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Plastic Extrusion Working Principle

Essai Industries Co.,Ltd | Updated: Oct 11, 2017

Plastic extruded plastics are thermoplastics - 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, the motor input energy - the motor to overcome the resistance of the viscous melt The friction heat generated in the cylinder when the screw is rotated - is all Plastic, the most important heat source, small system, low speed screw, high melt temperature plastic and extrusion coating applications except.

For other operations of plastic extrusion, it is important to realize that the cylinder heater is not a major heat source in operation and that the effect of plastic 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 die temperatures should normally be the desired melt temperature or close to this temperature unless they are used for a specific purpose like glazing, fluid distribution or pressure control.

The basic mechanism of plastic extrusion is simple - a screw rotates in the cylinder and pushes the plastic forward. The screw is actually a bevel or slope, wound on the center layer. The purpose of plastic extrusion 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: the friction between the solid particles (feed) on the cylinder wall and the friction between them when the screw rotates a few laps (feed zone); The adhesion on the cylinder wall; the flow resistance of the melt when it is pushed forward.

In plastic extrusion equipment, most single screws are right-hand threads, like screw and bolts used in carpentry 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. In other bite double screws, the two screws are rotated in the same direction and must therefore have the same orientation. However, no matter what kind of situation there is a thrust bearing that absorbs the backward force.

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. The typical reduction ratio is between 10: 1 and 20: 1.

During the plastic extrusion process, there may be three deceleration stages in some slow-running machines and the maximum speed may be as low as 30 rpm or less (60: 1). The other extreme is that some very long twin screws 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. When the plastic is extruded, either increase the screw speed beyond the previous limit or reduce the maximum speed to allow the system to run at a greater percentage of the maximum speed. This will increase the amount of energy available, reduce amps and avoid motor problems.