How does a twin screw extruder work?

Components of twin screw extruder

The twin screw extruder consists of 5 systems

  • Extrusion system

The extrusion system consists of a screw and a barrel.

Screw: The screw in the twin-screw extruder cannot form a closed meshing line, which is a non-sealed type. The screw is self-locking and driven by gears.

Barrel: It is a kind of metal cylinder with heat resistance, wear resistance, corrosion resistance, and other properties. The cooperation of the barrel and the screw can crush, plasticize and compress the plastic in the barrel.

Intermeshing counter-rotating twin-screw and conical twin-screw extruders generally use integral barrels, and a few large extruders use segmented combined barrels.

  • Transmission structure

The function of the transmission system is to drive the screw and supply the torque and speed required by the screw during the extrusion process. It is usually composed of a motor, a reduction box, and a bearing.

  • Feeding system

The feeding system adopts a forced metering feeding method to feed the barrel.

The screw that rotates and conveys the raw material is driven by a DC motor through the output shaft of a worm gear reducer. The speed and size of the raw material conveyed by the screw are determined by the working speed of the twin screw of the twin-screw extruder, the temperature of the barrel, the melt pressure of the product, and the amount of the product used. The threaded rod in the feeding device has threads, which can be single-ended or double-ended. Generally, single-threaded threads are more commonly used.

  • Heating and cooling structure

Heating and cooling are the basic conditions for the working of the extruder. Generally, the method of resistance heating the barrel is the main method.

  • Control System

The control system controls the operation and stops of the twin-screw extruder.

Brief diagram of structure:

Brief diagram of structure

Material Flow Process

Theoretically, the material flow process can be divided into four sections: 

  1. feeding of the extruder
  2. transport of mass
  3. flow through the die
  4. exit from the die and subsequent downstream processing

During material processing, the mass is transformed mainly by the shear force, pressure, cooling rate, shaping, and residence time.

Extrusion Channel

Conventionally, the extrusion channel is divided into three parts: 

  1. the feed zone
  2. the transition zone 
  3. the metering zone

The material processing time in the extruder is termed the residence distribution time.

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