Density separation, also called float-sink separation, sorts shredded plastics by whether they float or sink in a liquid, usually water. Plastics lighter than water, mainly polyethylene and polypropylene, float. Heavier plastics such as PET and PVC, along with sand, glass and metal, sink.
It is a simple, low-cost step built into most washing lines. Flakes pass through a float-sink tank, where the floating polyolefin fraction is skimmed off and the sinking fraction is removed from the bottom. Density separation cannot split polymers of similar density, for example HDPE from PP, so it is combined with NIR sorting or manual sorting upstream.
