Screening is the most underestimated engineering decision in a plastic washing line. Buyers spend weeks comparing crusher rotor diameters and friction washer speeds, then specify the flake sieve in a single line of the quotation: “one vibrating screen included.” Six months later the same buyer is standing in front of a blinded screen deck at two in the morning, watching good flake ride over the discharge lip into the fines bin, while the extruder downstream jams its screen changer every forty minutes. The line is running, but the material it produces cannot be sold at the grade the business plan assumed.
Screening is not “adding a vibrating screen.” Flake geometry, moisture, static charge and bulk density differ enormously between a rigid PET bottle flake, a limp PE film flake and a stringy PP woven filament. Choosing the wrong screen family for that geometry produces blinded apertures, carryover of good material, torn screen cloth and, ultimately, a purity level that cannot enter food-grade or fiber-grade markets. The flake sieve is where a washing line either converts mechanical cleaning into measurable purity, or quietly throws that purity away.
This guide treats flake screening as a discipline in its own right: what purification actually means, where sieves belong in the process sequence, how to match screen family to flake type, how to size aperture and screening area, how to defeat blinding, and what purity each end market realistically demands. Polyretec, a Wanplas factory, has built plastic recycling equipment since 2010 and formalized the brand in 2017, with more than one hundred project references and service coverage across more than fifty countries, and the screening logic described here is the same logic our engineers apply when configuring a washing line for a specific waste stream.
What Purification Really Requires in a Washing Line
Purification in a washing line is not a single function. It is four distinct separation duties that happen to share the same conveyor route, and only three of them belong to a sieve. Confusing them is the root cause of most disappointed purity expectations.
Size classification removes anything outside the target dimensional window: oversized flake that escaped the crusher screen, long strips, agglomerated lumps, and undersized fragments that behave like dust downstream. This is pure geometry, and a screen deck does it very well.
Foreign material removal targets objects that differ from the product in size or shape: stones, glass shards, wood splinters, metal fragments, paper, textile threads, caps and ring collars. Some of these leave with the oversize fraction, some with the undersize fraction, and some need density or magnetic help.
Fines removal strips out the powder generated by cutting, friction washing and pneumatic conveying. Fines are chemically identical to the product but behave like a different material in the extruder: they oxidize faster, they carry more surface moisture per unit mass, they raise ash content and they blind melt filters. A screen deck with a correctly chosen bottom aperture, supported by air classification, handles this duty.
Material separation distinguishes polymers from one another: PVC from PET, PP from PE, colored from natural, opaque from clear. No sieve does this. Density separation in a float tank or hydrocyclone, electrostatic separation, optical sorting and, in some cases, hot-wash chemistry are the only tools that address it. A screen upstream of these processes makes them far more effective by presenting a narrow, uniform size distribution, but it cannot substitute for them.
| Purification duty | Typical target contaminant | Performed by a sieve? | Supporting equipment required | Realistic removal efficiency |
|---|---|---|---|---|
| Size classification | Oversize flake, long strips, lumps, agglomerates | Yes — primary duty | Crusher screen upstream sets the feed distribution | 95–99 percent of out-of-window material |
| Foreign material removal (size-differentiated) | Stones, glass, wood, caps, textile threads | Partly | Float tank, hydrocyclone, manual pre-sort | 70–92 percent depending on size overlap |
| Fines and dust removal | Cutting powder, abraded surface material, paper pulp | Yes, with air assistance | Zigzag air classifier, cyclone, bag filter | 85–97 percent below the fines cut point |
| Material separation by polymer | PVC in PET, PP in PE, multi-layer fragments | No | Float-sink tank, hydrocyclone, electrostatic separator, optical sorting | Not applicable to screening |
| Metal removal | Ferrous fragments, aluminum neck rings, wire | No | Magnetic drum or grate magnet, eddy current separator | 96–99.5 percent ferrous, 85–95 percent non-ferrous |
| Color separation | Colored flake in a clear stream | No | Optical sorting equipment | Not applicable to screening |
The practical consequence of this table is a specification discipline. When a customer states a purity target — say, fewer than fifty parts per million of foreign polymer for fiber-grade recycled PET — the screening system is responsible for the size-related share of that target, while density and optical stages own the polymer-identity share. Writing the acceptance test so that each stage is measured on the contaminant class it can actually influence prevents the familiar dispute in which the screen supplier and the sorter supplier each blame the other for a failed sample.
A second consequence concerns sequencing. Because sieves narrow the size distribution, every downstream separation improves when screening is done first. A float tank separates PET from PP reliably when both are 10 mm flakes; it separates far less reliably when the stream contains 40 mm PP film fragments that trap air, plus 2 mm fines that follow the water flow regardless of density. Screening is therefore not only a purification step in its own right — it is the enabling step that lets the more selective processes reach their rated efficiency.
Where Sieves Sit in the Washing Line
A well-designed washing line contains several screening points, not one. Each point has a different target contaminant, a different aperture, and often a different screen family. Treating screening as a single machine placed somewhere near the end of the line is the most common layout error we correct when auditing existing installations.
The first screening point sits immediately after the wet crusher. Its job is to catch what the crusher failed to reduce: uncut bottle bodies, long handle strips, rope, and any tramp object that survived the rotor. This is a coarse duty with a large aperture, and it protects everything downstream from mechanical shock. The second point follows friction washing, where grit, sand and detached label pulp must leave before they enter the float tank and contaminate the water circuit. The third point sits at the float tank exit, grading flake before dewatering so that the squeezing dryer receives a uniform feed. The fourth point follows the dryer, breaking and removing the agglomerated clumps that mechanical dewatering inevitably produces. The final point precedes extruder feeding or big-bag packing, where a fine safety screen protects the screw and the melt filter.
| Position in line | Target contaminant | Typical aperture | Preferred equipment type | Wet or dry | Why it matters |
|---|---|---|---|---|---|
| After wet crusher | Uncut bodies, long strips, rope, tramp objects | 30–60 mm scalping deck | Static grid, wedge wire screen or heavy-duty linear vibrating screen | Wet | Protects friction washers and pumps from jamming |
| After friction washing | Sand, grit, label pulp, fiber, fines | 2–4 mm bottom deck | Linear vibrating screen with spray bars, or wedge wire dewatering screen | Wet | Keeps abrasives out of the float tank and water loop |
| After floating tank | Undersize fragments, residual fines before dewatering | 3–6 mm | Wedge wire dewatering screen or circular vibrating screen | Wet | Reduces water load on the dryer and stabilizes moisture |
| After squeezing dryer | Agglomerated clumps, fused lumps, residual fines | Top 20–30 mm, bottom 2–4 mm | Two-deck linear vibrating screen or centrifugal sieve | Dry | Prevents lumps from bridging in silos and feed throats |
| Before extruder feeding or packing | Residual dust, oversize survivors, foreign fragments | Top 15–25 mm, bottom 1.5–3 mm | Circular vibrating screen or centrifugal sieve with magnet | Dry | Protects screw, barrel and melt filter; sets final saleable purity |
| Air classification stage (parallel duty) | Label film, paper, lightweight flakes | Not aperture-based | Zigzag air classifier with cyclone | Dry | Removes low-density contaminants a screen cannot catch |
Two design notes follow from this layout. First, wet screening points should always be fitted with spray bars. A screen deck running in a wet section without water assistance simply pastes fines onto the mesh; with 0.2 to 0.4 cubic meters of water per ton of throughput sprayed onto the deck, the same mesh runs open for entire shifts. Second, the final safety screen should never be sized for maximum separation efficiency. Its aperture should be generous enough that it passes essentially all product and only rejects genuine outliers, because a blinded final screen at the extruder feed point stops the whole line, whereas a blinded intermediate screen only degrades quality.
Buyers frequently ask whether all these points are necessary at 500 kg/h. For a clean industrial scrap stream with consistent geometry, a two-point layout — after the crusher and after the dryer — is defensible. For post-consumer material, and particularly for bale-fed bottle streams with variable contamination, each omitted screening point transfers its workload to a machine that was not designed for it, and the cost shows up as filter changes, screw wear and rejected sample lots rather than as a line item in the equipment budget.
