Assembling the screw elements in a co-rotating twin screw extruder is like assembling building blocks. It is highly flexible and customisable. To achieve optimal mixing, it’s essential to consider several factors, including material properties, feeding methods, and exhaust mechanisms. When dealing with complex material systems, matching the correct screw combination becomes vital. Each screw element performs a specific function, and different process stages require distinct combinations.

The main types of screw elements include conveying, shearing, and mixing, as illustrated in Figure 1. These elements are usually classified according to their structure and characteristics. The most commonly used types are conveying elements, mixing and dispersing elements (such as toothed discs and kneading blocks) and shearing elements.

Under identical operating conditions, different screw elements primarily differ in their distribution, mixing, and conveying capacities, as shown in Table 1.

Conveying Elements

Conveying elements can be divided into forward and reverse conveying screw elements. The key difference is that forward elements push material in the direction of extrusion, while reverse elements act against the extrusion direction. This reverse action increases material retention time in the barrel, thereby boosting filling, material pressure, and mixing efficiency.

When setting conveying screw elements, several characteristics should be considered, including depth, lead, flight thickness, and clearance, as illustrated in Figure 2. The primary function of these elements is to transport material, with a shorter local residence time in the barrel.

Among these characteristics, lead is the most crucial factor. The larger the lead, the higher the extrusion output, resulting in shorter material residence times, which can reduce mixing quality, as shown in Table 2.

In general, large-lead screw elements are primarily used in scenarios where high throughput is emphasized, such as when dealing with heat-sensitive materials that require minimal residence time to prevent degradation. They are also used near exhaust ports to increase material surface area for effective degassing.

When a balance between conveying and mixing is desired, medium-lead screw elements are typically chosen. These elements are often used in combinations where the lead gradually decreases, providing both conveying and pressurizing functions. Small-lead screw elements are primarily applied in the feed and melting zones to enhance pressure and melting efficiency, while also improving mixing and ensuring the system’s stability.

This approach to assembling screw elements ensures that twin-screw extruders can handle a wide range of materials and processes, providing flexibility and efficiency in industries like plastics, pharmaceuticals, and more.