Mechanical recycling is how almost every tonne of recovered plastic in the world is actually processed. It is also the process most often described in six words and then left alone, which is why buyers arrive at an equipment quotation without knowing which stage their material will break.
This article walks the six stages in the order the material meets them, and for each one answers three questions — what physically leaves the stream here, which single variable decides whether the stage works, and which materials force you to reinforce it. Those six stages are also the shape of a recycling equipment catalogue, which is not a coincidence.
Six stages, six different contaminants. Nothing after stage one can raise the grade of the material.
What Mechanical Recycling Actually Is
Mechanical recycling recovers a polymer by physically cleaning and reshaping it rather than by changing its chemistry. Sorting, size reduction, washing, separation, drying and pelletizing remove everything in the waste stream that is not the target polymer, then reform what remains into a pellet a manufacturer can meter into a machine.
The word that carries the meaning is physically. No bond in the polymer backbone is deliberately broken and no monomer is recovered. A polyethylene molecule that enters a washing line as part of a milk bottle leaves it as the same molecule in a pellet. Everything the line does is either removal or reshaping.
Two consequences follow from that, and they explain most of what looks strange about how recycling plants are built.
First, the process is subtractive, so the grade of the output is capped at the first stage. Sorting is the only point at which the composition of the stream changes in your favour. Washing removes surface contamination, separation removes the wrong density, filtration removes solids — but none of them can turn a mixed bale into a single-polymer one. Everything downstream of sorting preserves quality; it cannot create it.
Second, the one thing the process spends is heat history. Melting the polymer to pelletize it consumes a measurable amount of the material’s remaining service life, and the cost differs by polymer. Polyolefins tolerate several cycles. PVC consumes stabiliser on every pass. PET loses intrinsic viscosity. That single melt is why a recycler tries to do as much of the work as possible cold, and why the pelletizing stage sits last.
Industry bodies define the route in similar terms. The Plastics Europe description of mechanical recycling is a useful reference point for the scope of the term as regulators and brand owners currently use it.
The Six Stages and What Each One Removes
Each stage exists because a specific class of contaminant cannot be removed by any other stage. Metal cannot be washed out. Glue cannot be screened out. Water cannot be filtered out of a melt. Reading a recycling line as six contaminant-removal operations, rather than six machines, is what makes the configuration decisions obvious.
Each row names one thing that can only be removed here, and the one setting that decides whether it is.
| Stage | What physically leaves the stream here | The variable that decides success | Materials that force you to reinforce it |
|---|---|---|---|
| 1 · Sorting | Non-target polymers, metal, wood, textile, stone, whole rejected items | How much of the stream a detector or a hand can actually identify | Mixed post-consumer bales, dark and black items, small formats |
| 2 · Size reduction | Nothing yet — this stage creates the flake everything else is sized around | Screen aperture, and rotor geometry against the material’s form | Film and woven fabric, thick-wall pipe, bulky lumps |
| 3 · Washing | Soil, sand, product residue, adhesive, printing ink, oils, label paper | Temperature, chemistry and mechanical shear at the flake surface | Agricultural film, oily containers, heavily glued labels |
| 4 · Separation | Everything with the wrong density — grit, metal fines, PET and PVC fragments, and film in a rigid stream | The density cut point of the medium and residence time in the tank | Mixed polyolefin streams, filled compounds, laminated packaging |
| 5 · Dewatering and drying | Water, in two forms — free surface water and moisture held in folds and pores | The moisture target the next stage actually needs | Film, woven fabric, hygroscopic polymers, anything going straight to extrusion |
| 6 · Pelletizing | Fine solid contamination in the melt, plus volatiles and residual moisture | Melt filtration area and the number of degassing zones | Post-consumer streams, printed film, mixed regrind |
Read down the third column and a pattern appears. Only two of the six controlling variables are set by the machine you buy. The other four — what the detector can see, what the contamination actually is, what density the contaminants have, and what moisture the next process needs — are set by the material and by the destination. This is why equipment suppliers ask for a sample before they quote, and why a line specified from a capacity figure alone tends to be wrong.
