Basic principles of twin-screw extrusion of plastics

01. Mechanical principles

The basic mechanism of extrusion is simple—a screw turns in a barrel and pushes the plastic forward. The screw is actually an incline or ramp that winds around the center layer. Its purpose is to increase pressure in order to overcome greater resistance. As far as an extruder is concerned, there are 3 kinds of resistance to overcome:

The friction of solid particles (feed) against the barrel wall and the mutual friction between them during the first few revolutions of the screw (feed zone);

Adhesion of the melt on the cylinder wall;

The internal flow resistance of the melt as 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. Thus, the axial force on the screw is balanced, and if it's putting a lot of forward push on the plastic melt it's also giving an equal backward push to something at the same time. Here, the thrust it exerts is on the bearing behind the feed inlet - the thrust bearing.

Most single screws have a right-hand thread, like the screws and bolts used in woodworking and machinery. If viewed from the rear, they rotate in opposite directions, as they try to unscrew the barrel backwards as far as they can. In some twin-screw extruders, the two screws counter-rotate and cross each other in the two barrels, so one must be right-handed and the other must be left-handed. In other intermeshing twin screws, the two screws rotate in the same direction and must therefore have the same orientation. However, in either case there are thrust bearings that absorb the rearward force, and Newton's principles still apply.

 

02. Thermal principle

Extrudable plastics are thermoplastics - they melt when heated and solidify again when cooled. Where does the heat to melt plastic come from?

Feed preheat and barrel/mold heaters may play a role and are very important at start-up, however, motor input energy —frictional heat generated in the barrel when the motor overcomes the resistance of the viscous melt to rotate the screw—is the most important source of heat for all plastics,

Exceptions are small systems, slow screw speeds, high melt temperature plastics and extrusion coating applications.

As with all other operations, it is important to realize that the barrel heater is not the primary source of heat in the operation and thus contributes less to extrusion than we might expect (see Principle 11). Rear barrel temperature may still be important because it affects meshing or the rate at which solids are conveyed in the feed. Die and die temperatures should generally be at or near 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, changes in screw speed are achieved by adjusting the motor speed. The motor usually turns at full speed around 1750rpm, but that's too fast for an extruder screw. If turned 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 reduction ratios are between 10:1 and 20:1. The first stage can use either gears or pulley blocks, but the second stage uses both gears and the screw is positioned in the center of the last large gear.

In some slow running machines (such as twin screw for UPVC), there may be 3 deceleration stages and the maximum speed may be as low as 30rpm or lower (ratio up to 60:1). At the other extreme, some very long twin screws for agitation can run at 600rpm or faster, thus requiring a very low deceleration rate and a lot of deep cooling.

Sometimes the deceleration rate is mis-matched to the task - 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 beyond the previous limit or reduces the maximum speed allowing the system to run at a greater percentage of maximum speed. This will increase available energy, reduce amperage and avoid motor problems. In both cases, depending on the material and its cooling needs, the output may increase.

 

04. The feed acts as a coolant

Extrusion transfers the power of a motor—and sometimes a heater—to cold plastic, converting it from a solid to a melt. The input 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 transfer from the hot front to the rear and controlled heating. Even when the front heat is held by viscous friction and the barrel heat input is not required, it may be necessary to turn on the after heater. The most important exception is the slotted feed barrel, almost exclusively for HDPE.

The screw root surface is also cooled by the feed and insulated from the barrel walls by the plastic feed pellets (and the air between the pellets). If the screw stops suddenly, the feed also stops, and the surface of the screw gets hotter in the feed zone because heat moves backwards from the hotter front end. This may cause sticking or bridging of particles at the root.

 

05.In the feed zone, sticks to the barrel & slides onto the screw

To maximize solids delivery in the feed zone of an extruder with a smooth barrel, the particles should stick to the barrel and slide onto the screw.

If particles stick to the root of the screw, there is nothing to pull them down; channel volume and solids intake are reduced.

Another reason for poor adhesion at the roots is where the plastic may heat up and produce gels and similar contamination particles, or it may adhere intermittently and break off with changes in output speed.

Most plastics slide naturally at the roots because they are cold when they come in, and the 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 that have sticking properties that are desired in end use.

