Screw Combination of Co-rotating Parallel Twin Screw Extruder

1. Number of feeding ports

The screw of the twin-screw extruder is a building block combination. The combination of the entire screw should be selected according to the specific formula, material characteristics, mixing requirements and operating conditions. In addition, before the screw combination, it is necessary to determine the number of feeding ports used according to the modification method.

(1) Solid filler filling

When adding a solid filler to a resin for filling and modification, generally only one feeding port is needed. At this time, the combination of the screw and the barrel can be regarded as a combination. However, if the filler is seriously worn on the barrel, two feeding ports can be considered. The first feeding port is filled with resin, and the second feeding port is added with filler. That is, when the resin is melted, the filler is added, so that the melt can wet and cover the solid filler, so that it does not directly contact the screw and barrel, and reduces the wear of the latter. If the amount of filler added is so large that it is difficult to completely add it from one feeding port, a second feeding port or even a side feeding port can also be considered.\

(2) Alloy blending

To make plastic alloys by blending two polymers with different viscosities, it is necessary to know the ratio of the two components. If the proportion of low-viscosity materials is small, one feeding port can be used, that is, a section is used. If the proportion of the two components is close to the same, two feeding ports can be used. The high-viscosity material is added at the first feeding port, and the second feeding port is set near the melting point, and the low-viscosity material is added at a higher rate. If the proportion of low-viscosity materials is high, even three feeding ports can be used. This is to consider that if a higher proportion of low-viscosity material is added from the same feeding port with high-viscosity material, the former acts as a lubricant, which will prevent the shearing element from inputting energy to the high-viscosity material, so that it cannot be melted. The use of the above method is a specific application of the isoviscosity principle that is often followed when making blends.

2. Screw combination

After the number of feeding ports is determined, the selection and combination of various screw elements in each section can be carried out. Since each segment plays a different role in the entire extrusion process, the combination of each segment should also be different. The basic principles of screw combination are shown below.

(1) A large lead thread should be used at the feeding port to ensure smooth feeding.

(2) A small lead thread should be used in the melting section to build up pressure, so as to compress and melt the material, a kneading block with a staggered angle of 90° can be set to balance the pressure, or a kneading block with a staggered angle of 30° can be used for preliminary distribution and mixing of materials. The kneading blocks should be set from the middle of the melting section, and note that the kneading blocks should be arranged at intervals.

(3) The main purpose of the mixing section is to shear, refine and disperse the material particles. The setting of the threaded elements in this section is very complicated and requires the designer to have rich practical experience. In this section, kneading blocks with staggered angles of 45° and 60° are mainly used to enhance shearing, supplemented by special elements such as tooth-shaped elements or "S"-shaped elements. However, it should be noted that the kneading and shearing elements cannot be set too much, and they cannot be arranged too closely to avoid excessive shearing. In addition, in order to enhance the conveying capacity of this section of material, screw conveying elements should also be arranged at intervals, that is, the kneading block and the screw conveying elements are staggered from each other.

(4) A reverse thread element or reverse kneading block should be set before the exhaust port or vacuum port; A large lead thread element should be provided at the exhaust port or vacuum port; Small lead threaded elements should be provided after the exhaust port or vacuum port.

(5) In the homogenization section, the thread lead should be gradually reduced to achieve pressurization, and at the same time, attention should be paid to the use of single thread and wide thread to improve the discharge capacity and avoid material leakage.

Now take the screw with two feeding ports and one exhaust port as an example to analyze the basic idea of screw combination.

Parts of the first to second feeding ports:

The main function of this part is to melt and plasticize the resin. The first part of the screw corresponds to the feeding port. The screw element here has a large pitch, a deep groove, and a large volume of the screw groove (larger than the subsequent screw elements) to accommodate the added low bulk density material. The next section of screw should gradually compact the loose material, so screw elements with gradually smaller screw groove volume should be used (by changing pitch, groove depth, screw width, etc.), If the material needs to be mixed in this section, a section of toothed threaded elements can be added, followed by a section of equidistant and equal-depth forward threaded elements to build up pressure. A set of kneading discs can be connected after the positive screw element, the purpose is to promote the melting and plasticizing of the material (the material should be basically melted and plasticized here); If the reverse threaded element is connected, the purpose is to melt, build pressure and form a seal to facilitate feeding at the second feeding port at the rear.

The second feeding port to the exhaust port part:

The main function of this part is to add the second component (such as glass fiber and second resin, etc.) and mix it with the material fed from the feeding port. The threaded element at the beginning of this part is aligned with the second feeding port, and should have a larger volume of the screw groove, so that the material in the screw groove is in a half-filled state of no pressure, so as to facilitate the addition of the material. After that, the conveying element can be connected, and then the toothed element or the reverse thread element or the kneading disc element (the helical direction formed by the staggered angle can be forward or reverse), so as to facilitate the mixing of materials added from the two feeding ports and build up high pressure before the subsequent exhaust port, a seal is formed.

From the exhaust port to the die part:

Its main function is to exhaust, further mix, and extrude the mixed material from the die at constant pressure, quantitative and constant temperature. The beginning of this section is the threaded element facing the exhaust port, and its groove volume should be large to facilitate surface renewal. Downstream of the exhaust port, a forward screw conveying element can be connected, but the pitch should be gradually reduced (even if the volume of the screw groove is reduced), so that the material gradually fills the screw groove and builds up pressure. If it is desired to further enhance the mixing effect, a segment of toothed elements or kneading disc elements can be combined. In the last section of the screw, there should be equidistant and equal-depth forward conveying screw elements, so that the material can be conveyed to the die at a constant rate, constant pressure and constant temperature.

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