Sieve Types and Selection Logic
Six screen families cover essentially all flake screening duties in plastic recycling. They differ in how they present material to the aperture, and that difference — not nominal capacity — is what should drive selection. A linear vibrating screen conveys and stratifies; a circular vibrating screen tumbles and re-presents; a trommel rotates and lifts; a centrifugal sieve forces material through with rotating paddles; an air classifier ignores size and works on the ratio of drag to weight; a static wedge wire deck does nothing but let gravity and water do the work.
Linear vibrating screens are the workhorse of flake screening. Twin counter-rotating vibration motors produce a straight-line throw at 40 to 50 degrees, which both conveys material along the deck and lifts it clear of the mesh so apertures stay open. They tolerate moisture well, accept multiple decks, and handle rigid flake at high tonnage. Their weakness is limp material: PE film flake lies flat on the deck, forms a mat, and blinds the mesh unless the throw is aggressive and the bed thin.
Circular vibrating screens, driven by a single unbalanced motor, tumble material rather than conveying it in a straight line. The tumbling action re-orients elongated particles repeatedly, giving each one several chances to find an aperture. This makes them attractive for stringy PP woven filament, but the same tumbling makes residence time hard to predict and throughput per square meter lower than a linear machine.
Trommel screens — rotating perforated drums — are the most forgiving family. They accept wet, sticky, tangled and irregular feed that would stall a vibrating deck, and they are almost immune to catastrophic blinding because the drum wall constantly changes orientation. The trade-off is footprint and efficiency: a trommel needs length, and its separation sharpness is lower because material near the drum axis never contacts the screen surface.
Centrifugal sieves use rotating paddles inside a cylindrical screen basket to fling material outward through the apertures. They deliver high throughput in a small footprint and cope with slightly damp material, which makes them the standard choice for post-dryer fines removal. They are aggressive, however, and can generate additional fines from brittle flake, which is a real concern with heavily degraded post-consumer PET.
Air classifiers, usually of the zigzag type, are not sieves at all in the geometric sense, but they occupy the same position in the process logic. Material falls through a zigzag channel against a rising air stream; particles whose drag exceeds their weight rise and leave overhead, and heavier particles fall. This is the only practical way to separate label film and paper from rigid flake of the same nominal size.
Static grids and wedge wire screens have no drive at all. Material slides across a sloped surface of parallel profiled bars, water and fines pass through the slots, and the flake continues to the discharge. They are cheap, silent and maintenance-light, which makes them ideal as scalping and dewatering elements, but their separation efficiency is modest and they need the material to be wet enough to flow.
| Screen family | Applicable flake type | Moisture tolerance | Typical capacity (kg/h) | Aperture range (mm) | Blinding risk | Power (kW) | Footprint | Maintenance level |
|---|---|---|---|---|---|---|---|---|
| Linear vibrating screen | PET bottle flake, HDPE and PP rigid flake, PVC rigid flake | High — handles surface-wet flake | 300–6000 | 1.5–60 | Medium | 1.5–7.5 | Medium, long and narrow | Medium |
| Circular vibrating screen | PP woven filament, mixed rigid flake, final safety screening | Medium | 200–3000 | 1.0–40 | Medium-High with film | 1.1–4.0 | Small, compact circular | Low-Medium |
| Trommel / rotary drum screen | PE film flake, wet tangled material, bale-fed pre-screening | Very high — handles sludge-level wet feed | 500–8000 | 5–80 | Low | 2.2–11 | Large, requires length | Low |
| Centrifugal sieve | Dry PET and HDPE flake, post-dryer fines removal | Medium — up to roughly 3 percent surface moisture | 500–5000 | 1.0–12 | Low-Medium | 4.0–22 | Small, vertical or horizontal | Medium-High |
| Air classifier / zigzag separator | Label film in PET flake, paper, dust, lightweight fragments | Low — feed must be dry | 300–4000 | Not aperture-based | None (no screen surface) | 5.5–30 including fan | Tall, vertical column | Low |
| Static grid / wedge wire screen | Wet scalping, dewatering, grit removal | Requires wet feed to flow | 500–10000 | 0.5–50 slot width | Very High if feed dries out | 0 (gravity) | Very small | Very Low |
The selection logic that follows from this comparison is straightforward once flake geometry is treated as the primary variable. Rigid, roughly equidimensional flake with predictable bulk density goes to a linear vibrating screen. Limp, low-bulk-density film goes to a trommel, or to a linear screen with a very thin bed and aggressive throw. Elongated filament goes to a circular screen where tumbling gives it repeated presentation angles. Dry fines removal at high tonnage in a tight building goes to a centrifugal sieve. Anything that differs in density rather than size goes to an air classifier. Anything wet and coarse goes over a wedge wire deck first, because it is the cheapest way to remove water and grit before energy-consuming machines see the material.
One further consideration deserves emphasis: decks are cheaper than machines. A two-deck or three-deck linear screen performs two or three separations in one footprint, with one drive and one foundation. When a process audit shows that a line needs both a 25 mm scalping cut and a 3 mm fines cut, specifying a single two-deck machine rather than two single-deck machines usually saves capital, floor area and installed power, at the cost of slightly more complex screen changing.
Aperture Selection by Flake Type
Aperture selection is where theory meets the specific waste stream. The governing rule is simple to state and frequently violated: the top deck aperture must be larger than the largest acceptable flake, and the bottom deck aperture must be smaller than the smallest acceptable flake. Everything between those two numbers is product. When either boundary is set too tightly, good material is rejected; when either is set too loosely, contamination passes.
Flake type dictates how much margin those boundaries need. Rigid PET bottle flake produced by a wet crusher with a 12 mm rotor screen typically presents a distribution centered around 8 to 14 mm, with a tail of thin curved fragments from the shoulder and base of the bottle. A 20 mm top deck and a 2 mm bottom deck capture that distribution cleanly. PE film flake is a completely different problem: nominal size may be 20 to 60 mm, but the material is limp, it folds, it bridges across apertures, and a fragment that measures 45 mm flat can pass through a 20 mm hole edge-on if the throw presents it that way. PP woven filament is the hardest of all, because a 60 mm long, 2 mm wide filament will pass a 3 mm bottom deck lengthwise and be lost with the fines unless the deck is chosen and inclined correctly.
| Flake type | Typical flake size | Recommended top screen aperture | Recommended bottom screen aperture | Fines cut point | Screen material and thickness |
|---|---|---|---|---|---|
| PET bottle flake (hot-washed) | 8–14 mm | 18–22 mm square or round perforation | 2.0–2.5 mm | Below 2.0 mm classified as fines | 304 stainless perforated plate, 1.5–2.0 mm thick |
| PET bottle flake (food-grade route) | 8–12 mm, narrow distribution required | 14–16 mm | 3.0–4.0 mm | Below 3.0 mm removed to control ash and moisture | 316L stainless perforated plate, 2.0 mm thick |
| PE film flake (post-consumer) | 20–60 mm, limp, bridging-prone | 60–80 mm, or trommel drum 50–70 mm | 6–10 mm | Below 6 mm, mostly paper and grit | Perforated plate 2.0–3.0 mm, or PU screen panel |
| PP woven flake (filament-shaped) | Length 30–80 mm, width 1.5–4 mm | 40–60 mm, elongated slot preferred | 4–6 mm round hole, not slot | Below 4 mm | Perforated plate with round holes, 2.0 mm thick |
| HDPE rigid flake (bottles, drums) | 8–15 mm | 20–25 mm | 2.5–3.0 mm | Below 2.5 mm | 304 stainless perforated plate, 2.0 mm thick |
| PVC rigid flake (profile, pipe scrap) | 6–12 mm, abrasive | 16–20 mm | 2.0–2.5 mm | Below 2.0 mm | Wear-resistant perforated plate 3.0 mm, or PU panel |
| Mixed rigid regrind (industrial scrap) | 5–20 mm, variable | 25–30 mm | 2.0–3.0 mm | Below 2.0 mm | 304 stainless woven mesh or perforated plate, 2.0 mm |
Three refinements to this table repay careful attention. The first concerns hole shape. Round holes are dimensionally conservative: a particle must be smaller than the hole in two axes to pass. Slotted or elongated holes let long thin particles through lengthwise, which is exactly what you want when the goal is to remove filaments and exactly what you must avoid when filaments are the product. For PP woven flake, using a round-hole bottom deck instead of a slotted one is the difference between keeping and losing several percent of saleable material every shift.
