The splicing and assembly of threaded elements of co-rotating twin-screw extruders is as flexible as building blocks. While meeting the mixing needs, multiple factors such as raw material performance, feeding method, and exhaust method must also be taken into consideration. Facing complex material systems, it is crucial to match a specific screw combination. Different screw elements have different functions, and different process sections also require different threaded screw combinations. Today we start by understanding screw elements and study the combination principles and characteristics.

Figure 1

In general, threaded elements are classified according to their different structures and characteristics. Commonly used threaded elements include the following types, namely conveying elements, mixing and dispersing elements (toothed discs, kneading blocks), shearing elements, etc. As shown in Figure 1.

Under the same operating conditions, the difference between different threaded elements mainly lies in the distributed mixing and conveying capabilities, as shown in Table 1.

Table1 Differences in the role of theaded elements

Conveying threaded elements are divided into forward-conveying elements and reverse-conveying elements. The main difference is that the action direction of forward-threaded elements is the same as the extrusion direction, and the reverse direction is the opposite. The reverse effect can hinder the forward transportation of materials. Its main function is to extend the residence time of materials in the barrel, thereby increasing the filling degree and material pressure, and greatly promoting the mixing effect.

Conveying threaded elements are divided into forward conveying elements and reverse conveying elements. The main difference is that the action direction of forward threaded elements is the same as the extrusion direction, and the reverse direction is opposite. The reverse effect can hinder the forward transportation of materials. Its main function is to extend the residence time of materials in the barrel, thereby increasing the filling degree and material pressure, and greatly promoting the mixing effect.

Large lead

Small lead

Generally, there are the following scenarios, mainly with large-lead threaded components: occasions where the conveying function is mainly used and high extrusion volume is emphasized; for heat-sensitive materials, it is necessary to minimize the stay time of the material in the barrel to reduce the degradation of materials; considering the combination structure, it is generally used at the exhaust port to increase the surface area of the material and is conducive to devolatilization and exhaust.

When emphasizing the mixing performance, medium-lead threaded elements will be selected, which are mainly used in combinations with gradually decreasing leads to play the role of transportation and pressurization. Small-lead threaded elements are mainly used in the conveying section and the melting section to achieve pressurization and improve the melting effect. At the same time, it can also improve the mixing degree and the stability of the extrusion system.

The shear element mainly refers to the kneading block, which is used to provide higher shear force and has the ability to distribute and disperse mixing. The main parameters are the number of heads, thickness and staggered angle. They are always used in pairs or in series. There is a staggered angle between adjacent kneading blocks, that is, a staggered angle. Only then can the materials in the extruder barrel be mixed and exchanged. Multiple adjacent kneading blocks can be combined to form a spiral angle, along with the rotation of the screw, promotes the mixing and exchange of materials along the axis of the threaded element, as shown in Figure 3.

Commonly used angle parameters are 30°, 45°, 60° and 90°. Different parameters have different functions and effects. When the threaded elements are in the forward direction, the larger the stagger angle, the lower the conveying capacity, which increases the residence time of the material in the barrel and enhances the mixing quality. However, there is a problem of material leakage between the threaded elements. Mixing quality includes distribution mixing and dispersion mixing. Distribution mixing will increase as the angle increases. Research shows that dispersion mixing shows different trend characteristics. It is best when the angle is 45°, followed by 30°, the worst is 60°; and when the angle of the threaded element is reversed, the greater the staggered angle, the worse the polymer mixing effect.

The thickness generally ranges from 7 to 19 mm and can be customized according to the use requirements. It is closely related to the mixing effect. As the thickness increases, the shear force generated also increases, and the mixing effect is weaker; conversely, the mixing effect is better. Regarding the distribution mixing effect and the dispersion mixing effect, the distribution mixing effect will become smaller as the thickness increases, and the dispersion mixing effect will increase as the thickness increases. As the thickness decreases, the effective flow rate of the material in the extruder barrel distributed to the axial direction increases, and at the same time, the effective flow rate in the radial direction decreases.

Whether it is a conveying element or a shearing element, the parameter involved is the number of heads, which mainly includes single heads, double heads, and three heads, as shown in Figure 4.

When the rotation direction of the threaded element is forward, the more heads there are, the smaller the extrusion conveying capacity will be, the smaller the torque generated, and the worse the mixing ability will be, but the shearing effect will be enhanced; When the rotation direction is reversed, the greater the number of heads, the greater the extrusion conveying capacity and the worse the mixing capacity.

Single-head element: Larger thickness can minimize material leakage; smaller capacity than double-head screw; maximum conveying efficiency.

Double-head element: As a conventional element for conveying elements in co-rotating twin-screws; it has smaller shear force than three-head elements; it is used for solid feeding, melt transportation, and exhaust. It is heated evenly and is well self-cleaning.

Three-head element: higher shear force, mainly used for melting, dispersing and mixing. It can make the pressure and temperature distribution of materials in the barrel more flexible, and the exhaust gas devolatilization effect produced is good, but the output is low.

Generally speaking, mixing elements refer to toothed elements (including straight teeth and helical teeth), which are threaded elements with grooves on the helical edges, as shown in Figure 5. The main function of the groove structure is to connect adjacent grooves and promote the mixing of materials, ultimately achieving the effect of melt homogenization and promoting longitudinal mixing of materials. However, because the screw rib is grooved, its conveying capacity and pressure-building ability are reduced. At the same time, this will also increase the filling degree of the material in the screw groove and increase the material residence time.

