How Does Extruder Screw Elements Influence the Performance of Twin Screw Extrusion Machines

The extruder screw, playing an important role in the extrusion machine performance, consists of different types of extruder screw elements, such as conveying elements, shearing elements, mixing elements. These screw elements also come in sub-types, including single head screw elements, double head screw elements, and triple-head screw elements, which exert different functions on plastic material blending, mixing and componding. Among the various types of twin-screw extruders, the intermeshing co-rotating twin-screw extruder is widely used in the plastics industry. Hence, in this article, Granuwel, your trusted manufacturer & supplier of extrusion machines and extruder screw elements will take you through different types of extruder screw elements that usually used in intermeshing co rotating twin screw exdruders and how extruder screws elements influence the plastic extrusion process.

Intermeshing Co-Rotating Twin-Screw Extruder Characteristics

The screws of the intermeshing co-rotating twin-screw extruder have the following main characteristics:
(1) The two screws rotate parallel and in the same direction, generating a uniform shearing action between their intermeshing parts and the barrel. The intensity of this shearing action can be adjusted through screw combination and spacing design.
(2) The geometric shape of the screw elements and their co-rotating nature endow the screws with excellent material distribution and mixing capabilities, suitable for compounding operations. After the material enters the barrel and is softened, due to the opposite directions of the twin screws at the intermeshing point, one screw pulls the material into the intermeshing gap while the other pushes it out. Thus, the material is transferred from one screw to the other in an “∞” motion. This motion has a significant relative velocity at the intermeshing point, which is highly beneficial for material mixing and homogenization. Moreover, the intermeshing area has a very small gap, and the material moves in the opposite direction to the screw rotation, resulting in a high shearing action that achieves uniform plasticization.
(3) The co-rotating twin-screw extruder has reactive capabilities and acts as a dynamic reactor. Materials can undergo a series of chemical reactions after melting in the barrel, such as polymerization and grafting. Reactive extrusion processing is mainly used for: polymerization reactions of monomers or oligomers (free radical polymerization, addition polymerization, condensation, and copolymerization reactions); controlled crosslinking and degradation of polyolefins; graft modification of polymers (functionalizing or polarizing the polymer to achieve material modification and the preparation of compatibilizers); and forced blending modification of various materials. It also includes physical modification of materials, such as filling, compounding, toughening, and reinforcing.
(4) There are many types of screw elements, including conveying elements, kneading elements, shearing elements, reverse-thread elements, and pressure-increasing thread elements, each serving different functions. For the common building-block screws, various elements can be combined like building blocks according to the material processing needs, and optimized design can be applied to accommodate the processing of various material formulations.

Types and Characteristics of Building-Block Twin-Screw Extruder Screw Elements

Under normal circumstances, based on different structures, the common types of screw elements include conveying elements, shearing elements, mixing and dispersing elements, etc.

(1) Conveying Elements 

Conveying elements are divided into forward conveying thread elements and reverse conveying thread elements. The main difference is that the action direction of the forward conveying element is the same as the extrusion direction, while the reverse is the opposite. The reverse action can hinder the forward conveying of the material, mainly to extend the material’s residence time in the barrel, thereby improving the filling degree and material pressure, greatly promoting the mixing effect

When setting the conveying screw elements, it is important to consider characteristics such as depth, lead, screw ridge thickness, and clearance, as shown in Figure 1. Its main function is to convey material, and the material’s residence time in the conveying screw element part is relatively short. Among all characteristic parameters, lead is the most critical factor. The larger the lead of the screw element, the higher the extrusion volume, and the shorter the material residence time, but such action will reduce the mixing quality.

Figure 1: Conveying Elements of Extruder Screw

Note: D = screw outer diameter, d = screw bottom diameter, P = pitch, L = lead

There are generally the following scenarios, mainly using large lead screw elements: occasions that emphasize high extrusion volume with conveying action; heat-sensitive materials that need to minimize material residence time in the barrel to reduce material degradation; considering the combination structure, generally applied at the exhaust port, increasing the material surface area is conducive to degassing.

When emphasizing mixing performance, medium lead screw elements are selected, mainly applied to combinations where the lead gradually decreases, serving the purpose of conveying and pressurizing.

Small lead screw elements are mainly used in the melting section to achieve pressurization and improve melting effects, while also increasing the degree of mixing and the stability of the extrusion system.

(2) Shearing Elements

Shearing elements mainly refer to kneading blocks, which provide high shearing force and have the ability to distribute and disperse mixing. The main parameters include the number of heads, thickness, and staggering angle. They are always used in pairs or in series, with an angle between adjacent kneading blocks, i.e., the staggering angle. The intermeshing blocks on the two screws are close to each other, forming a “grinding disk” structure. Materials are mixed and exchanged inside the barrel, and multiple adjacent kneading blocks combined together can form a spiral angle, which, with the rotation of the screws, promotes material mixing and exchange along the axis of the screw element.

