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 around the central layer. The aim is to increase pressure in order to overcome greater resistance. As far as an extruder is concerned, there are three kinds of resistance to be overcome: the frictional force of solid particles (feed) on the cylinder wall and the mutual frictional force between them when the screw rotates in the first few circles (feed area); the adhesion force of melt on the cylinder wall; and the internal logistics resistance when the melt is pushed forward.

Newton once explained that if an object does not move in a given direction, then the force on the object is balanced in that direction. The screw does not move axially, although it may rotate transversely and rapidly near the circumference. Therefore, the axial force on the screw is balanced, and if it exerts a large forward thrust on the plastic melt, it also exerts the same backward thrust on an object. Here, the thrust applied is applied to the thrust bearing behind the inlet and outlet.

Most single screw are right-handed threads, such as those used in woodworking and machinery. If viewed from behind, they rotate backwards, as they try to spin the cylinder backwards. In some twin-screw extruders, 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 occlusion twin screw, two screw rotates in the same direction and must have the same orientation. However, whenever there are thrust bearings that absorb backward forces, Newton's principle is still applicable.

2. Thermal Principle

Extrudable plastics are thermoplastics - they melt when heated and solidify again when cooled. Where does the heat from melting plastics come from? Feed preheating and barrel/die heaters may work and are important at start-up, but motor input energy, the friction heat generated in the barrel when the motor overcomes the resistance of the viscous melt and rotates the screw, is the most important heat source for all plastics, except for small systems, low speed screw, high melt temperature plastics and extrusion coating applications.

For all other operations, it is important to recognize that the barrel heater is not the main heat source in operation, and thus has less effect on extrusion than we expected (see Principle 11). Rear cylinder temperature may still be important because it affects the speed of solid conveying in the teeth or feed. Die and die temperatures should generally be the desired melt temperature or close to that temperature, unless they are used for specific purposes such as polishing, fluid distribution or pressure control.

3. Deceleration Principle

In most extruders, the change of screw speed is realized by adjusting the motor speed. The motor usually rotates at a full speed of about 1750 rpm, but this is too fast for an extruder screw. If rotated at such a fast speed, too much friction heat will be generated and the plastic residence time is too short to produce a uniform, well-stirred melt. The typical deceleration rate is between 10:1 and 20:1. In the first stage, both gears and pulleys can be used, but in the second stage, gears are used and the screw is positioned at the center of the last big gear.

In some slow-running machines (such as twin-screw for UPVC), there may be three deceleration stages and the maximum speed may be as low as 30 rpm or lower (the ratio is 60:1). At the other extreme, some very long twin-screw mixers can operate at 600 rpm or faster, requiring a very low deceleration rate and a lot of deep cooling.

Sometimes the deceleration rate mismatches the task -- there's too much energy to use -- and it's possible to add a pulley block between the motor and the first deceleration stage of changing maximum speed. This either increases the screw speed beyond the previous limit or reduces the maximum speed to allow the system to operate at a greater percentage of the maximum speed. This will increase available energy, reduce amperes and avoid motor problems. In both cases, output may increase depending on material and cooling requirements.

4. Feed as coolant

Extrusion transfers the energy of the motor, sometimes a heater, to cold plastics, thereby converting it from solids to melts. The surface temperature of the input material is lower than that of the cylinder and screw in the feeding area. However, the surface of the cylinder in the feeding area is almost always above the plastic melting range. It is cooled by contacting the feed particles, but the heat is transferred from the front end to the back and maintained by controlled heating. Even when the heat at the current end is maintained by viscous friction and does not require heat input from the cylinder, a post-open heater may be required. The most important exception is the trough feed drum, which is almost exclusively used for HDPE.

The screw root surface is also cooled by the feed and insulated from the cylinder wall by the plastic feed particles (and air between the particles). If the screw stops suddenly, the feed stops, and the screw surface becomes hotter in the feed area because the heat moves backwards from the hotter front end. This may lead to adherence or bridging of particles at the root.

