The high speed and high output of the extruder enable investors to obtain higher output and high returns with lower investment. The efficiency of
plastic extruders is mainly reflected in high output, low energy consumption, and low manufacturing cost. Precision can increase the gold content of the product, such as multi-layer co-extruded composite film, which requires precision extrusion, and as an important means to achieve precision extrusion, the melt gear pump must be intensively developed. In terms of functions, screw extruders have been used not only for extrusion molding and mixing of polymer materials, but also for applications such as food, feed, electrodes, explosives, building materials, packaging, pulp, and ceramics. In addition, the "one-step extrusion process" in which the kneading granulation and the extrusion molding process are combined into one is also worthy of attention. The large-scale extrusion molding equipment can reduce the production cost, which is more obvious in large-scale twin-screw granulating units, blown film units, and pipe extrusion units. The following are some of the principles that
plastic extruders need to be aware of.
1. Structural principle
The basic mechanism of extrusion is simple – a screw rotates in the barrel and pushes the plastic forward. The screw is actually a bevel or slope that wraps around the center layer. The aim is to increase the pressure in order to overcome the large resistance. In the case of an extruder, there are three kinds of resistances to be overcome: the friction between the solid particles (feed) on the wall of the cylinder and the mutual friction between the coils before the rotation of the screw (feeding zone); Adhesion on the wall of the barrel; the internal flow resistance of the melt as it is pushed forward.
Newton has explained that if an object does not move in a given direction, the force on the object is balanced in this direction. The screw does not move in the axial direction, although it may rotate laterally rapidly near the circumference. Therefore, the axial force on the screw is balanced, and if it applies a large forward thrust to the plastic melt, it also applies an identical rearward thrust to the object. Here, the thrust applied is the bearing acting on the thrust bearing behind the feed port.
Most single screws are right-handed threads, like screws and bolts used in woodworking and machinery. If they look from the back, they are turning in the opposite direction because they try to spin out the barrel as far as possible. In some twin-screw extruders, the two screws rotate in opposite directions in two cylinders and cross each other, so one must be right-handed and the other must be left-handed. In other occlusal twin screws, the two screws rotate in the same direction and must have the same orientation. However, in either case, there is a thrust bearing that absorbs the backward force, and Newton's principle still applies.
2. Thermal principle
The extrudable plastics are thermoplastics - they melt when heated and solidify again upon cooling. Where does the heat of the molten plastic come from? The feed preheating and the barrel/die heater may work and are very important at start-up, however, the motor input energy - the motor overcomes the resistance of the viscous melt when the screw is turned Friction heat in the barrel – the most important source of heat for all plastics, except for small systems, low speed screws, high melt temperature plastics and extrusion coating applications.
For all other operations, it is important to recognize that the barrel heater is not the primary source of heat in operation, and therefore the effect on extrusion is less than we expected (see Principle 11). The post-cylinder temperature may still be important because it affects the rate of solids transport in the teeth or in the feed. The die and mold temperatures should generally 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.
3. Deceleration principle
In most extruders, the change in screw speed is achieved by adjusting the speed of the motor. The motor typically rotates at full speed of approximately 1750 rpm, but this is too fast for an extruder screw. If it is rotated at such a fast speed, too much frictional heat is generated and the residence time of the plastic is too short to produce a uniform, well-stirred melt. Typical deceleration ratios range from 10:1 to 20:1. The first stage can be either gear or pulley, but the second stage uses gears 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 3 deceleration stages and the maximum speed may be as low as 30 rpm or lower (ratio of 60:1). At the other extreme, 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 deceleration rate matches the task incorrectly - there will be too much energy to use - and it is possible to add a pulley block between the motor and the first deceleration phase that changes the maximum speed. This either increases the screw speed above the previous limit or lowers the maximum speed allowing the system to operate at a greater percentage of maximum speed. This will increase the available energy, reduce the amperage and avoid motor problems. In both cases, the output may increase depending on the material and its cooling needs.
With the advent and rapid development of computer technology, some new extruder control systems have been computer controlled, and some auxiliary devices can be omitted. For example, instruments and other controllers in the temperature control system can be omitted. And the computer accepts all readings and controls these readings. It is foreseeable that with the further development of computers and extruders, a large number of extrusion molding equipment will quickly adopt computer control of all processing parameters to achieve fully automated control of extrusion molding.
