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 Cutting plastic scraps


Why different plastic scraps need different cutting methods
One of the most expensive step of a recycling process is cutting, and this for quite many reasons:
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 as possible.
The cutting machine should be chosen according to:
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 into scraps.
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 things:
- Two or three shafts slow rotating shredder.
- Single shaft shredders with or without grid
- 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.
This kind of machine does a very good job if material you should shred is not very contaminated and if size don't matter.
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 quite.
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 produce anything.
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 applications.
There are two categories for this machine.
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 size.
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 longer.
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 than ever.
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 of throughput.
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 industry.
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 to:
- 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 be adjusted.
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.