Different extrusion tool concepts can be used for the extrusion of rotationally symmetrical semi-finished product geometries (e.g. pipes, hoses, jackets, blown films, blow molds). Typical applications are the use of spiderleg dies, screenpack dies, heart curve but also the use of spiral mandrel distributors. From a price point of view, spiral mandrel dies are often the most expensive variant. But is a purchase really worthwhile?
The task of an extrusion die is to transfer the melt provided by the extruder into the product form. In the case of rotationally symmetrical hollow bodies such as the above-mentioned product geometries, this can only be done if there is a geometric body inside the tool to whoch the melt can “flow around”. The core of the product remains free and a hollow body is created.
In extrusion dies, this body surrounded by the melt is called a mandrel. The fixing of this mandrel in the flow volume of the melt is done in a spiderleg die via metallic webs (spiderlegs). In a basket tool, the quill is realized over a surface designed to form a sieve (e.g. cylindrical surfaces with many melt bores – sieve-like). The heart curve tool, on the other hand, fixes the quill centrally and allows the melt to flow laterally around the mandrel in a heart shape. Thus, each tool has different special features from the design and process engineering point of view.
In spiral distributors, the melt is distributed by helical grooves which are milled into the mandrel and continuously decrease in depth. The melt therefore flows primarily through the spiral for a certain time, but also increasingly exits the spiral and flows axially in the extrusion direction through an outer gap between mandrel and cylinder – gap width increases continuosly). This results in a continuous overlapping of different flow fronts, which gives the spiral distributor special properties with regard to its distribution effect of the melt. (A more detailed functional description of the flow behavior in a spiral distributor follows elsewhere).
The melt is fed exclusively into the spirals via a so-called “primary distributor” upstream of the spiral mandrel distributor. As primary distribution 3 essential geometries are used, these are:
- Star predistribution
- 2^n distribution
- Length-balanced distribution
- (combined distribution system e.g. with coat-hanger distributors)
In star predistribution (left figure), the melt flow provided by the extruder is split in a star shape and each helical groove is directly connected to a bore of the predistributor. The melt flows through these holes directly into the helical grooves. The advantages of star predistribution are a simple die production, short flow paths and short dwell times of the melt. Disadvantages are that the production of coextrusion tools becomes complex, since many different bores are necessary and they must not cross. Further disadvantages are that there is no longer a free centre, which makes this distribution variant unsuitable for coating applications.
An alternative option for feeding the helical grooves is based on the 2 to the power of n principle (middle figure). Milled melt channels over the circumference divide from 1 channel to 2 channels. These 2 channels share again, so that 4 channels are created, then 16 and so on (until the number of spirals is reached).
This distribution principle offers the great advantage (milled into the outer circumference of the mandrel) that the centre of the tool remains free so that media can be passed through it (e.g. compressed air) or even jackets are possible (cables, pipes, etc.). Disadvantages are, for example, the more complex die production as well as the longer flow paths and thus longer residence times (rinsing times). (see figure in the middle)
Length-balanced distribution systems (right figure) can also be milled on the circumference of a cylinder and offer similar advantages to the 2 to the power of n distributors. The functional principle of this predistribution is dealt with separately in another article.
When are spiral mandrel dies really recommended?
Due to the special flow situation in a spiral mandrel die, such extrusion tools offer these advantages in particular (with correct design):
- Avoiding the formation of weld lines
- Very good wall thickness distribution over the circumference
- Moderate pressure losses
Spiral distributors are particularly suitable where high demands are placed on the product quality of the extrudate hose. By distributing the melt and combining different melt fronts, almost complete avoidance of flow or weld seams in the product is possible. Such weld lines primarily occur where split melt flow fronts are brought together again – which happens, for example, with a spiderleg die behind each individual bar(spiderleg). At this point of confluence, the macromolecules of the plastic lie linearly side by side without the individual molecules being able to “loop” together. As a result, there is a mechanical weak point in the product, some of which can also be detected as an optical defect.
Another advantage is the ability to achieve a very uniform wall thickness distribution due to a very uniform melt exit speed from the mould over the gap circumference. This avoids thick spots or thin spots in the product, which in turn can lead to further problems, such as piston rings in films, see illustration.
A further advantage is that spiral distributors with good rheological design can still achieve moderate pressure losses and short flushing times despite very high product quality. The dwell times of the melt are higher than with other tool concepts such as the spiderleg dies or the screen basket tools, but spiral distributors usually meet these criteria very well. (At least so good that even thermally very sensitive materials such as PVC are occasionally processed on spiral distributors.)
All in all, the spiral mandrel distributor is regarded today as the optimum tool concept when very high product qualities are required. In conclusion, however, it should be noted that the individual design of the spiral distributor ultimately determines whether the positive effect can be achieved.
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