- Why different plastic scraps
need different cutting methods
most expensive step of a recycling process
is cutting, and this for quite many
- First is the fact most, if not all,
plastics are abrasive by themselves
and they wear out any kind of steel.
- Second because we are talking here
about dirty plastics and dirt doesn't
help to keep blades sharp.
- Third because during blades replacement
all the system is down and this cost
a lot of money.
- So let's see what's available on
the market to make cutting as less expensive
- The cutting machine should be chosen
- The kind of plastic to cut (if soft,
hard, heavy, light etc.)
- The size of flakes or chips.
- If the process is wet or dry.
- The capacity of the machine.
- The kind and quantity of contamination
- If it is possible to make some kind
of accumulation in the process.
- Forget about strange machinery under
development and let's go to the real
- - Two or three shafts slow rotating
- - Single shaft shredders with or
- - Granulators, all kind.
- There are other machinery available
for cutting like guillotines, alternative
cutters etc but these are for specific
applications and we may put some specific
pages for these.
- TWO OR THREE SHAFTS
- This kind of machine does a very
good job if material you should shred
is not very contaminated and if size
- They have a scissor type cutting
and blades are provided with hooks to
get the material.
- Rotation speed is something around
20-25 RPM so low heath, low noise, high
capacity, very low maintenance etc.
- So, the perfect machine ? Not
- First because cut pieces can be
any size in length and therefore difficult
to handle after this, second because
it is difficult to dose the material
to the blades and this means the machine
overload and goes automatic reverse
and, while going backwards, it doesn't
- This happens specially after a while
when the machine gets hot, because of
friction done by cutting, so, in our
opinion, this is not the best machine
to go with, unless for some specific
- SINGLE SHAFT SHREDDERS
- There are two categories for this
- First is the one without grid and
it is more a "tearing" machine
than a cutting one.
- Having very contaminated materials
like agricultural film, full of sand,
the only way to go is with a machine
that practically doesn't have blades
but only some "hooks" tearing
film till it breaks in pieces of some
- Normally after this machine, one
kind of pre-washing is provided to get
rid of most of dirt and making the material
good for further precise cutting in
some other machine.
- The machine we like the most instead,
is the "multiple blades" single
shaft shredders and this is why:
- The rotor is slowly rotating, at
least compared with granulators, and
this makes life of blades to last much
- Is equipped with a grid therefore
size of flakes is stated.
- It has a pusher that, with a good
software, doses the scraps to the rotor
and this makes the machine to delivery
a pretty steady amount of cut material.
- Blades are easy to replace and maintenance
time is affordable.
- Even if one or more blades are gone,
for some reason, the machine works as
well and you can continue to work till
maintenance time arrives, so little
to none downtime.
- Because of "slow" rotation
speed, if a piece of something hard
gets to the blades, it may break few
of them but nothing else happens and
you can restart it in a very short time.
- Of course if you haven't fed a war
tank with the gun and everything else.
- In nowadays, many manufacturers
are battling on a patent about the shape
of the rotor, that's been a big improvement
by the way, and performances are really
increased making this machine more attractive
- The only two weak points, because
there are always weak point, is the
cutting is not precise as it should
be handling PET bottles for example
and second it doesn't like much hollow
bodies or light pieces, this in terms
- Last, but not least, the fact the
machine cost a certain amount of money
so you better be sure it can handle
your scraps the proper way.
- Granulators are the most common
machines used in the plastic recycling
- Granulators are nothing but a rotor
with blades spinning into a chamber
with a grid at the bottom.
- If this sounds easy, in practice
the story is a little bit different.
- Granulators can be different, and
thus performing in a different way according
- - Speed of the rotor
- - Angle of rotating blades with
respect to fix blades
- - The area of the grid
- - Shape of the rotor
- There are two important things when
choosing a granulator:
- First is the weight of the machine
because it needs to be very strong construction;
we'll not go in technical details about
why this but, weight matters, specially
in the long run.
- Second is the way blades are hold
onto the rotor and how easy they can
- Remember that the guy replacing
blades when dull is surely not an aerospace
engineer and gets bored to do this job
so many times so, an easy way for blades
adjusting helps himself and your production.
- Production rate, in fact will be
higher and, may be more important, flakes
will have a sharp edge that helps the
washing process as well.
- So, if the machine is well painted
and has a lot of chrome plated parts
it is very nice, but you better give
it a look inside and check the important
parts of it.
- Most of manufactures say the time
to replace blades is from two to three
hours; it may be true if you do it yourself
with the aid of a strong man besides
you but the standard time, for a standard
worker is anything between 6 and 8 hours,
if everything goes smooth.