Double-stage (cascade system) granulating lines in the plastic recycling are intended for granulation of shredded in the form of films and plastic scrap, strongly printed (highly contaminated) and higher humidity of plastic waste such as: PP, PE, LLDPE, LDPE, PS or ABS. Depending on the materials being processed, the double-stage lines may have different equipment configurations.
The cascade system of the granulating line consists in a special arrangement of 2 connected extruders and its configuration in one system with the other devices of the granulation line. The specificity of granulation in this system assumes a milder treatment of the material to obtain better quality granules in respect of plasticity, purity and humidity. This becomes possible by equipping the second extruder with a clearly lower motor power and shorter screw length of the plasticizing system and thus achieving a longer material path extrusion at a lower coagulation rate.
Unlike a conventional single-screw extruder line, better quality parameters in the use of double-stage granulating lines are achieved by using multiple (from 3 to 5) vacuum degassing systems. A special part for them is the degassing chamber, which is installed at the transition between extruders and is adapted to work with a much larger surface area of the material than in a standard degassing system in the extruder. The appropriate purity of the processed material is also obtained by using double filtration in the form of 2 screen changers, installed both at the transition between extruders and at the outlet from the second extruder.