- The different sorting methods
for plastic scraps
in this field did big steps ahead lately.
- Of course we're talking about automatic
- NIR detectors can sort all kind
of different plastics, together with
- Easy, isn't it ? OK, lets
go to see the back face of the medal.
- Let's divide also this page into
- Plastics coming from garbage selection:
- For this purpose we are keen to
approve this way of doing because the
NIR detectors can handle a pretty good
quantity of scrap material and
because separation is, in practice,
something like 92-95% the machine pays
for itself in not a long time.
- Please note we said 95% top as separation
efficiency. (actually like manufacturers
- So, if after this you go into a
washing and separation process, you'
will remove the rest of contamination
pretty easily with some exceptions.
- The HDPE bottles stream will have
some (little) film (LDPE), some PET
and PVC bottles and practically nothing
else, separation in any washing line
will easy and the product at the only
HDPE and the 2% LDPE that, may be remains
inside, doesn't bother at all.
- Almost the same applies to PET bottles
that require further sorting anyway.
(we'll talk about this later)
- As far as LDPE film concern here
- Together with film there will be
HDPE bottles, and this not big deal,
some PET bottles as, and this can be
handle quite easily as well, some PS
trays not easy to remove by sink-float
tanks but, the most difficult and messy
materials will be sponges, all kind
of foamed materials and last but not
least, cleaning pads, a piece of cloth,
some cotton fabric and craps like these.
- OK, you'll say, it will be matter
to put on more NIR device to sort out
the 95% of the 5% that was left before.
- If for some reasons the machine
didn't recognize in the first separation
step, there is a reason and, more than
likely, it will not recognize it in
the second step, the third and so on.
- So, efficiency of a second machine
will be way less than 95 % and up to
you to state if it is convenient to
go this way or do it in some different
- What we suggest in this case is
to improve the efficiency of separation
into the washing line to get rid of
most of contamination (all contamination
separation will never happen) without
pretending to get a pellet good for
film or even close to this; pellets
will be good for injection molding,
very thick sheet and applications like
- For more information go to Mix
Plastics Washing Line.
- As far as HDPE
bottles, not much to say, simply
because also if there is some contamination
left, a good washing line will take
care of all of it and the product at
the end will be absolutely pure.
- Now, let's go to the PET bottles
- If what's coming is supposed to
be only PET bottles, here what you'll
- HDPE bottles
- Aluminum cans
- Little glass (normally broken already)
- Little PS in form of trays, cups
- PVC bottles (it is in the last place
for a reason)
- If you find something else, just
start complaining with your supplier,
doesn't matter what the price of bottles
- HDPE bottles don't bother at all,
unless it is 50% of the total amount,
but this never happens.
- As far as Aluminum cans, they can
be sorted out by three different methods:
- First is electrostatic separation
and, because it is a costly device,
it worth having it if the amount of
cans is pretty high and what you get
from the aluminum itself pays off for
- Second is what everybody does, that
is manual sorting; Aluminum is easy
to recognize even for a sleepy person
in the middle of the night and shouldn't
be a big problem anyway.
- Third is to wait till it gets to
the metal detector and remove it when
the conveyor belt stops; this, of course,
if the amount of Aluminum is very little.
- There are also continuous detectors
scanning Aluminum flakes and sorting
them out but this should be put at the
end of the line only for security purposes
and, if it is discarding too muck, because
it removes also quite many PET flakes
together with the Aluminum one,
you better find a better way to sort
it out at the beginning of the line.
- Glass, in pieces, will be automatically
removed by the trammel.
- If a whole glass bottles is into
the bale of PET bottles, call your supplier
and smash it on his head. He will remember.
- PS is unfortunately pretty common
into PET bottles scrap bales.
- It is easy to remove because it
doesn't have the shape of a bottle in
- If you go with the positive sorting,
you'll never find a piece at the
end of the line while with negative
sorting, personnel should care about
- It sinks like PET and it stays with
it all the way till the end.
- Some of it will be lost by the rinse-dryers
and dryer because a lot more brittle,
but some will remain and will contaminate
your final product.
- Now let's go to the last one, not
the last in order of importance:
- PVC is the most dangerous contaminant
for the PET, and everybody knows this.
- Being the ideal quantity equal to
zero but a fair amount that can be tolerated
is 10 PPM (parts per million) so let's
see if there is any way to reach this
- We said NIR automatic detectors
are doing a good job separating different
plastics so let's run some numbers:
- The average quantity of PVC found
into bottle bales is about 1% -AVERAGE-
- Even taking in consideration this
as maximum quantity, we do have as starting
point 10.000 PPM (1%)
- With the NIR detector, with a sorting
capacity of 95%, that's the best case,
we therefore remove 9.500 PPM but what
remains is 500 PPM that's 50 times more
than what we need.
- As we said at the beginning of this
page, if the NIR detector didn't recognize
the PVC bottle in the first chance,
it will be difficult it will do it in
a second one.
- This can happen because of many
factors but here we'll be talking about
the fact we still have PVC into our
PET stream and not way it is there.
- Another good sorter is the (old)
x-ray detector; old because it was the
very first detector used in plastic
recycling and not widely used any more
because it recognize only PVC.
- But is isn't this exactly what we
- An x-ray detector has an higher
sorting capacity that can go up to 99%
and because it uses a different scanning
method, more than likely it will discard
PVC bottles the first machine didn't
- And 1% of the 500 we had before
makes 5 PPM that's a good number indeed,
meaning that even if contamination at
the beginning has a peak up to 2% we
still are on the safe side.
- Everything almost perfect, till
the time you get the offer of the two
scanners and start scratching your head
because we are talking about expensive
- Let's see instead what's available
to get to the same result without spending
a couple of 200.000 dollars or more.
- (We forgot to mention colors sorting
but this shouldn't be big deal for anybody)
- Polarized and UV light is something
available at a way cheaper price (if
we do not consider the people involved)
- Polarized light is nothing but a
white light going through a piece of
plastic sheet that polarize the light
(let the light to go in only one direction)
and if operator wears a pair of polarized
glasses, will easily recognize PVC bottles,
that turn dark, from PET remaining clear.
easy ? Wait till the end.
- UV light is the second chance to
sort out PVC bottles.
- You build up a box, because it shouldn't
be any other light with different frequency
than UV range, and your PVC bottle will
turn rainbow color while PET bottles
- Let's go to the other face of the
- In a 1.000 Kg/hour system, because
the average weight of our bottles is
35 grams each, we'll be talking about
29.000 bottles per hour to scan.
- Because scanning, doesn't matter
if using polarized light or UV, should
be "mono-layer" meaning
bottles cannot be piled up on the conveyor
belt and because it takes a certain
time to check every single bottle, we'll
be talking about a lot of persons and/or
multiple conveyor belts to get the job
- Like everything in this world, there
is also something in between, meaning
a first manual sorting (positive sorting
if possible) and a final automatic machine
for the final control.
- This actually what most of everybody
does and it has been proven to be the
cheapest and safer way to go.
- Few more words about what we said
in the pre-washing section; if bottles
will go through an hot water trammel
and specially into the hot air trammel
for bottles shrinking, PVC bottles will
turn slightly brown and manual sorting
will be easier and faster.
- On the other side, energy consumption
will be pretty high.
- We will not go to any conclusion
here; the best we can say is each system
requires its own sorting method depending,
as you sure understood, by quite many
- And only yourself know your reality.