The choice of the right extruder size is based on many different factors and can have a decisive influence on the achievable product quality and also on the achievable productivity of the production. General statements are often not correct and must therefore be thoroughly questioned before they are implemented in day-to-day production.
When does the question of the right extrusion system actually arise?
In many companies the situation is that the products to be extruded should not only be manufactured on a single line specially selected for this purpose, but that, depending on availability, one and the same product must be manufactured on different extrusion lines. The lines are then equipped with the appropriate extrusion dies as required and production is started with production parameters that match the line.
In such cases it happens that identical products are sometimes produced on a (e.g.) 45 mm extruder and on another day on a 75 mm extruder. It is imperative that very different processes are used, so that the product properties can vary greatly in some cases. If quality problems arise, the question is often raised as to the extent to which the extruder size can influence the product.
Another situation in which the question of the optimum extruder size regularly arises is when investing in new lines. Here a distinction must be made between whether the new line is to be purchased specifically for a continuous runner, i.e. whether it is to be optimally dimensioned for a special product, or whether the new line is to be suitable for a certain range of products, i.e. whether it is to be used as flexibly as possible.
In all cases, the choice of the right extruder size is an important task.
Extruder size, how is it described?
The size of an extruder can be described by the three main factors:
the screw diameter in mm
the screw length in multiples of the diameter
the drive power in kW
An extruder of type 45/35 (40kW) thus has a screw diameter of 45mm and a screw length of 1,620mm (45mm x 36). The drive power in this case is 40 kW.
Extruder size, relevant data:
The indicated screw diameter refers to the outer diameter of the screw (flight diameter). The usual dimensions for classic single-screw extruders for production are between 30mm and 150mm, but there are both smaller and larger systems.
It is not possible to make a general statement about the throughput that an extruder with a certain screw diameter can achieve, since the throughput that can be achieved can always depend on many other parameters such as the screw design, the material to be processed and other parameters such as the plant design or the drive power.
In practice, deep-cut screws with a large free volume (so-called throughput-intensive screws) can achieve higher throughput rates (with low pressure build-up requirements) than flat-cut screws with a smaller free volume (so-called pressure-intensive screws). With regard to the screw length, it can be roughly stated that longer systems improve the homogenizing and mixing performance of the extruder, but can lead to degradation due to the longer residence times of sensitive materials. In addition, the time and material required for “rinsing” the system increases, for example when changing the colour.
The drive power of the extruder can also be a limiting factor, but in practice it is usually oversized rather than undersized.
The approaches presented in the following are therefore only intended to assist in the selection of an extruder size and do not represent general criteria. Nevertheless, we would like to try to define a small selection aid here.
Extruder selection:
For the selection of an extruder different aspects should be considered in the selection:
Desired throughput:
The throughput required in the application gives a rough idea of the approximate dimensions of the extruder. The following table shows a rough summary for throughput rates of different screw diameters with standard screw geometry (3 zones), in each case related to realistic maximum throughputs for processing PEHD and PPH.
Below is a diagram of the relationship between screw diameter and maximum achievable throughput, based on experience. This diagram is not general and not transferable, but serves only to illustrate the large dependencies.
(Special applications such as high-speed extrusion concepts with enormously high screw speeds (high-speed extrusion >1m/s peripheral speed) are not taken into account here).
On the basis of such information, which can usually be obtained from the various extruder manufacturers, a rough selection can be made as to the size of the screw diameter required to achieve the maximum throughput required. However, it is important that such data are always material-specific and cannot be easily transferred to other materials.
A further aspect that is of decisive importance when selecting an extruder is not only the throughput, but also the peripheral speeds of the screw occurring at the respective speed.
If, for example, an output of 400kg/h is planned, this can be achieved in the following configurations (exemplary data, not generally valid):
It is logical that a comparatively small extruder requires a higher screw speed to achieve throughput than a larger extruder. The screw speed, however, also directly increases the peripheral speed of the screw. If one now considers that high peripheral speeds can damage the material due to the high friction and shear in the barrel, it quickly becomes clear that gentle processing of the material is more feasible on slow-running lines. At the same time, however, the residence time of the melt in the system increases with larger extruders, so that sensitive materials can be thermally damaged.
The order of magnitude of the permitted circumferential speeds of the respective material can usually be found in the processing instructions of the raw material manufacturer.
Typical values for permitted circumferential speeds are in this order of magnitude:
With knowledge of the desired output of the extruder and considering the processing specifications of the material manufacturer, it is then possible to make a size selection for the extruder.
The following table and diagram show the resulting peripheral speed for different speeds and screw diameters:
Example (fictitious values, not universal):
A throughput of 300kg/h is to be achieved with a polycarbonate (PC) material. A critical peripheral speed of 0.3 m/s for this material can be taken from the manufacturer’s processing instructions. A 60 mm and a 75 mm extruder are available.
