Do You Know The 11 Important Principles of Plastic Extrusion?

01 Mechanical Principle

The basic mechanism of extrusion is simple, a screw turns in a barrel and pushes the plastic forward. The screw is actually an inclined plane or ramp that wraps around a central layer. Its purpose is to increase pressure in order to overcome greater resistance.

For an extruder, there are three kinds of resistance that need to be overcome: the friction of solid particles (feed) on the barrel wall and the mutual friction between them during the first few turns of the screw (feed zone); the adhesion of the melt on the barrel wall; and the internal logistics resistance of the melt when it is pushed forward.

Newton explained that if an object is not moving in a given direction, then the forces on the object are balanced in that direction. The screw does not move axially, although it may rotate rapidly laterally near the circumference. Therefore, the axial forces on the screw are balanced, and if it applies a large forward thrust to the plastic melt, it also applies an equal backward thrust to something. In this case, the thrust it applies is on the bearing behind the feed port - the thrust bearing.

Most single screws have right-hand threads, like the screws and bolts used in woodworking and machinery. If you look at them from behind, they are counter-rotating because they are trying to screw back out of the barrel. In some twin-screw extruders, the two screws rotate in opposite directions in the two barrels and cross each other, so one must be right-handed and the other must be left-handed. In other interlocking twin screws, the two screws rotate in the same direction and must have the same orientation. However, in either case, there are thrust bearings to absorb the backward force, and Newton's principle still applies.

02 Heat principle

Extrudable plastics are thermoplastics—they melt when heated and solidify again when cooled. Where does the heat for the molten plastic come from? Feed preheat and barrel/die heaters may play a role and are very important at startup, but motor input energy—frictional heat generated in the barrel as the motor turns the screw against the resistance of the viscous melt—is the most important heat source for all plastics, except for small systems, slow screw speeds, high melt temperature plastics, and extrusion coating applications.

For all other operations, it is important that the barrel heaters are not the primary heat source in the operation and therefore have less effect on extrusion than we might expect. Post-barrel temperature may still be important because it affects the meshing or solids transport rate in the feed. Die and mold temperatures should generally be at or close to the desired melt temperature unless they are used for a specific purpose like glazing, fluid distribution, or pressure control.

03 Deceleration principle

In most extruders, the speed of the screw is varied by adjusting the speed of the motor. The motor usually runs at a full speed of about 1750 rpm, but this is too fast for an extruder screw. If it runs that fast, too much frictional heat is generated and the residence time of the plastic is too short to produce a uniform, well-mixed melt. Typical reduction ratios are between 10:1 and 20:1. The first stage can use either gears or pulleys, but the second stage uses gears and the screw is located in the center of the last large gear.

In some slow running machines (such as twin screws for PVC), there may be 3 reduction stages and the maximum speed may be as low as 30 rpm or less (ratio of 60:1). At the other extreme, some very long twin screws used for mixing can run at 600 rpm or faster, thus requiring a very low reduction ratio and a lot of deep cooling.

Sometimes the reduction ratio is not matched to the task correctly - there will be too much energy that cannot be used - and it is possible to add a pulley set between the motor and the first reduction stage that changes the maximum speed. This will either increase the screw speed beyond the previous limit or reduce the maximum speed to allow the system to run at a greater percentage of the maximum speed. This will increase the available energy, reduce amperage and avoid motor problems. In both cases, the output may be increased depending on the material and its cooling needs.

04 Feed acts as coolant

Extrusion is the transfer of energy from the motor - and sometimes the heater - to the cold plastic, thereby converting it from a solid to a melt. The incoming feed is cooler than the barrel and screw surfaces in the feed zone. However, the barrel surface 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 heat transferred backward from the hot front and by controlled heating. Even when the front heat is retained by viscous friction and no barrel heat input is required, a post heater may be required. The most important exception is the grooved feed barrel, which is almost exclusively used for HDPE.

The screw root surface is also cooled by the feed and insulated from the barrel wall by the plastic feed particles (and the air between the particles). If the screw stops suddenly, the feed stops, and the screw surface becomes hotter in the feed zone because the heat moves from the hotter front end to the back. This can cause particles to stick or bridge at the root.

05 In the feeding area, it sticks to the barrel and slides onto the screw

In order to maximize the solid particle delivery in the smooth barrel feed zone of a single screw extruder, the particles should stick to the barrel and slide onto the screw. If the particles stick to the root of the screw, there is nothing to pull them down; the channel volume and the amount of solid inlet are reduced. Another reason for poor adhesion at the root is that the plastic may heat up here and produce gel and similar contamination particles, or it may stick intermittently and break off with changes in output speed.

Most plastics naturally slide on the roots because they are cold when they enter and friction has not yet heated the roots as hot as the barrel walls. Some materials are more likely to stick than others: highly plasticized PVC, amorphous PET, and certain polyolefin copolymers whose adhesion properties are desirable in some end uses.

