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 Pre-Washing plastic scraps


First, let's see what is useful to pre-wash and what's useless divided by scrap category:
(You can go directly to the one is of interest to you)
What is common to all pre-washing system is a GOOD Water filtration
MIX PLASTICS sounds very dirty because coming from garbage selection and consequently, it is.
Pre-washing is provided mainly when granulator blades can wear rapidly because of the amount and type of dirt, but here a single shaft shredder is the machine to use and wearing is not critical.
As we said it its dedicated page, the problem here is mainly separation and two, sometimes three, sink-float tanks are required to deeply separate, at least, olefins from sinkable plastics.
Just because the best way to achieve results is using clean water, each tank should "separated" from the following one by a washing/drying step so, while material goes forward, water goes backwards.
This apply to almost all systems, surely to all "no chemical" washing lines but, in this particular case, it is very important.
Back to the pre-washing matter, it goes by itself this system already has a pre-washing and nothing else is needed, at least from cleaning point of view. (while the separation problem remains)
On FILM WASHING LINES the matter can be different.
For film scraps coming from collection we do not suggest a pre-washing treatment of any kind because amount of dirt is generally little and granulator should handle this pretty easily.
If contamination is not this "little" but still scraps are coming from scraps collection, we do suggest a single-shaft shredder that will last very many days without problems.
If film scrap is agro-film, silage film or stuff like that, the pre-washing step should be seriously taken into consideration.
The film we just mentioned is clearly contaminated by sand, hearth, stones, you name it and any blade of any cutting machine wears out in no time therefore a pre-washing of some sort is kind of compulsory.
Specially for film, there are many ways to carry out this depending by which material you're feeding in and what you need at the end. (pellets to blow film, pellets for pipes or profiles extrusion, blow molding, injection molding, etc.)
For agro-film, for example, a low energy consumption wet trammel will do the job and removes 95% of dirt just spraying some water in a tumbling, perforated screen; with a washer/dryer just after it, dirt will be almost completely gone and for the rest of the washing line it will be like eating a piece of cake.
In case of silage film, where contamination is not only sand but also grass, hay, leaves and other floating contamination, it will be better to use a pre-washing machine that makes friction as well to release most of this dirt.
This machine will consume more energy but it is the only way to get rid of floating contamination.
HDPE bottles washing, most of the times, doesn't require pre-washing.
Of course, unless scrap bottles are not coming from a landfill pick up or something similar to this and unless we are not talking about oil jars.
If this is the case, we do suggest to go with a double shaft shredder that with little energy opens and shred the bales of bottles (or lose) and doses pieces to a pre-washing machine that will remove surface dirt, most of paper labels and most of soap, milk oil or whatever dirt is inside. (the most, not all the dirt)
After this a wet granulator will easily handle what's left into it and a final deep washing will do the rest.
This, once again, only in the case bottles are more than dirty because for every machine you add to the system, there will be a bigger investment cost, maintenance, energy etc. (it means, do not complain if at the end the system costs too much !)
For the pre-washing of the PET bottles washing line, we need a little bit of your patience to go through what it means.
Let's make an introduction first:
It is quite important the way PET bottles are cut; this effects the way line will perform, the amount of fines, the amount of PET flakes floating on to of water and some other few things.
Therefore a sharp edge flakes will make the washing line to run much better than a jagged one.
All this at the end means blades of granulator should be sharp all the time. Period.
As everybody knows, PET is abrasive by itself, paper isn't less and dirt doesn't help much so how we do it ?
A lot depends, once again, by the way bottles are arriving at the very beginning.
If you operate in a country where it never rains, we do suggest to skip the pre-washing  process all together and put a second granulator so you can switch from one to the other and have the line running 24/7 with no downtime.
Why this ? Because a second dry granulator cost less than the pre-washing, takes less space, consume less energy, is more productive than the wet process, granulator will never rust and all these good things.
After this pre-washing of flakes is much easier, more effective and trouble-free.
But we should be talking about pre-washing, isn't it ?
Because most of countries have rain, snow etc. pretty often and bottles are already wet, most of the times, we do suggest to go with a wet process all the way.
After bales-breaker, so after bottles are in single form, we go into a trammel with continuous hot water spray to detach all labels and get rid of most of them, together with some glue and most of surface dirt.
Keeping water very hot the process has the big advantage to turn PVC bottles opalescent, so way easy to recognize and remove from the PET stream.
More than this, being in a country where electricity doesn't cost a fortune, a second trammel, running with hot air, can get to the point to shrink all bottles, making the average thickness higher and easier to wash, dry etc..
Does this sound good ? Yes it does also because, back to PVC bottles, they will definetly get brown and very easy to recognize and remove.
After all this, your bottles will be pretty clean, shrunk, labels and dirt free and without any PVC content so granulator blades will last very long and its maintenance cost not high at all.
On the other side you should keep in mind that hot water cost a bit and hot air even more so you should run the numbers in the country where you are and see if this is convenient to you.
Each piece of the equipment cost money from investment and operating costs point of view.
Keep this always in mind.