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

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

⑵ 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 polymer, while continuously and uniformly feeding the molten material into the molding system. The barrel’s length is generally 15 to 30 times its diameter, designed to ensure 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 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 along a defined flow path, 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 (flow‑diverting 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.

Extruders are classified, based on the angle between the die‑head flow direction and the screw’s central axis, into angled heads (with a 120° angle) and straight‑angle heads. The head housing is bolted to the extruder body; inside the head, the die includes a die‑core holder, which is secured at the feed end by nuts. A die core is mounted in front of the core holder, and both the core and its holder feature centrally located holes for passing the core wire. A pressure‑equalizing ring is installed at the front of the head to balance pressure. The extrusion‑and‑sheathing section consists of a die‑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 sheath. External heating and temperature‑sensing 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; as a result, the motor power increases, and the gearbox size must also be upgraded accordingly. However, higher screw speeds correspond to lower gear ratios. For gearboxes of the same physical size, those with lower ratios have larger gear module dimensions, which in turn enhance the gearbox’s load‑carrying capacity. Consequently, the increase in the gearbox’s volume and weight does not scale linearly with the motor power. If one uses extrusion output as the denominator and divides by the gearbox’s weight, the high‑speed, high‑efficiency extruder yields a smaller value, whereas the conventional 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 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, the die neck, and the 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 uses 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 system.