The extruders mainly used in the domestic plastics industry can be divided into two main categories: single-screw extruders and twin-screw extruders. These two types of extruders are widely used in the plastics field due to their unique characteristics. But what are the advantages of each type, and what are the differences between them? Furthermore, how should we select appropriate screw parameters for different raw materials? We will explore these questions one by one.
A single-screw extruder, as its name suggests,
refers to an extruder containing only one screw. The barrel of this type of extruder is carefully designed with three different effective lengths, divided according to the screw diameter, pitch, and depth. Typically, these three lengths evenly occupy the entire barrel, each accounting for approximately one-third.
The first section: From the last thread at the feed nozzle to a certain point, this section is called the conveying section. In this area, although the material is not fully plasticized, it needs to be preheated and compressed. Previous theories considered the material at this point to be a loose mass; however, research has shown that it is actually a solid plug. After being compressed, the material takes on a plug-like shape, thus its primary function is to convey the material.
The second stage: Following the conveying stage is the compression stage. In this area, the volume of the screw channel gradually decreases, while the temperature needs to be raised to a level where the material can be plasticized. During this stage, the material is compressed, with a compression ratio of up to 3:1 (i.e., compressed from the width of the conveying stage to the width of the compression stage). After compression, the material enters the next stage.
The third stage: The metering stage is the final stage of the material's journey. Here, the material must maintain its plasticizing temperature and be accurately and quantitatively delivered to the die head. The metering stage operates similarly to a metering pump, ensuring precise supply of molten material. Temperature control in this stage is crucial; it must not fall below the plasticizing temperature and is typically slightly above it.
