Modern Plastics Worldwide - March 2010 - dae uptlax

EXTRUSION
Figure 2. A black mark around one of the four holes in the back
of the screw can serve as a counting mark.
test melt index on other materials to see if they are held more
closely than this one. Other tests can be useful, too, but this is
one of the easiest. It is also possible, of course, that something
is changing in your plant, but unlikely that it would happen
with this resin and no others.
Q: What’s the best screw design for highly filled materials?
AG: Screw design is a controversial topic, as so many people
have different designs that they developed and promote. A lot
depends on the extruder, as a long
machine allows more to happen in
the screw. A lot also depends on
how much filler is “high,” and on
whether you have fillers already
dispersed in a compatible resin, or
are adding them in powder form
and need a lot more mixing.
If I already have an extruder, I would avoid the question
(and the need to buy a new screw) by trying the desired mixture
on a system I have, selecting a line with some past history of
good mixing if possible. I would also pay attention to:
a) Power requirements: Fillers will increase viscosity and maybe
get too close to maximum power available. Some remedies for
that include running hotter feed and/or hotter barrel (less heat
needed from the motor = less power draw, and even a hotter
die might help if it didn’t overheat the melt and slow down
production rate), or changing speed range with retrofit pulleys
“People who run pressure pipe
will know what I mean, or
if they don’t, they should.”
to get more power from the existing system.
b) Mixing quality: Better dispersion may mean less filler needed
to get desired properties. Water cooling of the screw, or installation
of a static mixer after the screens, or special mixing breaker
plates, or a tighter screen pack, or changing the carrier and loading
level of a concentrate are all ways to improve mixing.
If I can’t make adequate product from the existing system,
the problems I encounter would indicate for me what I could
do next. I can accept the idea of a new screw, but only if I
clearly see why that screw would solve these problems, and
what past experience that design had with high filler loadings.
A computer simulation using viscosity data at extrusion shear
rates (in the clearances as well as channels) would be helpful,
too, and to me is essential for advance planning of large (hence
expensive) screws.
Q: How can you determine temperature offset due to shear?
AG: Shear heating is the increase in a melt thermocouple reading
because of the friction of the melt against the thermocouple,
which can raise the displayed reading by as much as 10 deg C
(18 deg F). Usually it is not as great, and usually it is ignored, as
what matters most is the consistency in melt temperature, rather
than the absolute value. However, where chemical reactions
(foaming, cross-linking) are involved, or where the material is
in serious danger of degradation, a more accurate reading of
melt temperature is desired. This is done by measuring the rise
at a given set of conditions (known resin, flow rate, and viscosity)
and subtracting it from the displayed value.
You can measure the rise by stopping the machine suddenly,
and watching the melt temperature quickly drop to the real
value as the shear heating stops suddenly, but the mass does
not move and the thermocouple can record its true temperature
at the point of measurement. This isn’t as easy as it sounds,
because sudden stoppage requires management of what is
coming out of the die at full speed (if it is slowed down before
stopping, you won’t get a true reading of the shear heating).
Further, it may be dangerous to start up again without bringing
the screw speed down to zero
and raising it as usual during a
startup. Most modern machines
have controls that prevent such a
restart “in gear,” but it may have
been disabled or not have been
there in the first place. It may be
most convenient to run this test at
the end of a run, when you have to shut down anyway.
Q: What is the value of flush-mounted thermocouples for
measuring melt temperature?
AG: Extruders don’t normally have flush-mounted melt thermocouples
in the adapter or die. The only ones I’ve seen are the
rather common and rather useless combination gauges, where
a thermocouple is inside the pressure gauge at the screw tip.
There was an infrared gauge sold 20-30 years ago (Vanzetti),
but it quietly disappeared.
36 MARCH 2010 • MODERN PLASTICS WORLDWIDE plasticstoday.com/mpw