Twin Screw Extruder Working Principles

The extrusion process of twin-screw extruders also involves solid conveying, melting and kneading, metering conveying, and pressure building. What is extruding? Extruding is a manufacturing process that transforms raw materials into a continuous profile by forcing them through a die. Unlike single-screw extruders, twin-screw extruders generally use quantitative feeding, and the extrusion process also involves venting design issues. It is generally believed that compared with single-screw extruders, twin-screw extruders have a narrower residence time distribution. This is because twin-screw extruders have self-cleaning capabilities and possess a certain degree of positive displacement conveying capacity, which is different from single-screws that rely purely on friction for conveying. Since most screw channels are in an unfilled state during twin-screw operation, their solid conveying and melting mechanisms are also different from single-screw extruders. What is extruding's role in this context? It's the fundamental process that enables the transformation of raw materials through these complex mechanisms.

1.4.2.1 Counter-Rotating Twin-Screw Working Mechanism

Counter-rotating twin-screw extruders can achieve both horizontal and vertical sealing functions, making them excellent positive displacement devices. What is extruding in this context? It's the process where materials are transformed through these specialized mechanisms. Materials are enclosed in several separate C-shaped chambers, as shown in Figure 1-64. When the screw rotates, the C-shaped chambers move one pitch toward the die, thereby achieving the purpose of conveying materials. For outward counter-rotating twin-screw extruders, during feeding, materials are easily drawn into the meshing gap under gravity, friction, and the meshing action of screw flights and channels. In the meshing zone, materials are subjected to grinding and rolling between screw flights and channels, an action similar to rolling on a calender, hence the term "calendering effect" for twin screws.

When thick materials fill the meshing gap, a back pressure is formed that separates the axes of the two screws, causing axial bending deformation of the screws. Increased deformation will reduce the gap between the screw and barrel, accelerating wear between them. The degree of wear is extremely sensitive to changes in screw speed and overload. Excessive deformation will also cause the screw to scrape the barrel, resulting in severe damage to the surfaces of the screw and barrel in a short period. Therefore, inward counter-rotating twin screws can only operate at low speeds, generally between 8~50r/min.

In recent years, with continuous improvements in design levels, counter-rotating twin-screw extruders have also been pursuing higher speeds, higher output, and higher efficiency. Janssen (1978) researched the melt conveying mechanism of counter-rotating twin-screw extruders. In China, Professor Geng Xiaozheng and his research group have conducted extensive and in-depth studies on the twin-screw extrusion process using finite element methods since 1987, greatly enriching the positive displacement pump conveying theory.

Pressure build-up in counter-rotating twin screws occurs only when materials fill the screw channels near the die end. Due to their positive displacement conveying mechanism, the extrusion process exhibits pressure pulsation characteristics, which can be mitigated by using multi-start threads. Under normal processing conditions, the number of filled screw channels adapts to the die resistance. The greater the die resistance, the longer the filled length of the screw channels, and the pressure gradient along the axis remains constant. When the feeding amount is constant and only the screw speed is increased, it is found that the filled length of the screw channels decreases, and the axial pressure gradient of the screw increases, but the die pressure remains unchanged.

The melting process of intermeshing counter-rotating twin-screw extruders differs from that of single-screw extruders. The lower screw channel space in the plane where the screw axes lie is called the lower meshing zone, and the upper space is the upper meshing zone. Experiments have found that the melting zone moves downstream in the extrusion direction as the screw speed increases. There are differences in the melting process between conventional thread and kneading block combinations. The lower meshing zone is completely filled with materials, and the materials in contact with the inner surface of the barrel first melt to form a melt film. In the later stage of melting, a "sea-island" structure is formed with the melt as the continuous phase and residual solid particles suspended in it, with the residual solid phase mainly concentrated near the advancing screw flight.

What is extruding's melting mechanism in counter-rotating twin screws? The melting mechanism of intermeshing counter-rotating twin-screw extruders depends on the structural form of the screw elements and the conveying mechanism. In thread elements, there is a significant difference between the upper and lower meshing zones, with melting mainly occurring in the lower meshing zone, and the main energy source is heat conduction from the barrel. In the melting process of toothed discs, the melting processes in the upper and lower meshing zones are basically the same, with the main heat sources being barrel heat conduction and viscous dissipation heat. Leakage flows in the side gaps and calendering gaps play a very important role in the melting process, promoting material exchange and the progress of the melting process. The "sea-island" melting efficiency is higher than that of the solid bed model in single-screw extruders.