The second refinement concerns open area. Perforated plate with 12 mm holes on a 16 mm pitch gives roughly 50 percent open area; the same 12 mm holes on a 20 mm pitch give barely 32 percent. Two plates with identical nominal aperture can therefore differ by more than half in effective capacity. When comparing quotations, aperture alone tells you nothing about throughput — open area percentage is the number that matters, and it should be stated explicitly in the technical annex of the purchase contract.
The third refinement concerns the food-grade route. A narrower size distribution matters more than a large size window when the flake is destined for food-contact recycled PET, because the decontamination step that follows — whether solid-state polycondensation or a vacuum-assisted extrusion route — depends on predictable diffusion path lengths. Flake that varies from 2 mm to 20 mm cannot be decontaminated uniformly at a single residence time. This is why the food-grade line in the table above uses a tighter window than the general-purpose line, deliberately sacrificing yield for consistency.
Screen Media Engineering
The screen surface is a consumable, and it is the component that determines both separation quality and operating cost. Four media families dominate flake screening, and each represents a different position on the trade-off between open area, wear resistance and blinding resistance. No medium wins on all three, so the choice is always a considered compromise driven by the specific duty.
Woven wire mesh in 304 stainless offers the highest open area of any medium, often 55 to 70 percent, which makes it the most productive choice per square meter. It is also the most fragile: wires work-harden under vibration, and a single oversize tramp object can tear a panel that then bleeds oversize material into the product for hours before anyone notices. Upgrading to 316L improves corrosion resistance in caustic hot-wash environments and in lines using recycled process water with elevated chloride, at a moderate cost premium and no loss of open area.
Perforated plate trades open area for robustness. A 2 mm stainless plate with round holes gives 30 to 50 percent open area but tolerates impact, resists tearing, holds its aperture dimension precisely over its whole service life, and can be re-tensioned or even locally repaired. For post-dryer decks handling flake that has already been through several impact-heavy stages, perforated plate is almost always the right answer.
Polyurethane screen panels change the physics entirely. The aperture walls flex under vibration, so particles that would wedge in a rigid aperture are ejected rather than trapped. Open area is the lowest of the four families, typically 25 to 40 percent, because the polyurethane webs between apertures must be thick enough to carry load. In exchange, service life against abrasive material can be several times that of stainless mesh, and blinding resistance is excellent. For PVC regrind, for glass-contaminated streams and for any duty where blinding is the dominant failure mode, polyurethane panels justify their higher purchase cost within one or two mesh-change cycles.
Wedge wire, sometimes called profile bar or slot screen, consists of V-profile bars welded to support rods, forming slots that widen downward. The widening geometry is the key feature: a particle that enters the slot cannot wedge, because the passage grows behind it. Wedge wire is therefore the standard for wet dewatering duties where fines and water must pass while flake continues, and it is nearly maintenance-free. Its limitation is that slots are directional, so it should not be used where elongated particles must be retained.
| Screen medium | Wear resistance | Blinding resistance | Open area | Typical service life (hours) | Cost level | Best-suited duty |
|---|---|---|---|---|---|---|
| 304 stainless woven wire mesh | Low-Medium | Low | 55–70 percent | 800–2000 | Low | Dry, non-abrasive fines cut where throughput per square meter is critical |
| 316L stainless woven wire mesh | Medium | Low-Medium | 55–70 percent | 1200–2800 | Medium | Hot caustic wash sections, high-chloride recycled process water |
| Perforated plate (stainless, 1.5–3 mm) | Medium-High | Medium | 30–50 percent | 3000–8000 | Medium | Post-dryer decks, impact-loaded scalping, dimensional stability |
| Polyurethane screen panel | High | High | 25–40 percent | 6000–15000 | High | Abrasive PVC regrind, glass-contaminated feed, chronic blinding duties |
| Wedge wire / profile bar screen | Very High | Very High | 20–35 percent | 10000–25000 | Premium | Wet dewatering, grit removal, static scalping under water spray |
| Rubber screen panel | High | High | 25–35 percent | 5000–12000 | High | Noise-sensitive installations with coarse abrasive feed |
Service life figures in this table assume moderate abrasivity, which is the usual case for post-consumer packaging flake after washing. Streams that carry residual sand, glass fragments or mineral filler shorten every figure substantially — a 304 mesh that lasts 1800 hours on clean PET flake may last 400 hours on unwashed post-consumer film containing field soil. This is the strongest argument for placing an effective grit removal stage upstream: the sand you remove early is sand that does not consume screen media, pump impellers and dryer rotors for the rest of the line’s life.
A practical note on tensioning. Woven mesh must be tensioned correctly and re-tensioned after the first eight to twenty-four hours of operation, because the weave settles. Under-tensioned mesh flaps, which destroys separation accuracy, accelerates wire fatigue and generates noise; over-tensioned mesh tears at the clamp rail. Perforated plate and polyurethane panels avoid this class of problem entirely, which is one reason operations teams with limited maintenance staffing often prefer them despite the lower open area.
The Blinding Problem and Anti-Blinding Design
Blinding — the progressive plugging of screen apertures — is the dominant failure mode in flake screening, and it is the single most common reason a screen that passed factory acceptance underperforms in the field. A blinded deck does not announce itself with an alarm. Capacity falls quietly, oversize carries over into the product, and the operator responds by increasing feed rate, which makes the situation worse.
Three mechanisms cause blinding in plastic recycling, and they need different countermeasures. Wet adhesion occurs when surface moisture creates capillary bridges between flake and mesh. It is the reason a screen that runs perfectly on dry regrind blinds within twenty minutes on the same material at 5 percent surface moisture. Electrostatic attraction is the specialty of dry PE and PP film flake, which accumulates charge during pneumatic conveying and then clings to a grounded steel mesh with surprising tenacity. Fiber entanglement is the mechanism behind PP woven and textile-contaminated streams, where filaments hook through apertures, catch on each other and build a mat that no amount of vibration will clear.
The countermeasures form a hierarchy. Bouncing balls in a sub-deck compartment are the cheapest and most widely used: rubber or silicone balls trapped between the screen and a coarse retaining grid are thrown upward by the vibration and strike the underside of the mesh, dislodging wedged particles. They work well on wet adhesion and moderately on static, and they add almost nothing to the machine cost. Ultrasonic deblinding applies high-frequency mechanical energy directly to the mesh through a transducer and resonator ring, which is extremely effective on fine apertures below 1 mm where balls cannot reach, though it is rarely needed above 2 mm. Knocker or rapping devices strike the deck frame periodically; they are crude but effective on fiber mats. Polyurethane panels provide inherent anti-blinding through aperture wall flexure, as discussed above. Increasing deck inclination from a nominal 10 degrees to 15 to 25 degrees increases the tangential velocity component, which sweeps the surface clean at the cost of reduced screening efficiency, because particles spend less time over each aperture. Reducing bed depth — by widening the deck or splitting the feed — attacks the root cause, since a thick bed prevents particles from reaching the mesh at the right angle and presses blinded particles more firmly into apertures.
| Blinding cause | Typical stream | Recommended countermeasure | Effectiveness | Cost impact | Side effect to manage |
|---|---|---|---|---|---|
| Wet adhesion (capillary bridging) | Post-friction-washer flake, 4–10 percent surface moisture | Bouncing ball deck plus water spray bars, or move duty to wet screening | High | Low | Spray water must be recovered and filtered |
| Electrostatic attraction | Dry PE and PP film flake after pneumatic conveying | Static eliminator bar at feed point, humidity control, polyurethane panels | Medium-High | Low-Medium | Ionizer bars require periodic cleaning |
| Fiber and filament entanglement | PP woven, textile-contaminated post-consumer streams | Round-hole plate instead of mesh, knocker device, steeper inclination | Medium | Low | Steeper deck lowers separation efficiency |
| Near-size particle wedging | Any stream with a peak close to the aperture dimension | Polyurethane panel with flexing aperture walls, or shift aperture away from the size peak | High | Medium-High | Reduced open area lowers rated capacity |
| Sticky residue (adhesive, sugar, oil) | Label adhesive, beverage residue, lubricant-contaminated scrap | Improve hot caustic wash upstream; add mesh wash-down cycle between shifts | Medium | Low | Requires scheduled downtime for wash-down |
| Excessive bed depth | Any deck fed above rated tonnage or with poor feed spreading | Feed spreader plate, wider deck, split feed across two machines | Very High | Medium | Additional footprint or a second machine |
| Fine aperture plugging below 1 mm | Dust classification and micro-fines cuts | Ultrasonic deblinding system on the deck | Very High | High | Transducer and generator require spare stocking |
In practice the most cost-effective anti-blinding investment is rarely a device. It is feed control. A screen fed intermittently by a surge-prone conveyor will alternate between a starved deck and an overloaded one, and the overloaded intervals do the damage. Installing a small buffer hopper with a variable-speed feeder ahead of the screen, so the deck sees a steady, evenly spread bed at 70 to 85 percent of rated capacity, typically halves blinding incidents without touching the screen itself. Polyretec configures washing line screening stations with this control philosophy by default, because it costs less than the bouncing ball decks it makes unnecessary.