5. In the feeding area, stick to the cylinder and slide to the screw.

In order to maximize the solid particle transport in the feeding area of the smooth cylinder of a single screw extruder, the particles should adhere to the cylinder and slide onto the screw. If the particles stick to the root of the screw, nothing can pull them down; the volume of the passage and the entrance volume of the solid are reduced. Another reason for poor root adhesion is that plastics may be heated here to produce gels and similar contaminated particles, or adhere intermittently and interrupt with the change of output speed.

Most plastics slide naturally at the root because they enter cold and friction has not yet heated the root to the same heat as the wall. Some materials are more likely to adhere than others: highly plasticized PVC, amorphous PET, and some polyolefin copolymers with adhesion properties desired in final use.

For the cylinder, it is necessary for the plastic to stick here so that it can be scraped off and pushed forward by the screw thread. There should be a high friction coefficient between the particles and the cylinder, which in turn is strongly influenced by the temperature of the rear cylinder. If particles do not adhere, they simply rotate in place without moving forward - that's why smooth feeding is not good.

Surface friction is not the only factor affecting the material. Many particles never touch the cylinder or screw root, so there must be friction and mechanical and viscous linkage within the particles.

The grooved cylinder is a special case. The trough is in the feeding area, the feeding area and the rest of the cylinder are thermally insulated and deeply water-cooled. Threads push particles into the groove and form a high pressure within a fairly short distance. This increases the occlusion allowance of the same output with lower screw speed, thereby reducing the friction heat generated by the front end and lowering the melt temperature. This may mean that cooling limits faster production in blown film production lines. The trough is especially suitable for HDPE, which is the most slippery plastics besides perfluorinated plastics.

6. Material costs the most

In some cases, material costs can account for 80% of production costs - more than the sum of all other factors - with the exception of a few products of particular importance for quality and packaging, such as medical catheters. This principle naturally leads to two conclusions: processors should reuse as many corners and scraps as possible to replace raw materials, and adhere to tolerances as strictly as possible to avoid deviations from the target thickness and product problems.

7. Energy costs are relatively unimportant

Although the attractiveness of a factory is at the same level as the real problems and rising energy costs, the energy required to run an extruder is still a small part of the total production cost. This is always the case, because the material cost is very high, the extruder is an effective system, if too much energy is introduced, then the plastic will soon become very hot and cannot be processed normally.

8. The pressure at the end of the screw is very important.

This pressure reflects the resistance of all objects downstream of the screw: filter screen and contamination crusher plate, adapter conveyor pipe, fixed agitator (if any) and die itself. It depends not only on the geometry of these components, but also on the temperature of the system, which in turn affects the resin viscosity and passing speed. It does not depend on screw design except when it affects temperature, viscosity and throughput. For safety reasons, it is important to measure temperature - if it is too high, the die and die may explode and injure nearby people or machines.

Pressure is beneficial to agitation, especially in the final area of the single screw system (metering area). However, high pressure also means that the motor has to output more energy - hence higher melt temperature - which can set the pressure limit. In twin screw, the two screw biting each other is a more effective agitator, so no pressure is needed for this purpose.

In the manufacture of hollow components, such as pipes made by spider moulds with brackets positioned to the core, high pressure must be generated within the moulds to assist in the reorganization of separated logistics. Otherwise, the products along the welding line may be weak and may have problems in use.

9. Output = displacement of the last thread /- pressure flow and leakage

The displacement of the last thread is called positive current, which depends only on the geometry, speed and melt density of the screw. It is regulated by pressure flow, which actually includes the effect of reducing the resistance of output (indicated by the maximum pressure) and any over-biting effect in the feed of increasing output. Leakages on threads may be in either direction.

It is also useful to calculate the output of each rpm, because it represents any decrease in the pump capacity of the screw at a given time. Another related calculation is the output per horsepower or kilowatt used. This represents efficiency and is able to estimate the productivity of a given motor and driver.