What is a twin-screw extruder?

Twin-screw extruder is a kind of extruder which is developed on the basis of the single-screw extruder and has some connection with a single-screw extruder, but there are many differences. This article will introduce a twin-screw extruder in detail.

Here is the list of contents

• Background of the birth of twin-screw extruder

• Structure of twin-screw extruder

• The working principle of twin-screw extruders

Background of twin-screw extruder

With the development of the polymer processing industry, there are more and more and higher requirements for polymer material molding and mixing process, and the single-screw extruder cannot meet these requirements in some aspects. For example, in the single-screw extruder for filling modification and glass fiber reinforced modification, etc., the mixing and dispersion effect is not ideal. In addition, single-screw extruders are particularly unsuitable for the processing of powdered materials. In order to meet the requirements of the mixing process in polymer processing, especially the processing of hard PVC powder, twin-screw extruders have been developed in Italy since the late 1930s, have been improved and perfected for more than half a century, and have been greatly developed. Currently, twin-screw extruders are widely used in polymer processing, accounting for 40% of all extruders. Hard PVC pellets, pipes, profiles, and sheets are almost all processed and molded by twin-screw extruders. As continuous mixers, twin-screw extruders have been widely used for polymer compounding, filling, and reinforcement modification, and also for reactive extrusion. In the last 20 years, the development of polymer compounding and reactive extrusion technology has further contributed to the increase in the number and type of twin-screw extruders.

Structure of twin-screw extruder

The twin-screw extruder is composed of several parts, such as the drive unit, the feeding unit, the barrel, and the screw, and the role of each part is similar to that of the single-screw extruder. The difference from the single-screw extruder is that the twin-screw extruder has two parallel screws placed in the same barrel. There are many different forms of twin-screw extruders, the main difference being the screw structure. The screw structure of a twin-screw extruder is much more complex than that of a single-screw extruder because the screw in a twin-screw extruder has issues such as the direction of rotation, degree of engagement, etc.

The working principle of twin-screw extruders

Although the structure of a twin-screw extruder is very similar to that of a single-screw extruder, the working principle is very different. In a twin-screw extruder, the material is added by a charging device (usually a dosing device) and reaches the head die by the action of the screw. During this process, the movement of the material varies depending on the way the screw engages and the direction of rotation.

In the non-engaging twin-screw extrusion system, the material flows in various ways, in addition to the form of movement toward the head. Since the two screws do not engage, there is a large radial clearance between them and there is a large leakage flow. The main flow modes are as follows.

Flow occurs because the relative positions of the screw ribs of the two screws are staggered, i.e., the material pressure on the thrust surface of one screw is greater than the material pressure on the dragging surface of the other screw.

Material from the higher pressure screw thrust surface to the other screw dragging the surface of the flow, while with the rotation of the screw, in the gap between the two screws the material is constantly stirred and is constantly taken away and renewed, especially in the process of anisotropic rotation, the material is obstructed at A, resulting in the flow.

Multiple forms of material flow, including various flows due to the interaction between the two screws, increase the mixing and shearing of the material. However, this twin screw has no self-cleaning effect and is generally used only for blending, and is not suitable for the production of PVC profiles.

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