Twin-screw extruders rely on the interaction between the rotating screws and the material, and through a combination of structural design and process control, efficiently process and shape materials. They are currently a core piece of equipment in polymer material processing.
Screw Structure and Rotation Mechanism
Twin-screw extruders have two rotation methods: co-rotation and counter-rotation.
1. Co-rotation: Both screws rotate in the same direction (right-handed), their threads interlock, and the combined thrust results in very high shear force. This is suitable for processing materials requiring high shear force.
2. Counter-rotation: The two screws rotate in opposite directions (one left-handed and one right-handed). Their threads may or may not interlock, resulting in offsetting thrust and good sealing. This structure is more suitable for processes requiring good sealing.
Material Conveying and Melting Process
The material goes through five key stages in the barrel:
1. Feeding Section: Material enters the screw channel from the hopper, smoothly fed by the large-lead screw. At this stage, the screw channel is not full. This stage mainly ensures a stable supply of material.
2. Melting Section: The barrel heating and screw shearing work together to gradually soften and melt the material, with the remaining solid continuing to melt. Temperature control is crucial.
3. Mixing Section: Parts like the kneading block shear and refine the additives (such as pharmaceuticals and fillers) and melt, mixing them evenly. This stage directly affects the quality of the final product.
4. Venting Section: The large-lead screw allows for a smaller material load, and vacuum is used to expel gases and volatiles, minimizing air bubbles in the finished product.
5. Metering Section: The small-lead screw applies pressure, ensuring the melt steadily fills the screw channel, and finally, it is extruded from the die to form the final product. Accurate product dimensions are critical at this stage.
