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Extruders are the final machine of any scraps recycling system.
 
Because it is the last one, it should handle all the problems the previous machines didn't solve.
 
Special extruders can handle materials with some contamination, moisture, etc. but don't relay on this machine to solve the problems left behind by other machinery.
 
The better quality flakes you feed the higher quality you'll get.

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 Pelletizing of plastic scraps

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Pelletizing lines starting from recycled materials need a chapter by its own because the matter is pretty complicated and not easy to explain in a short way.
First part of the line is the extruder.
There are hundreds of companied around the world manufacturing extruders with very different characteristics but not many somehow specialized for handling plastic scraps, specially coming from washing and separation lines.
Extruders in this case MUST have at least a venting unit, two better, and a good mixing action while melting the polymer.
Single screw or twin ?
This the main question for everybody that doesn't have an answer yet.
Of course it is easy to say a long co-rotating twin screw extruder with two venting makes a perfect job but, on the other side, what about investment, operating and maintenance costs ?
Does a twin screw extruder build pressure enough for effective filtration ?
Should extruder have barrel with anti-wearing steel coating or it's a waste of money ?
Which is the ratio power/Kg of single versus twin screw extruders ?
After you answer all these questions, you got the right machine.
Actually there is not a fix answer to all this, meaning the choice is different case by case because a lot depends on the material you are dealing with, the characteristic of the incoming material and which quality you're trying to get at the end or, in other words, the final application of the pellet itself.
Because of the high level of competition in this field, technology is going forward quite fast and new developments are achieved very often.
On the other side, do not expect any miracle even from the very best extruder around because if you feed garbage, in the very best case, you get a nice looking pellet that's still garbage anyway and doesn't have any selling price.
The meaning of this is to not complain with the extruder manufacturer if your pellet is not good as you expected but check what you're feeding into first and try to get a better quality chip first.
As far as Polymer Filtration, there is another dedicated page.
Few words about die-face cutters and pelletizers.
A good die-head of both die-face cutters and flat head for pelletizers make a lot of difference in production rate so this is another important choice to make.
Be careful !