Sifting How a Plansifter Increases the Quality of Plastic Granules

Author / Editor: Daniel Baumann / Tobias Hüser

To increase the quality of master batches, a sifting process can separate plastic granules that are too big from the “good” product. But sifting problems occur frequently, so a thorough analysis was required, to find a technique that achieves the desired results. By taking four facts into account, Fuchs Machines developed a universal plansifter with convincing results.

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The Fuchs Siftomat plansifter separates granules with a high precision.
The Fuchs Siftomat plansifter separates granules with a high precision.
(Picture: Fuchs Maschinen; © tibeta -

The sifting problem lies in the fact that plastic granules do not behave in the same manner as many other materials. In fact, most of the sifting machines do not operate in a satisfactory way when used for sifting plastic granules. The problem is that granules, which are too long and which should be sifted out, remain with the desired end product. The reason is that the sifting surface is too big, or that the selected sifting technique is inadequate.

Often both reasons apply. In most cases, the bigger the sifting surface, the higher the quality or the throughput of the sifted material. However this will not be the case, when plastic granules, which are too long, are sifted out. The reason for this being that with increased sifting surface, the probability of having a too “long” granule falling vertically through a “hole” in the sieve increases. In addition to this, by using inappropriate sifting techniques (for instance a vibrating sieve), these granules will increasingly “jump” into a vertical position and fall through a hole of the sieve.

To understand the problem, Fuchs Machines discovered the following facts about the sifting of coarse particles:

  • If possible, the thickness of the product-layer should be constant on the whole surface of the sieve at the start of the sifting process (in this case a perforated plate) and in the order of 2 to 5 cm. It is then possible to avoid long granules getting into a vertical position and falling through a hole.
  • The thickness of the layer should ideally remain constant throughout the whole sifting process. If this cannot be guaranteed, for instance with long sifting surfaces, some of the granules will show a tendency for jumping over the remaining surface in a non-controlled manner, increasing the probability of having vertically positioned granules falling through a hole in the sieve. The sifting surface should therefore not be too big and well adapted to the corresponding throughput of the sieve.
  • The sieve movement should not contain vertical components. These would only exa-cerbate the “jumping effect”.
  • The frequency of the sieve movement should be as low as possible. The best sifting results are obtained with low-speed sieve movements and the “jumping effect” of the granules gets worse with increasing frequency. A certain movement is necessary to guaranty the conveying of the sifting material, especially on the “fine” sieve.

The Right Sifting Conditions

By taking these findings into account, it became clear that the Fuchs Siftomat plansifter offers good conditions for sifting plastic granules, as the sifting movement has no vertical component. Additionally, the rectangular shape of the sifter has the advantage that, compared to circular units, the sifting width does not increase during the process.

Additionally, the developer subdivides the sifting surface, the perforated plate, into several segments with perforated and imperforated plates. Thus, it is possible to adjust the surface of the perforated plates to the required capacity by inserting only as many perforated segments as necessary. The rest of the segments are imperforated and sieve surfaces that are too large can be avoided.

The frequency of the plane circular sifter movement is kept as low as possible, without interfering with the conveying of the material to be sifted. The low frequency accelerates the process and prevents the long granules from getting into a vertical position and falling through the holes into the “good” products. By using a frequency converter the process can be adapted to the specific requirements and optimised.

For high throughputs requiring a fast transport of the material to be sieved, the slope of the sieve can be increased. Additionally, the sieve case and the sieves are modularly built. For cleaning the sieve case and changing the sieves, for instance in order to process other plastic granules, the sieves and the spacer frames can be removed and replaced very easily.


In order to avoid problems, the sifting surface should be as highly adapted as possible to the sifting capacity. Excessive frequencies of the sifting movement have a negative effect on the sifting quality. The thickness of the material layer on the sieve should be kept constant, in order to avoid ”jumping“ of the granules. The sifting movement should not have vertical components, which exacerbates the “jumping effect”.

* Daniel Baumann is CEO of Fuchs Machines Ltd, Granges-Paccot/Switzerland.