Extrusion
Blow Molding
> Inspect
all machine ventilation systems to ensure adequate air flow.
> Check
operating manual of the machine for other startup checks and requirements
that have
to be met such as motor load (amperage) readings.
Startup
The
following startup procedure provides a general review since each line has
specific
requirements.
When starting up a new extrusion setup, start the screw rotation at
about 5
rpm. Gradually look into the air gap between the feed throat and throat
housing
and make sure the screw is turning. Screws have been installed without their
key in
place, or the key has fallen out during installation. Also make sure that
antiseize
material is applied to the drive hub, to help installation and removal. Also if
the key
is left out and the drive quill is turning and the screw is not, the screw will
not
gall to
the drive quill.
Details
for operating the machine are based on what the plastic being processed
requires,
such as temperature settings, screw rpm, etc. available from
the
material supplier and=or experience. Startup procedures involve certain
precautions:
> Starting
with the front and rear zones die end and feed section, one should set
heat
controllers slightly above the plastic melting point and turn on the heaters.
Heatup
should be gradual from the ends to the center of the barrel to prevent
pressure
buildup from possible melt degradation.
> Increase
all heaters gradually, checking for deviations that might indicate burnedout
or
runaway heaters by slightly raising and lowering the controller set point to
check if
power goes on and off.
> After
the controllers show that all heaters are slightly above the melt point, adjust
to the
desired operating temperature based on experience and=or lastic
manufacturer’s
recommendation,
checking to ensure that any heat increase is gradual,
particularly
in the front=crosshead.
> The
time required to reach temperature equilibrium may be 30 to 120 min,
depending
on the size of the extruder. Overshooting is usually observed with
on=off
controllers.
> Hot
melts can behave many different ways, so no one should stand in front of the
extruder
during startup, and one should never look into the feed hopper because of
the
potential for blowback due to previous melt degrading, and so on.
> After
set temperatures have been reached, one puts the plastic in the hopper and
starts
the screw at a low speed such as 2 to 5 rpm; some plastics, such as nylon,
may
require 10 to 20 rpm.
> The
processor should observe the amperage required to turn the screw; stop the
screw if
the amperage is too high, and wait a few minutes before restart.
> When
working with a melt requiring high pressure, the extruder barrel pressure
should
not exceed 7MPa (1,000 psi) during the startup period.