1.4.2.2 Co-Rotating Twin-Screw Working Principles

Overall, co-rotating intermeshing twin-screw extruders are similar to single-screw extruders, but they have many operating variables: feeding is an independent variable; the configuration of twin screws is also very complex, with variable thread element combinations, and the screw configuration is also an independent variable; there are meshing zones and gap zones in twin screws, making the movement law of materials much more complex. Therefore, extrusion processes with different configurations of twin screws are also different.

In terms of meshing principles, it is impossible for co-rotating intermeshing twin-screw extruders to achieve complete horizontal and vertical sealing. The screw channel width must be designed to be larger than the screw flight width. Currently, widely used self-cleaning co-rotating intermeshing twin-screw extruders have narrower screw flights and wider screw channels, with a spiral "∞" shaped channel between the two screws, presenting a leaf-like structure in the meshing zone, as shown in Figure 1-65.

In terms of conveying mechanism, co-rotating twin screws have both positive displacement conveying and frictional drag conveying. The larger the screw flight width, the more obvious the positive displacement conveying effect, as shown in Figure 1-60. The positive displacement conveying capacity of Figure 1-60(d) is greater than that of Figure 1-60(c). In recent years, based on visualization experimental technology, research on solid conveying and melting processes has made certain progress, deepening people's overall understanding of the co-rotating twin-screw extrusion process.

Referring to Figures 1-24 to 1-26, research on solid conveying by Professor Geng Xiaozheng's research group at Beijing University of Chemical Technology shows that there are differences in the solid conveying mechanisms between granular materials and powder materials: the positive displacement conveying dominates the granular material conveying process, and only a few particles move along the screw channel direction. When using offset feeding, the bottom of the screw channel is filled, but the conveying capacities of the two screws are different. For example, with right screw offset feeding, the left screw has greater conveying capacity. When performing quantitative feeding and conveying of powder materials, the filling degree of the left screw is higher than that of the right screw. At small feeding amounts, the lower part of the left screw is filled while the right screw is not filled. When the feeding amount increases to the point where both screws are filled, an "8"-shaped flow along the screw channels of the two screws appears.

Overflow feeding conveying shows that the threads are completely filled with materials at this time. The forward thread elements with small lead have higher bulk density of materials and stronger feeding capacity. When designing the screw configuration for overflow feeding, the compression degree of materials should be compatible with the conveying capacity; otherwise, blockage may occur.

The melting process is a very important stage in the extrusion process of co-rotating twin-screw extruders and also the most energy-consuming stage. Due to the ever-changing screw configurations, quantitative research is very difficult. Research on this aspect gradually increased in the 1990s, and Todd (1993), Cumy (1995), White (1995), Potent (1996), and others have done very meaningful work. Professor Geng Xiaozheng's research group proposed a new idea of melting sub-zones based on previous research. Different melting sub-zones obtain energy in different ways, some mainly through convective heat transfer, and others mainly through frictional heat generation. What is extruding's melting process in this context? It's the transformation of solid materials to molten state through these complex energy transfer mechanisms.

The research on the melt conveying mechanism of co-rotating twin screws has now been fully understood with the help of numerical simulation technologies based on finite element and finite volume methods. For self-cleaning co-rotating intermeshing twin-screw extruders with horizontal sealing and vertical opening, the positive displacement conveying capacity is related to the degree of vertical opening. The smaller the opening degree, the stronger the positive displacement conveying capacity, as shown in Figure 1-60. For currently commonly used self-cleaning twin-screw extruders with narrow screw flights, melt conveying is mainly based on drag flow along the "∞"-shaped spiral channel shown in Figure 1-56, with the melt movement direction as shown in Figure 1-60(e). The velocity vector diagram in the fully filled screw channel is shown in Figure 1-66, and the pressure distribution in the screw channel is shown in Figure 1-67.