Extrusion process of twin screw extruder
DATE：2021/7/15 8:45:34 / READ： / SOURCE：This station
Extrusion process of twin screw extruder
After the two screws of the intermeshing co rotating twin screw extruder are kneaded together, three gaps will be formed. In addition, there are four gaps between the top of the screw rib and the inner wall of the barrel: 1. The gap between the top of the screw rib and the inner wall of the barrel.Radial clearance (also known as calendering clearance) is the clearance between the top surface of a screw thread and the bottom (root diameter) of another screw groove. Because the gap is like the gap between calender rollers, it is also called calender gap. 3. In some cases, the side wall of the screw edge is not perpendicular to the bottom of the screw groove, but forms an angle. It is located between the two screw adjacent screw edge side, approximate to tetrahedron, so it is called tetrahedral gap. If it is a rectangular thread, the two screws mesh together, so-called tetrahedral gap will not be formed. The tetrahedral gap is long near the plane passing through the axes of the two screws, and short and wide at the end of the meshing zone (near the inner wall of the barrel). Antirust paper coater www.handern.com
4-side clearance is the clearance between the sides of two screw ribs. Because the screw groove is longitudinally open, there is a channel between the two screws from the feeding port to the machine head. When the material is added to the screw from the feeding port, it will be transported forward along the screw groove to the lower wedge area under friction drag, where the material is compressed to a certain extent. If the screw edge is narrower than the screw groove, the screw edge of the other screw will not block the material forward path. Because the two screws have a velocity gradient of equal size and opposite direction in the wedge-shaped area, it is impossible for the material to enter the meshing area and continue to move around the same screw. Instead, the material is lifted by another screw and transported forward along the groove of the other screw under the friction drag of the barrel surface. When the material moves to the upper wedge area, the process is repeated, but it has moved a certain distance in the axis direction. Therefore, from the macro point of view, the material is around the two screws, along the spiral ∞ channel, until it reaches the die. In the co rotating twin-screw, there is no tendency to collect materials at any point on the ∞ channel around the two screws. There is no calendering effect between the two screws of co rotating twin screw, so there is no force to separate the two screws. The material makes the screw float in the center of the barrel, so it can keep a small gap between the two screws and between the screw and barrel without serious wear. Antirust paper coater www.handern.com
In order to improve its targeting ability, the co rotating twin-screw is usually made into a close meshing and conjugate type.
In this method, the side of one screw edge sweeps the side of the other screw in the tangential direction at a constant speed; In the so-called calendering gap, the top of the screw edge of one screw sweeps the root of the other screw in the tangential direction at a constant speed. These two "sweeping" movements prevent the material from remaining and sticking in the side gap and calendering gap. This is particularly important for thermosensitive materials, and makes it possible for the materials to have uniform and rapid color change, leaving no residue. Antirust paper coater www.handern.com
Because there are several gaps between the meshing screws, the material will flow in these gaps under the action of axial pressure gradient and tangential pressure gradient. The flow in the gap between the top of the screw rib and the inner wall of the barrel is mainly composed of pressure term and drag term. The flow in the tetrahedral gap is mainly affected by the tangential pressure gradient. Tetrahedral gap is very important for co rotating twin-screw. Under the action of tangential pressure gradient, the material flows from one screw groove to another screw groove through tetrahedral gap. There is no calendering effect because the top of the rib of one screw and the root of the groove of the other screw slide through each other in the opposite speed direction, which means that the melt is subject to constant shear rate and the particles in the melt are subject to uniform shear stress. Antirust paper coater www.handern.com
If the spiral groove area and meshing area are analyzed together, the case is: the material layer with low shear stress at the bottom of the spiral groove of one screw is scraped off by the screw edge of another screw in the meshing area and gathered in the high shear area of this screw (barrel wall area). On the contrary, the material layer with high shear stress at the barrel wall area is brought to the bottom of the spiral groove of another screw in the meshing area. In addition, there is no significant low shear stress region in the co rotating twin screw, which makes it possible to plasticize uniformly and mix well. Antirust paper coater www.handern.com
The reason is that the velocity gradient at the top of the spiral groove is larger than that at the bottom of the spiral groove. Antirust paper coater www.handern.com
Under the condition that the feed rate is less than the maximum theoretical output, the increase of the feed rate makes the reverse pressure flow decrease, thus reducing the average shear stress of the material in the spiral groove, so the shear effect of the material is small. The results show that the screw speed has a significant effect on the average shear stress of the material in the screw groove, and the average shear stress increases with the increase of screw speed. The average shear stress of the material in the groove of the large lead screw is greater than that of the small lead screw. The average shear stress of the material in the groove of double thread and three thread is similar, but the average shear stress of the material in the single thread is much less than the former two. Antirust paper coater www.handern.com
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