Figure 2: Shearing Elements of Extruder Screw

Staggering Angle, as shown in Figure 2, α, commonly used parameters include 30°, 45°, 60°, and 90°, with different parameters having different effects. When the screw element is forward, the larger the staggering angle, the lower the conveying capacity, which extends the material’s residence time in the barrel and enhances the mixing quality.

Thickness, as shown in Figure 2, t, generally varies from 7 to 19 mm, customized according to usage requirements, closely related to the mixing effect. As the thickness increases, the generated shearing force also increases, and the mixing effect becomes less noticeable; conversely, the mixing effect is better.

Whether it is a conveying element or a shearing element, the parameter involved is the number of heads, mainly including single-head, double-head, and triple-head, as shown in Figure 3.

Figure 3: Single, Double, and Triple-head Shearing Elements of Extruder Screw

When the screw element rotation is forward, the more heads there are, the lower the extrusion conveying capacity, the smaller the generated torque, and the worse the mixing ability, but the shearing action is enhanced; when the rotation is reverse, the more heads indicate a greater extrusion conveying capacity and worse mixing ability.

Single-head Screw Element

A larger thickness can minimize material leakage (backflow); it has a smaller capacity than a double-head screw; the conveying efficiency is the highest.

Double-head Screw Element

As a conventional conveying element in co-rotating twin-screws; it has less shearing force than a triple-head element; used for solid feeding, melt conveying, degassing, and melt conveying. Mainly used for extrusion, it has the characteristic of uniform heating and good self-cleaning performance.

Triple-head Screw Element

It has higher shearing force and is mainly used for melting, dispersion, and mixing. It allows the material’s pressure and temperature distribution in the barrel to be more flexible, producing good degassing and volatilization effects, but the output is lower.

(3) Mixing Elements 

Generally, mixing elements refer to toothed elements (including straight and helical teeth), which are screw elements with grooves on the screw ridge, as shown in Figure 4. The main function of the groove structure is to connect adjacent screw grooves and promote material mixing with each other, ultimately achieving melt homogenization and promoting material longitudinal mixing effects. Because the screw ridge is grooved, its conveying capacity and pressurizing capacity are somewhat reduced, but this also increases the material filling degree in the screw groove and increases the material residence time

Figure 4: Mixing Elements (Refer to toothed elements, (including straight and helical teeth)

The number and shape of the teeth of the mixing element are key to the requirements of mixing. The shape of the teeth mainly serves to disturb the flow of material, which can accelerate the uniformity of the material. The more teeth there are, the more noticeable the mixing effect, but in actual use, it is necessary to pay attention in time to whether the shearing is excessive and causes unnecessary damage to the material molecules.

Combination Application of Building-Block Twin-Screw Extruder Screw Elements

Under normal circumstances, a complete set of screws can be divided into five sections according to different functions: conveying section, melting section, mixing section, degassing section, and homogenizing section, as shown in Figure 5.

Figure 5: Five Sections of Extruder Screws (includingconveying section, melting section, mixing section, degassing section, and homogenizing section)

Conveying section

Mainly for conveying material, while also preventing material from overflowing the feeding port; configured with large lead conveying elements.

Melting section

Through heat transfer and friction shearing, the material in the barrel is completely melted and uniform. Configured with small lead conveying elements.

Mixing section

Single-component or multi-component materials are exchanged with each other, and the best state is to achieve complete mixing; configured with multiple groups of shearing elements with different staggering angles and mixing elements.

Degassing section

Mainly to discharge moisture and low molecular weight substances and other impurities outside the material system to achieve purification; configured with large lead conveying elements.
Homogenizing section:
Mainly for conveying and pressurizing, increasing the density of the material fluid at the extruder outlet to a certain degree, while also making the mixing more complete, ultimately achieving stable extrusion. Configured with small lead conveying elements.

Summary

In the design and application of twin-screw extruder, screw and screw components are the core components that determine its performance. Nanjing Granuwel Machinery Co., Ltd. with its deep technology accumulation and innovation ability in the field of extruders, to provide customers with a series of high-performance twin-screw extruder solutions. Our twin-screw extruders meet the high precision and high efficiency requirements of the plastics processing industry with their excellent mixing efficiency, precise melting control and stable conveying performance. Through the precise calculation and optimization of screw geometric parameters, combined with advanced processing technology and material science, Nanjing Granuwel Machinery Co., Ltd. ensures the reliability and durability of extruders under various process conditions. Our products not only lead in technology, but also show our professional commitment in quality control and service support. We look forward to working with you to drive innovation and development of plastics processing technology.