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The extrusion process

The basic mechanism of extrusion is very simple:

1: a screw in the cylinder to rotate and the plastic push. Screw is actually aslant or slope, winding on the core layer. The aim is to increase thepressure in order to overcome the larger resistance. A extruding machines,there are 3 kinds of resistance to be overcome: solid particles (feed) on the cylinder wall friction and the rotation of the screw front laps (feeding zone)mutual friction between them; the melt in the barrel wall adhesion; meltpushed forward when the resistance of the internal logistics.

Newton has explained that, if an object is not to a given direction, then theobjects on the force is in this direction balance. Not to screw axial movement, although it may be in the circumferential near horizontal fast rotation. Therefore, the axial force on the screw to be balanced, and if it is applied to the plastic melt is a great forward thrust it also to a certain object exerts a same backward thrust. Here, it is the role of applied thrust bearingin the feed opening behind the thrust bearing.

The majority of single screw is right hand thread, as the use of woodworkingand machine screw and bolt. If viewed from behind, they are reverse rotation, as they try to back out of the cylinder rotation. In a twin screw extruder, screw in two two cylinder rotates reversely and cross each other,so one must be to the right side, another must be left to the. In otherocclusal twin screw, two screw with the same direction of rotation and therefore must have the same orientation. However, no matter what thesituation is to absorb thrust bearing backward force, the Newton principle is still applicable.

The 2 heat principle

Extrudable plastics is thermoplastic - they when heated melting andsolidification again on cooling. Melt the plastic heat come from? Feed preheating and cylinder / die heater may play a role and at startup time is very important, but the friction heat energy input, motor - motor overcomesviscous melt resistance rotational screw generated in the cylinder body -- isthe most important source of all plastics, except for small systems, low speed screw, high melt temperature and extrusion of plastic coatingapplication.

For all other operations, recognizing the main source of heat tube heater is not in operation is very important, and thus on the extrusion effect than weexpected to be small (see the eleventh principle). After the barreltemperature may still be important, because it affects the tooth or into thesolid material in the conveying speed. Die and mould temperature should be generally melt temperature want or close to this temperature for a specific purpose, unless they like polishing, fluid distribution or pressure control.

3 reduction principle

In the majority of the extruder screw speed, change is achieved by adjusting the motor speed. Motor is usually rotating at full speed of about 1750rpm,but that of an extruder screw was too fast. If the rotation is so fast, it will produce too much friction heat and plastic or residence time is too short for preparation of uniform, good stirring melt. The typical speed reduction ratio between 10:1 to 20:1. The first stage can use gear can also block, but the second stage with gear and screw positioning at the end of a big gear center.

In some slow running machine (double screw for example for UPVC), there may be 3 deceleration phases and the maximum speed may be low to 30rpm or less (ratio 60:1). At the other extreme, some for stirring speedrunning double screw long can be 600rpm or faster, thus requiring a verylow reduction rate and many deep cooling.

Sometimes the deceleration rate and task matching error -- there will be too much energy can not be used and may increase a pulley block between the first deceleration motor and change the maximum speed. This either that the screw speed increased to more than the previous limit or reduce thepercentage of maximum speed allows the system to run at maximum speedgreater. This will increase the available energy, reduce and avoid the motoramperage. In the two case, according to the material and the coolingneeds, the output may increase.

The 4 feeding as coolant

Extrusion is the motor of the energy - sometimes - transfer heater to coldplastic, so as to take it from the solid into the melt. The input feed ratiofeeding area in the barrel and screw surface temperature low. However, tothe surface of the cylinder body material in the area is almost always on theplastic melting range. It cooled by and feed grain contact, but the heat from hot front end to the back end of the heat transfer, which can control the heating and maintaining. Even the current end heat by viscous friction and does not need to keep the cylinder heat input, may need to open the heater. The most important exception is the groove into the barrel, almostexclusively for HDPE.

Screw root surface is also feed cooling and plastic feed particles (andbetween the particles from the air) on the cylinder wall insulation. If the screw stop suddenly, feed also stopped, and because heat movingbackward from the front end surface of the screw is more hot, more hot inthe feed zone. This may be caused by particles in the roots of the adhesionor bypass.

5 in the feed zone, stick to the barrel slid into the screw

In order to make a single screw extrusion solid particle machine smoothcylinder feeding zone transport capacity reaches the maximum, the particlesshould stick on the cylinder body and slide screw. If the particles are attached to the screw root, nothing can pull them down; inlet channelvolume and solid reduces. In another reason root adhesion is not goodplastic may here thermal refining and produce gel and similar pollutionparticles, or with the change of the output speed