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Do you know the 5 segments of the screw in a twin screw extruder?

The segmentation of the screw and its function

The screw is generally divided into 5 sections: intake and conveying section, melting section, mixing section, exhaust section and homogenization section.

  1. Intake and conveying section, conveying materials, preventing overflow.
  2. Melting section, which fully melts and homogenizes the material through heat transfer and frictional shear.
  3. Mixing section, the material component size is further refined and uniform, forming an ideal structure, with distribution and dispersion of mixed function.
  4. Exhaust section, discharge water vapor, low molecular weight substances and other impurities.
  5. The homogenization (measurement) section, convey and pressurize, to establish a certain pressure, so that the material at the die mouth has a certain density, at the same time, further mixing, and finally achieve the purpose of smooth extrusion granulation.

Screw arrangement segmentation and temperature setting

1. Conveying section

A. Screw arrangement:

  • Deep groove forward thread
  • Medium screw groove large lead forward thread, and the screw groove volume changes from large to small, that is, the lead of screw thread gradually changes from large to small.

B. Temperature setting

  • It should not be too high, affecting the transportation and shearing of materials in this section; It should not be too low, the screw is stressed too much or stuck.
  • Generally slightly close to melting, arranged in a gradient.

2. Melting section

A. Screw arrangement:

The purpose of the material in this section is to make the processed material obtain the energy required for physical change and partial chemical change, so that the distribution between the components is uniform and initially dispersed, so that the components are homogeneous and the viscosity is close.

  • Generally, the material is required to withstand large shear and barrel heat transfer to melt.
  • Generally, kneading blocks are set, cutting elements or reverse threads, and paying attention to the arrangement and matching between phases.

B. Temperature setting

1. Glass fiber series, the temperature is too low, the resin is semi-melted, and the glass fiber coating is poor in the back section; The temperature is too high, the resin flow increases, the temperature refining and shear action becomes smaller, and even high temperature degradation occurs. The setting principle is as follows:

  • Depending on the base material and glass fiber content;
  • Deduct the heat input of screw shearing, slightly higher than the melting point range of the base material;
  • The melt flow condition of the back section of the melt section (i.e. the glass fiber inlet).

2. Filling series, (provide strong shear to make the filler, fully dispersed), the melting section is 10~20 °C higher than the melting point of the base material (as much as possible), so that the material is fully melted and evenly distributed.

  • Flame retardant series, (protect flame retardant), the temperature should be low, especially white materials, as far as possible to reduce;
  • Glass fiber reinforced flame retardant series, set temperature between the previous two, based on the basic melting point of the material;
  • Alloy series, based on the melting temperature of two groups, while considering the proportion of components and the thermal sensitivity of the components, etc., adjust the temperature appropriately.

3. Mixing section

A. Screw arrangement

The purpose of the material in this section is: 1. Refine and disperse to form the ideal size and structure. 2. Pay attention to protect the ideal structure of the finished product from being destroyed.
There are generally two typical ideas: 1, enhanced, two-headed and three-headed combination; 2. The high shear and high shunt flow of both distribution and dispersion are based on the kneading block and the thread block as the auxiliary high shear.

B. Temperature setting

1. Glass fiber series, the temperature is too low, the material flow performance is poor, the viscosity is large, the friction becomes large, the heat generation is high, and there will be local overheating; The temperature is too high, the resin degrades, the shear is small glass fiber dispersion deteriorates, and its setting principle:

  • It varies according to the base material and glass fiber content;
  • Slightly higher than the melting point range of the base material;
  • It is determined by the gloss of the finished strip.

2. Filling series, (provide strong shear to make the filler, fully dispersed), the mixing section is 10~20 °C higher than the melting point of the base material (as much as possible), so that the material is fully melted and evenly distributed, to ensure that the mixture is a fluid state.

  • Flame retardant series, (protect flame retardant), the temperature should be low, especially white materials, as far as possible to reduce。
    Glass fiber reinforced flame retardant series, temperature setting based on the basic melting of the material as the basis, protect the flame retardant.
  • Alloy series, it is adjusted according to the melting temperature of the component, taking into account the proportion of the component and the heat sensitivity of the component.

4. Exhaust section

At the inlet of the general exhaust port, a reverse thread reverse kneading block is set up to seal the melt to establish high pressure; Use large lead thread elements to form a low fullness and melt layer, so that the material exposes a free surface or adopts multi-head small lead threads to increase the speed of melt surface renewal, which is conducive to gas discharge and volatilization ——The general idea: reverse thread (R-LH) or reverse rod block (KG) + conveying thread + large lead or multi-head small lead thread.

5. Homogenization (measurement) section

A. Screw arrangement

The lead of the thread block is gradually small or the screw groove is gradually reduced to achieve pressurization, reduce the length of the back pressure section, and pay attention to the use of single-head thread and wide screw thread to improve the discharge capacity and avoid material exposure.

B. Temperature setting

In order to reduce the temperature appropriately, but the high temperature of the die is conducive to discharging.

On the basis of the temperature of the melting section, the temperature is appropriately reduced, and the principle is determined according to the reduction of the gloss.

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