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This is one of the best way we know to increase efficiency in separation of different plastic materials.
 
Hydrocyclone is a static device where flakes are spinning into a tube by means of water flow created by a pump.
 
Separation occour because the two forces, centrifugal and centripetal increase the difference of specific weight.
 
As simple as this.

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 Hydrocyclone for separation of plastic scraps

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Hydrocyclones are widely used for separation in plastic recycling.
You surely know the very common cyclone used everywhere.
Well, hydrocyclone is nothing but a cyclone working with water and not with air.
Because of this, a little more attention should be paid to inlet speed and outlet sizes because these parameters can change the performance of this device but, the principle is just the same.
Hydrocyclone uses the centrifugal force of water, spinning inside the cylinder, to push heavier than 1 material to the side while less than 1 material gets centripetal force and it stays in the centre, going to another outlet.
The spinning of water increases the difference of specific weight.

Theory is very good indeed; let's go to practice.
To get the maximum available speed we'll need a high pressure water pump, and this is easily available but, and it is a big but, impeller should handle not only water but water and rigid plastic flakes so, your supplier will start to have a question mark on his face.
Another very important point of the system is the size of the inlet pipe; of course the smaller the size the highest the speed.
Yes, but flakes size should be taken into consideration as well.
On top of everything, the smaller the flow, the bigger the residence time (into vertical cylinder) to allow different materials to separate.
Solution to all this is, of course, a compromise among inlet size, pressure, flow rate and % of flakes into water.
When you find the right relation among all those, you're set.
To make a long story short, there are a lot very detailed books talking about hydrocyclones but no one, always as far as we know, clearly defines all parameters to be taken into consideration when rigid materials, with a pretty good size, are involved.
If you like to get more information we will be glad to tell you as much as we know about it and about experience we got with different materials, sizes and whatever else involved in the process.
We did make an example talking about separation between PET flakes and olefins but, of course, hydrocyclones can be used to separate different specific weight plastics of any kind.