Vibration Parameters That Actually Matter
A vibrating screen is a tuned mechanical system, and four parameters determine whether it separates or merely shakes: amplitude, frequency, vibration direction angle and deck inclination. Suppliers rarely publish all four, and buyers rarely ask, which is why two machines with identical screen area and motor power can perform completely differently on the same feed.
Amplitude is the peak-to-peak displacement of the deck, typically 3 to 8 mm in flake screening. Large amplitude throws material high, breaks up the bed, and clears apertures — which is what wet, sticky or matting material needs. Small amplitude keeps particles closer to the surface, giving each one more presentation opportunities per meter of travel, which is what fine dry powder classification needs. Amplitude is set by the eccentric weight setting on the vibration motors and can be adjusted on site, usually in 10 percent increments of the available weight range.
Frequency is the rotational speed of the exciters, generally 900 to 1500 rpm for flake duties. Frequency and amplitude are coupled through the acceleration they produce, so they should be tuned as a pair rather than independently. High frequency with low amplitude suits fine, dry, free-flowing material; low frequency with high amplitude suits coarse, wet, cohesive material. Running a wet PET flake deck at 1500 rpm and 3 mm amplitude, a setting borrowed from a mineral fines application, is a reliable recipe for a blinded screen.
Vibration direction angle applies to linear machines and describes the angle between the throw line and the deck surface, normally 40 to 50 degrees. A shallower angle conveys material faster and reduces residence time; a steeper angle lifts more and conveys less. When a linear screen discharges product too quickly to separate properly, the fix is often to steepen the direction angle by repositioning the motor mounting rather than to buy a longer machine.
Deck inclination ranges from horizontal to 25 degrees. Horizontal decks maximize residence time and separation sharpness; inclined decks maximize throughput and self-cleaning. Wet duties and blinding-prone materials benefit from 15 to 25 degrees; high-precision fines cuts on dry material prefer 0 to 10 degrees.
These parameters combine into the throw index, sometimes called the vibration intensity index, expressed as Kv = Aω2 sinθ / (g cosα), where A is single amplitude, ω is angular frequency in radians per second, θ is the vibration direction angle, α is the deck inclination and g is gravitational acceleration. Kv describes how vigorously material is thrown clear of the deck between impacts. Below roughly 3.0 the bed slides rather than hops and blinding becomes likely; above roughly 5.0 particles fly so far that they skip apertures and separation efficiency collapses while wear accelerates. The practical working window for plastic flake is 3.3 to 4.5, with the lower half of that range for precision fines cuts and the upper half for wet or matting material.
| Material condition | Amplitude (mm) | Frequency (rpm) | Direction angle | Deck inclination | Target Kv | Engineering rationale |
|---|---|---|---|---|---|---|
| Wet PET flake, 6–10 percent surface moisture | 6–8 | 900–1100 | 45–50 degrees | 15–20 degrees | 4.0–4.5 | High throw breaks capillary bridges and clears apertures |
| Dry PET flake, final safety screening | 3–4 | 1300–1500 | 40–45 degrees | 0–5 degrees | 3.3–3.8 | Long residence time and gentle handling limit fines generation |
| PE film flake, limp and matting | 7–8 | 950–1150 | 45–50 degrees | 18–25 degrees | 4.2–4.8 | Aggressive lift prevents mat formation on the deck |
| PP woven filament | 5–7 | 1000–1250 | 45 degrees | 10–18 degrees | 3.8–4.4 | Tumbling re-orientation gives filaments repeated aperture presentation |
| Dry fines and dust classification below 1.5 mm | 2–3 | 1400–1500 | 40 degrees | 0 degrees | 3.0–3.5 | Low throw keeps fine particles in contact with the mesh |
| Abrasive PVC regrind | 4–6 | 1000–1200 | 40–45 degrees | 12–18 degrees | 3.6–4.2 | Moderate intensity limits media wear while staying self-cleaning |
| Post-dryer agglomerated lumps | 6–8 | 950–1200 | 45–50 degrees | 15–20 degrees | 4.0–4.6 | High energy breaks soft clumps as well as classifying them |
Commissioning discipline matters as much as the numbers. Amplitude should be verified with a vibration amplitude card at four corners of each deck, not assumed from the motor nameplate; a difference of more than 10 percent between corners indicates a spring, mounting or weight-synchronization problem that will eventually crack the deck frame. Motor bolt torque must be re-checked after the first eight hours and then weekly, because vibration motors loosen their own mounting bolts more reliably than any other component on a washing line.
Capacity Sizing Formula
Screening area is calculated, not guessed, and the arithmetic is simple enough to do during a specification meeting. The working relationship is A = Q / (q × k1 × k2 × k3), where A is the required screening area in square meters, Q is the feed rate in kilograms per hour, q is the base unit capacity in kilograms per hour per square meter for the given aperture, and k1, k2 and k3 are correction factors for aperture size, moisture content and flake shape respectively.
The base unit capacity q depends chiefly on aperture and bulk density. For plastic flake with a bulk density in the 250 to 400 kg per cubic meter range — which covers most washed rigid flake — a working value of 200 to 320 kg per hour per square meter applies at apertures around 3 mm, rising to 600 to 900 at apertures around 20 mm, and falling to 80 to 140 at apertures near 1 mm. These figures assume a properly spread bed at the recommended Kv and a screen operating below 85 percent of its hydraulic limit.
The correction factors capture what makes real material differ from the ideal. k1, the aperture size factor, ranges from about 0.7 for very fine cuts below 2 mm to about 1.3 for coarse scalping above 25 mm, reflecting the fact that fine apertures pass material far less readily than their open area alone suggests. k2, the moisture factor, is 1.0 for dry material below 1 percent surface moisture, 0.85 at 3 to 5 percent, 0.7 at 6 to 10 percent, and as low as 0.5 for material discharged directly from a float tank without dewatering. k3, the flake shape factor, is 1.0 for equidimensional rigid flake, 0.8 for curved or dished flake such as bottle shoulder fragments, 0.6 for limp film flake, and 0.45 to 0.55 for elongated woven filament.
A worked example makes the arithmetic concrete. A line processes 1200 kg/h of hot-washed PET flake at 7 percent surface moisture, requiring a 3 mm fines cut. Base capacity q at 3 mm is taken as 280 kg per hour per square meter. The aperture factor k1 at 3 mm is 0.85; the moisture factor k2 at 7 percent is 0.7; the shape factor k3 for curved bottle flake is 0.8. The denominator becomes 280 × 0.85 × 0.7 × 0.8 = 133 kg per hour per square meter, so A = 1200 / 133 = 9.0 square meters. That is a substantial deck — roughly 1500 mm wide by 6000 mm long, or more realistically two decks of 1500 by 3000 mm operating in parallel, or a single machine with the fines duty split across a lower deck fed by a pre-classified stream. The instructive part of this example is how far the correction factors move the answer: ignoring moisture and shape would have suggested 5.0 square meters, and the resulting machine would have blinded within an hour of the first wet shift.
| Line capacity (kg/h) | Recommended deck size (mm × mm) | Effective screen area (m²) | Number of decks | Motor power (kW) | Typical duty |
|---|---|---|---|---|---|
| 500 | 1000 × 3000 | 3.0 | 2 | 2 × 0.75 | Scalping plus fines cut for rigid flake |
| 800 | 1200 × 3500 | 4.2 | 2 | 2 × 1.1 | Post-dryer classification, PET or HDPE |
| 1000 | 1200 × 4000 | 4.8 | 2 | 2 × 1.5 | Post-dryer classification with fines removal |
| 1500 | 1500 × 4500 | 6.75 | 2–3 | 2 × 2.2 | Combined scalping, product cut and fines cut |
| 2000 | 1800 × 5000 | 9.0 | 3 | 2 × 3.0 | High-tonnage bottle flake line |
| 3000 | 2000 × 6000, or two parallel 1500 × 4500 | 12.0 | 3 | 2 × 3.7 | Large bottle-to-flake plant |
| 4000–6000 | Two parallel 1800 × 6000 machines | 21.6 combined | 3 each | 4 × 3.7 | Industrial-scale PET flake production |
Two sizing habits protect a project. First, size for the wettest and most awkward material the line will realistically see, not for the clean sample used in the pre-sale trial; the correction factors above are precisely where optimism becomes expensive. Second, leave 15 to 20 percent area margin. Screen area is comparatively cheap at purchase and impossible to add later without rebuilding the platform, whereas the cost of an undersized deck is paid every shift for the life of the plant.
