Plastic extruders can be classified into single-screw extruders, twin-screw extruders and multi-screw extruders according to the number of screws they have. At present, single-screw extruders are the most widely used and are suitable for the extrusion processing of general materials. The twin-screw extruder features less heat generated by friction, more uniform shearing of the material, greater conveying capacity of the screw, more stable extrusion volume, longer residence time of the material in the barrel, and uniform mixing. The conical twin-screw extruder features forced extrusion, high quality, wide adaptability, long service life, low shear rate, less material decomposition, good mixing and plasticizing performance, and direct powder molding. It is equipped with temperature control and vacuum exhaust devices. It is applicable to the production of products such as pipes, plates and special-shaped materials. Single-screw extruders play a significant role both as plasticizing and granulation machinery and as forming processing machinery. In recent years, single-screw extruders have witnessed considerable development.
At present, the large-scale single-screw extruders for granulation produced in Germany have a screw diameter of up to 700mm and a production capacity of 36 tons per hour. The main sign of the development of single-screw extruders lies in the development of its key component – the screw. In recent years, a large amount of theoretical and experimental research has been conducted on screws. Up to now, there are nearly a hundred types of screws, including common ones such as separation type, shear type, barrier type, diversion type and wavy type, etc. From the perspective of the development of single-screw extruders, although single-screw extruders have been relatively perfected in recent years, with the continuous development of polymer materials and plastic products, more distinctive new types of screws and special single-screw extruders will emerge.
Overall, single-screw extruders are developing towards high speed, high efficiency and specialization. The twin-screw extruder has excellent feeding characteristics and is suitable for powder processing. Compared with the single-screw extruder, it has better mixing, venting, reaction and self-cleaning functions. Its superiority is more evident when processing plastics with poor thermal stability and blends.
In recent years, twin-screw extruders have made significant progress abroad. Various types of twin-screw extruders have been serialized and commercialized, and there are many manufacturers. They can be roughly classified as follows:
(1) According to the relative positions of the two axes, they can be classified as parallel or conical.
(2) According to the meshing procedure of the two screws, they can be classified into meshing type and non-meshing type.
(3) According to the rotation direction of the two screws, they can be classified as co-rotating and counter-rotating. Among the counter-rotating screws, there are further distinctions of inward and outward rotation.
(4) According to the rotational speed of the screw, it can be divided into high-speed and low-speed types.
(5) According to the structure of the screw and the barrel, it can be divided into integral and combined types.