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Twin Screw Extruder Vs Single Screw Extruder

Polymer Flow Mechanism

The polymer flow mechanism in a twin-screw extruder is different from that in a single-screw extruder and largely depends on the degree of meshing. In a single screw machine, the polymer flow is towed-style. In the twin-screw machine, there is also a volumetric flow mechanism which depends on the meshing degree of the screw. 

It occurs most fully in a counter-rotating extruder that engages with each other. 

This mechanism does not occur in non-meshing screws. In general, the closer the meshing is, the different the transmission mechanism is. The non-meshing extruder is basically similar to the single-screw extruder, so it does not have as good transportability as the tight meshing screw.

Due to the complex flow properties, the twin-screw extruder has an excellent performance in the aspect of mixing and devolatilization, heat exchange and rapid polymer plasticizing.

The positive displacement of the meshing zone makes the twin-screw extruder particularly suitable for processing materials that are difficult to feed and thermosensitive materials, such as PVC.

The Mixing Effect Of Twin Screw Extruder Is Better

There are two screws meshing and rotating in the twin-screw extruder to complete the forced transportation and plasticizing of the plastic. 

The raw material is stable and uniform under the action of twin-screw meshing and shearing in the barrel, and the raw material is plasticized and mixed evenly. 

Among them, the clearance at the meshing place of the co-directional twin screw is very small, the velocity direction of the screw edge and the screw groove is opposite, and the relative speed is large, so the meshing area has a very high shear speed and a large shear force, and the mixing effect is much better than that of single screw extruder and counter-rotating twin screw extruder.

CWT-Plus-Twin-Screw-Extruder
Twin Screw Extruder

The Exhaust Performance Of Co-rotating Twin Screw Extruder Is Better

Due to the mutual meshing of the two screws, the shearing process of the material at the meshing place constantly updates the surface layer of the material and improves the exhaust effect, so that the co-directional twin-screw extruder has better exhaust performance than the exhaust single-screw extruder. 

A threaded conveying element with large lead is used in the exhaust section, and the ring dam or reverse thread element is usually used to build pressure between the exhaust section and the first metering section. The melt beyond the ring dam enters the exhaust section, the pressure is released and the volatile can escape. 

On the other hand, the surface renewal effect of the material of single screw exhaust extruder is small, so the exhaust effect is poor.

The Screw Is Interchangeable

The flow of twin-screw extruder is complex, so the simple design equation which can be used for single-screw extruder is difficult to be used for twin-screw extruder.
Because of this complexity and lack of predictability, the screw of a twin-screw extruder is usually provided as an interchangeable element.

Processing Capacity And Energy Consumption

The twin screw extruder has the advantages of high output, fast extrusion speed and low energy consumption per unit output.

The Self-cleaning Ability Of Twin Screw Extruder Is Better

The residual material in the barrel of the twin-screw meshing rotary machine can be cleaned automatically.

Operational Convenience Degree

The single screw is easy to operate and the process control is simple; the twin screw operation is relatively complex and the process control requirements are high.

Single Screw Extruder Is Economical

Twin-screw extruder is more expensive, complex and difficult to manufacture than single-screw extruder. 

The single screw extruder has the advantages of simple design, low price and wide application.

Single-Screw-Extruder
Single Screw Extruder

Different Applications

Because the structure and principle of twin-screw extruder is different from that of single-screw extruder, their application advantages are also different. 

The material transportation of single screw extruder mainly depends on friction, which limits its feeding performance, and it is difficult to add powder, paste, glass fiber and inorganic fillers. When the head pressure is high, the countercurrent increases and the productivity decreases.

Single screw is suitable for plasticizing extrusion of polymer and extrusion processing of granular materials. 

Applications:

  • Pipe extrusion
  • Sheet extrusion
  • Profile extrusion
  • Modified compounding

Twin screw has good mixing and plasticizing ability, and the residence time of material in the extruder is short, so it is suitable for powder processing. 

Applications:

  • Foam extrusion
  • PET extrusion
  • Sheet direct extrusion
  • Compounding extrusion

Advantages Of Twin Screw Extruder

The twin-screw extruder has good feed characteristics and is suitable for powder processing. When the raw material is extruded and plasticized in the machine barrel, the friction heat produced is less, the residence time in the machine barrel is short, and the output of extruded products is high. Powdered raw materials can be directly used for extrusion products, and the plasticizing and mixing quality of raw materials is relatively stable. It has better mixing, exhaust, reaction and self-cleaning functions than a single screw extruder. It shows its superiority when processing plastics and blends with poor thermal stability.

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