How HDPE Drums Are Washed and Reprocessed into Pellets

HDPE recycling turns rigid polyethylene — drums, bottles, crates and sheet offcuts — into washed flake or pellets. Unlike PET, where the purity grade decides the line, HDPE is decided by the form the material arrives in. Wall thickness and residue set the front end long before the washing stages matter.

That is the practical difference a first-time buyer keeps running into. A 200-litre chemical drum and a milk bottle are the same polymer with the same density and the same melt behaviour, and they need almost nothing in common at the head of the line. Quotes for “an HDPE recycling line” vary enormously for exactly this reason: the seller is guessing at a form you have not specified.

Why HDPE Sits Between Film and PET

HDPE is one of the easier streams to process, and it is worth understanding why before looking at the hard cases.

It floats. At a density below water it separates cleanly from PET, PVC, glass, metal fines, sand and waterlogged paper in a single tank, which removes a whole class of contamination that PET lines have to fight with sorting belts. It is also thermally forgiving: unlike PET it does not hydrolyse, so residual moisture at the extruder does not destroy the batch. And it moves in volume — HDPE is consistently one of the larger recovered rigid streams in the US EPA plastics material-specific figures, which is why the buyer market for flake and pellet is stable.

The polymer, in other words, is the easy part. What is not easy is that HDPE arrives in a wider range of physical shapes than any other stream a recycler handles. The sections below are organised by that shape, because that is what your machine list responds to.

Drums and IBC Containers — The Form That Sets Your Front End

Drums are the hardest form, and for two reasons that have nothing to do with each other.

The first is residue, and it is not a machine problem. A drum is defined by what was in it. Food-grade and detergent drums are a washing question. Drums that held solvents, agricultural chemicals or anything classified as hazardous are an intake question, governed by the rules on what can be handled as ordinary waste at all — in the United States that framework starts with the EPA definition of solid and hazardous waste. No wash stage makes that decision for you. Decide what you will accept at the gate, because a load you should have refused contaminates the water circuit, not just the batch.

The second is wall thickness. Drum and IBC wall is far heavier than bottle wall, and feeding it straight into a granulator is how rotors get damaged and knives get chipped. The standard sequence is to reduce first and crush second: a twin-shaft shredder tears drums into coarse pieces at low speed and high torque, and only then does the material go into a crusher for final sizing. Twin-shaft suits this because it does not need uniform output — it needs to survive the steel bung rings, handles and cage fragments that arrive with the drum.

What it looks like when the gate decision goes wrong. A single load of drums with unrecorded contents does not spoil one batch of flake — it spoils the water. Whatever was inside dissolves or emulsifies into the wash circuit, and from there it reaches every subsequent batch until the circuit is drained, cleaned and refilled. That is lost production, a tank of effluent you now have to characterise before disposing of it, and flake already made that day which you cannot honestly certify. Recyclers who have been through it once start asking for the previous contents in writing, which is cheaper than any filtration.

Bottles and Containers — Caps, Labels and Colour

Bottles are the volume form, and the problems are all about what is attached to them.

Colour is the largest single price line in HDPE. Natural bottles — milk and water containers with no pigment — sell substantially above mixed-colour detergent and household bottles, because a natural pellet can be coloured by the buyer and a mixed pellet cannot be un-coloured. Colour cannot be washed out. If your feedstock mixes them, either sort at intake or accept the lower grade permanently — a decision made at the sorting belt, not by any machine downstream of it.

Caps are a polymer problem disguised as a mechanical one. Most HDPE bottle caps are PP, and both PP and HDPE float. The sink-float stage that removes almost everything else cannot tell them apart, which is why PP content appears on HDPE flake specifications and why cap removal has to happen mechanically before the crusher rather than chemically in the tank.

Labels come off by friction, not by chemistry. A dry label-removal stage strips 80–90% of labels and caps while the bottles are still whole, and the sequence matters as much as the equipment: a label on an intact bottle is one large object, whereas the same label crushed alongside the bottle becomes thousands of fragments the same size as the flake. After crushing, a high-speed friction washer takes off the adhesive and surface soil that survived. Wet crushing on a heavy-duty crusher to 12–20 mm keeps the dust down and starts cleaning during size reduction.

Get that order wrong and there is no recovery later in the line. Crushing labelled bottles turns one removable object into thousands of paper and film fragments sized like the flake itself. Screens cannot separate them because they are the same size, sink-float cannot because wet paper and PE film do not sort reliably against each other, and the friction washer only removes what is on the surface. The fragments travel all the way to the melt filter, where they become screen changes rather than a quality problem you can wash out. Dry label removal ahead of the crusher costs one machine; skipping it costs a permanent grade.

Sheet, Board and Thick Lumps — The Industrial Offcuts

Post-industrial HDPE inverts the usual difficulty. It is the cleanest material you will handle — often no labels, no residue, single colour, known resin — and mechanically the most demanding.

Extruder purge lumps, board offcuts and thick sheet do not shatter; they load the rotor continuously. A crusher sized for bottles will stall or overheat on the same tonnage of solid lumps, and the failure is usually a jam rather than gradual wear. Pre-shredding is not optional at this end of the range, and rotor geometry matters more than nameplate power.

Pipe and profile offcuts are their own case, because length rather than thickness is the obstacle. Long sections bridge above a normal hopper and never reach the cutting chamber. A single-rotor shredder built for long pipe is designed around that geometry, feeding lengths in end-on instead of relying on gravity.

The compensation is that clean industrial HDPE often skips the wash line entirely and goes from size reduction straight to the extruder, which changes the economics of the plant.