Fines and Dust Removal
Fines are the contaminant that recyclers most often tolerate and most often regret. Because they are the same polymer as the product, they look harmless in a purity assay that only counts foreign material. Their damage appears downstream, in the extruder and in the finished pellet.
The mechanisms are well understood. Fines have an enormous surface-to-volume ratio, so they carry disproportionate surface moisture into the feed throat, which flashes to steam in the barrel and produces bubbles, voids and surface defects in the pellet. That same surface area accelerates thermo-oxidative degradation, lowering intrinsic viscosity in PET and raising melt flow rate in polyolefins, which shows up as reduced mechanical properties in the finished article. Fines also carry the highest share of inorganic contamination — abraded label paper, mineral filler, soil residue — so they drive up ash content, the parameter most likely to fail a fiber-grade specification. Finally, fines blind melt filters. A stream carrying 2 percent fines can shorten screen changer intervals from eight hours to under one, and each change costs melt, energy and, on non-continuous changers, line stoppage.
Effective fines removal uses two complementary mechanisms rather than one. Screening removes fines by size, using a bottom deck at the cut point established in Table 4. Air classification removes them by aerodynamic behavior, which catches the flat, low-mass particles that pass a size test but behave like dust. The combination reaches removal efficiencies of 92 to 97 percent, whereas either alone typically plateaus around 80 to 88 percent. A zigzag column placed after the post-dryer screen, with its cyclone recovering the light fraction and a bag filter polishing the exhaust, is the standard configuration.
Air volume estimation is straightforward. For a zigzag classifier handling washed rigid flake, an air demand of 2500 to 4000 cubic meters per hour per ton per hour of throughput covers most cases, with the upper end applying to film flake where the terminal velocity difference between product and contaminant is small. Cyclone inlet velocity should sit between 15 and 20 meters per second for efficient collection, and the bag filter should be sized for an air-to-cloth ratio of no more than 1.5 to 2.0 cubic meters per minute per square meter to keep pressure drop and bag wear reasonable. Under-sizing the dust extraction is a false economy that reappears as an unhealthy working environment and as fugitive dust settling on every surface in the building.
| Throughput (kg/h) | Screen fines cut | Zigzag classifier air volume (m³/h) | Cyclone diameter (mm) | Fan power (kW) | Expected residual fines |
|---|---|---|---|---|---|
| 500 | 2.0–2.5 mm bottom deck | 1500–2000 | 500 | 4.0 | Below 0.6 percent |
| 1000 | 2.0–3.0 mm bottom deck | 3000–4000 | 630 | 7.5 | Below 0.5 percent |
| 1500 | 2.5–3.0 mm bottom deck | 4500–6000 | 800 | 11 | Below 0.4 percent |
| 2000 | 3.0 mm bottom deck plus centrifugal sieve | 6000–8000 | 900 | 15 | Below 0.3 percent |
| 3000 | 3.0 mm bottom deck plus centrifugal sieve | 9000–12000 | 1100 | 22 | Below 0.3 percent |
| 4000–6000 | Dual-stage screening plus centrifugal sieve | 12000–20000 | Two units, 1100 each | 2 × 18.5 | Below 0.25 percent |
One caution about centrifugal sieves in fines duty: they remove fines efficiently and they also create them. Rotating paddles impact flake at tip speeds of 15 to 25 meters per second, and brittle material — degraded post-consumer PET, filled PP, weathered agricultural film — fractures under that impact. When the incoming stream is already brittle, reduce paddle tip speed to the lower end of the range, increase basket clearance, and verify by sieve analysis that the machine is subtracting more fines than it generates. On a genuinely brittle stream, a gentler linear deck with an air classifier will outperform a centrifugal sieve on net fines content even though its nameplate capacity is lower.
Label and Film Removal
Label material is the defining purity problem of PET bottle recycling. A polypropylene or polyethylene sleeve label is the same nominal size as the flake it accompanies after crushing, so no screen aperture can separate them. It is a different polymer, so it melts at a different temperature, forms gels, causes haze and destroys the mechanical properties of a fiber or sheet product. And it arrives in enormous quantity: a typical post-consumer bottle bale carries 3 to 8 percent label material by weight, sometimes more where full-body shrink sleeves dominate the local market.
Three physical differences make separation possible. Labels are thinner, so their area-to-mass ratio is far higher, which is what an air classifier exploits. Labels are less dense — PP at roughly 0.90 g/cm³ and PE at 0.92 to 0.96 g/cm³ against PET at 1.38 g/cm³ — which is what a float-sink tank exploits. And label material accepts and holds electrostatic charge differently from PET, which is what electrostatic separation exploits.
A practical purification sequence layers these mechanisms. Dry pre-washing with a label remover strips the majority of loose sleeve labels from whole bottles before crushing, typically removing 60 to 85 percent of label mass at very low energy cost. What remains, now cut into flake, goes through hot caustic friction washing, which detaches adhesive-bonded paper labels and softens residual glue. The float-sink tank then removes the bulk of the remaining polyolefin label flake by density. Finally, a zigzag air classifier on the dried flake catches the thin fragments that survived, because a 0.04 mm sleeve fragment and a 0.35 mm bottle wall flake have very different terminal velocities even when both measure 10 mm across. Where the target is food-contact recycled PET, an electrostatic stage or optical inspection stage can be added to reach the lowest residual levels.
| Incoming label content | Bottle stream type | Required process combination | Achievable residual polyolefin | Suitable end market |
|---|---|---|---|---|
| Below 2 percent | Pre-sorted deposit-return stream, labels largely removed | Dry label remover plus float tank | 150–400 ppm | Strapping, non-critical sheet |
| 2–4 percent | Sorted post-consumer bottles, partial sleeve labels | Dry label remover, hot wash, float tank, air classifier | 40–120 ppm | Fiber grade, monofilament |
| 4–6 percent | Mixed post-consumer with full-body sleeves | Dry label remover, hot caustic wash, float tank, two-stage air classification | 25–60 ppm | Fiber grade, sheet grade |
| 6–8 percent | Heavy sleeve market, mixed color | Full sequence plus electrostatic separation | 15–40 ppm | Sheet grade, bottle-to-bottle after decontamination |
| Above 8 percent | Unsorted mixed bales | Manual pre-sort plus full sequence plus optical inspection | 10–30 ppm | Food-grade route with approved decontamination process |
| Paper labels with adhesive | Regional streams using wet-glue paper labels | Hot caustic wash at 80–90 degrees Celsius plus friction washing plus fines screening | Cellulose below 50 ppm | All grades, subject to ash verification |
Air classifier tuning is where most of the achievable improvement lies, and it is a two-sided optimization. Raising air velocity lifts more label material, but it also lifts thin PET flake from bottle shoulders and base sections, so yield falls. The correct operating point is found empirically: run the classifier at increasing air velocity, sample the light fraction each time, and stop at the velocity where PET loss in the light fraction reaches the economic limit — typically 0.5 to 1.5 percent of throughput depending on the value differential between flake and reject. A recirculation loop that re-processes the light fraction through a second, gentler classification pass recovers much of that PET and is well worth its modest capital cost on lines above roughly 1000 kg/h.
Metal and Heavy Contaminant Removal
Metal contamination is the fastest route to catastrophic damage in a washing line and in the extrusion equipment that follows it. A single steel bolt entering a crusher can destroy rotor knives and screen; an aluminum neck ring surviving to the extruder will mark the melt filter and, in worst cases, score the screw. Metal removal therefore uses redundancy by design: multiple stages, multiple mechanisms, placed so that no single failure sends metal downstream.