Sorting Decides the Ceiling for Everything Downstream
Sorting is the only stage that changes what the stream is made of. Bales are broken open, obvious foreign objects are pulled by hand, ferrous metal is lifted magnetically, and near-infrared detection ejects polymers that do not belong. Everything after this point works on whatever composition sorting left behind.
A sorter removes what it can identify. Two categories are invisible to it, and both leave with the product.
The sequence itself is unremarkable. What matters is knowing where it is blind, because those blind spots become the contamination specification of your output whether you plan for them or not.
Carbon black is invisible to near-infrared detection. The sorter identifies polymers by the infrared light they reflect, and carbon black absorbs it. A black polypropylene tray therefore registers as nothing, is not ejected into the PP stream, and reports to residue. Nothing later in the line can recover it, because the line never knew it was there.
Small items fall through before they are ever presented to a detector. Screening decks that remove glass fines and grit also remove caps, cutlery and small lids. This is why a stream of large containers behaves so much better than a nominally identical stream of small ones.
Bonded multi-material items sort as whichever polymer is on the outside. A laminated pouch presents one surface to the detector and carries a second polymer inside the wall, so it is ejected into the stream matching its skin and behaves as a contaminant from that point on.
Where the material is clean and consistent enough, sorting is built into the line rather than bought separately. SUHUI publishes two stages of optical sorting inside the PET bottle sorting washing line, because a post-consumer bottle bale carries PVC and other bottle polymers that must leave before the hot caustic stage rather than after it. Film and woven streams are handled differently — a magnetic separator removes ferrous debris and optical sorting is offered as an option on the PP PE soft film recycling washing line for mixed-material feedstock rather than being standard.
Size Reduction Sets the Flake the Rest of the Line Has to Handle
Size reduction removes nothing. Its job is to convert bulky, variable waste into a flake with a controlled top size, because every stage after it — wash tanks, friction washers, separation, dewatering, extruder feed — is designed around a particular flake dimension and stops behaving predictably outside it.
Three different targets, each set by what the next machine needs rather than by the shredder.
The published targets across SUHUI’s lines make the logic visible. Film goes to 10–20 mm through an anti-tangle crusher on the soft film washing line. Rigid HDPE packaging goes to 10–15 mm on the HDPE rigid milk bottle recycling washing line. Clean dry flake destined for pelletizing is granulated finer still, to 3–8 mm, so that it feeds an extruder at a stable mass rate.
Two things decide whether this stage works.
Screen aperture is the actual output control. The rotor cuts, but only fragments small enough to pass the perforated screen beneath the chamber leave the chamber. Changing the screen changes the flake, which is why the same plastic crusher covers several output specifications. A finer screen buys uniformity and costs throughput, energy and fines.
Rotor geometry has to match the material’s form, not its polymer. Film and woven fabric wrap around a conventional rotor and stall it, which is why film and jumbo bag lines lead with anti-tangle and anti-winding designs rather than standard granulators. Thick-wall pipe and bulky lumps need torque instead of speed, which is what separates a shredder from a crusher. That distinction is worked through in our comparison of plastic granulator vs shredder vs crusher, and the machine range is set out on the crusher and shredder category page.
The cost of over-reducing is fines. Material cut below the screen deck of downstream equipment leaves with the wash water and the dust extraction, and it never appears in a specification sheet. It is one of the quieter places yield disappears between the weighbridge and the bagging station.
Washing Is Four Different Operations Sharing One Word
Washing is not a stage, it is a sequence of four operations with different physics, and specifying one when the contamination needs another is the most common configuration error in the whole route. Pre-washing, hot caustic washing, friction washing and rinsing each remove a different class of contaminant.
