The working principle of a screw-type plastic extruder

The working principle of a screw-type plastic extruder relies on the pressure and shear forces generated by the rotation of the screw, which enable the material to undergo thorough plasticization and uniform mixing before being shaped through the die. Therefore, sometimes a single extruder can simultaneously accomplish a series of processes—including mixing, plasticization, and shaping—thus enabling continuous production.

　　The working principle of a screw-type plastic extruder relies on the pressure and shear forces generated by the rotation of the screw, which enable the material to undergo thorough plasticization and uniform mixing before being shaped through the die. Therefore, sometimes a single extruder can simultaneously accomplish a series of processes—including mixing, plasticization, and shaping—thus enabling continuous production.

　　In addition, the operating principle of a plunger-type extruder relies primarily on using plunger pressure to force the previously plasticized material through the die orifice, thereby achieving the desired shaping. After the material in the barrel has been extruded, the plunger retracts; once a new batch of plasticized material is added, the process resumes for the next cycle. This production technique is classified as discontinuous, and it essentially fails to provide thorough mixing and blending of the material. Moreover, this method requires pre-plasticization of the material beforehand; therefore, in actual production practice, it is seldom employed. It is typically reserved for plastics with extremely poor flowability or very high viscosity—such as nitrocellulose plastics—where it is particularly suitable for the forming and processing of these materials.

　　Add water or an appropriate liquid to the powdered material and continuously mix it. The well-mixed material is then extruded through a multi-hole die or a metal mesh under high extrusion pressure. Typically, after placing the material into a cylindrical container, it is extruded using a screw. With the use of variable-frequency technology, the extrusion pressure can be precisely controlled, allowing for the selection of the most suitable linear extrusion speed. The principle of single-screw extruders: A single screw is generally divided into three sections along its effective length, with the lengths of these three sections determined by the screw diameter, pitch, and depth of the flight. These sections are usually divided in equal thirds.

　　The last thread section at the feed port is called the conveying section: At this point, the material doesn't need to be plasticized, but rather preheated and compacted under pressure. Early extrusion theory used to assume that the material here was a loose bulk material; however, subsequent studies have confirmed that the material in this section is actually a solid plug—meaning that after being subjected to compression, it behaves like a solid plug. Therefore, its sole function is simply to complete the conveying task.

　　The second section is called the compression section. At this stage, the volume of the screw channel gradually decreases from large to small, and the temperature rises to the point where the material becomes plasticized. The compression here occurs between the conveying section—section three—and section one; this ratio is known as the screw’s compression ratio (3:1). Some machines may have variations, but once the material is fully plasticized, it moves on to the third section.

　　The third section is the metering section. Here, the material is maintained at the plasticizing temperature and is precisely and quantitatively conveyed—much like a metering pump—into the die head. At this stage, the temperature must not fall below the plasticizing temperature; typically, it’s slightly higher.