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The three major principles of the twin-screw extruder
AddDate:2018-05-02 From:Xielong

Extruder screw is a common device in the plastics industry. Twin-screw extruder also belongs to one of them, and it is developed on the basis of single-screw extruder because of its outstanding feeding function and mixing. Features such as plasticizing, venting, and extrusion stability are now widely used in the extrusion of extruded products. What are the three major principles of attention for twin-screw extruders? Next, please see the detailed introduction below!


First, the structural principle


The basic mechanism of the extrusion process is simply a screw that rolls in the barrel and pushes the plastic forward. Extruder screw barrel structure is a slope or slope wrapped around the center layer, its intention is to add pressure in order to overcome greater resistance. As far as the extruder is concerned, there are three kinds of resistance requirements to overcome during operation: First is the conflict force, which contains the collision force between the solid particles (feed) against the cylinder wall and the first few turns of the screw (feed zone) between them. There are two kinds of conflict forces; the second is the adhesion of the melt on the cylinder wall; and the third is the resistance to the internal flow of the melt as it is advanced.


According to Newton's theorem, if an object is stationary in a certain direction, then the object is balanced in the direction of force balance in this direction. With regard to a screw that moves circumferentially, it has no axial movement, that is to say the axial force on the screw is in an equilibrium condition. So, assuming that the screw exerts a large forward thrust on the plastic melt, it also applies a same-size, but backward thrust to another object. Obviously, the thrust it exerts acts on the thrust bearing behind the feed port.


Most single-screws are right-handed threads, assuming that they are rolling backwards when viewed from behind, and they spin out of the barrel backwards through a rotational motion. In some twin-screw extruders, the two screws roll backwards in the two cylinders and cross each other, so it is necessary to be a right-facing, one-left, occlusal twin screw, and the two screws are Rolling in the same direction, it is necessary to have the same orientation. However, regardless of the situation, thrust bearings that accept backward forces are still compatible with Newton's theorem.


Second, the principle of temperature


Extrudable plastics are thermoplastics that melt when heated and re-condense when cooled. Therefore, heat is required during the extrusion process to ensure that the plastic reaches the melting temperature. So where does the melting heat of plastic come from? The primary floor load preheating and barrel/mold heaters may function and are very important at start-up. In addition, the motor enters energy, which is the heat of collision in the barrel when the motor overcomes the resistance of the viscous melt rolling screw. The most important source of heat for all plastics is, of course, small systems, low speed screws, high melt temperature plastics and extrusion coatings.


In operation, it is important to realize that the cartridge heater is not actually the primary source of heat, and its effect on extrusion is less than our estimate. The rear barrel temperature is more important because it affects the speed of solids transport in the teeth or in the feed. In general, the die and mold temperature should reach or approach the melt temperature, except for a certain intent (such as light, fluid distribution, or pressure control).


Third, the principle of deceleration


In most extruder screws, changes in screw speed are accomplished by adjusting the speed of the motor. The drive motor typically rolls at approximately 1750 rpm at full speed, which is too fast for an extruder screw. Assuming that it rolls at such a fast rate, too much heat of collision will occur, and the uniform, well-mixed melt cannot be prepared because the residence time of the plastic is too short. The typical reduction ratio should be between 10:1 and 20:1. The first stage can use both gears and pulleys. However, the second stage is best to use gears and locate the screw in the center of the final gear.


With respect to some slow-running machines (such as twin-screws for UPVC), there may be three deceleration stages, and the maximum speed may be as low as 30 rpm or less (a ratio of 60:1). On the other hand, some long twin screws used for mixing can operate at 600 rpm or faster, thus requiring a very low deceleration rate and more deep cooling.


If the deceleration rate is misaligned with the job, too much energy will be wasted. It may then be desirable to add a pulley block between the motor and the first deceleration phase that changes the maximum speed, which either adds or exceeds the previous limit of the extruder screw speed or reduces the maximum speed. This increases the available energy, reduces the current value, and avoids motor failures. In both cases, the output may increase due to the material and its cooling requirements.

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