4. Feeding as a coolant
Extrusion transfers the energy of the motor, sometimes the heater, to the cold plastic, converting it from solid to melt. The input feed is lower than the barrel and screw surface temperatures in the feed zone. However, the surface of the barrel in the feed zone is almost always above the melting range of the plastic. It is cooled by contact with the feed particles, but the heat is retained by the heat transferred back to the hot front end and controlled heating. Even after the current end heat is held by the viscous friction and no barrel heat input is required, the post heater may be required. The most important exception is the slotted feed barrel, which is almost exclusively for HDPE.
The screw root surface is also cooled by the feed and is insulated from the barrel wall by the plastic feed particles (and the air between the particles). If the screw suddenly stops, the feed also stops, and as the heat moves back from the hotter front end, the screw surface becomes hotter in the feed zone. This can cause adhesion or bridging of the particles at the roots.
5. The pressure at the end of the screw is very important
This pressure reflects the resistance of all objects downstream of the screw: the filter screen and the contaminated shredder plate, the adapter transfer tube, the fixed stirrer (if any), and the mold itself. It depends not only on the geometry of these components but also on the temperature in the system, which in turn affects resin viscosity and throughput. It does not depend on the screw design, except when it affects temperature, viscosity and throughput. For safety reasons, measuring temperature is important – if it is too high, the die and mold can explode and harm nearby people or machines.
The pressure is advantageous for agitation, especially in the last zone of the single screw system (metering zone). However, high pressure also means that the motor has to output more energy - and thus the melt temperature is higher - which can dictate the pressure limit. In a twin screw, the engagement of the two screws with each other is a more efficient agitator, so no pressure is required for this purpose.
In the manufacture of hollow parts, such as tubes made from spider-centered spider molds using brackets, high pressure must be created within the mold to aid in the recombination of separate streams. Otherwise, the product along the weld line may be weak and problems may occur during use.
6. Output = displacement of the last thread +/- pressure flow and leakage
The displacement of the last thread is called positive flow and depends only on the geometry of the screw, the screw speed and the melt density. It is regulated by the pressure stream and actually includes a drag effect that reduces the output (indicated by the highest pressure) and any overbiting effect in the feed that increases the output. The leak on the thread may be in either of two directions.
It is also useful to calculate the output per rpm (rotation) as this represents any drop in the pumping capacity of the screw at a time. Another related calculation is the output per horsepower or kilowatt used. This represents efficiency and is capable of estimating the production capacity of a given motor and drive.
7. Shear rate plays a major role in viscosity
All common plastics have shear-reducing properties, meaning that the viscosity becomes lower as the plastic moves faster and faster. This effect of some plastics is particularly noticeable. For example, some PVCs increase the flow rate by a factor of 10 or more when the thrust is doubled. On the contrary, the LLDPE shear force does not drop too much, and the flow rate only increases by a factor of three to four when the reasoning is doubled. The reduced shear reduction effect means high viscosity under extrusion conditions, which in turn means more motor power is required. This can explain why LLDPE operates at a higher temperature than LDPE. The flow rate is expressed in shear rate, approximately 100 s-1 in the screw channel, between 100 and 100 s-1 in most die profiles, and greater than 100 s-1 in the gap between the threads and the wall and some small die gaps. Melt coefficient is a commonly used measure of viscosity but is reversed (eg flow/thrust rather than thrust/flow). Unfortunately, the measurement is not a true measurement in an extruder with a shear rate of 10 s-1 or less and a very fast melt flow rate.
8. The motor is opposite to the cylinder, and the cylinder is opposite to the motor.
Why is the control effect of the cylinder not always the same as expected, especially in the measurement zone? If the cylinder is heated, the viscosity of the material layer at the wall of the cylinder becomes smaller, and the motor needs more energy to operate in this smoother cylinder. less. The motor current (amperage) drops. Conversely, if the barrel cools, the melt viscosity at the wall of the barrel increases, the motor must rotate more forcefully, the amperage increases, and some of the heat removed by the barrel is returned by the motor. Usually, the barrel regulator does have an effect on the melt, which is what we expect, but there is no regional variable in any place. It is best to measure the melt temperature to really understand what happened.
The eighth principle does not apply to the die and the mold because there is no screw rotation there. This is why external temperature changes are more effective there. However, these changes are uneven from the inside out, unless it is stirred in a fixed agitator, which is an effective tool for melt temperature change and agitation.
These principles can help produce high quality products and use equipment more efficiently.
