Filtration - a procedure |
Polymer melts contain foreign substances and inhomogeneities which badly effect extrudate depending on dimension and quantity. Such contamination and inhomogeneity lead, for example, to breakage of fibres while spinning, hazes in blown film, changes of resistance in cable coating, or weak spots in pressure pipes.
Filtration in process engineering is defined as a means of trapping liquid and hard pieces in fluids by means of a filtration medium.
Filtration has met two challenges. On the on hand, the recycling of post-consumer material and manufacturing waste have gained economic importance.
On the other hand, the constantly increasing quality standards necessitate the filtration when processing an ever increased range of plastics. An example is the production of polyamide or polycarbonate pellets for optical applications such as mobile displays. Filtration is very important in compliance to high quality standards and essential for high quality production. |
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Rheological design of Screen ChangersTo define the size of a screen changer we need to know the extruder output, polymer and grade, shear rate, expected contamination level and target pressure lost.
This data is fed into our latest computer modelling programme to indicate both the size of the screen changer but also the optimum flow channel dimensions.
The example shows the flow in a K-SWE/RS backflush screen changer.
It clearly shows that the biggest part of the calculated pressure loss is produced by the filter element in front of the breaker plate. The inlet geometry of the screen changer interface to downstream components has to be designed to minimise shear the melt. For this, KREYENBORG offers adaptors taking into account the flow behaviour of the polymer to avoid high shear rates at the inlet into the screen changer. |
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