The twin-screw extruder can be divided into an opposite-direction twin-screw and a co-rotating
twin-screw according to the direction of rotation of the two screws. Let us look at the difference between the two twin-screw extruders and the single-screw extruder:
1. Differences in working principle between co-rotating twin-screw extruder and single-screw extruder
The structure and function of the co-rotating twin-screw extruder are similar to those of a single-screw extruder, but there are significant differences in the working principle. Mainly in the following seven aspects:
1) Forced delivery. Engaging the twin-screw in the same direction, the two screws move in opposite directions at the meshing position, one screw pulls the material into the meshing gap, and the other screw pushes the material out of the gap, resulting in material from a screw channel Going to another screw channel, the material is forced to convey along the screw in the direction of the head.
2) Homogenization and mixing. The co-rotating twin-screw has a small clearance at the meshing point, the speed direction of the spiral rib and the groove is opposite, and the relative speed is large, so the meshing zone has a high shearing speed, the shearing force is large, and the mixing effect is much better than the
Single Screw Extruder and counter-rotating twin-screw extruder.
3) Self-cleaning. The co-rotating twin-screw extruder has a relatively high shear speed due to the opposite direction of the speed of the screw and the groove of the meshing zone, and can scrape off any accumulated material adhering to the screw. Good self-cleaning effect, so that the residence time of the material is very short, and it is not easy to cause local degradation and deterioration.
4) Plasticization of materials. The size of the screw gap has a great influence on the quality of material plasticization. The smaller the gap, the greater the shear force, but the amount of material passing through is reduced; the larger the gap, the greater the amount of material passed, but the shear force is reduced.
5) Compression of materials. The method of compressing materials in the same direction twin-screw extruder is much more, and the comprehensive effect is good.
6) Feeding method. The co-rotating twin-screw extruder requires uniform and quantitative feeding, and adopts a metered hunger feeding method.
7) Exhaust. Due to the hunger feeding, a large lead thread conveying element can be used, so that the groove is in an underfill state and is in a zero pressure state, so that the exhaust section can be set.
2. The main difference between the counter-rotating twin-screw extruder and the single-screw extruder
What is the input principle of the counter-rotating twin-screw extruder?
In a counter-rotating twin-screw extruder, the material is conveyed like a positive displacement in a gear pump. The counter-rotating twin-screw extruder has a low rotation speed, and the material shearing heat is small, and the material is not easily decomposed. Therefore, it has low requirements on the thermal stability of the material, and is particularly suitable for the molding processing of heat-sensitive materials (such as PVC). Compared with the co-rotating twin-screw extruder, the counter-rotating twin-screw extruder has higher conveying efficiency, better exhaust effect and melting effect, although the dispersing and mixing effect of the material is worse than that of the Tongxiang Shuangxuan extruder. However, the ability to establish a stable head pressure is stronger, so it is more suitable for direct extrusion of products. In addition, the use of a counter-rotating twin-screw extruder instead of a single-screw extruder can eliminate the pre-plasticizing granulation process and directly extrude the PVC powder to reduce the cost of the product. The counter-rotating twin-screw extruder is mainly used for extruding PVC pipes. Profiles, plates and granulation. Parallel counter-rotating twin-screw extruders have also been applied on large HDHDPE pipe production lines.
Counter-cone twin-screw extruders have traditionally been used for the extrusion of polyvinyl chloride products, but in recent years there have been examples of the application of an isotropic conical twin-screw extruder to the extrusion of polyolefin pipes. Practice has shown that Basic success has been achieved. The purpose of this approach is to save energy. A 65-piece 132umum conical twin-screw extruder with a drive motor power of only 37kW. When extruding polyethylene tubing, the pipe size and output can be equivalent to a single-screw extruder of 75mm or even 80mm. A single screw extruder of 75mm or 80mm has a minimum drive power of 90kW. There is already a ready-made experience in the extrusion of products with parallel counter-rotating twin-screw extruders. The use of anisotropic conical twin-screw extruders in China is still due to the cost of equipment. Parallel counter-rotating twin-screw extruders The manufacturing cost is higher than that of the anisotropic conical twin-screw extruder.
This method of use is of course not a simple alternative. After all, the melt flow properties of polyethylene melt and polyvinyl chloride are significantly different, so it is necessary for an anisotropic conical twin-screw extruder for extruding polyvinyl chloride products. Redesigning the screw can be used to extrude polyethylene; and the nose should be modified accordingly. However, some people in China have tried to extrude products with co-rotating twin-screw extruders (including cone double and flat double), but some progress has been made, but Xiaobian believes that from the perspective of energy saving, the same direction and the opposite twin screw are not greatly Differences, while establishing a stable head pressure in a co-rotating twin-screw extruder is more difficult, and it is not as easy as using a counter-rotating twin screw directly.
