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The Important Principle Of Plastic Extrusion

Essai Industries Co.,Ltd | Updated: Jul 04, 2017

  The important principle of plastic extrusion

  1. mechanical principles

  The basic mechanism of extrusion is simple - a screw rotates in the cylinder and pushes the plastic forward. The screw is actually a slope or slope, wound on the center layer. The aim is to increase the pressure in order to overcome the greater resistance. In the case of an extruder, there are three kinds of resistance to overcome: plastic extrusion the friction between the solid particles (feed) on the cylinder wall and the friction between them when the screw rotates several times (feed zone); The adhesion on the cylinder wall; the flow resistance of the melt when it is pushed forward.

  Newton has explained that if an object does not move in a given direction, then the force on the object is balanced in this direction. The screw does not move in the axial direction, although it may rotate horizontally around the circumference. Thus, the axial force on the screw is balanced, and if it exerts a large forward thrust on the plastic melt, it also applies an equal back thrust to an object. Here, the thrust applied by it is on the bearing-thrust bearing that is behind the inlet.

  Most single screws are right-hand threads, like screw and bolts used in woodworking and machines. If they are from the back, they are reverse, because they have to try to pull out the cylinder. In some twin-screw extruders, the two screws in the two cylinder reverse rotation and cross each other, so one must be right, the other must be left. plastic extrusion In the other bite pairs of twin screws, the two screws are rotated in the same direction and must therefore have the same orientation. However, no matter what the situation has to absorb the backward force of the thrust bearing, Newton's principle is still applicable.

  2. The principle of heat

  The extrudable plastic is a thermoplastic - they melt upon heating and re-solidify upon cooling. Where does the heat of molten plastic come from? Feed preheat and cylinder / mold heater may work and are very important at start-up, however, plastic extrusion the motor input energy - the motor to overcome the resistance of the viscous melt The friction heat generated in the barrel when turning the screw - is all Plastic the most important heat source, small system, low speed screw, high melt temperature plastic and extrusion coating applications except.

  For all other operations, it is important to realize that the cylinder heater is not the primary heat source in operation, and thus the effect of extrusion is less than we expected. The post-cylinder temperature may still be important because it affects the rate at which the solids are fed in the feed or in the feed. The die and mold temperatures should normally be the desired melt temperature or close to this temperature unless they are used for a specific purpose like glazing, fluid dispensing or pressure control.

  3. The principle of deceleration

  In most extruders, the change in screw speed is achieved by adjusting the motor speed. The motor is usually rotated at full speed of about 1750 rpm, but this is too fast for an extruder screw. If it is so fast, it will produce too much frictional heat and the residence time of the plastic is too short to prepare a homogeneous, well-agitated melt. plastic extrusion The typical reduction ratio is between 10: 1 and 20: 1. The first stage can be either a gear or a pulley block, but the second stage is geared and the screw is positioned at the center of the last large gear.

  In some slow-running machines (such as twin screws for UPVC), there may be three deceleration phases and the maximum speed may be as low as 30 rpm or less (60: 1). The other extreme is that some of the long twin screws used for agitation can run at 600 rpm or faster, thus requiring a very low deceleration rate and a lot of deep cooling.

  Sometimes the rate reduction matches the task - there will be too much energy to use - and it is possible to add a pulley between the motor and the first deceleration phase that changes the maximum speed. plastic extrusion This either increases the screw speed to exceed the previous limit or reduces the maximum speed to allow the system to run at a greater percentage of the maximum speed. This will increase the availability of energy, reduce amps and avoid motor problems. In both cases, the output may increase depending on the material and its cooling needs.