Chapter 3 Filtration
Starting up a spinning line is a complex operation and one wants to minimize the frequency. It also is time-consuming to replace spinneret plates and filter beds. Inclusion of particles, whether they are thermally degraded polymer or small chips of metal, can lead to imperfections along the filament and potential points where the filament may break. Worse still, the particle may become attached to the entrance to a tiny spinneret hole and this could reduce the flow to that hole and thus the diameter of the resulting filament; this smaller filament will cause the yarn to take up dye differently and the resulting non uniformity will show up in the final fabric. Another problem with partial blockage of flow is that the blockage may be nearly complete, leading to breaking of the filament, which is usually only a very slight problem, since there are so many filaments, although gradual oozing of the melt through the nearly blocked hole can be a very major problem. It becomes a major problem because the polymer, as it oozes out the hole, will gradually travel to neighboring holes and interfere with the filaments from them as well. The only solution for this problem is to shut down the line and remove and clean the spinneret plate.
Since there can be up to 1000 holes in a spinneret plate, the large number means that the filtration process must be completed to very exacting standards. Similarly, because one wants to operate the line for long periods of time between shutdowns, one wants to use a filter medium which can trap a large number of particles without causing excessive pressure drop or blinding of the filter. This brings us to the first part of filtration, deciding between filter screening and entrapment among small particles.