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The flow and mainly in the coaxial

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The extrusion
process using extrusion head for pelletizing can be performed as cold or hot
process (49; 50;
85).
Flow system of the extrusion head for cold pelletizing is similar to the flow
system of the longitudinal slit extrusion head. It consists of the inlet,
circular main distribution, and rectangular auxiliary flow channels as well as
dies (52).
From several up to couple hundred circular dies are usually placed in one or
two rows of a rectangular plate and enable to shape the extrudate in the form
of rods of a specific diameter (86; 87; 88). Those rods are
right away cooled in a bath of water or in water stream and then cut into
pellets. Extrusion heads for hot pelletizing are the combination of the
longitudinal extrusion head and the pelletizer. These extrusion heads at the
same time extrudes multiple small diameter identical polymer rods and cut them
into short pellets (89; 90;
91).
Length of pellets can be adjusted by changing the rotational speed of the
pelletizer rotor. Pelletizing extrusion heads can by divided into the extrusion
heads for pelletizing in air and in water, depending on the medium used during
extrudate cutting and pellet cooling. Cutting and cooling in water is used for
polymers of a big thermal capacity, at higher rates of polymer flow and mainly
in the coaxial systems. Cutting and cooling in air is used for polymer of small
thermal capacity and at lower polymer flow rates. Water pressure can be so high
that it enables to replace the cutting blades and used water pressure for
polymer 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 low
watt-hour efficiency and requires additional drying of pellets (59). Extrusion head for
hot pelletizing can be divided into coaxial pelletizing, when the axis of the
pelletizing head correspond with the axis of the die. When axis are shifted the
extrusion process is called eccentric pelletizing (52).

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