Optimum processing of Desmopan® is only possible
on screw-type injection molding machines. Well plasticated,
homogeneous melts can be produced with single-flighted, three-zone
screws of normal length. If a high plasticizing capacity
(throughput) is needed, longer screws can be used (Fig. 1).
|Fig. 1 Injection molding screw for processing
Due to the high shear stress,
short-compression-zone screws are unsuitable. The high
plasticisizing energy needed for TPU requires a high torque for
the screw drive. Insufficient torque leads to fluctuations in the
screw speed and thus to non-uniform
To a limited extent, higher
cylinder temperatures may yield better results, although there is
a risk of the material overheating. The nozzle and cylinder head
should be designed in such a way that there are no dead corners in
which the material can become lodged and thus become thermally
Accurate temperature control for the
cylinder and nozzle heating system is
Care must be taken to ensure that
the nozzle is heated evenly over its entire length. Only in this
way can local overheating or possible "freezing"
of the melt be prevented.
Molten Desmopan® is
neither corrosive nor abrasive. For this reason, there is no need
for any special steel alloy or armor-plating of the screw.
|Fig. 2 Injection molded gear shift knobs