Four operations, four mechanisms. Substituting one for another leaves the contaminant in the flake.
| Operation | Mechanism | What it removes | What it cannot touch |
|---|---|---|---|
| Pre-washing | Agitation in ambient water, with heavy contaminants settling out | Loose soil, sand, grit, stones, free product residue | Anything bonded to the flake surface |
| Hot caustic washing | Temperature plus alkaline chemistry dissolving or saponifying the contaminant | Adhesive, oils and fats, protein residue, some label substrates | Contamination absorbed into the polymer, and pigment |
| Friction washing | High-speed mechanical shear between flakes and against a screen | Softened glue, fibre, paper pulp, loosened surface film | Anything the chemistry has not already released |
| Rinsing | Displacement with clean water | The detergent and caustic the previous stages added | Nothing else — this stage exists to undo the last two |
The published temperatures follow the contamination rather than the polymer. PET bottle bales run a two-stage hot caustic wash at 80–90°C, because bottle glue and residual beverage sugars need both heat and alkalinity. Rigid HDPE dairy packaging runs 60–80°C. Film runs 60–80°C as well, and the soft film line lists it as an optional stage.
That word optional is worth dwelling on, because the specification proves it. Both consumables on the SHW-series film line are published with a lower bound of zero — nothing for steam, nothing for chemistry, at the bottom of the range. A clean post-industrial scrap can therefore be run cold on the same machine that consumes the top of both ranges on soiled mulch film. Heat and chemistry are a variable cost driven by contamination, not a fixed property of the polymer.
Friction washing is the operation most often assumed to be interchangeable with hot washing, and it is not — it supplies shear, not chemistry, and it only removes what the chemistry has already loosened. Its rotor speed, residence time and water flow are covered separately in our article on the friction washer.
One consumable decides whether the whole wash section is affordable, and it is not the chemistry. Water leaves the tanks carrying everything the stage removed, so treating and returning it is a permanent operating requirement rather than an environmental extra. The HDPE rigid line publishes a reduction in fresh water demand of up to 70% from closed-loop treatment, and the film compacting line publishes 60–80%.
Separation Is Where the Line Admits What It Cannot Do
Separation sorts the washed flake by density rather than by chemistry. A float-sink tank lets polymers lighter than water rise and everything heavier sink, which removes the contamination that survived washing because it was never on the surface in the first place — grit, metal fines, and fragments of the wrong polymer.
Water sets the cut point at 1.0 g/cm³. Everything the tank can do follows from that one number.
The physics is simple and the consequences are not. In plain water the cut point sits at 1.0 g/cm³. Polypropylene at 0.90–0.92 g/cm³ and polyethylene at 0.92–0.97 g/cm³ both float. PET near 1.38 g/cm³ and PVC sink, along with sand, glass and metal fines.
That single fact explains the different maturity of two recycling streams. On a PET bottle line the tank does a large part of the separation for nothing — the PET flake sinks while polyolefin caps and label fragments float, and both fractions come out usable. On a polyolefin line the tank removes the heavies but cannot split PP from PE, because both sit on the same side of the cut.
The cut point is not fixed, though. Dosing saline into the tank raises the density of the medium and shifts the boundary upward, so materials that both float in plain water can be made to separate — a technique the film washing line specification names explicitly. Its limits are practical rather than theoretical. The dosing has to be held steady, the brine has to be managed inside the water loop, and closely spaced density bands still produce a blurred split.
Three situations defeat density separation entirely, and it is worth naming them before a line is specified. A filled compound does not have its polymer’s density — talc-filled polypropylene can sink where unfilled PP floats. A bonded multilayer laminate has no single density at all, so it cannot be assigned to either stream. And a flake with entrapped air floats regardless of what it is made of, which is why residence time and agitation in the tank are part of the specification rather than an afterthought.
Dewatering and Drying Split One Job Between Two Machines
Water leaves the flake in two completely different ways, and the split between them is an energy decision rather than a process one. Force alone removes what is loose on the surface, and it does so almost for free. Heat has to remove the rest, at a cost per kilogram many times higher.
Mechanical removal is cheap and stops early. Thermal removal is expensive and is the only way to finish.
