The conveying principle of twin-screw extruders
Twin-screw extruders are commonly used to melt and pump a polymer through a die which is designed with a required shape. The materials in a twin-screw extruder are mixed with resin and additives like color concentrate and then are transformed into smooth fusant after being subjected to heating, pressing and shearing.
Basic understanding of twin screw extruders
Disadvantages of single-screw extruders: Since the transmission of the plastic materials is through friction, there is a limitation when it comes to the feeding performance. Some materials like paste or powder possess difficulties in mixing. This makes using the machine in some processes unsuitable.
The twin-screw extruder is developed on the basis of the single-screw extruder. Its main parts include the conveying unit, the feeding unit, the barrel and the screw. Each part plays a role similar to that of the single-screw extruder. As the name indicated, the twin-screw extruder has two screws while the single-screw extruder only has one.
Advantages of twin-screw extruders
- short residence time distribution
- Remarkable mixing capability
- high level of process flexibility
- higher convective heat transfer
- lower dispersion of shear rates and strain
- accurate control of temperature profile
- saving power
The twin screw extruders can force the material forward due to the positive displacement pumping and shear force generated at the intermeshing area. Theoretically, positive displacement conveying is independent of the rheological properties of the medium being conveyed, i.e., the frictional nature and viscosity of the medium have no effect on the conveying characteristics.
The conveying principle of twin screw corotating intermeshing extruders
The cross section of the screw channel can be closed while the longitudinal direction of the screw channel should be open because in a closed arrangement the screw flights in the longitudinal direction are closed at intervals. The size of the channel is determined by the purpose of use. The larger the longitudinal opening, the more the positive displacement conveying capacity is lost and the greater the effect of frictional and viscous drag. The intermeshing corotating twin-screw extruder transport the material through positive displacement pumping and frictional and viscous drag forces.
Therefore, the conveying function of the intermeshing corotating twin-screw extruder is between that of the single-screw extruder and the intermeshing corotating twin-screw extruder which is closed in both longitudinal and transverse directions.
The conveying principle of twin-screw non-intermeshing corotating extruders
Nonintermeshing systems are open lengthwise and crosswise. Their design does not provide a positive displacement action for pumping the materials forward. These types of extruders depend on friction for extrusion, just like single screw extruders.
The conveying principle of intermeshing counter-rotating twin-screw extruders
The intermeshing twin-screw extruders can be designed to achieve varying degrees of positive displacement conveying with this twin screw. The better the longitudinal and transverse closure of the screw groove, the better the positive displacement conveying.
The conveying principle of conical twin-screw extruders
The conveying principle of the conical twin-screw extruder is the same as that of the intermeshing corotating twin-screw extruder. If the screw groove is closed in both longitudinal and transverse directions, its conveying is positive displacement conveying; if the screw groove is open in both longitudinal and transverse directions, it will lose part of the positive displacement conveying ability but improve the mixing effect.