The following are important principles to keep in mind regarding extrusion. They should help save money, yield higher quality products, and use equipment more efficiently.
1. The mechanical principle.The basic mechanics of extrusion are simple—a screw turns in a barrel and pushes the plastic forward. A screw is really an inclined plane, or ramp, wound around a central core. The intent is to multiply the force so that a great resistance can be overcome. In the case of an extruder, there are three resistances to overcome: the rubbing of solid particles (the feed) against the barrel wall and each other in the first few turns of the screw (the feed zone); the adhesion of the melt to the barrel wall; and resistance to flow within the melt as it is pushed forward.
Sir Isaac Newton explain-ed that if a thing is not moving in a given direction, the forces on it are balanced in that direction. The screw does not move in an axial direction, although it may be turning rapidly in the cross direction around the circumference. So, the axial forces on the screw are balanced,and if it is pushing forward with great force on the plastic melt it must be pushing backward on something with equal force. In this case, it is pushing on a bearing behind the feed entry called the thrust bearing.
Most single screws are right-hand thread, like the screws and bolts used in carpentry and machinery. They turn counter -clockwise, if viewed from behind, as they try to screw themselves backward out the barrel. In some twin -screw extruders, two screws turn in opposite directions in a double barrel and intermesh, so that one must be right -handed and the other left-handed. In other intermeshing twin -screws, both screws turn in the same direction and therefore must have the same orientation. In all cases, however, there are thrust bearings to take the backward force, and Newton’s principle still applies.
2. The thermal principle.Extrudable plastics are thermoplastics—they melt when heated and become solid again when cooled. Where does the heat to melt the plastics come from? Feed preheating and barrel/die heaters may contribute, and are critical at startup, but motor energy input — frictional heat generated inside the barrel as the motor turns the screw against the resistance of the viscous melt—is by far the most important source of heat for all except very small systems, slow -moving screws, high -melt -temperature plastics, and extrusion -coating applications.
For all other operations, it is important to realize that the barrel heaters are not the primary source of heat during operation, and therefore have less effect on extrusion than we might expect. The rear barrel temperature may remain important because it affects bite, or the rate of solids conveying in the feed. Head and die temperatures should normally be at or near the desired melt temperature, unless they are used for a specific purpose such as gloss, flow distribution, or pressure control.
3. The speed reduction principle.In most extruders, screw speed is changed by modifying motor speed. Motors typically turn at around 1750 rpm at full speed, but this is much too fast for an extruder screw. If it were turned that fast, it would generate too much frictional heat, and the residence time of the plastic would be too short to prepare a uniform, well -mixed melt. A typical reduction ratio is between 10:1 and 20:1. The first stage may use either gears or a pulley set, but the second stage always uses gears and the screw is set in the center of the last, big gear.
In a few slow -moving machines (such as twins for UPVC), there may be three stages of reduction, and the top speed may be as low as 30 rpm or less (with ratios up to 60:1). On the other extreme, some very long twins used for compounding may run at 600 rpm or more, so that a very low reduction ratio is needed, as well as a lot of intense cooling.
Sometimes the reduction ratio is mismatched to the job—there is power going unused—and it is possible to add a set of pulleys between the motor and the first reduction stage to change the top speed. This either increases screw speed beyond the prior limits or reduces top speed to allow the system to run at a greater percentage of that top speed. This increases available power, reduces amperage, and avoids motor problems. In both cases, output may be increased, depending on the material and its cooling needs.
Post time: May-04-2017