Twin screw extruders are increasingly used in industrial production, especially in plastics industry, which is inseparable from twin screw extruders. In order to meet processing requirements of various polymers, people have constantly optimized and improved it, which greatly improved performance and productivity of twin screw extruder. Today we will talk about most commonly used twin screw extruder.
1. Classification of twin screw extruders
There are many types of screw extruders. In addition to twin screw extruders, there are single screw and three screw extruders. The main twin screw extruders used are non-engaging counter extruders, intermeshing counter extruders and co-rotating extruders, most commonly used of which are intermeshing twin screw extruders.
Figure 1.1. Twin Screw Extruder
Second, screw element
The screws of co-rotating twin screw extruder are divided into solid materials conveying section, melt mixing section and melt conveying section according to different functions. According to requirements of each functional section, different structural components of "building block". selected for a reasonable combination to meet requirements of product's production. Screw elements mainly include threaded elements, kneading block elements and toothed disk elements.
Figure 2-1. Mutual engagement of co-rotating twin screw extruder elements
2.1 Threaded components
Table 2-1 Classification of screw elements
It is worth noting that reverse thread element does not have ability to feed and is a resistance element. Therefore, when designing and selecting reverse threaded components, amount of pressure drop must be taken into account in order to facilitate transport of materials from die.
2.2 Kneading Block Component
The kneading block element is commonly known as "shear block", which can provide high shear force and play a role in mixing and dispersing materials. Usually elements of kneading block can be single or combined.
In a twin screw extruder, effect of axial mixing of kneading blocks depends on value of displacement angle (K), mainly kneading blocks with K=30°, 45°, 60°, 90°, arranged in a checkerboard pattern. kneading block with angle K=90° can move and mix material strongly in longitudinal direction, but throughput of axial movement is zero.
Figure 2-2 Mixing unit components
Scallop element 2.3
A toothed element consisting of one or a series of toothed discs is a good hybrid element including TME, ZME and SME.
Figure 2-3 Gear Disc Components
Third, Advantages of Unidirectional Twin Screw Extruder
Co-rotating twin screw extruders use screw elements to feed polymer materials, which provides two advantages: uniform shear rate distribution, high shear strength of kneading block elements and intensive mixing. It is not available in other screw extruders, especially single screw extruders. In addition, extruder also has following advantages:
Higher adaptability: The use of building block structure according to different material characteristics and special process requirements allows you to optimize and combine screw components to meet requirements of polymer processing;
Good self-cleaning property: The gap between two screws of co-rotating twin screw extruder is very small, and bonding area scratches each other when screws rotate, has a good self-cleaning effect;
Narrow residence time distribution: Screw diameter φ, screw speed n and extruder length-to-diameter ratio L/D can be adjusted at any time according to material processing properties. .
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