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):
- 60 mm Extruder at 220 rpm –> peripheral speed: 0,691 m/s
- 75 mm Extruder at 160 rpm –> peripheral speed: 0,628 m/s
- 90 mm Extruder at 110 rpm –> peripheral speed: 0,518 m/s
- 105 mm Extruder at 80 rpm –> peripheral speed: 0,440 m/s
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.
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.
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