set up, need to designers have a wealth of practical experience. In this section, the main use of staggered angle of 45 ° and 60 ° of the pinch block to strengthen the shear, assisted by special elements such as tooth-shaped elements or “S” shaped elements.
  4. Reverse threaded elements or reverse pinch blocks should be set before the exhaust port or vacuum port, large lead threaded elements should be set at the exhaust port or vacuum port, and small lead threaded elements should be set after the exhaust port or vacuum port.
  5. In the homogenization section, the thread lead should be tapered to achieve pressure increase and reduce the length of the back pressure section, and at the same time, pay attention to the use of single-ended threads and wide-pronged threads to improve the discharge capacity and avoid bubbling material.

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