5. Filter changer

It is always necessary to filter the melt at the extruder exit to retain any foreign matter in order to :

• prevent contamination of the sheet or plate, especially when very high levels of ground material are involved;

• avoid clogging the material channels in the coextrusion box and die;

• to eliminate the risk of damaging the chrome-plated polishing-calendering rollers.

Fine-mesh and/or very fine-mesh sieves are used, inserted against cylindrical plates with holes. When degassing occurs, as the sieve clogs and the pressure upstream of the sieve increases, the second stage of the screw, whose pumping capacity decreases, fills up with more and more material until the degassing zone backs up, at which point the filter has to be changed....