Plastic Recycling: Float-Sink Density Sorter

Feed Stream

Process Controls

Feed rate 6 fl/s

Tank fluid density 1.00 g/cm³ (water)

Separation Result

Float 0Sink 0

Float purity

–

Sink purity

–

Flakes processed

Misdirected

How it works

Real materials-recovery facilities separate mixed shredded plastics with sink-float tanks. A flake of density ρ_p submerged in a fluid of density ρ_f feels buoyancy and gravity; the net force per unit volume is:

F_net/V = (ρ_f − ρ_p)·g   (Archimedes' principle)
ρ_f > ρ_p → flake floats (F_net > 0, rises)
ρ_f < ρ_p → flake sinks  (F_net < 0, falls)

Common resin densities (g/cm³): PP 0.90–0.91, HDPE 0.94–0.97, PS 1.04–1.09, PVC 1.16–1.45, PET 1.34–1.39. In plain water (ρ = 1.00) PP and HDPE float while PET, PVC and PS sink together — which is exactly why real plants raise or lower the bath density with salt or additives to pull one more resin out of the sinking fraction at a time, cascading through several tanks. Tune the Tank fluid density slider here to see the float/sink boundary move: push it above 1.05 and PS crosses over to float; push it toward 1.35 and even PET starts to float, leaving only PVC behind.

Each flake in the tank experiences buoyant force, gravity, and a damped drag term (F_drag = −k·v) so it settles to a terminal velocity rather than oscillating forever, matching how flakes drift to a stable float or sink layer in a real tank.

Floating fraction Sinking fraction

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