The 60 mm extruder would probably be able to achieve this throughput at a screw speed of approx. 170 rpm. The circumferential speeds would be approx. 0.5 m/s.
The 75 mm extruder could achieve the throughput at speeds of approx. 100 rpm. Peripheral speeds of approx. 0.39 m/s would occur.
Both extruders would therefore not be suitable for processing the material with the desired throughput.
On the 60 mm extruder, the maximum peripheral speeds would be reached at approx. 85 rpm. At this speed a throughput of approx. 115 kg/h would be realistic.
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On the 75 extruder, the maximum circumferential speeds would be reached at approx. 75 rpm, so a throughput of approx. 190 kg/h would be feasible.
Consideration of the drive power
A further aspect that still has to be considered when selecting or dimensioning the extruder is the design of the drive power. The first indication for the design of the drive power is the amount of energy required to heat the material to be processed to processing temperature.
If, for example, a PC is to be heated from ambient temperature to 270°C, a desired throughput of 200 kg/h would require approx. 25 kW of purely thermal power.
Due to the losses that occur in an extruder and the additional energy required to build up the pressure, today’s extruders are usually equipped with at least 2.5 times this power.
Consideration of the residence time
Residence time is the time a plastic particle remains in the system from its entry into the extruder to its exit from the extruder. The longer the residence time, the more likely it is that the material in the system will be damaged and thermally degraded. In principle, it is advisable to extrude with the shortest possible residence times, unless the required homogenising capacity is not sufficient.
For a very rough calculation of the residence time, the quotient of the volume of the extruder filled with melt and the volume flow of the melt can be formed according to basic physical laws.
If, for example, it is assumed that there is a volume of 3,000 cm³ filled with melt in the extruder and is produced with a throughput of 100 kg/h, the average residence time (at an assumed density of 1,000 kg/m³) is approx. 108 seconds. (you can find an excel- tool for residence time calculation in our download section)
To determine the approximate residence time, however, it is necessary to describe the design of the screw and extruder as well as the degree of filling of the system as precisely as possible. The given calculation is therefore only suitable as a rough estimate.
In general, however, it can be said that the higher the temperatures of the melt, the shorter the residence times should be. Here, too, the processing instructions of the material manufacturer are an important point of reference.
Conclusion:
When selecting or designing an extruder, various aspects have to be taken into account, all of which are interdependent and generally have to be considered very material-specifically.
A universal extruder that can meet all requirements in the same quality is simply impossible. Each extruder dimensioning is a compromise of different sizes and must therefore be considered specifically for the application.
Nevertheless, it is important to have other aspects (circumferential speed, residence time) in mind in addition to the pure attainability of a desired throughput. In practice, neglecting these aspects repeatedly leads to considerable quality differences in the production of supposedly identical products when these are produced on different systems.
Gearbox and motor selection are important keys to the successful and economically optimized single-screw extrusion process. That is, the motor and gearbox should be selected such that the extruder operates at near 70% motor current load and about 70% of the maximum normal screw speed. This will allow a rate increase in the future and the extrusion of resins that are more viscous while minimizing the capital costs for the motor and gearbox.
If the gearbox and motor are not selected properly for the application, then the rate of the line can be limited by the extruder by either operating at maximum screw speed or maximum motor current. For most applications, either the most expensive segment of the line or the cooling process should be the rate limiting section of the line, and not the extruder.
This article focuses on setting the maximum screw speed for the extruder based on the maximum motor speed, gearbox reduction and belt sheaves, if used. Motor size depends on the application.
Original equipment manufacturers (OEMs) are typically very good at sizing motors and gearboxes for extrusion applications. But the purchaser has the final decision on the acceptance of the design. I have seen a very large number of extrusion applications that were set up optimally. That is, the motor and gearbox provided the proper speed and torque range for the screw at the target rate, discharge pressure and discharge temperature. An optimal design should also include a future rate increase of up to 20%.
In the last 35 years, however, I have optimized nearly 30 extruders that were configured with less than ideal motors and gearboxes. Most of these cases were because the extruders were purchased for a particular application and then repurposed for a new application that required a different torque and speed for the screw. But there were the occasional specifications of a poor motor and gearbox on new equipment.
I have optimized nearly 30 extruders that were configured with less than ideal motors and gearboxes.
Extrusion applications operate with different discharge temperatures. For polyethylenes (PEs), typical discharge temperatures and screw speeds for some of the most common applications are provided in the accompanying table. The discharge temperatures are controlled in general by the metering channel depth, screw speed and resin viscosity. For example, a cast film process requires that the discharge temperature be near 250oC. The temperature is typically obtained by using a screw with a metering channel depth that is about 6 mm for a 100-mm diameter extruder and a screw speed of about 100 rpm.