For the barrel, it is necessary for the plastic to adhere here so that it can be scraped off and pushed forward by the screw flights. There should be a high coefficient of friction between the pellets and the barrel, which in turn is strongly affected by the rear barrel temperature. If the pellets do not adhere, they just turn in place and do not move forward - this is why smooth feeding is not good.

Surface friction is not the only factor that affects feed. Many particles never touch the barrel or screw root, so there must be friction and mechanical and viscosity linkage within the particle.

The grooved barrel is a special case. The grooves are in the feed zone, which is thermally insulated from the rest of the barrel and heavily water-cooled. The threads push the pellets into the grooves and create a very high pressure over a relatively short distance. This increases the bite allowance at lower screw speeds for the same output, resulting in less frictional heat generated at the front and lower melt temperatures. This can mean faster production in cooling-limited blown film lines. Grooves are particularly suitable for HDPE, which is the most slippery of the common plastics except for perfluorinated plastics.

06 Materials cost the most

In some cases, material costs can account for 80% of production costs—more than all other factors combined—except for a few products where quality and packaging are particularly important, such as medical tubing. This principle naturally leads to two conclusions: processors should reuse scrap and waste as much as possible to replace raw materials, and adhere to tolerances as tightly as possible to avoid deviations from target thickness and product problems.

07 Energy costs are relatively unimportant

Despite the appeal of a plant and the real problems with rising energy costs, the energy required to run an extruder is still a small fraction of the total production cost. This is always the case because the material cost is very high, the extruder is an efficient system, and if too much energy is introduced the plastic will quickly become too hot to process properly.

08 The pressure at the end of the screw is important

This pressure reflects the resistance of everything downstream of the screw: the filter screen and contamination breaker plate, the adapter delivery tube, the fixed agitator (if any), and the die itself. It depends not only on the geometry of these components but also on the temperature in the system, which in turn affects the resin viscosity and throughput rate. It does not depend on the screw design, except when it affects temperature, viscosity, and throughput. Measuring the temperature is important for safety reasons - if it is too high, the die head and mold may explode and injure nearby personnel or machinery.

Pressure is beneficial for mixing, especially in the final zone (metering zone) of a single-screw system. However, high pressure also means more power output from the motor - and therefore higher melt temperatures - which can dictate the pressure limit. In a twin-screw extruder, the two intermeshing screws are a more efficient agitator, so pressure is not needed for this purpose.

When making hollow parts, such as tubes, using spider molds with brackets to hold the core in place, very high pressure must be generated inside the mold to help the separated streams reunite. Otherwise, the product along the weld line may be weak and may cause problems during use.

09 Output = displacement of the last thread +/- pressure flow and leakage

The displacement of the last flight is called positive flow and depends only on the screw geometry, screw speed and melt density. It is regulated by the pressure flow and actually includes the effect of resistance (represented by the maximum pressure) which reduces the output and the effect of any overbite in the feed which increases the output. Leakage on the flight may be in either direction.

It is also useful to calculate the output per rpm (revolutions) as this indicates any decrease in the screw's pumping capacity over 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 drive.

10 Shear rate plays a major role in viscosity

All common plastics have a shear reduction property, meaning that the viscosity decreases as the plastic moves faster and faster. Some plastics show this effect more strongly. For example, some PVCs will increase their flow rate by a factor of 10 or more when the thrust is doubled. In contrast, LLDPE does not shear much, increasing its flow rate by a factor of only 3 to 4 when the thrust is doubled. Reduced shear reduction means higher viscosity under extrusion conditions, which in turn means more motor power is needed.

This explains why LLDPE runs hotter than LDPE. Flow is expressed as shear rate, which is about 100s-1 in the screw channel, between 100 and 100s-1 in most die dies, and greater than 100s-1 in the flight-wall gap and some small die gaps. The melt coefficient is a common measure of viscosity but is inverted (i.e. flow/thrust instead of thrust/flow). Unfortunately, it is measured at shear rates of 10s-1 or less and may not be a true measurement in extruders with very fast melt flow rates.

11 The motor and the cylinder are opposite to each other

Why is it that barrel control does not always work as expected, especially in the measurement zone? If the barrel is heated, the layer of material at the barrel wall becomes less viscous, and the motor requires less energy to operate in this smoother barrel. The motor current (amperage) drops. Conversely, if the barrel is cooled, the melt viscosity at the barrel wall increases, the motor must turn harder, the amperage increases, and some of the heat removed through the barrel is returned by the motor. In general, barrel conditioners do have an effect on the melt, which is what we expect, but the effect is not as great anywhere as the regional variation. It is best to measure the melt temperature to really understand what is happening.

Rule 11 does not apply to the die and mold, because there is no screw rotation there. That is why external temperature changes are more effective there. However, these changes are from the inside out and therefore not uniform unless they are stirred in a stationary agitator, which is an effective tool for melt temperature changes as well as stirring.

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