In plastic extrusion equipment, the plastic extruder is typically referred to as the main machine, while the downstream auxiliary equipment that works in conjunction with it is called the auxiliary machine. The plastic extruder (the main machine) can be paired with a wide range of auxiliary extrusion units—such as those for producing pipes, films, monofilaments, flat tapes, strapping, extruded mesh, sheets or plates, profiled sections, pelletizing, and cable jacketing—to form various plastic extrusion production lines, enabling the manufacture of diverse 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 core component of the plastic extruder—the main machine—is the extruder itself, which comprises 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.
⑵ 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 feeding the molten material into the molding system. Generally, the barrel’s length is 15 to 30 times its diameter, designed to ensure thorough heating and complete plasticization of the polymer.
⑶ 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 a sight glass 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 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 a well‑designed flow channel 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 mitigate pressure fluctuations during extrusion, a 均压环 (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.
Based on the angle between the die‑head flow direction and the screw’s central axis, die heads are classified into angled die heads (with a 120° angle) and straight‑angle die heads. The die head housing is bolted to the machine frame, and the die inside features a die‑core holder that is secured at the feed end of the die head with nuts. A die core is mounted in front of the die‑core holder, and both the die core and its holder have 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. 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 contribute to increased manufacturing costs.
For extruders with the same screw diameter, high‑speed, high‑efficiency models consume more energy than conventional ones; consequently, the motor power must be doubled, and the gearbox size must be correspondingly increased. However, a higher screw speed entails a lower reduction ratio. With gearboxes of identical dimensions, those featuring a lower reduction ratio have larger gear module sizes and greater load‑carrying capacity compared to units with higher ratios. As a result, the increase in the gearbox’s size and weight does not scale linearly with the rise in 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 standard extruder produces 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, the die neck, and the die head. The heating system uses external heaters to warm the plastic within the barrel, raising its temperature 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 thermal degradation, scorching, or difficulties in shaping. Barrel cooling is typically implemented either by water or by air; for small and medium‑sized extruders, air cooling is generally preferred, whereas large units often employ water cooling or a combination of both. Screw cooling is mainly achieved through central water cooling, with the aim of enhancing solid‑material conveyance, stabilizing extrudate output, and improving product quality. However, cooling at the hopper serves two purposes: first, to reinforce 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 mechanism.