For this application, the motor and gearbox (and belt sheave if used) would provide a maximum screw speed of about 120 rpm. For extrusion coating applications, the discharge temperature is typically 300oC and the screw speed during extrusion would be near 220 rpm, and the screw would have a metering channel depth of about 3 mm. The motor and gearbox combination for cast film would not be suitable for extrusion coating because the extruder could only run at the maximum screw speed of 120 rpm, providing about half the required rate. Moreover, the slower screw speed would likely not obtain a discharge temperature of 300oC, even for a screw with a 3-mm deep metering channel.
Recall from high school physics that power for a rotating shaft or screw is equal to the torque multiplied by the rotation speed of the screw. The motor current is directly proportional to the motor torque. A convenient method to estimate the power inputted via the screw is by using this equation:
where P is the power that is required by the screw in hp, Pmax is the name-plate power (hp) for the motor, A is the motor current (amperage) observed during the extrusion, Amax is the name-plate motor current at full load, RPM is the screw speed during extrusion, and the RPMmax is the maximum screw speed that the extruder is capable of running (base speed).
It is easy to see that if the maximum screw speed is 200 rpm and the screw speed used for the extrusion is 100 rpm, the highest power that can be inputted to the screw is half the power from the motor. The maximum screw speed is simply the maximum motor speed divided by the gearbox reduction and belt sheave ratio. The maximum torque at the screw depends on the motor size and the speed reduction by the gearbox and belt sheave.
A gearbox built for extrusion coating was used as a replacement box on a cast film line. The gearbox and motor combination set the maximum screw speed to 200 rpm. The cast film process, however, operated at about 95 rpm, limiting the torque available to the screw. That is, the motor current was running at 98% of the maximum current during operation. The redesign of the screw was complicated because of the lack of torque. The design provided an acceptable rate, but the extrudate temperature was too high.
Typically, a screw designer will increase the metering channel depth to decrease the extrudate temperature to an optimal level. When the metering channel depth is increased, the specific rate increases and the motor current needed for the process will increase. For this line, decreasing the discharge temperature via screw design was not possible because the motor was already running at the maximum motor current.
Purchasing and upgrading extrusion systems should always design flexibility into the line that enables rate increases, alternative resins and process optimizations.
Recently, I examined several cast film extruders that were repurposed on extrusion coating lines. As previously discussed, the processes operate at different temperatures with PE cast film at about 250oC and extrusion coating at 300oC. The extruder had a maximum screw speed of 70 rpm. The motor was operating with 45% of maximum motor current. A new design can provide the proper extrudate temperature at low rate or a higher rate with an extrudate temperature that is too low. The compromise was to design at a low rate that met the high extrudate temperature requirement.
Gearboxes and motors on cooling extruders used in tandem foam sheet lines must be selected carefully. A schematic of a tandem foam sheet line is shown in the accompanying figure. The first extruder melts the resin and mixes in a physical blowing agent such as supercritical carbon dioxide. The discharge is typically about 235oC for polystyrene (PS). The resin, however, is too hot to foam. The cooling extruder decreases the PS mixture to about 140oC. The cooling extruder is larger in diameter than the first extruder, and the screw rotates very slowly and has channels that are very deep.
A new tandem foam sheet line was designed and installed with a conventional slotted screw in the cooling extruder. This screw runs at a relatively high specific rate and a screw speed of about 18.2 rpm. The motor and gearbox combination provided a maximum screw speed of 20 rpm from a 150 hp motor. This screw, however, could not decrease the material temperature consistently, resulting in a low quality and unusable foam.
A high-performance screw was designed and built that would increase the foam quality and the rate. The high-performance screw operated at a much higher specific rate, causing the screw speed to operate at 11.5 rpm for the same rate. At 11.5 rpm, only about 58% of the motor power could be delivered to the screw. The motor and gearbox were not capable of supplying enough power to the high-performance screw.
There are often belt drives between a gearbox and a motor that can be used to either increase the torque to the screw or increase the maximum speed to the screw. The belt drive sheaves were changed on the cooling extruder such that the new maximum speed was 15 rpm, enabling the input of 77% of the motor power to the screw. Changing the belt sheaves to a maximum screw speed of 15 rpm will decrease the service life of the gearbox because the maximum torque was increased on both the input and output shafts. The OEM should be contacted when a belt drive is changed to make sure that the service life and safety of the gearbox are not compromised.
Purchasing and upgrading extrusion systems should always design flexibility into the line that enables rate increases, alternative resins and process optimizations. Many of these upgrades will require additional screw speed and/or torque. A properly specified gearbox and motor speed will enable additional torque and screw speed to satisfy these optimizations.
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