Double screw generally divided into 5 sections: conveying section, melting section, mixing section, exhaust section, homogenization.
1. Conveying section: conveying materials to prevent overflow.
2. Melting section: make the materials fully melt and homogenize by heat transfer and friction shear.
3. Mixing section: further refine and uniform the size of materials and components to make them forming an ideal structure with the function of distribution and dispersion mixing.
4. Exhaust section: discharge water vapor, low molecular weight substances and other impurities.
5. Homogenization (metering) section: convey and pressurize to establish a certain pressure and make the materials have a fixed density at the die mouth; at the same time mix the materials so finally achieving the purpose of extrusion granulation.
Distinction between distribution and dispersion
1. The distribution and mixing of double screw make the melt divided and recombined so that the spatial distribution of each component is uniform.This step can be mainly realized by the displacement flow under the stress of separation, tension (compression and expansion alternately), distortion, and re-orientation of fluid activity.
2. By dispersing and mixing, the components are broken into particles or the incompatible two components can be in the required range of size which can be realized mainly by shear pressure and tensile stress are .
Extruder screw elements in the threaded form
1. Expression: for example ,"56/56" transport block, the former "56" means the lead is 56 mm, and the latter "56" means the length is 56 mm.
2. Large lead, with pitch witjin1.5 D~2 D.
3. Small lead, with pitch about 0.4 D.
4. The usage: with the increase of lead, the extrusion quantity of double screw increases, the residence time of material decreases and the mixing effects decrease.
5. The choice of large lead thread is mainly matching to transportation which is beneficial to increase the production. Heat sensitive polymer can shorten the stopping time and reduce the degradation. Using the exhaust (also choose shallow groove) can increase surface area so that being conducive to exhaust and volatilization.
6. The occasion of choosing extruder screw elements is in medium lead thread which is mainly being used to mixture or in the gradually reduced combination of different working sections for transportation and pressurization.
7. The occasion of choosing extruder screw elements is in small lead thread which is generally used in a gradually reduced combination which used in conveying section and homogenizing metering section to improve pressure, the degree of melting and mixing, and the extrusion stability.