In plastic extrusion equipment, the plastic extruder is typically referred to as the main machine, while the downstream equipment that works in conjunction with it—known as the plastic extrusion molding machine—is called the auxiliary machine.

Plastic extruders (main machines) can be paired with a wide range of auxiliary equipment for plastic molding—such as pipe, film, rod, monofilament, flat‑filament, strapping, mesh, sheet (plate), profile, pelletizing, and cable‑jacketing units—to form diverse plastic extrusion production lines that manufacture an array of plastic products. Consequently, plastic extrusion machinery remains, both today and in the future, one of the most widely used types of equipment in the plastics processing industry.

The main unit of a plastic extruder is the extruder itself, which consists of an extrusion system, a conveying system, and a heating–cooling system.

I. Extrusion System

The extrusion system comprises a screw, a barrel, a hopper, a die head, and a mold. The plastic is plasticized into a uniform melt within the extrusion system and, under the pressure generated during this process, is continuously forced through the die head by the screw.

⑴ Screw: The screw is the primary component of an extruder, directly determining the extruder’s application scope and production capacity. It is manufactured from high-strength, corrosion-resistant alloy steel.

(2) Barrel: A metallic cylindrical component, typically fabricated from heat‑resistant, high‑strength alloy steel or a composite steel pipe with an alloy‑steel liner. The barrel works in conjunction with the screw to crush, melt, plasticize, degas, and compact the plastic, while continuously and uniformly feeding the molten material into the molding system. Generally, the barrel length is 15–30 times its diameter, with the aim of ensuring thorough heating and complete plasticization of the polymer.

(3) Hopper: The bottom of the hopper is equipped with a shut-off device for adjusting and interrupting the material flow, while the side of the hopper features an inspection port and a calibrated metering device.

⑷ The Die Head and Mold: The die head consists of an alloy‑steel inner sleeve and a carbon‑steel outer jacket. It houses the forming mold, which converts the rotational motion of the plastic melt into a uniform, straight‑line flow, guiding it smoothly into the die sleeve while imparting the necessary molding pressure. Within the barrel, the plastic is plasticized and compacted; it then passes through a porous filter plate and the die neck into the die‑forming mold. The core and sleeve are properly matched to create an annular gap whose cross‑section continuously decreases, causing the molten plastic to form a continuous, dense tubular sheath around the core wire. To ensure an optimal flow path within the die head and eliminate dead zones where plastic might accumulate, a分流套筒 (diverter sleeve) is often installed. Additionally, to minimize pressure fluctuations during extrusion, a pressure‑equalizing ring may be provided. The die head is also equipped with mechanisms for mold alignment and adjustment, facilitating precise centering of the core and sleeve.

II. Transmission System

The function of the drive system is to power the screw, providing the torque and rotational speed required during the extrusion process; it typically consists of an electric motor, a reducer, and bearings.

Moreover, under the premise of essentially identical structure, the manufacturing cost of a reducer is roughly proportional to its external dimensions and weight. A larger size and greater weight imply higher material consumption during production, as well as the use of larger bearings, both of which contribute to increased manufacturing costs.

For extruders with the same screw diameter, high‑speed, high‑efficiency models consume more energy than conventional ones. However, a higher screw speed corresponds to a lower reduction ratio. With gearboxes of the same size, a lower reduction ratio results in larger gear module dimensions and greater load‑carrying capacity. Consequently, the increase in the gearbox’s size and weight does not scale linearly with motor power. If we use extrusion output as the denominator and divide by the gearbox’s weight, the high‑speed, high‑efficiency extruder yields a smaller value, while the standard extruder yields a larger one.

III. Heating and Cooling Unit

Heating and cooling are essential conditions for the plastic extrusion process to proceed.

⑴ Extruders typically employ electric heating, which is categorized into resistance heating and induction heating. Heating elements are installed in various sections of the extruder, including the barrel, die neck, and die head. The heating system raises the temperature of the plastic within the external heating cylinder, thereby achieving the temperature required for the processing operation.

⑵ The cooling system is designed to maintain the plastic within the temperature range required by the process. Specifically, it removes excess heat generated by shear friction during screw rotation, thereby preventing overheating that could lead to thermal degradation, scorching, or difficulties in shaping. Barrel cooling is categorized into water cooling and air cooling; for small and medium‑size extruders, air cooling is generally preferred, whereas large units typically employ water cooling or a combination of both. Screw cooling primarily utilizes central water cooling, aiming to enhance solid‑material conveyance, stabilize extrudate output, and improve product quality. Meanwhile, cooling at the hopper serves two purposes: first, to strengthen the transport of solid material and prevent plastic pellets from becoming sticky and clogging the feed throat due to temperature rise; second, to ensure the proper operation of the drive system.