Grinding is the size-reduction step in which sorted plastic waste is cut into small, uniform pieces by a granulator, a machine with rotating knives and a screen that sets the particle size. The output is called regrind or flakes.
Grinding usually follows coarse shredding. A shredder tears bales and bulky items into rough pieces; a granulator then cuts those pieces down to a consistent size, typically 8 to 12 mm. Small, even flakes wash more thoroughly, dry faster and feed steadily into an extruder. Wet grinding, with water sprayed into the cutting chamber, begins the cleaning process and keeps the blades cool. Rigid plastics such as HDPE bottles and polypropylene parts are the materials most commonly ground.
