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.
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.
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