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Some important principles to be paid attention to in the work of twin screw extruders

Dec 12, 2017



Some important principles to be paid attention to in the work of twin screw extruders

1. principles of structure

For the basic mechanism of the extrusion process, it is simply that a screw rotates in the cylinder and pushes the plastic forward. The screw structure is a slope or slope winding on the center layer, the aim is to increase the pressure to overcome the larger resistance. On the extruder, when there are three kinds: one is the need to overcome the resistance of friction, it contains solid particles (feed) on the cylinder wall friction and screw rotation before laps (feed) mutual friction between them two; two is the melt in the cylinder wall adhesion; three logistics the internal resistance of the melt when pushed forward.

According to the Newton theorem, if an object is at rest in a certain direction, then the object is in the equilibrium state of the force equilibrium in this direction. For the circumferential motion of the screw, it has no axial motion, that is to say, the axial force on the screw is in the state of balance. So if the screw exerts a great forward thrust on the plastic melt, it also applies the same size to the same object but backward thrust at the same direction. It is obvious that the thrust it exerts is on the thrust bearing behind the inlet. Most of the single screws are right-handed threads. If they look back from the back, they rotate backward, and they spin the cylinder back through the rotation movement. In some twin screw extruders, the two screws move opposite to each other in two cylinders and cross each other, so they must be a right direction and a left direction. For the twin screw, the two screws must rotate in the same direction, so they must have the same orientation. However, the thrust bearing, which bears the backward force in any case, still conforms to the Newton theorem.

2. temperature principle

The extruded plastic is hot plastic, which melts at heating and solidifies again during cooling. As a result, heat is needed in the extrusion process to ensure that the plastic can reach the melting temperature. So where does the heat of the melting plastic come from? First, loadometer feed preheating and cylinder / mould heater may play a role in and start is very important, in addition the electrical input energy, the motor to overcome the friction heat viscous resistance of the melt generated when the screw rotation in the cylinder body, is also the most important source of all plastic, except when small, low speed, high melt screw system the temperature of plastic extrusion coating and application. In operation, it is important to realize that the tube heater is not the main source of heat, and its effect on extrusion is smaller than we expected. The temperature of the rear cylinder is very important because it affects the speed of the solids transport in the teeth or in the feed. Generally speaking, except for some specific purpose (such as glazing, fluid distribution or pressure control), the temperature of die and mold should reach the desired temperature or near the temperature of melt.

3. deceleration principle

In most extruders, the change of screw speed is achieved by adjusting the speed of motor. The driving motor usually rotates at full speed of about 1750rpm, which is too fast for an extruder screw. If rotating at such a high speed, it will cause too much friction heat. It will not be able to prepare even and well mixed melt due to the short residence time of plastic. The typical reduction rate should be between 10:1 and 20:1. The first stage can be either gear or pulley block, but in the second stage, it is better to use gear and locate the screw in the center of the last big gear. For some slow running machines (for example, twin-screw for UPVC), there may be three deceleration stages, the maximum speed may be low to 30rpm or lower (ratio to 60:1). On the other hand, some long twin-screw used for stirring can run at 600rpm or faster, so a very low deceleration rate and more deep cooling is needed. If the deceleration rate is wrong with the job matching, there will be too much energy to be wasted. It may be necessary to add a pulley block between the motor and the first deceleration stage of the maximum speed, which either increases the screw speed or even exceeds the previous limit, or decreases the maximum speed. This can increase energy, reduce current value and avoid motor failure. In these two cases, output will increase due to material and cooling needs.