Where Sink-Float Does the Work, and Where It Does Not

A sink-float separation tank is the highest-value single stage on an HDPE line, and understanding its blind spot is what keeps your flake spec honest.

What it removes: PET fragments, PVC, glass, sand, metal fines, stones and saturated paper. All of these sink while HDPE floats, and they leave in the bottom discharge without any operator judgement.

What it cannot remove: anything else that floats. PP caps, PP labels and LDPE film all ride out on the same surface as the HDPE, because the density gap between polyolefins is too small for a water tank to resolve. This is the same limitation that makes PP and PE mixtures hard to separate anywhere in the industry, and the practical answer is always upstream sorting rather than a cleverer tank.

PET is the instructive contrast: there the deciding factor is purity grade rather than form, and sink-float is blind to PVC, the contaminant that matters most. That case is worked through in PET flake grades and what separates Grade A from Grade C.

Whether the tank needs to sit in a hot circuit depends on what is stuck to the material. Detergent bottles and clean crates often do not need it; oil residue, adhesive and heavy soil do. Hot washing at 85–95 °C with 1–3% alkali is the most expensive stage on the line to run, so it is worth reading when hot washing is genuinely necessary and when cold is enough before specifying it into a quote.

From Flake to Pellet

Clean HDPE flake is a saleable product in its own right, and plenty of operations stop there. The decision to pelletize is a market decision, not a technical one: pellets reach injection and blow moulders directly, flake mostly reaches compounders and traders.

If you do pelletize, HDPE is undemanding compared with PET. A single-screw pelletizing machine for rigid plastics with melt filtration handles it — there is no need for the twin-screw and decontamination stages a bottle-to-bottle PET route requires. What still matters is what goes in:

Moisture. A centrifugal dryer at 1500 rpm brings flake from the wash line down to 0.5–2%, and a hot-air stage finishes below 1%. HDPE tolerates residual moisture better than PET, but water still shows up as voids and surface defects in the strand.

Melt index consistency. Blow-moulded bottle HDPE and injection-moulded crate HDPE are different grades. Mixing them gives a pellet with an averaged, unpredictable melt index, which is exactly what a buyer’s incoming test catches. Segregating by source is worth more than any extra machine.

Filtration. The screen changer stops paper fibre, wood and unmelted contamination reaching the die. Size it for the dirtiest feedstock you plan to run.

What Line Each Form Needs

Putting the four forms side by side, the differences concentrate at the front end and mostly disappear after washing.

All four converge on the same back end: a washing line for rigid plastics where washing is needed, then drying and pelletizing. Power draw scales with throughput rather than with form — a 500 kg/h line sits around 150 kW installed and a 3000 kg/h line around 500 kW. What each additional stage adds to the investment is broken down in what actually adds up in plastic recycling plant cost. The Association of Plastic Recyclers design guide is worth reading from the opposite direction: it lists what package designers are asked to avoid, which previews what will otherwise arrive on your belt.

What a Rigid Line Costs to Run at Each Size

HDPE is the cheapest of the common streams to process, and the figures are worth having because they are the benchmark every other material gets compared against.

Three things in that table decide how a rigid project should be sized.

The energy curve is finished by about 900 kg/h. It falls steeply from 400 to 138 kWh per tonne across the first three tiers, then goes 128 and back up to 131. The top two tiers exist for plants that need the tonnage, not for plants chasing unit cost.

Labour keeps improving after energy stops. From 8.0–12.0 down to 1.6–2.6 people per tonne per hour, and unlike energy it is still improving at the top of the range. In a high-wage economy that is where the case for the larger tiers actually sits.

Rigid is the benchmark other streams are measured against. At 1,500 kg/h it runs 128 kWh and 5.7–7.1 m³ per tonne. A PET line at the same tonnage is 200 kWh, and a film line 253–400 kWh on 13.3–20.0 m³ of water. If you have a choice of stream, this is the cheapest one to run.

Checking a Load Before You Commit to It

Everything above assumes you already know what is in the load. Often you do not, and the seller’s description is optimistic. Two checks cost nothing and can be done on the weighbridge.

A bucket of water settles the density question. Take a handful of pieces from several places in the bale, cut or snap them small, and drop them in water. What sinks is what your sink-float tank will pull out: PET, PVC, glass, grit, metal fines and waterlogged paper. What floats is what you are buying, plus everything else that floats. If a visible fraction sinks, you are not buying HDPE at the quoted percentage, and the difference is yield you will pay to transport, wash and then discard.

The wall tells you the front end. Squeeze and flex the heaviest pieces in the load. If the thickest wall is drum or IBC section rather than bottle section, the quotation you have been given for a bottle line is the wrong quotation — that material needs shredding before it reaches any crusher, and adding a shredder afterwards is far more expensive than specifying one at the start.

What neither check can tell you is the PP fraction, because caps, closures and PP crates float exactly as HDPE does. That one has to be counted by eye at the sorting belt, which is the recurring theme of this material.

What a Buyer Tests When Your Flake Arrives

The reason to care about all of the above is that the buyer’s incoming inspection is short, cheap and decisive. Nearly every rejection traces back to a decision made at the front of the line rather than at the extruder.

Two of those six — melt index and colour — cannot be corrected by anything downstream. They are set the moment two feedstocks are tipped onto the same belt. The others are equipment questions with equipment answers. How buyers actually run these checks, and what they do with the result, is covered in how buyers grade and test plastic regrind, and which of them will pay for a given output is mapped in where recycled granules are actually sold.

Frequently Asked Questions