- Extrusion lines for the
pelletizing of plastic scraps
line is the one at the end of many
plastic recycling lines.
- And because of this it receives
all the problems accumulated into the
recycling process and we try to make
ourselves a little more clear with this:
- If scraps have not been washed well,
separated, dried and all the other steps
a recycling line is supposed to do,
the final extruder is supposed
to deliver a good pellet anyway.
- This to tell you that if the extruder
doesn't perform well, in most cases
is not the extruder fault but the problem
should be found in some earlier stage
of the process.
- A pelletizing should have these
characteristics in all cases:
- A good venting device with a powerful
vacuum pump to increase the degassing
effect; if venting are two even better.
- An oversized screenchanger because
sometimes, if material has not been
washed well, and this will happen,
you need to get production anyway and
a big screenchanger helps a lot.
- The length (the ratio between Length
of the screw and Diameter) and screw
design can vary according to materials
and there are very many theories about
- Theory because feeding a scrap that
never has fixed characteristic as far
as MFI, melting point etc, can
be only a guess.
- Let's talk about the extruder itself;
there are two main categories of extruders
and some other only listed.
- The most common, most used, most
simple extruder is the single screw
- Twin screw are getting more and
more popular we'll see why.
- Other extruders are satellite, disk
and other built for special purposes
- Which is the main difference between
single and twin screw extruder ?
- On our only personal opinion the
two are called with the same name but
are two completely different machines:
- While single screw extruder is a
pump to all extents, twin screw is a
mixer, specially the co-rotating one.
- Now, out of pump you cannot pretend
to have a good mixing, even if the L/D
of the machine is over 50, and this
has been demonstrated quite many times
while, on the other side, productivity
is high, maintenance is low together
with investment cost and mainly because
to operate this machine is pretty simple
- Twin screw extruders, as we said
before, are mixers instead and perform
in a very different way; shearing effect
is much lower so the RPM of screw can
be much higher, the power of main motor
is a little less than the one used by
the single screw, and more important
than anything else, the material
at the end of the machine is a compound
and not only a melt.
- So why not everybody uses it ? First
is the investment related because for
the same production rate price is normally
twice as much, and there are quite many
reasons for this, it needs a good technician
for the assembling of the screws
to give them a proper design, and if
a piece of something breaks you should
call a leasing company to buy the
- Moreover, because recycled materials
need a deep filtration, and thus pressure
to achieve it, while single screw are
limit-less, twin screws have a low limit
because of screws structure and some
mechanical limits as well.
- So, where to go ?
- Difficult to say and, like in most
of cases, it just depends on the material
you have, which throughput you are looking
at and the quality of final product.