As long as it involves extrusion, these 11 basic principles must be followed!

I. Mechanical Principles

The basic mechanism of extrusion is very simple - a screw rotates in the cylinder and pushes the plastic forward. The screw is actually an inclined plane or slope, wound around the central layer. The purpose is to increase the pressure in order to overcome greater resistance. For an extruder, there are three types of resistance that need to be overcome: the frictional force between solid particles (feed) and the barrel wall, and the mutual frictional force between them during the first few turns of the screw (feed zone). The adhesion of the melt on the cylinder wall; The internal flow resistance of the melt when it is pushed forward.

Newton once explained that if an object does not move in a given direction, then the forces on this object are balanced in that direction. The screw does not move axially, although it may rotate rapidly laterally 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 simultaneously exerts the same backward thrust on an object. Here, the thrust it applies acts on the bearing behind the feed inlet - the thrust bearing.

Most single-screw screws have right-handed threads, like the screws and bolts used in woodworking and machinery. If viewed from behind, they are rotating in the opposite direction because they are trying their best to spiral out of the cylinder backward. In some twin-screw extruders, two screws rotate in opposite directions and cross each other in two barrels, so one must be rightward and the other must be leftward. In other interlocking twin-screw systems, the two screws rotate in the same direction and thus must have the same orientation. However, in either case, there are thrust bearings that absorb backward force, and Newton's principle still applies.

Ii. Heat Principle

Extrudable plastics are thermoplastics - they melt when heated and solidify again when cooled. Where does the heat for melting plastic come from? Feed preheating and cylinder/mold heaters may play a role and are very important at startup. However, the motor input energy - the frictional heat generated in the cylinder when the motor overcomes the resistance of the viscous melt to rotate the screw - is the most important heat source 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 cylinder heater is not the main heat source in the operation, and thus its effect on extrusion may be smaller than we expected (see Principle 11). The temperature of the rear cylinder may still be important as it affects the meshing or the conveying speed of solid materials in the feed. The temperature of the die head and mold should usually be the desired melt temperature or close to it, unless they are used for a specific purpose such as polishing, fluid distribution or pressure control.

Iii. Principle of Deceleration

In most extruders, the change in screw speed is achieved by adjusting the motor speed. The motor usually rotates at full speed of approximately 1750rpm, but this is too fast for an extruder screw. If it rotates at such a fast speed, too much frictional heat will be generated and the retention time of the plastic will be too short to prepare a uniform and well-stirred melt. The typical deceleration ratio is between 10:1 and 20:1. In the first stage, either gears or pulley blocks can be used, but in the second stage, gears are used and the screw is positioned at the center of the last large gear.

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

Sometimes the deceleration rate is mismatched with the task - there will be too much energy that cannot be used - and it is possible to add a pulley block between the motor and the first deceleration stage where the maximum speed is changed. This either increases the screw speed beyond the previous limit or reduces the maximum speed, allowing the system to operate at a greater percentage of the 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 requirements.

4. The feed serves as a coolant

Extrusion is the process of transferring the energy of the motor - sometimes that of the heater - onto cold plastic, thereby converting it from solid to melt. The input feed is lower than the surface temperature of the cylinder and screw in the feeding area. However, the surface of the cylinder in the feeding area is almost always above the melting range of the plastic. It is cooled by coming into contact with the feed particles, but the heat is retained by the heat transferred from the hot front end to the rear and the controllable heating. Even when the heat at the front end is maintained by viscous friction and no heat input from the cylinder is required, the rear heater may need to be turned on. The most important exception is the trough-type feed cylinder, which is almost exclusively used for HDPE.

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

Five. The feed adheres to the cylinder or slides onto the screw

To maximize the solid particle conveying capacity in the feeding zone 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 off. The volume of the channel and the inlet volume of solids are reduced. Another reason for poor adhesion at the root is that the plastic may be thermally smelted here and produce gel and similar contaminant particles, or adhere intermittently and be interrupted with changes in output speed.

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

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

Surface friction is not the only factor affecting the feed. Many particles never come into contact with the cylinder or the root of the screw, so there must be friction and mechanical and viscosity interlocking within the particles.

The grooved cylinder is a special case. The trough is in the feeding zone. The feeding zone is thermally insulated from the rest of the cylinder and is deeply water-cooled. The thread pushes the particles into the groove and creates a very high pressure over a relatively short distance. This increases the biting tolerance of the same output at a lower screw speed, thereby reducing the frictional heat generated at the front end and lowering the melt temperature. This might imply that cooling limits faster production in the blown film production line. The groove is particularly suitable for HDPE, which is the smoothest common plastic after fluorinated plastic.