The published sequences make the division explicit. Film goes through two mechanical stages to reach 12% moisture, then a thermal stage with cyclone separation to reach 5%. Rigid and PET flake, holding far less water to begin with, come out of the same thermal stage below 1%.
The target is not a quality metric. It is set by whatever happens next.
| Destination for the flake | Published moisture target | Why that number |
|---|---|---|
| Bagged film flake for sale or pelletizing | Below 5% | Enough to prevent caking and biological activity in storage, and to feed a vented extruder |
| Rigid and PET flake for sale or pelletizing | Below 1% | Rigid flake holds less surface water, and buyers weigh and pay for what is in the bag |
| Moisture-sensitive rigid blends before extrusion | Below 0.5% | Steam bubbles and surface defects appear in the pellet above this |
| PET flake before twin-screw compounding | Below 0.02% | Above it, water attacks the polymer chain at melt temperature rather than simply boiling off |
Film is the awkward case, and it is awkward for a geometric reason. A film flake has an enormous surface area for its mass and folds over on itself, so it carries far more water into the dryer than a rigid flake of the same weight and holds it in places centrifugal force cannot reach. That is the entire reason the plastic film squeezer granulating machine exists — a high-torque screw mechanically compresses water and air out of washed film and densifies it in the same pass, covering 300–750 kg/h and displacing part of the thermal drying load. Support equipment around a line is grouped on the auxiliary machine page.
Pelletizing Is the Only Stage That Changes the Polymer
Everything before this point is cold work on a solid. Pelletizing melts the material, and that melt does three jobs at once — it filters out solid contamination too fine for any wash stage, it removes volatiles and residual moisture under vacuum, and it converts loose flake into a dense granule an extruder or moulding machine can meter.
The melt is the last chance to remove anything, and the first time the polymer pays a price.
Melt filtration is the part buyers under-specify most often. On virgin material a screen pack is insurance. On recycled material it is a working process stage that has to be sized against the actual contamination load, which is why the rigid plastic granulating pelletizing line specifies multi-stage filtration with continuous or discontinuous screen changers, and why the film compacting line offers four filter types from a simple two-position plate up to an automatic self-cleaning unit. Undersized filtration announces itself as climbing melt pressure and shortening screen intervals long before it shows up as a rejected batch.
Degassing is the second job. Detergent residue, printing ink solvent, trapped water and degradation products all turn to vapour in the barrel, and with nowhere to go they end up as voids and silver streaking in the granule. Vented barrels exist for this. Published configurations on the PP PE film compacting pelletizing line carry either one vacuum exhaust zone or two, with a second extruder available where a single pass is not enough.
The cut method is chosen against the melt, not against the polymer name. Strand pelletizing suits a melt with enough strength to be drawn through a water bath. Water ring and underwater cutting suit soft or sticky melts and give the most uniform granule. Published output sits at 2–5 mm across the rigid and film lines and 2–4 mm on the PVC granulating pelletizing line, which uses die-face hot cutting with air-cooled cyclone conveying because PVC does not tolerate a water bath the way polyolefins do. Single-stage against double-stage is its own decision entirely, set out in our article on plastic recycling pelletizing.
Which Stages Your Material Forces You to Reinforce
No recycling line strengthens all six stages equally, because no material stresses all six. The configuration question is therefore narrow — given this material in this condition, which two or three stages carry the load, and which can be specified at the standard level without risking the output grade.
