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Extrusion lines for the pelletizing of plastic scraps
 
Pelletizing 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 this subject.
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 one.
Twin screw are getting more and more popular we'll see why.
Other extruders are satellite, disk and other built for special purposes only.
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 and safe.
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 spare.
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.