The toothed disc on the screw has staggered areas and non-staggered areas, which can not only promote material distribution and mixing, but also generate shear force perpendicular to the flow direction of the material flow, and have both distribution and dispersion mixing effects.

In addition, because the gap between the toothed elements is very small, it will generate a high shear rate, which is conducive to the dispersion and mixing of the material components. If the toothed disc is a straight disc with straight teeth, the material mainly relies on the pressure difference to pass through the toothed element, but the premise is that a certain pressure must be formed upstream.

If the mixing element is a combination of straight disc and helical teeth, and the inclination direction of the helical teeth is consistent with the direction of the positive thread element, then this mixing element will have a certain forward conveying effect in addition to the above distribution and dispersion effects. If the inclination direction of the helical teeth is opposite, the reverse force here will play a certain role in building pressure. For straight disc toothed elements,the tooth tips of the mixing element and the extruder barrel may stagnate and decompose the material due to the low material flow rate. The inner walls brush against each other, while the inner surface of the barrel in the non-staggered intervals is not brushed by the teeth.

The number and shape of the teeth of the mixing elements depend critically on the mixing requirements. The shape of the teeth mainly plays a role in disturbing the flow of materials and can accelerate the uniformity of the materials. The more teeth, the better the mixing effect. However, in actual use, it is necessary to pay attention to whether excessive shearing will cause unnecessary damage to the material molecules.

In recent years, with the development of polymer blending, modification, toughening and other mixing processing techniques and the deepening of understanding, more and more types and quantities of fillers are added to polymer compounds, which places higher requirements on mixing and processing equipment and processes.Therefore, in the field of polymer mixing processing technology, the research on mixing mechanisms and formula ratios, the development of mixing equipment and process control engineering technology, and the formulation of optimal PID parameters are inextricably linked..

Generally, the screw is divided into five sections: conveying section, melting section, mixing section, exhaust section and homogenizing section, as shown in Figure 6

Conveying section: mainly transports materials, and also needs to prevent materials from overflowing the feeding port.

Melting section: through heat transfer and friction shear, the material in the barrel is completely melted and evenly distributed.

Mixing section: single-component or multi-component materials are exchanged with each other. The best state is to achieve complete distribution and dispersion mixing.

Exhaust section: mainly emit and devolatilize impurities outside the material system such as moisture and low molecular weight substances to achieve the purpose of purification.

Homogenization section: Mainly for the purpose of transportation and pressure building, to increase the density of the material fluid at the outlet of the extruder to a certain extent, and at the same time to make the mixing more complete, and finally achieve the purpose of smooth extrusion.

The difference between distribution (distribution) mixing and dispersion mixing: Distribution mixing can re-divide and combine materials to achieve the purpose of full exchange of components; Dispersion mixing breaks the components into tiny particles, or disperses the sizes of two incompatible components to a desired range, mainly through shear pressure and tensile stress.

The principle of screw combination is to ensure the effect of mixing and extrusion. The function of the conveying section in the twin-screw extruder is mainly to convey the material to the next stage. Therefore, the threaded elements at the conveying end are mainly spiral elements; The function of the melting section is to melt the material and achieve the dispersion and mixing effect. The shearing effect produced by the kneading block element and the reverse thread element will make the material dispersed and mixed better, so the threaded element mainly used in the melting section is the kneading block and reverse threaded elements.

However, the conveying effect of the two is weak, and some forward threaded elements should be added to ensure the conveying effect of the melting section; The mixing section mainly distributes and mixes different materials. The special structures of the mixing elements and kneading blocks enable them to have good distribution mixing and dispersion mixing effects. Among them, the distribution mixing effect of the mixing elements is better,both are the mainly used threaded elements in the mixing section.Therefore, it is very important to optimize the combination of threaded elements in the melting section and mixing section.

I.

Large-lead threaded components should be used at the feeding port to increase the surface area of the material and make it smoother for the material to enter the extruder.

II.

In order to build up pressure in the melting section, small-lead threaded elements should be used to fully compress and melt the material. Kneading blocks with staggered angles of 90° and 30° are mainly used to balance the pressure and distribution mixing effect. In addition, it should be noted that the kneading blocks should be installed at intervals starting from the middle of the melting section.

III.

The main purpose of the mixing section is to shear, disperse, and fully mix the material particles. The design of the threaded elements in this section is relatively complex and generally needs to be adjusted based on the actual situation. There is rarely a one-step solution. To enhance the shearing effect, this section mainly uses kneading blocks with staggered angles of 45° and 60°. At the same time, toothed elements, 90° kneading blocks, or other new elements that mainly function as mixing elements need to be arranged at intervals to enhance the dispersion effect. However, it should be noted that the kneading blocks and shearing elements should not be too numerous or too close to avoid side effects caused by excessive shearing. In addition, several threaded conveying elements should be designed at intervals to push the material forward.

IV.

The exhaust port is generally divided into a natural exhaust port and a vacuum exhaust port. It is better to set up a reverse element before the exhaust port, which can be a reverse threaded element or a reverse kneading block. At the same time, a threaded element with a larger lead is installed at the exhaust port, and a threaded element with a smaller lead is installed immediately after the exhaust port. This combination can depolarize the small components in the material as much as possible.