Magnetic separation handles ferrous material. A permanent magnetic drum at the infeed conveyor discharge removes loose steel before the crusher; overband magnets serve the same duty on wider belts. Grate magnets or magnetic rod cartridges installed in flake chutes, particularly at the final screening station and immediately before extruder feeding, catch the fragments that escaped earlier stages. Neodymium magnets with a surface field strength of 10000 to 12000 gauss capture even weakly ferrous stainless fragments, which conventional ferrite magnets miss entirely.
Eddy current separation handles non-ferrous metal, principally aluminum from neck rings, closures and foil seals. A rapidly rotating magnetic rotor inside a non-conductive shell induces currents in conductive particles, generating a repulsive force that throws them forward over a splitter. Efficiency depends strongly on particle size — above 5 mm results are excellent, below 3 mm they degrade sharply — so an eddy current unit should be placed after screening, where the size distribution has been narrowed, rather than before it.
Gravity and density mechanisms complete the set. Stones, glass and ceramic fragments are denser than any common polymer and settle reliably in a float-sink tank or a hydrocyclone. Sand and grit leave through the bottom deck of a wet screening station and through the sludge discharge of the float tank. A dedicated stone trap at the wet crusher discharge, essentially a settling chamber with a scraper, is inexpensive insurance for streams collected from unpaved yards.
| Stage position | Equipment | Target contaminant | Typical removal efficiency | Notes on placement |
|---|---|---|---|---|
| Infeed conveyor, before crusher | Permanent magnetic drum or overband magnet | Loose steel, wire, fasteners | 95–99 percent of ferrous | Mandatory — protects rotor knives and crusher screen |
| Wet crusher discharge | Stone trap and settling chamber | Stones, glass, ceramic, heavy grit | 80–92 percent | Scraper discharge avoids manual cleaning downtime |
| After friction washing | Wet screening bottom deck | Sand, grit, mineral fines | 85–95 percent below the cut point | Spray bars essential to keep the deck open |
| Float-sink tank | Density separation | Residual glass, stones, metal fragments, PVC in PET | 90–97 percent for high-density contaminants | Water density control determines the separation sharpness |
| After final screening | Grate magnet or magnetic rod cartridge | Small ferrous fragments, work-hardened stainless | 92–98 percent | Second mandatory magnet stage; requires scheduled cleaning |
| Before packing or extruder feeding | Eddy current separator | Aluminum neck rings, foil, non-ferrous fragments | 85–95 percent above 5 mm | Place after screening so particle size is uniform |
| Extruder feed throat | Magnetic rod insert plus melt filtration | Final ferrous safety catch | Backstop stage | Melt filter is protection, not a separation stage |
The sequencing rule worth committing to memory is that magnetic separation belongs both before size reduction and after final screening. Before the crusher, it protects the machine; after the final screen, it protects the customer. Installations that fit only one magnet almost always fit the first and omit the second, which is exactly backwards from a product quality standpoint, since fragments generated by wear inside the line itself — knife chips, worn screen wire, bolt heads — enter the stream after the infeed magnet has already done its work.
Purity Targets by End Application
Screening specifications should be written backwards from the end market, because purity requirements vary by more than an order of magnitude between applications, and over-specifying is as costly as under-specifying. A strapping producer who pays for food-grade purity is buying quality that the product cannot monetize; a fiber producer who accepts strapping-grade flake will be rejecting spun packages for weeks.
Food-contact recycled PET sits at the top of the hierarchy. Beyond a foreign-polymer limit typically below 10 parts per million, the entire process route must be evaluated and approved under the relevant regional framework — an EFSA opinion in the European Union, an FDA no-objection letter in the United States — and traceability of input material becomes as important as the mechanical purity itself. Screening contributes to this by delivering a narrow, uniform size distribution that lets the decontamination step operate at a validated residence time, and by holding fines and ash within limits.
Fiber-grade recycled PET, the largest single outlet for washed bottle flake, tolerates around 50 parts per million of foreign polymer, but is unusually sensitive to two other parameters: moisture, because residual water hydrolyzes PET during spinning and collapses intrinsic viscosity, and ash, because inorganic particles break filaments at the spinneret. Both are screening-adjacent parameters, controlled through fines removal and grit screening rather than through polymer sorting.
Strapping grade is the most forgiving PET outlet, accepting up to roughly 200 parts per million of foreign polymer and higher color variation. Polyolefin grades follow their own logic: recycled HDPE for non-pressure pipe applications is judged mainly on melt flow rate consistency and gel count, recycled PP for injection molding on impact retention and odor, and recycled PE film grade on the absence of unmelted contaminants that cause film breaks.
| End application | Max foreign polymer (ppm) | Max metal (ppm) | Max fines (percent) | Moisture (percent) | Required screening stages |
|---|---|---|---|---|---|
| Food-grade recycled PET (bottle-to-bottle) | Below 10 | Below 5 | Below 0.2 | Below 0.5 before decontamination | Five stages plus air classification, electrostatic or optical inspection, approved decontamination process |
| Fiber-grade recycled PET | Below 50 | Below 15 | Below 0.5 | Below 0.8 | Four stages plus air classification and dual magnet positions |
| Sheet-grade recycled PET | Below 80 | Below 20 | Below 0.6 | Below 1.0 | Four stages plus air classification |
| Strapping-grade recycled PET | Below 200 | Below 40 | Below 1.0 | Below 1.2 | Three stages plus single air classification pass |
| Recycled HDPE pipe grade | Below 300 | Below 30 | Below 0.8 | Below 0.5 before extrusion | Three stages plus float separation and melt filtration |
| Recycled PP injection grade | Below 500 | Below 50 | Below 1.0 | Below 0.6 | Three stages plus float separation |
| Recycled PE film grade | Below 800 | Below 50 | Below 1.5 | Below 0.8 | Two to three stages plus fine melt filtration downstream |
Two practical observations about this table. First, the metal limits are tighter than most buyers expect, and they are the limits most often missed, because metal contamination is episodic rather than continuous — a stream can run clean for a week and then deliver one contaminated bale that fails an entire production lot. This is why redundant magnet positions matter more than incremental magnet strength. Second, the moisture column is a screening responsibility even though no screen measures moisture, because residual fines carry disproportionate surface water. Cutting fines from 1.2 percent to 0.4 percent typically lowers measured flake moisture by 0.1 to 0.2 percentage points at no additional drying energy, which is often the difference between meeting and missing a fiber-grade moisture clause.
Polyretec Flake Screening Equipment
Polyretec, a Wanplas factory, builds flake screening stations as engineered modules rather than catalog add-ons. Each screening station is configured around the flake geometry, moisture condition and purity target of the specific project, using the sizing method described earlier in this guide. The screening range covers washing lines from 500 kg/h through 6000 kg/h, matching the full capacity span of the Polyretec washing line program, and every unit is designated by nominal throughput so that line documentation stays readable on site.
Three screening families are built in-house. Linear vibrating screens, designated PLS, are the general-purpose classifiers, supplied as single, double or triple deck machines with twin vibration motors, adjustable eccentric weights and a bolted deck frame that allows screen media to be changed without dismantling the machine. Centrifugal sieves, designated PCS, are the high-throughput dry fines removers, with a horizontal basket, adjustable paddle clearance and a quick-release end cover for basket changes. Trommel screens, designated PTS, are the tolerant coarse classifiers for wet and film-rich streams, with a bolted drum segment construction so a worn section can be replaced without removing the whole drum.
