now let's look at the inside of an
extruder to see how it works
the plastic raw material in this hopper
travels by gravity to the feed throat
and drops on to the rotating screw the
rotation of the screw conveys the
plastic forward through the heated
barrel
as the plastic has conveyed forward
along the screw the channel depth
decreases forcing the plastic through a
smaller area the combination of
compression and screw rotation causes
friction which generates heat this is
called shear heating this heat along
with the heat from the barrel heating
system melts the plastic by the time the
plastic is conveyed to the end of the
screw it should be well mixed and at a
proper temperature and pressure needed
by the die the plastic is called a melt
at this stage beyond the screw there can
be a screen pack
the screen pack is a set of wire mesh
filters that the plastic must flow
through the screens are typically from
20 to 100 mesh which indicates the wires
per inch the screen pack is supported by
a breaker plate which is a sturdy metal
plate with many holes for the plastic to
pass through the screen pack filters
contaminants from the melt
the screen pack must be set in place
with a coarsest screen next to the
breaker plate so each screen can support
a finer one otherwise the pressure
difference may break the fine wires
contaminating the product and possibly
doing permanent damage to the die