Sifting plastic granules and especially master batches aims to separate granules that are too big and grains that are too fine, from the ”good” product. The sifting process tends to increase the quality of the granule.
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. Problem: granules, which are too long and which should be sifted out, remain with the desired end product. Reason: sifting surface is too big, or that the selected sifting technique is inadequate.
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, these granules will increasingly ”jump”, into a vertical position and fall through a hole of the sieve.
FUCHS discovered the following facts:
The thickness of the product-layer should be constant on the whole surface of the sieve at the start of the sifting process 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 exacerbate the ”jumping effect”.
The frequency of the sieve movement should be as low as possible. It is a fact that the best sifting results are obtained with low-speed sieve movements and the ”jumping effect” of the granules gets worse with increasing frequency.
The plansifter Siftomat offers the best possible 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.
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. The slope of the sieve can also be adapted