Common engineering features run across all three families. Product-contact surfaces are stainless steel, because carbon steel in a wet caustic environment contaminates flake with iron oxide long before it fails structurally. Vibration motors are mounted on machined pads with locking hardware and marked torque values. Deck frames use a bolted modular design so that a customer can move from a two-deck to a three-deck configuration as the product mix changes. Screen media is supplied in interchangeable panel sizes so a plant can stock one panel inventory across several machines rather than a separate set for each.
| Model | Type | Screen area (m²) | Capacity (kg/h) | Layers | Aperture range (mm) | Motor power (kW) | Amplitude (mm) | Dimensions L × W × H (mm) |
|---|---|---|---|---|---|---|---|---|
| PLS-500 | Linear vibrating screen | 3.0 | 300–600 | 1–2 | 1.5–40 | 2 × 0.75 | 3–7 | 3400 × 1350 × 1550 |
| PLS-800 | Linear vibrating screen | 4.2 | 500–900 | 2 | 1.5–50 | 2 × 1.1 | 3–8 | 3900 × 1550 × 1650 |
| PLS-1000 | Linear vibrating screen | 4.8 | 700–1200 | 2 | 1.5–50 | 2 × 1.5 | 3–8 | 4400 × 1550 × 1700 |
| PLS-1500 | Linear vibrating screen | 6.75 | 1100–1800 | 2–3 | 1.5–60 | 2 × 2.2 | 4–8 | 4900 × 1850 × 1900 |
| PLS-2000 | Linear vibrating screen | 9.0 | 1600–2400 | 3 | 1.5–60 | 2 × 3.0 | 4–8 | 5400 × 2150 × 2050 |
| PLS-3000 | Linear vibrating screen | 12.0 | 2400–3600 | 3 | 1.5–60 | 2 × 3.7 | 4–8 | 6400 × 2350 × 2200 |
| PCS-800 | Centrifugal sieve | 1.6 basket area | 500–900 | 1 | 1.0–8 | 5.5 | Not applicable | 2300 × 1000 × 1450 |
| PCS-1500 | Centrifugal sieve | 2.6 basket area | 1000–1700 | 1 | 1.0–10 | 11 | Not applicable | 2900 × 1200 × 1600 |
| PCS-3000 | Centrifugal sieve | 4.4 basket area | 2200–3400 | 1 | 1.0–12 | 18.5 | Not applicable | 3600 × 1450 × 1800 |
| PTS-1000 | Trommel / rotary drum screen | Drum 800 × 3000 | 600–1200 | 1–2 zones | 5–60 | 3.0 | Not applicable | 3800 × 1300 × 1900 |
| PTS-2000 | Trommel / rotary drum screen | Drum 1000 × 4000 | 1300–2400 | 2 zones | 5–80 | 5.5 | Not applicable | 4900 × 1550 × 2150 |
| PTS-4000 | Trommel / rotary drum screen | Drum 1200 × 5000 | 2800–4500 | 2–3 zones | 5–80 | 7.5 | Not applicable | 6000 × 1800 × 2400 |
Capacity figures in this table are stated as ranges rather than single numbers, and the range is deliberate. The lower bound reflects wet, film-rich or filament-shaped material with correction factors near the pessimistic end; the upper bound reflects dry, equidimensional rigid flake. When Polyretec engineers size a screening station for a specific project, they apply the A = Q / (q × k1 × k2 × k3) calculation to the customer’s actual material, and where a sample is available they run a screening trial before confirming the model.
Configuration options that are frequently specified include bouncing ball anti-blinding decks, water spray bar assemblies for wet screening positions, static eliminator bars at the feed point for film streams, polyurethane panel decks for abrasive material, dust extraction hoods with flanged connections to a central system, and stainless steel enclosure covers for installations where fugitive dust control is a workplace requirement. Frequency converter control on the vibration motors, allowing operators to trim throw intensity from the line control panel rather than by changing eccentric weights, is available across the PLS range and is worth specifying on lines that process several different materials in campaign production.
Polyretec Washing Line Modules That Feed the Sieve
A screening station is only as good as the material presented to it. Half the screening problems reported from the field trace back not to the screen but to the machine immediately upstream: a crusher producing an uncontrolled size distribution, a friction washer leaving grit in the stream, a float tank overflowing fines onto the discharge screw, or a dryer discharging clumped flake at 8 percent moisture instead of 3. Specifying the sieve without specifying its feed condition is how projects end up with a correctly sized screen that cannot perform.
Polyretec builds the complete upstream chain, which allows the interface conditions between machines to be defined at the design stage rather than discovered during commissioning. The wet crusher sets the flake size distribution through rotor screen selection, and running it wet suppresses dust and reduces knife wear. The friction washer applies high-speed mechanical scrubbing with counter-current water to strip surface soil, adhesive residue and detached label pulp. The float-sink tank separates by density with paddle-assisted residence time control. The squeezing dryer performs mechanical dewatering by centrifugal action and a compression screw, typically bringing surface moisture from over 20 percent down to 1 to 3 percent depending on flake geometry. The air classifier removes what remains of the light fraction.
| Equipment | Capacity range (kg/h) | Power (kW) | Outlet flake condition | Interface to sieve |
|---|---|---|---|---|
| Wet crusher (rotor 600–1200 mm) | 500–6000 | 37–160 | Size set by rotor screen, 8–60 mm, surface moisture above 25 percent | Feeds coarse scalping deck or trommel; rotor screen choice defines the sieve top aperture |
| Friction washer (horizontal, high-speed) | 500–5000 | 15–55 | Soil and adhesive removed, grit liberated, moisture 15–25 percent | Feeds wet screening deck with spray bars for grit and fines removal |
| Float-sink tank (paddle type) | 500–6000 | 4–15 | Density-separated flake, saturated, mixed size distribution | Feeds wedge wire dewatering screen ahead of the dryer |
| Hot washer with caustic dosing | 500–4000 | 22–75 plus heating | Adhesive and oil removed, labels detached, elevated temperature | Requires 316L media on any screen deck downstream |
| Squeezing dryer (centrifugal plus compression screw) | 500–5000 | 22–90 | Surface moisture 1–3 percent, some agglomerated clumps | Feeds two-deck classification screen: clumps overhead, fines under |
| Thermal drying silo with hot air circulation | 500–4000 | 18–60 | Surface moisture below 1 percent, dry and static-prone | Feeds final safety screen; static eliminator recommended |
| Zigzag air classifier with cyclone | 300–4000 | 5.5–30 | Label film and dust removed, dry | Operates in series with the final screen, before packing or feeding |
The interface discipline that matters most is the crusher rotor screen. Because the crusher screen sets the upper tail of the size distribution, choosing a 12 mm rotor screen and then specifying a 30 mm top deck on the classification screen means the top deck does essentially nothing except add pressure drop and maintenance. Conversely a 20 mm rotor screen feeding a 14 mm top deck will reject a substantial fraction of good flake as oversize. These two apertures must be selected together, with the sieve top deck typically 1.4 to 1.8 times the crusher screen aperture for rigid flake.
Polyretec washing lines serve two principal application families. The food-grade PET bottle washing line, offered from 500 kg/h to 6000 kg/h and configured for different flake grades, produces flake for fiber spinning, sheet extrusion, strapping and, with an approved decontamination route, bottle-to-bottle applications. The PP and PE soft plastic crushing and washing line, offered from 500 kg/h to 1500 kg/h, handles post-consumer and post-industrial film, woven bags and agricultural film, feeding compounders, film producers and injection molders. Screening configurations differ substantially between these two families, which is precisely why the screening station should be quoted as part of the line rather than sourced separately.
Wet Screening versus Dry Screening
Whether to screen in the wet section or after drying is one of the few genuinely strategic choices in screening layout, and the honest answer is that a well-designed line does both, at different points, for different reasons.
Wet screening carries three decisive advantages. Water suppresses dust completely, which removes an entire category of workplace exposure and housekeeping cost. Water eliminates electrostatic charge, so film flake that would cling to a dry mesh flows freely. And water actively assists separation by washing fines through the aperture rather than relying on mechanical action alone, which is why a wet deck with spray bars can achieve a sharper fines cut than a dry deck of the same aperture. The disadvantages are equally clear: everything screened wet must subsequently be dried, the water must be recovered, filtered and recirculated, and the sludge fraction becomes a waste stream requiring its own handling.