| Material and condition | Stages that carry the load | Why | Published SUHUI configuration |
|---|---|---|---|
| Post-consumer PET bottle bales | Sorting and washing | Bales carry other bottle polymers and heavy glue and sugar loads | PET bottle sorting washing line, 500–3,000 kg/h, two optical sorting stages, two-stage hot caustic at 80–90°C, final moisture below 1% |
| Rigid HDPE dairy and detergent packaging | Washing and water treatment | Product residue and label adhesive dominate, and water cost decides the margin | HDPE rigid milk bottle recycling washing line, 300–2,500 kg/h, 10–15 mm flake, hot caustic at 60–80°C, closed loop cutting fresh water by up to 70% |
| Woven PP sacks and FIBC bulk bags | Size reduction and dewatering | Woven fabric winds on a rotor and holds water in the weave | PP jumbo bag recycling washing line, 300–2,000 kg/h, anti-winding shredder, two-stage dewatering below 12% then drying below 5% |
| Agricultural and packaging film | Washing and dewatering | Soil load is high and the flake geometry holds water | PP PE soft film recycling washing line, SHW300 to SHW2000, 250–2,000 kg/h, 10–20 mm flake, final moisture below 5% |
| Mixed rigid regrind for pelletizing | Pelletizing | Fine contamination and mixed melt flow behaviour meet at the extruder | Rigid plastic granulating pelletizing line, 300–3,000 kg/h, 3–8 mm feed, multi-stage filtration, 2–5 mm pellets |
| PVC pipe, profile and cable scrap | Pelletizing, on thermal control rather than filtration | Stabiliser is consumed on every heat cycle, so residence time is the constraint | PVC granulating pelletizing line, 3–6 mm feed, 2–4 mm pellets, die-face hot cutting with air cooling |
The third column is the argument for describing a stream by its form and condition before naming its polymer. Two materials sharing a resin code — a clean post-industrial PP offcut and a soiled woven PP sack — load entirely different stages. The equipment routes are set out on the plastic recycling machine overview, and the buyer-side walkthrough is our guide to the plastic recycling line.
Frequently Asked Questions
What is mechanical recycling?
Mechanical recycling recovers plastic by physically cleaning and reshaping it rather than by altering its chemistry. Sorting, size reduction, washing, density separation, drying and pelletizing remove everything that is not the target polymer, and the polymer chain itself survives the process unchanged apart from the heat history of one melt.
Does mechanical recycling change the plastic itself?
No bond in the polymer backbone is deliberately broken, so the molecule that entered as a bottle leaves as the same molecule in a pellet. The one thing the route does spend is heat history — a single melt at the pelletizing stage, which costs a measurable amount of the material’s remaining service life.
What are the six stages of mechanical recycling?
Sorting, size reduction, washing, density separation, dewatering and drying, and pelletizing. Each removes a different class of contaminant that no other stage can address, so the sequence cannot be reordered and a weak stage exports its error into everything downstream of it.
What limits the grade a mechanical recycling line can reach?
Sorting, and nothing downstream. Every later stage protects the grade the sorter handed it — washing removes surface contamination, separation removes wrong-density fragments, filtration removes solids — but none of them can raise it. A bale sorted to two resins cannot leave as a single-resin pellet however good the equipment behind it is.
Why does mechanical recycling need so much water?
Because contamination is removed by dissolving and shearing it off the flake surface, and both need water as the medium. Closed-loop treatment recirculates it rather than discharging it, and published reductions in fresh water demand run to 70% on rigid lines and 60–80% on film compacting lines.
Can mechanical recycling separate PP from PE?
Not in a plain water float-sink tank. Polypropylene at 0.90–0.92 grams per cubic centimetre and polyethylene at 0.92–0.97 both float, so the tank cannot discriminate. Saline dosing can shift the cut point, but reliable separation usually has to happen upstream through near-infrared sorting or source separation.
How many times can plastic be mechanically recycled?
There is no single number, because the limit is set by the polymer and by how much heat history each cycle spends. Polyolefins tolerate several passes, PVC consumes stabiliser on every melt, and PET loses intrinsic viscosity. Tracking how many cycles a batch has seen matters more than the count itself.
Turning These Six Stages Into a Line
- Plastic Recycling Washing Line — stages three to five bought as one machine group, with a build per material
- Plastic Recycling Pelletizing Line — stage six on its own, where melt filtration and degassing get specified
- Plastic Recycling Line Guide — whether the six stages arrive as one line or as three separate purchases
- Plastic Recycling Machine Overview — the same route drawn as a machine catalogue instead of a stage sequence
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