One Stage, Two Directions

Float-sink separation is the simplest idea in a washing line: put material in a bath, and density sorts it. Everything lighter than the liquid rises and is drawn off; everything heavier settles and is cut from the bottom. What makes it interesting is that the stage has no fixed opinion about which of those two streams is the product.

Set the two published line configurations side by side and the difference is structural, not cosmetic:

In the bottle line the tank is a guard against the wrong polymer: a PET flake with a PP cap fragment welded to it behaves differently in the dryer and shows up later as a colour or spec problem. In the rigid line the tank is a guard against the wrong material full stop: metal, stone and dense plastics arrive in the bale and have to leave before granulation.

The Direction Is Set by the Polymer, Not the Tank

The bath is a density gate set at roughly the density of water. Which side of it a particle lands on is a property of the polymer, so the answer is known before the machine is chosen:

Two consequences follow, and both matter more than the table. First, a mixed rigid stream almost always contains polymers on both sides of the gate, so the stage is never only cleaning the product — it decides which fraction the plant is willing to lose. Second, the same tank serves two different jobs, which is why an enquiry that names no product polymer cannot be answered precisely.

The table is a starting point, not a specification. Filled and mineral-loaded grades are denser than the base polymer, foamed articles can float when the polymer would sink, and multi-layer packaging behaves as a composite rather than as its label. Where the feed is mixed, the cut is set on a sample, not the resin name.

Why the Stage Sits After Washing

Both published sequences put the density stage in the same place: after size reduction and washing, before dewatering. The rigid line runs feed, crushing, friction washing, float-sink separation, dewatering, drying. The bottle line runs de-baling, de-labeling, crushing, hot washing, density separation, then dewatering and drying. That ordering is not a layout preference.

Soil is the reason. A film or flake carrying compacted soil is heavier than the polymer it is made of, and a particle that should rise can travel with the sink cut and leave the plant as waste. Washing first removes the mass that changes buoyancy, and it also releases the labels and adhesive that would otherwise travel with the flake into the bath and foul the water. That is the same reasoning behind the crusher-to-washer handoff covered in our flake-size article: the stages only work in the order and the size window they were designed for.

There is a second, quieter reason. Washing is where the material is already in water. Putting the density stage immediately after it means the flake arrives wet, at temperature if hot washing is used, and without an extra dewatering and re-wetting step in between.

What a Wrong Cut Costs

The two failure modes pull in opposite directions, which is why the cut cannot be set by rule of thumb.

On a PET line, an incomplete float cut leaves cap and ring fragments in the flake. They are the same size as flake, they pass through the same screens and they do not disappear in drying; the cost appears downstream, in granulation and in the colour and spec of the pellet. An over-aggressive sink cut has the opposite cost: PET flake is carried out with the heavies and the plant pays for material it already washed.

On a PP/PE rigid line the logic mirrors it. PS, ABS and PVC belong in the sink cut, and if they are not taken out they contaminate a polyolefin stream that has no other stage capable of removing them — density is the only property that separates them from the product. But tighten the same cut too far and light flake is dragged under by the agitation and leaves with the rejects. Yield loss and spec failure sit on either side of one setting.

This is also why the published line tables describe capacity and water but never a cut point: the cut point is a property of the feed, not of the tank.

Where the Tank Sits in the Water Loop

The density stage is the largest open body of water in most washing lines, so it is also where the water circuit announces itself. The published line series runs from 3-5 t/h on WL-500 to 12-20 t/h on WL-5000, and both line pages state that the configuration includes a closed-loop water system with settling, filtration and water treatment. The tank is the part of that loop where solids have the chance to settle out under control instead of travelling into the spray bars.

Machine-level figures are published at a different boundary and should not be added to the line figure. The FW-series friction washer table lists water use from 2-4 m³/h on FW-500 to 8-12 m³/h on FW-1200; that is one stage's spray and rinse demand, while the WL column is the demand of the whole configuration. Reading them as a sum double-counts, and reading the machine figure as a plant requirement understates the loop. Our six-stage line guide covers the same boundary question from the water side.

The dirty fraction of the loop is what came off the material in the two cuts — soil, labels, adhesive, fines. The cleaner the cut, the slower the loop loads up and the less make-up water the site needs.

Film Is the Case Where the Stage Changes Job Entirely

Film lines expose the limit of density separation, and the film washing line page says so in its opening line: film wraps around shafts and floats instead of sinking. When the product floats and the contamination also floats, a water bath separates very little.

That is why the film sequence is built differently — bale opening, anti-wrapping feed, wet size reduction, intensive washing, squeezing, thermal drying — and why light-fraction work is handed to air classification instead of a tank. Our air separation article covers that stage; the point here is the division of labour. A tank sorts by density relative to water. Air sorts by density relative to air, a far wider gate and the only one that lifts a light film away from a heavier one.

Reading a Feedstock Before Choosing the Stage

Because the cut follows the feed, the useful questions come before the machine list:

• What is the product polymer, and what else is in the bale? Rigid PP/PE bales carry PS, ABS and PVC closures; PET bales carry PP caps and sleeves. The second list is what the stage is for.
• Are caps removed upstream? On a bottle line, de-capping before the bath changes the float cut from a quality guard into a small trim stream.
• How much soil is in the feed? Heavy soil argues for more washing ahead of the tank, not for a deeper tank.
• What happens to each cut? The sink cut from a PET line and the float cut from a rigid line are both product-bearing somewhere; whether they are sold, re-processed or discarded changes the economics of the setting.
• What is the target moisture and the drying arrangement? The density stage passes wet material to dewatering, and the dewatering structure follows the material form — vertical centrifugal for rigid flake, horizontal screw press for film.

Those five answers turn “float-sink” from a stage name into a configuration. The published line tables then decide the model; the material decides the cut.

Frequently Asked Questions