Extrusion cylinders are generally divided into smooth barrel extruders (conventional extruders) and grooved extrusion cylinders (grooved barrel extruders, helibars, etc.). The differences between the different systems lie, as the name suggests, in the design of the extruder barrel. While the conventional extruder has a completely smooth cylinder, a grooved system has grooves on the inside of the cylinder. What advantages the different systems have and when they are used is the content of this article.
Upper illustration: Section through extrusion cylinder (schematic), left: plain tube without grooves; right: cylinder with axial grooves)
From a geometric and manufacturing point of view, extrusion cylinders can be divided into the two groups.
- smooth cylinders without grooves
- grooved cylinders (with different groove geometries)
Geometry, number, course and shape of grooves
The shape, the number, the course and the length of the grooves can be very different.
Rectangular-sharp-edged, rectangular-rounded, semi-circular, trapezoidal and other special shapes are used as possible groove shapes, which may also differ in width, depth and number over the inner circumference.
With regard to the course, purely axial grooves but also helical systems running over the inner circumference (helix) are established.
Illustration shows different geometric shapes which are used for grooves.
The length of the grooves can also vary greatly. Often the grooved area ends at the end of the feed zone and the groove depth then decreases continuously from an initial depth to the inner diameter of the cylinder. However, the depth of the groove does not always decrease continuously and runs out to the actual core diameter of the cylinder. In some cases, the grooves are also drawn through to the end of the extruder
What happens to the granules in the extruder when the cylinder is smooth or grooved?
One of the most important tasks of an extruder is to convey the plastic from the feed opening to the nozzle. In order for a screw-based system to be able to realize axial conveying, it is necessary to prevent the plastic mass in the screw flights from “rotating” and thus rotating on a circular path instead of being conveyed axially.
The screw-nut model can be used as an illustrative example. If a nut is on a screw and the screw is turned, the nut rotates at the same speed without moving it axially on the screw. However, if a resistance force (friction) acts on the nut, the situation occurs that the screw rotates more slowly and is simultaneously displaced in the axial direction. If a positive fit completely prevents rotation of the screw, pure axial movement of the nut occurs.
Illustration: Screw-nut model, as an illustrative example for conveying granulate in an extruder (according to: Chris Rauwendaal)
Three possible situations of granules movement within an extruder can therefore be transferred:
- The granulate is not conveyed axially, but follows a rotary path around the screw. This extreme condition would require that the frictional forces between granulate and screw are much greater than the frictional forces between granulate and barrel.
- The granulate is conveyed purely axially and the granulate does not move in a circular path like the screw. This extreme condition would require that the frictional forces between barrel and granulate are much greater than the frictional forces between granulate and screw.
- The granulate is conveyed axially, but there is also a rotating movement.
The third option usually occurs in practice in an extruder. The granulate is conveyed axially, but still moves on a circular path. The pellets are thus conveyed along a spiral path from the extruder inlet to the die. The second option can only occur with grooved barrel extruders and only if the circular movement can be completely prevented by the geometry of the grooves.
With the above explanations it is now understandable what effect a grooved feed zone can have. The grooved design of the inner surface of the cylinder allows granules to enter the grooves, whereby the (axial) grooves prevent the granules from moving in the circumferential direction. A rotational movement of the granulate is thus made more difficult and axial movement of the granulate is forced.
The exact processes within the grooves can be distinguished according to the depth of the grooves in relation to the size of the granulate. How exactly the design of the grooves can have an influence on the conveying follows in another article.
To simplify matters, it can be stated that a grooved-barrel extruder can increase friction (or even a kind of positive fit) due to the design of the inner cylinder surface, which increases the axial conveying effect and reduces the circumferential movement of the pellets.
Pressure build-up in smooth tube extruders and grooved sleeve extruders
Due to the above mentioned situation, there is a very different pressure build-up behaviour in smooth barrel extruders and grooved barrel extruders. In the smooth barrel extruder, the pressure is continuously built up over the length of the screw and reaches the maximum melt pressure in the last area of the extruder. In the grooved barrel extruder, on the other hand, the maximum pressure is reached early and is then reduced again in the further course of the extruder or extrusion die.
Illustration: Pressure build-up in a smooth tube extruder and grooved bush extruder (schematic)
A particular advantage of the grooved-barrel extruder is that the pressure build-up in the grooved-barrel extruder and thus also the mass flow is constant regardless of the counter-pressure of the die. This special advantage makes the grooved barrel extruder very suitable for certain applications in which the counterpressure varies (e.g. with adjustable nozzles for wall thickness control during blow molding).
However, due to the pressure build-up behaviour of the grooved barrel extruder, certain disadvantages also result, such as a reduced mixing capacity of the extruder. Because the maximum pressure prevails at the end of the system in a smooth tube extruder, a small part of the material already melted flows continuously back towards the feed zone (e.g. via the leakage gap of the screw), so that improved mixing can be achieved. In the grooved barrel extruder, the flow component resulting from the pressure is directed in the same direction as the conveying direction. This means that no additional mixing is required, so that additional mixing parts often have to be used.
Grooved barrel extruder or smooth tube extruder
Basically, both extruder concepts have certain advantages and disadvantages and have a similar, overlapping area of application. Therefore, it is not possible to classify precisely for many applications whether a grooved feed area is the medium of choice or a smooth extruder shows the better results. In many companies it is also common in practice that one and the same product can be produced on smooth barrel extruders as well as on grooved barrel extruders.
Grooved barrel extruder: advantages, disadvantages, areas of application and limits
- grooved-barrel extruder is considered a “solids conveyor”.
- higher mass throughputs tend to be possible
- good for material with low coefficient of friction (material does not rotate with the screw, conveying effect remains high)
- mass throughput should be as independent of back pressure as possible (e.g. with nozzle widths varying during extrusion, when using filters that become clogged, etc.)
- high conveying capacities are possible
- extruder operates with high consistency, mass flow rate depends only on the speed (similar to a pump)
- use is usually only possible with granulates (no recycled material, no edge trimming, etc.), since the system reacts very sensitively to fluctuations in bulk density and therefore the throughput constancy is endangered
- the use of additional melt pumps is usually not necessary
- usually cooling of the grooved bushing (usually water-cooled) is required, in some cases temperature control by means of temperature control units is also useful
- small to medium machine sizes available
- grooved barrel extruders were originally developed for the processing of PE and PP and are still mainly used for this purpose (but not exclusively)
- processing of very hard materials (e.g. many amorphous materials) conditionally possible, special experience necessary
- very high pressures can occur in some cases
- torque peaks can occur
- increased wear
- is considered as standard in the field of blown film extrusion and blow moulding
Smooth tube extruders: advantages, disadvantages, areas of application and limits
- smooth-barrelextruder is considered a “melt conveying machine”.
- mixing effect of the extruder is very good
- bulk density fluctuations are usually unproblematic, e.g. processing of recycled material, edge trimming, flakes, etc.
- very universal
- simple, cost-effective, long-lasting system
- no additional cooling necessary
- can be used if the specific advantages of the grooved bush extruder mentioned above are not necessary
- changes in back pressure lead to changes in throughput (e.g. with clogging filters)
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