Six. The cost of materials is the highest

In some cases, material costs can account for 80% of the production cost - more than the sum of all other factors - except for a few products where quality and packaging are particularly important, such as medical catheters. This principle naturally leads to two conclusions: Processors should reuse as many scraps and waste products as possible to replace raw materials and strictly adhere to tolerances as much as possible to avoid deviations from the target thickness and product issues.

Seven, energy costs are relatively unimportant

Although the appeal of a factory and the real problems and rising energy costs are at the same level, the energy required to run an extruder still accounts for a very small portion of the total production cost. The situation is always like this because the material cost is very high. The extruder is an effective system. If too much energy is introduced, the plastic will quickly become very hot and cannot be processed normally.

Eight. 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 contamination crushing machine plate, the adapter delivery pipe, the fixed agitator (if any), and the mold itself. It not only depends on the geometry of these components but also on the temperature in the system, which in turn affects the viscosity of the resin and the passage speed. It is not dependent on the screw design, except when it affects temperature, viscosity and flow rate. For safety reasons, measuring the temperature is very important - if it is too high, the die head and mold may explode and hurt people or machines nearby.

Pressure is beneficial for stirring, especially in the final area (metering zone) of a single-screw system. However, high pressure also means that the motor has to output more energy - and thus the melt temperature is higher - which can specify the pressure limit. In a twin-screw system, the interlocking of the two screws is a more effective agitator, and thus no pressure is required when used for this purpose.

When manufacturing hollow components, such as pipes made by using a spider mold with a support for core positioning, a very high pressure must be generated inside the mold to help the separated logistics recombine. Otherwise, the products along the welding line may be weak and problems may occur during use.

Ix. Output = Displacement of the last thread +/- pressure flow and leakage

The displacement of the last thread is called forward flow, which only depends on the geometry of the screw, the screw speed and the melt density. It is regulated by pressure flow and actually includes the resistance effect of reducing the output volume (indicated by the maximum pressure) and any overseizing effect in the feed that increases the output volume. Leakage on the thread may occur in either of the two directions.

It is also useful to calculate the output per rpm, as this indicates any decrease in the pumping capacity of the screw at a certain time. Another related calculation is the output per horsepower or kilowatt used. This indicates efficiency and can estimate the production capacity of a given motor and driver.

X. Shear rate plays a major role in viscosity

All common plastics have the property of shear force reduction, meaning that the viscosity decreases as the plastic moves faster and faster. This effect is particularly evident in some plastics. For instance, for some PVCs, the flow velocity will increase tenfold or more when the thrust is doubled. On the contrary, the shear force of LLDPE does not decrease too much. When the inference doubles, its flow velocity only increases by 3 to 4 times. The reduced shear force effect implies high viscosity under extrusion conditions, which in turn means that more motor power is required.

This can explain why the operating temperature of LLDPE is higher than that of LDPE. The flow rate is expressed in terms of shear rate. It is approximately 100s ⁻ ¹ in the screw channel, between 100 and 100s ⁻ ¹ in most die openings, and greater than 100s ⁻ ¹ in the gap between the thread and the barrel wall and some small die clearances.

The melt coefficient is a commonly used method for measuring viscosity, but it is reversed (for example, flow rate/thrust rather than thrust/flow rate). Unfortunately, its measurement may not be a true one in extruders where the shear rate is 10 seconds -1 or less and the melt flow rate is very fast.

Eleven. The motor is in opposition to the cylinder, and the cylinder is in opposition to the motor

Why is it that the control effect of the cylinder is not always as expected, especially in the measurement area? If the cylinder is heated, the viscosity of the material layer on the cylinder wall decreases, and the motor requires less energy to operate in this smoother cylinder. The motor current (amperage) has decreased. Conversely, if the cylinder cools down, the melt viscosity at the cylinder wall increases, and the motor has to rotate more forcefully, increasing the amperage. Some of the heat removed when passing through the cylinder is sent back by the motor. Generally, the cylinder regulator does have an effect on the melt, which is what we expect, but the effect anywhere is not as great as that of the regional variable. It is best to measure the melt temperature to truly understand what has happened.

The 11th principle does not apply to the die head and mold, as 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 they are stirred evenly in a fixed stirrer, which is an effective tool for melt temperature changes and stirring.