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 HD-LDPE film washing lines


What it should be considered setting up a film washing line
Film scraps out of olefins is the most common scrap that can be found on the market so one of the easy part of the job is sourcing the raw material.   Unfortunately the easy part of this job is only this one.
Into bales of film scraps, doesn't matter if your supplier did swear it was deeply sorted, you should expect what an unrestrained fantasy mind can think of and, may be, it's not enough.
The motto "do it yourself" is what should apply here, meaning you can forget to load the whole bale into the best and powerful shredder you just bought because is not going to work, at least for a long time.
Scrap bales need to be opened and sorted not only for that reason but removing of wood, foamed materials, an entire piece of carton etc, is compulsory if what you're looking for is a pellet good for film application.
Not to talk about steel or other metals that will be into bales, no question.
Therefore, manual sorting will not be good only to protect your shredder but also for the quality of your final product.
We always mention "shredder" and not granulator on purpose; again because you should expect everything and because we're talking about dirty material, we strongly suggest to go with a single shaft shredder rather than a granulator to save maintenance first and consequently (a lot of) downtime.
If you'll need a small size flake because of your force-feeder, you can always go into a granulator at the end of the line because, after material is clean and dry, your blades will last forever.
Of course we are talking about an industrial system working 24/7 because if you have spare time here and there the story can be very different.
So, we got our material shred, using a grid of about 50 mm in order to not create lots of fines during washing and drying.
Even if it is true shredder delivers material constantly, we like to go into a buffer silo anyway to be sure the quantity of flakes we feed to the line is what we want and, just in case, operator should state how much material goes to the line and not the shredder itself.
Buffer silo extraction screw speed can be adjusted to deliver to the line what operator think is the best for the efficiency of the line. (because it is a screw conveyor, dosing will be by volume and not by weight, remember it)
Separation into a sink-float tank should take place first to get rid of sinkable that surely is contamination.
A lot of guys out there call them "washing tanks" but you try yourself to dip a flake into water, leave it there for a couple of hours and check how clean it is after that.  You judge it yourself.
Anyway, after separation by flotation, giving to dirt the time to absorb water, material need to be washed, meaning removing dirt for its surface.
What we do is to go to a "centrifuge like" machine to apply a lot friction in presence of water, like my grandma was doing long time ago because, by the way, it still works.
The combination of a sink float tank and a washer is what separates and clean plastic film flakes.
Now, if your material has a little amount of dirt, one sink float tank and a washer will do the job, if it not enough, you should add another set to get it more clean, and so on.
No machinery manufacturer knows about your material so it is customer responsibility to show what will be the worse scrap coming to the line and to suggest what you need.
This means if you change your scrap characteristic, line will perform differently, may be better, may be not, up to you.
And this is normally why customers complain after putting a line in operation; may be it is a machinery supplier fault but, most of the time. material is not the one it was supposed to be, and nothing works or, if it does, it comes out to be too expensive, throughput too low, lower quality than expected and so on.
You may not like it but the sentence "shit in, shit out" is what applies here.
Back to film washing line, let's talk a bit about what effects the most production rate besides contamination.
This is the data you should know very well because it is what states the throughput of the line.
As we said before, starting from first screw conveyor extracting flakes from buffer silo, everything goes by volume and the fact you need production rate expressed in Kg/hour doesn't match.
In other words, if you feed a 40 micros average thickness film and you get, let's say, one ton per hour, with exactly the same line you'll get 750 Kg/hour with a 30 microns film, 500 Kg with a 20 microns and so on.
On the other side, problems (cutting and drying) will increase proportionally with the decrease of thickness so, not only you get less production but your machinery will not perform as well as they are supposed to.
We'll make another example for the "non conosseur" about drying.
Let's assume the centrifugal dryer leave one drop of water every one square meter of a 40 microns film that weight about 3,5 grams that represent .
Because the dryer removes water from a surface you'll find the same drop (same weight of water) on the same square meter of film that, this time is only 20 microns, therefore you have twice as much in term of moisture content.
Got the point ?
So, at the end, thickness is the issue and the line should be configured accordingly, keeping in mind film makers are daily trying to decrease thickness to save money and you'll be more and more in trouble.
And we, as machinery manufacturer, as well.
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