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

Extrusion system

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

⑴ Screw: The screw is the most critical component of an extruder, directly determining its 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‑pressure‑strength, wear‑resistant, and corrosion‑resistant alloy steel, or from a composite steel pipe with an alloy‑steel liner. The barrel works in conjunction with the screw to crush, soften, melt, plasticize, degas, and compact the plastic, while continuously and uniformly conveying the molten material to the molding system. Generally, the barrel’s length is 15 to 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 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 parallel linear motion, guiding it smoothly and uniformly 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 along designated flow channels, enters the die neck, and flows 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 mitigate pressure fluctuations during extrusion, a pressure‑equalizing ring may be incorporated. The die head is also equipped with mechanisms for mold alignment and adjustment, facilitating precise centering of the core and sleeve.

Extruders are classified, based on the angle between the die‑head flow direction and the screw’s central axis, into angled die heads (with a 120° angle) and straight‑angle die heads. The die head housing is bolted to the extruder body; inside the die head, the die assembly includes a die‑core holder, which is secured at the die head’s feed port by nuts. A die core is mounted in front of the die‑core holder, and both the die core and its holder feature central bores for passing the core wire. A pressure‑equalizing ring is installed at the front of the die head to balance pressure. The extrusion‑coating section consists of a die‑sleeve holder and a die sleeve; the position of the die sleeve can be adjusted via bolts and supports, allowing precise control of its relative alignment with the die core and facilitating uniformity in the thickness of the extruded coating layer. External heating and temperature‑measurement devices are also provided.

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 drive up manufacturing costs.

For extruders with the same screw diameter, high‑speed, high‑efficiency models consume more energy than conventional units; consequently, the motor power must be doubled, and the gearbox size must be correspondingly increased. However, higher screw speeds entail lower gear ratios. With a given gearbox size, a lower ratio results in larger gear module dimensions, thereby enhancing the gearbox’s load‑carrying capacity. As a result, the increase in the gearbox’s size and weight does not scale linearly with the motor power. If one uses extrusion output as the denominator and divides by the gearbox weight, the high‑speed, high‑efficiency extruder yields a smaller value, whereas the standard extruder yields a larger one.

In terms of unit output, the lower motor power and lighter weight of the reducer in high-speed, high-efficiency extruders mean that their per-unit‑output manufacturing costs are lower than those of conventional extruders.

Heating and cooling device

Heating and cooling are essential prerequisites 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, bringing it to the level required for the specific processing operation.

⑵ The cooling system is designed to maintain the plastic within the temperature range required by the processing conditions. Specifically, it removes excess heat generated by shear friction during screw rotation, thereby preventing overheating that could lead to plastic 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 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 components.