For the barrel, it is necessary for the plastic to stick here so that it is scraped off and pushed forward by the screw flight. There should be a high coefficient of friction between the pellets and the barrel, which in turn is strongly influenced by the temperature of the back barrel. If the particles don't stick, they just turn in place and don't move forward - which is why smooth feeds don't work well.

Surface friction is not the only factor affecting feed. Many particles never touch the barrel or screw root, so there must be friction and mechanical interlocking with viscosity inside the particles.

Fluted cylinders are a special case. The trough is in the feed area, which is thermally insulated from the rest of the barrel and deeply water cooled. The threads push the pellets into the groove and build up a 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 by the front end and lower melt temperatures. This can mean cooling limits faster production in blown film lines. The grooves are especially suitable for HDPE, which is the slipperiest of common plastics other than perfluorinated plastics.

 

06. Materials cost the most

In some cases, material costs can account for as much as 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 catheters. This principle naturally leads to two conclusions: converters should reuse as much scrap and scrap 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

While the allure and real problems of a plant are on the same level as 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, an extruder is an efficient system, and if too much energy is introduced the plastic can quickly become too hot to process properly.

 

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

This pressure reflects the resistance of everything downstream of the screw: screen and contamination breaker plates, adapter delivery tubes, stationary agitators (if present), 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 speed. It is independent of screw design, except where it affects temperature, viscosity and throughput. Measuring the temperature is important for safety reasons - if it's too high, the die and die could explode and injure nearby people or machinery.

Pressure is favorable for mixing, especially in the last zone (metering zone) of a single-screw system. However, high pressure also means more power is being put out by the motor – and thus a higher melt temperature – which can dictate the pressure limit. In a twin screw, the two screws intermeshing are a more efficient agitator, so no pressure is required for this purpose.

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

The displacement of the last flight is called positive flow and depends only on screw geometry, screw speed and melt density. It is regulated by the pressure flow, which actually includes the drag effect (indicated by the highest pressure) to reduce output and any overbite effect in the feed to increase output. Leaks on threads can go in either direction.

It is also useful to calculate the output per rpm (revolution) as this represents any decrease in the pumping capacity of the screw over time. Another relevant calculation is the amount of output per horsepower or kilowatt used. This indicates efficiency and enables an estimate of the production capacity of a given motor and drive.

10. Shear rate plays a major role in viscosity

All common plastics have a shear drop property, meaning that the viscosity of the plastic becomes lower as it moves faster and faster. This effect is particularly pronounced for some plastics. For example some PVCs will increase flow velocity by a factor of 10 or more when thrust is doubled. On the contrary, the shear force of LLDPE does not decrease too much, and its flow rate only increases by 3 to 4 times when it is reasoned to double. The reduced shear reduction effect means higher viscosity under extrusion conditions, which in turn means more motor power is required.

This could explain why LLDPE runs hotter than LDPE. The flow rate is expressed as a shear rate, which is about 100s-1 in the screw channel, between 100 and 100s-1 in most die mouths, and greater than 100s-1 in the gap between the thread and the barrel wall and some small die gaps.

Melt coefficient is a common measure of viscosity but is reversed (eg flow/thrust instead of thrust/flow). Unfortunately, it is measured at shear rates of 10 s-1 or less and may not be a true measurement in extruders with fast melt flow rates.

 

11. The motor is opposed to the barrel, and the barrel is opposed to the motor

Why is the control effect of the cylinder not always as expected, especially in the measurement area? If the barrel is heated, the material layer at the barrel wall becomes less viscous, and the motor needs less energy to run in this smoother barrel. Motor current (amperage) drops. Conversely, if the barrel cools, the viscosity of the melt at the barrel wall increases, the motor must turn harder, the amperage increases, and some of the heat removed as it passes through the barrel is sent back by the motor. Generally, the barrel regulator does have an effect on the melt, which is what we would expect, but nowhere as large as the zone variable. It is best to measure the melt temperature to really understand what is going on.

Principle 11 does not apply to dies and dies, since there is no screw turning there. That's why external temperature changes are more effective there. However, these changes are from the inside out and therefore not uniform unless homogenized in a stationary stirrer, which is an effective tool for both melt temperature changes and stirring.

Nanjing Haisi is a professional manufacturer of granulating extruders for coloring/dyeing, filling masterbatch production, as well as plastic modification and compounding , plastic recycling. Innovative machines are built as per the concept of durability, high efficiency and economical power consumption.