The screw—the heart of a plastic extruder—is central to one of the two key processes in plastic processing. The plastic extrusion process involves plasticizing a polymer within the extruder barrel, forcing it through a die, and then subjecting it to a series of subsequent operations—such as cooling, stretching, blow‑molding, coating, laminating, cutting, and winding—to produce a wide range of finished products, including common items like pipes and profiles. In a plastic extrusion line, the extruder serves as one of the primary forming units, providing a molten material with uniform temperature and composition to ensure stable, controllable product quality. In recent years, the extrusion production of various composite plastic pipes and plastic pipes reinforced with different metals and fibers has posed significant challenges to extruder design.

There are many ways to classify extruders. Based on the number of screws, they can be categorized as screwless, single-screw, twin-screw, or multi-screw; according to their spatial orientation, they are classified as horizontal or vertical; by screw speed, they are divided into standard-speed, high-speed, and ultra-high-speed types; depending on whether the barrel is vented, they are either vented or non-vented; and in terms of assembly structure, they are either integral or split‑type. In plastic pipe extrusion lines, the most commonly used extruder is the horizontal single-screw extruder. A single-screw extruder consists primarily of four subsystems: the extrusion system, the drive system, the heating/cooling system, and the control system.

As one of the key components of an extruder’s extrusion system, the screw is responsible for conveying solid plastics, plasticizing them, and delivering the molten polymer. Variations in its geometric configuration directly affect both the quality and throughput of the final plastic products, playing a decisive role in the extruder’s overall performance. When designing a screw, it is essential to account for a variety of factors to accommodate the distinct properties of different plastics. In the plastics industry, it is simply not feasible to rely on a single “universal” screw to efficiently produce a wide range of plastic products.

Generally, a screw can be divided into three sections: the feeding section, the compression (or plasticization) section, and the metering (or homogenization) section.

The feeding section primarily receives material supplied by the feeding system and then conveys it to the compression section. In the compression section, the material is compacted and melted; after plasticization, it is pushed by the screw into the metering section. Within the metering section, the material undergoes further plasticization, and under conditions of constant pressure and temperature, the screw extrudes an equal volume of uniformly plasticized melt toward the die.

As the primary component of an extrusion system, the screw must rotate at high torque within a high-pressure, high-temperature barrel. Consequently, the alloy steel used to manufacture screws must exhibit exceptional mechanical strength, withstand substantial torsional loads without deforming under elevated temperatures, and achieve a high surface hardness after heat treatment to resist wear caused by significant friction. Currently, in China, most screws are fabricated from 38CrMoAl alloy steel, which undergoes quenching and tempering to attain a hardness of HB 260–290. The surfaces are then subjected to nitriding, with a case depth of 0.3–0.6 mm and a surface hardness exceeding HV 740, while the screw’s brittleness is ≥2.

As the application fields of extruders continue to expand and technological advancements persist, the extruder market has maintained a steady upward trajectory. In recent years, efficiency and energy conservation have remained the dominant themes driving continuous improvements in the international plastics machinery industry. High speeds and high production rates enable investors to achieve substantial returns with relatively modest capital outlays. Looking ahead, both the extruder mainframe and 3D‑printing filament production lines are expected to evolve toward greater technological sophistication while seeing price reductions. In particular, single‑screw extruders will trend toward larger sizes, miniaturization, higher length‑to‑diameter ratios, increased throughput, and enhanced venting capabilities—making the development of screw–barrel assemblies that can meet diverse processing requirements a key focus of research and innovation.