Important Components of Extruder—screw

Perhaps many people have discovered that most of the extruders used in large-scale product production lines are single-screw structures, and very few are twin-screw structures. Upstream raw material suppliers, especially manufacturers of modified materials, functional masterbatches or color masterbatches, mostly use twin-screw extruders. Why is there such a difference? In this article, the editor will analyze and discuss the difference between single-screw extruders and twin-screw extruders.

1. Screw structure and working principle

Single screw extruder: a screw with different thread structures in different parts. The material is melted and transported forward through heating and rotation of the screw. The conveying process mainly relies on the rotation of the screw and the dragging of the material by the barrel.

Twin-screw extruder: Two screws can rotate in the same direction or in opposite directions. The conveying process is the mutual movement and meshing of the two screws to force the material to move. In addition, the rotation of the two screws causes the movement direction of the material unit to change all the time, which has a stronger shearing effect on the material. The powder/oil material can be well wrapped between the polymers, so that the material can be well blended.

2. Function and application

Single-screw extruder: The single-screw extruder has a relatively simple structure, and its functions are only melting, plasticizing and conveying. Its blending ability is very weak. Precisely because of its relatively simple structure, its extrusion speed and conveying capacity are stronger than those of a twin-screw structure, making it suitable for large-scale production equipment with high output.

Twin-screw extruder: Since the two screws of the twin-screw extruder rotate simultaneously, in addition to forced conveying, it also has a good blending effect. For material modification, especially for formulas that require the addition of powder/oil materials, the mixing effect of the twin-screw extruder is far superior to that of the single-screw extruder.

3. Difficulty of equipment manufacturing

Obviously, the manufacturing difficulty of twin-screw extruder is much higher than that of single-screw extruder, especially the thread matching and meshing degree of the two screws, which also place high demands on equipment manufacturers, and are also important factors affecting the blending effect of the extruder. It is understood that the current twin-screw equipment manufacturers divide the screw into two parts: the screw shaft and the thread knot, and design many thread knots with different structures. According to the application scenario, application materials, required effects, etc., different combinations of thread knots can change the direction of the material unit in the barrel, thereby meeting the mixing effects and application requirements of different materials.

4. Summary

Manufacturers who modify materials need to add various additives to mix with resins. Some of these additives are in oily form, some are in powdery form, and some are even incompatible with resins. This requires the excellent blending effect of the twin-screw extruder to disperse the additives evenly in the resin to achieve the desired effect. Product manufacturers use resin pellets (including modified materials in granular form), which do not require strong shearing for mixing, and do not require a complex twin-screw extruder.

In addition, although high shear can achieve better mixing effects, it is not a good effect on polymer materials. Under the dual effects of heating and high shear, polymer materials usually undergo a certain degree of degradation, and degradation means quality degradation for products. This is one of the reasons why general polymer material manufacturers use single-screw extruders. Even if there is a demand for the use of special modified materials (relatively small in overall usage), they can be purchased upstream without using their own production equipment for blending and modification. In this way, only a small amount of modified materials will be degraded, which can avoid more degradation of all materials in the entire production through the twin-screw extruder.

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