- How to dry flakes out of
a washing line
together with cutting, is one of the
difficult step of any plastic washing
- Drying is easy when talking about
tick flakes of any elastic material.
- For example drying HDPE flakes from
plastic drums is the easiest job in
this world because thickness is in the
range of two or three millimeters,
the material is elastic so you can put
it into any spin dryer and you get them
- The story becomes a little different
when talking about thin flakes out of
a film line, and for thin we mean 20
microns (of millimeter) or less, or
brittle materials like Polystirene,
- Let's see one problem at a time.
- This film first.
- If your centrifugal dryer leaves
one milligram of water per square
centimeter (or change the units but
the general concept remains) and your
flakes weighs one gram, moisture content
will be 0,1% and everybody is
- Because the centrifuge performs
the same way, doesn't matter the thickness
of your flakes, the same one milligram
of water per square centimeters means
10% if the flake weighs 0,1 gram.
- And a thin film flake can weigh
way less that this. Got the point
- We'll talk a little later on this
page about what can be done to dry flakes
to almost zero but we would like to
talk about centrifugal dryers first
and other mechanical drying machinery
- Most of everybody uses hot air for
final drying, and because hot air cost
a lot of energy, one matter is to start
from a moisture content of 2% and a
completely other one is a10% starting
- So, before sizing the hot air drying
system, you better care about the %
of moisture out of your dryer, whatever
- We do love centrifuges for quite
- First is the fact we can get a very
low moisture content, even with very
thin materials, and very proud of this.
- Second because any centrifuge, while
drying, it washes as well because it
is the combination of friction with water
- Third because even the most sophisticated
centrifuge is nothing but a rotor spinning
into a screen basket that means it is
an easy to maintain machine and cannot
give lots of problems.
- Because the purpose of a centrifuge
dryer is to dry, the fact many of these
machines are equipped also with the
"static centrifuge" make us
- If you still have water coming out
from the perforated cyclone (the real
name of the static centrifuge) it simply
means the centrifuge didn't make its
job, that was supposed to be removing
water from flakes. Isn't
- We don't like also to talk about
the "squeezing press" because
of simple reason: the concept is (almost)
right but to make one that works the
right way cost so much money nobody
is prepared to pay.
- So, as far as today, the very best
way we know to get plastic flakes dry,
is to go with a well performing centrifuge.
- Centrifuges can be very different
and the one that makes thin film dry
will make 30% fines running Polystirene
so diameter, shape of the paddles, size
of screen holes and may be something
else should change accordingly.
- After getting flakes mechanically
dry, for some plastics, we will need
further drying to get the best possible
- We'll talk about Crystalline Polymers
later in this chapter because they need
something very specific.
- For all other polymers, what we
do suggest, is to get a good extruder
with a venting, better with two, and
feed it straight with the flakes you
got after the centrifuge.
- This what we do in all cases but
one; dealing with film, doesn't matter
the thickness, washing and drying is
done with flakes of about 30-40 mm.
in size and any force feeding (we know
of) accepts only way smaller flakes
so further cutting is needed.
- Because any granulator will make
friction by cutting, meaning developing
heat, and then, by blowing, flakes lose
some more humidity and by the time they get
to the extruder, will be dry enough
- For all other situation, no hot
air is needed because a good centrifuge
MUST deliver a flake with less than
1% mixture and all extruders will be
able to easily remove it by venting.
- Crystalline Polymers, as we said,
need a chapter by its own.
- PET and PC in fact don't like the
mixture of water (moisture) and heat
together plus the fact they are hygroscopic
by themselves. (PA
performs the same way even not being
- So if you dry and leave them in
a storage place they will get moisture
from the air and the energy you have
been using to dry them is just thrown
- Drying should occur just before
extrusion and moisture content is measured
in PPM and not a %
- Both material need a dryer where
they stay for sometimes (few hours)
with or without vacuum and this because
they should release the moisture that's
inside the flakes, or pellets.
- If these polymers are not completely
dry, during extrusion IV (Intrinsic
Viscosity) will decrease a lot and polymer
loses its characteristics.
- Mechanical drying at the end of
the line should deliver a flake with
a moisture content of 0,5-0,6% and after
this it will be a dryer unit job to
decrease it to nothing (a good
dry flakes should have 20 PPM before