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Detailed use of thread element of parallel double screw plastic extruder

July 01,2023

Characteristics of parallel plastic twin screw extruder:
1. High speed and close relative motion speed in different positions of the engagement area (the two screws overlap in the cross-section diagram), so strong and uniform shear can be generated;
2. The geometry dictates that the longitudinal runners must be open to allow for the exchange of material between the two screws. During the exchange, the material that was originally at the bottom of one screw groove moves to the top of the other screw groove. The opening of the longitudinal channels also enables the opening of the transverse channels for the exchange of material between adjacent slots of the same screw. This gives the plastics of the co-rotating twin screw extruder a better distribution and mixing capability.
If there is a passage for material from the beginning of theplastic screw extruderto the end of the screw, where material can flow from one screw to the other (i.e. there is flow along the screw slot), this is called a longitudinal opening, otherwise it is called a longitudinal closure. In the engagement zone of two screws, if there is a passage across the screw, i.e. material can flow from another slot of the same screw, or material from one slot of one screw can flow to two adjacent slots of the other screw, it is called a transverse opening, otherwise it is called a transverse closure.
The conveying elements are open vertically and closed horizontally; the engagement blocks are open vertically and horizontally.

1. Transmission elements

There are three main points of conveying element: conveying direction, the pitch and thread heads number.
1) Delivery direction
Forward delivery: the material moves toward the device end
Reverse delivery: the material moves away from the discharge end of the device.
2) The pitch
Distance: rotate a cyclic axial length of the blade
The pitch can determine the material or fast or slow to control the fill degree.The reverse transport element can be used as a “limiting” element material within the screw.
▲ Large pitch (between 1.5 and 2 times diameter) features: maximum capacity; fastest transmission speed; lowest fill; for feeding and exhaust.
▲ Characteristics of medium pitch (1 x diameter): medium capacity; medium transmission speed; increased fill at downstream large pitch elements.
▲ Small pitch (0.25 to 0.75 x diameter): minimum capacity; slowest transmission; increase fill at the downstream medium pitch element; maximum fill at no pressure behind; heat propagation and pumping capacity.
3) Number of thread heads

▲ Features of single-head element: wide blade can minimize material leakage; smaller than double-head screw capacity; maximum extraction efficiency.
▲ Features of two elements: standard parts of same twin screw conveying element; less shear force than three elements; for solid feeding, melt transmission, exhaust and melt pumping.
▲ Three-head element features: maximum shear force; shallow channel depth; for melting, dispersed mixing.
4). Reverse element
Features: 100% fill in front; for melt seal (vacuum), large shear force input.
5). the SK element
Features: increase capacity; increase transmission capacity; mainly used for feeding (not self-cleaning)

2. Rodent block

★ Distribution mixing and dispersion mixing
▲ Distribution mixing
Distribution mixing is the process that makes the material uniform and does not change the size of the material, but only changes the relative position.
Distribution mixing makes melt segmentation and recombination, and makes the spatial distribution of each component uniform. It is mainly realized by displacement flow under stresses such as separation, stretching (generated alternately between compression and expansion), distortion, and reorientation of fluid activity.
▲ Discrete mixing

The dispersed mixing breaks the elelments into particles or brings the incompatible two components to the required range, which is mainly achieved by shear pressure and extension stress. To achieve the so-called “good” crushing mixing, both crushing and distribution mixing work together.
The engagement pieces are arranged smoothly or backward at certain angles (30° or 45° or 60° or 90°). In the two heads, the number of mesh was to shift the mesh by 180°, KB 30 ° = 7, KB 45 ° = 5, and KB 60 ° = 4.
The transport direction of the engagement block can be divided into forward, reverse and neutral.With the same angle of the mesh block, the thickness of the mesh can produce different effects, as compared in the figure as the below:

▲ KB 30 ° engagement block
The gap between mesh sheets may allow some mixing; the gap between channels is small; forward = small mixing; reverse = high pressure
▲ KB 45 ° engagement block
Greater mixing capacity than 30° mesh; less than 30° mesh; forward = moderate strength mix; reverse = medium to large pressure
▲ KB 60 ° engagement block
Stronger mixing ability than 45°; less delivery ability than 45°; forward = moderate mixing; reverse = moderate to low pressure
▲ KB 90 ° engagement block
No transfer capability; 100% fill (low pressure); strong mix

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