The extrusion system of the planetary extruder consists of a feeding section and a planetary section. The feeding section is equivalent to the feeding section of an ordinary single-screw extruder. The temperature control system of the planetary extruder includes the screw and barrel of the feeding section

Introduction

The planetary screw extruder is a planetary screw structure in which the spiral middle section (i.e., the plasticizing section) of the screw in the extruder or the entire screw (only the feeding section of the feeding part is not a planetary screw). When working, the central screw is the active screw, and its outer circle has multiple small-diameter screws meshing and rotating with it. These small-diameter screws can both rotate on their own and revolve around the central screw, because the outer periphery of the small screw is also meshed with a jacket with spiral teeth inside. This jacket is also a barrel, which is fixed to the front and rear barrels with bolts. The thread section of the screw is an involute tooth shape, and the spiral is a multi-start thread. The three parts in the figure below, the central screw, the planetary small diameter screw and the outer barrel with threaded teeth, form the mixing and plasticizing mechanism of the planetary screw. Their thread pitch, thread depth and tooth meshing angle of the vertical section are equal; the distance from the pitch circle of the outer spiral teeth is also equal; the center distance between each planetary screw is equal and larger than the diameter of the planetary screw to avoid interference during meshing transmission. These data are the necessary working parameters to ensure the normal meshing and rotation of this set of planetary screws.

The way the planetary screw mixes and plasticizes the raw materials is as follows: the raw materials pushed to the planetary section by the feeding section thread move to the extrusion of the front head of the screw under the continuous and strong push of the rear screw feed, and pass through the internal thread teeth between the center screw and the barrel and the teeth of the planetary small-diameter screw meshing with it; because the screws in the planetary screw are constantly rotating, the raw materials are subjected to strong extrusion, rolling and shearing and other forces in the gap between the meshing and rotating thread teeth, so that the raw materials are constantly turned and mixed in this section, and finally become molten, uniformly plasticized, and gradually pushed to the front section of the screw and extruded from the head.

Structural features

The planetary extruder is usually composed of a rotating system, an extrusion system, a feeding system and a temperature control system. Its extrusion system has a unique structure, and the planetary extruder is named after the characteristics of its extrusion system.

The extrusion system of the planetary extruder consists of a feeding section and a planetary section. Its feeding section is equivalent to the feeding section of an ordinary single-screw extruder. This section establishes a very low pressure, and the screw groove is only partially filled with material. The barrel of the feeding section is equipped with a water cooling system, and there is an insulation layer at the connection with the planetary section barrel to prevent the temperatures of the two sections of the barrel from affecting each other.

The most important part of the planetary extruder is the planetary section, which mainly plays the role of melting and plasticizing the material and mixing it. This section consists of a central screw, several planetary screws and a barrel with teeth in the wall. These three components are the three key parts of the planetary extruder. The central screw, the planetary screw and the barrel are all made of high tensile strength and wear-resistant alloy steel. The gap between the screw and the inner teeth of the barrel is generally 0.2-0.4mm. The number of planetary screws is proportional to the diameter of the planetary section, generally 6-18. The output is linearly related to the number of planetary screws. During operation, the rotating central screw drives the planetary screw to rotate on its own, and at the same time revolves around the central screw like a planet, so that the planetary screw #floats$ between the main screw and the inner teeth of the barrel. Since the planetary segment helical teeth have a helix angle of 45бу, an axial force will be generated when the main screw rotates, causing the planetary screw to move forward axially. Therefore, a thrust ring is provided at the end of the planetary segment barrel, and its inner diameter should be smaller than the diameter of the circle formed by the axis of the planetary screw. The surface of the thrust ring contacting the planetary screw is subject to great sliding friction, and its wear resistance is very critical, so the thrust ring is generally plated with a wear-resistant layer or made of cemented carbide to improve wear resistance. The planetary segment can be designed as a two-stage type, with a dispersion ring in the middle. The dispersion ring can increase the back pressure, strengthen the material shear, make the material start to melt earlier, and improve the mixing and exhaust performance.

The planetary extruder is an open system in the axial direction, and the pressure established by the planetary segment is relatively low. The increase in back pressure will reduce the output and increase the shear and melt temperature. If high-pressure extrusion is required, a melt pump or a second stage part is equipped in the die head. The second stage part can be a single-screw extruder or a counter-rotating twin-screw extruder, or a second planetary extrusion system. An exhaust device can be provided between the first stage and the second stage. The two stages of this two-stage planetary extruder use separate drive devices and can be controlled independently, thereby optimizing the processing process.

The feeding system of the planetary extruder generally uses a forced feeding device, which has a sufficiently large feeding rate to prevent bridging in the hopper and is particularly suitable for low-density materials (such as recycled materials). Since the bottom of the forced feeding device is cooled, heat-sensitive materials (such as PVC) can be prevented from being burned in the feeding device. A quantitative feeding device can also be used for feeding.

The temperature control system of the planetary extruder includes a water cooling circulation system for the screw and barrel of the feeding section and an oil heating circulation system for the planetary section. It can ensure that the planetary extruder has good temperature control performance.