The flow and mainly in the coaxial
The extrusionprocess using extrusion head for pelletizing can be performed as cold or hotprocess (49; 50; 85).Flow system of the extrusion head for cold pelletizing is similar to the flowsystem of the longitudinal slit extrusion head. It consists of the inlet,circular main distribution, and rectangular auxiliary flow channels as well asdies (52).From several up to couple hundred circular dies are usually placed in one ortwo rows of a rectangular plate and enable to shape the extrudate in the formof rods of a specific diameter (86; 87; 88). Those rods areright away cooled in a bath of water or in water stream and then cut intopellets.
Extrusion heads for hot pelletizing are the combination of thelongitudinal extrusion head and the pelletizer. These extrusion heads at thesame time extrudes multiple small diameter identical polymer rods and cut theminto short pellets (89; 90; 91).Length of pellets can be adjusted by changing the rotational speed of thepelletizer rotor.
Pelletizing extrusion heads can by divided into the extrusionheads for pelletizing in air and in water, depending on the medium used duringextrudate cutting and pellet cooling. Cutting and cooling in water is used forpolymers of a big thermal capacity, at higher rates of polymer flow and mainlyin the coaxial systems. Cutting and cooling in air is used for polymer of smallthermal capacity and at lower polymer flow rates.
Water pressure can be so highthat it enables to replace the cutting blades and used water pressure forpolymer cutting instead, as can be seen in Figure 24(52).It enables high polymer flow rate as 75.000 kg/h, but this solution has lowwatt-hour efficiency and requires additional drying of pellets (59). Extrusion head forhot pelletizing can be divided into coaxial pelletizing, when the axis of thepelletizing head correspond with the axis of the die. When axis are shifted theextrusion process is called eccentric pelletizing (52).