Dry screening reverses the balance. Separation efficiency per square meter is higher because there is no water film binding particles together, and downstream drying load is unaffected. Dry screening is also the only realistic option immediately before extruder feeding, where any added moisture would defeat the purpose of the dryer. Against this stand dust generation, electrostatic blinding, and the noise level of a dry deck running at full throw.
| Criterion | Wet screening | Dry screening |
|---|---|---|
| Dust emission | Effectively zero | Significant; requires hoods and extraction |
| Electrostatic blinding | Eliminated by water film | Common with PE and PP film flake |
| Fines removal sharpness | High — water carries fines through the aperture | Medium-High with air assistance |
| Capacity per square meter | Lower, roughly 60–80 percent of dry | Higher baseline |
| Downstream drying load | Increased; dryer must be sized accordingly | Unaffected |
| Water treatment requirement | Filtration, sludge handling and recirculation needed | None |
| Screen media wear | Lower — water lubricates and cushions | Higher, especially with abrasive fines |
| Suitability before extruder feeding | Not suitable | The only option |
| Capital cost level | Medium — includes spray bars, launders, pumps | Low-Medium — includes hoods and extraction |
| Operating cost level | Medium — pumping and water treatment | Low-Medium — extraction fan and filter media |
The combination that works for most washing lines assigns coarse scalping and grit removal to the wet section, where water is already present and free, and assigns final classification, fines cut and safety screening to the dry section after the squeezing dryer. This layout removes the abrasive and the bulky contamination before it can wear expensive machinery, then performs the precision separations under conditions where precision is achievable. For film lines specifically, where electrostatic blinding on dry decks is chronic, shifting one additional classification step into the wet section usually pays for the extra dewatering capacity it demands.
Maintenance and Wear Management
Screening equipment is mechanically simple, which leads plants to under-maintain it until a failure forces attention. The maintenance burden is genuinely low, but it is not zero, and the three components that matter — screen media, springs and bearings, and motor mounting hardware — all fail in ways that degrade product quality before they cause a stoppage.
Screen media replacement intervals depend on media type, material abrasivity and running hours. The practical approach is condition-based rather than calendar-based: inspect at fixed intervals and replace on evidence. Signs that a panel has reached end of life include visible wire thinning, enlarged apertures at the feed end where impact is highest, localized dishing between support bars, and any tear regardless of size. An enlarged aperture is the insidious failure, because the machine keeps running and product keeps flowing while the separation cut point silently drifts coarser.
| Media type | Low abrasivity (clean washed rigid flake) | Medium abrasivity (post-consumer with residual grit) | High abrasivity (unwashed film, filled PVC, glass-contaminated) | Inspection interval |
|---|---|---|---|---|
| 304 stainless woven mesh | 1800–2400 hours | 900–1400 hours | 300–600 hours | Weekly |
| 316L stainless woven mesh | 2400–3200 hours | 1300–1900 hours | 500–900 hours | Weekly |
| Perforated plate 2 mm | 6000–8000 hours | 3500–5000 hours | 1800–3000 hours | Every two weeks |
| Polyurethane panel | 12000–15000 hours | 8000–11000 hours | 5000–7000 hours | Monthly |
| Wedge wire deck | 20000–25000 hours | 14000–18000 hours | 9000–12000 hours | Monthly |
| Centrifugal sieve basket | 5000–7000 hours | 3000–4500 hours | 1500–2500 hours | Weekly, with paddle clearance check |
Beyond media, four checks form the core of a screening maintenance routine. Tension on woven mesh must be verified at commissioning, again after the first shift, and weekly thereafter; loose mesh is audible as a slapping sound distinct from normal deck noise. Support rubbers and springs carry the entire dynamic load, and a cracked or sagging spring changes the machine’s motion characteristics before it fails outright — compare free heights across all four corners monthly, and replace springs as a matched set rather than individually. Vibration motor bearings are the highest-duty bearings in the plant, running under continuous alternating load; grease according to the manufacturer interval, monitor bearing temperature, and treat any rise of more than 15 degrees Celsius above the established baseline as a replacement trigger. Motor mounting bolts require torque verification after eight hours, after one week, and monthly thereafter, using marked reference lines on bolt heads so a shifted bolt is visible at a glance.
| Interval | Task | Acceptance criterion |
|---|---|---|
| Every shift | Visual check of feed distribution and discharge streams; listen for abnormal noise | Bed evenly spread across deck width; no flake in the fines discharge |
| Every shift | Check magnet surfaces and clean accumulated ferrous material | Magnet face clear; captured material logged |
| Daily | Inspect screen surface for blinding, tears and material build-up | Open area visibly clear across the full deck |
| Daily | Check spray bar nozzles on wet decks | All nozzles flowing; no clogged or misdirected jets |
| Weekly | Verify mesh tension; check bouncing ball condition and quantity | No slapping sound; balls intact and free-moving |
| Weekly | Verify vibration motor bolt torque against marked reference | No bolt rotation from the mark; torque at specification |
| Monthly | Measure amplitude at four deck corners with an amplitude card | Corner-to-corner variation within 10 percent |
| Monthly | Compare spring free heights; inspect for cracks and corrosion | Heights matched within tolerance; no visible cracking |
| Monthly | Sieve analysis on product and fines streams | Cut point within specification; flake carryover below the agreed limit |
| Quarterly | Bearing temperature trend review; regrease per manufacturer schedule | Temperature within 15 degrees Celsius of the established baseline |
| Annually | Deck frame weld inspection; structural fastener audit | No cracks at weld toes or mounting brackets |
A monthly sieve analysis deserves particular emphasis because it is the only check that measures what the machine is actually for. Take a representative sample of the product stream and the fines stream, run a laboratory sieve stack, and plot the cut point against the specification. A drifting cut point diagnoses media wear, tension loss or amplitude decay long before any of those causes becomes visible, and it converts screening from a machine that is assumed to work into a process that is known to work.
Requirement to Model Selection Guide
The following table translates the engineering logic of this guide into concrete configurations for the four scenarios that account for the majority of enquiries Polyretec receives. Each row assumes the material has already passed through the appropriate washing sequence and that the screening station is being sized for the condition at its own inlet, not at the line inlet.
| Customer scenario | Recommended Polyretec screening configuration | Aperture combination | Layers | Dust and air handling | Notes |
|---|---|---|---|---|---|
| 1000 kg/h PET bottle flake, food-grade route | PLS-1000 wet deck after friction washing, plus PLS-1000 dry classification after the dryer, plus PCS-800 final fines removal | Wet deck 25 mm over 3 mm; dry deck 15 mm over 3.5 mm; centrifugal basket 3 mm | 2 plus 2 plus 1 | Zigzag classifier at 3000–4000 m³/h with 630 mm cyclone, 7.5 kW fan | 316L media in the hot wash section; dual magnet positions; narrow size window for uniform decontamination |
| 800 kg/h PE film flake, post-consumer | PTS-1000 trommel for wet pre-classification, plus PLS-800 dry classification with polyurethane panels | Trommel 60 mm; dry deck 60 mm over 8 mm | 2 zones plus 2 | Zigzag classifier at 3000 m³/h; static eliminator bar at the feed point | Trommel avoids matting; high amplitude and 20 degree inclination on the dry deck |
| 1500 kg/h PP woven bag flake | PLS-1500 three-deck linear screen with round-hole perforated plate, plus knocker device | 50 mm over 20 mm over 5 mm round hole | 3 | Zigzag classifier at 4500–6000 m³/h with 800 mm cyclone, 11 kW fan | Round holes only on the bottom deck; slots would lose filament to the fines stream |
| 500 kg/h mixed rigid regrind, industrial scrap | PLS-500 two-deck linear screen, plus grate magnet at discharge | 25 mm over 2.5 mm | 2 | Zigzag classifier at 1500–2000 m³/h with 500 mm cyclone, 4.0 kW fan | Perforated plate for impact tolerance; frequency converter control for material campaigns |
| 2000 kg/h PET flake, fiber grade | PLS-2000 three-deck classification plus PCS-1500 centrifugal fines removal | 20 mm over 12 mm over 3 mm; basket 3 mm | 3 plus 1 | Zigzag classifier at 6000–8000 m³/h with 900 mm cyclone, 15 kW fan | Ash and moisture control are the binding constraints, not foreign polymer |
| 3000 kg/h PET flake, strapping and sheet | PLS-3000 three-deck classification plus PCS-3000 centrifugal fines removal | 22 mm over 14 mm over 3 mm; basket 3 mm | 3 plus 1 | Zigzag classifier at 9000–12000 m³/h with 1100 mm cyclone, 22 kW fan | Wider purity tolerance allows a simpler air handling package |
| 1200 kg/h abrasive PVC regrind | PLS-1500 two-deck linear screen with polyurethane panels throughout | 18 mm over 2.5 mm | 2 | Enclosed hoods with 3500 m³/h extraction | Polyurethane extends media life several-fold against a highly abrasive stream |
Where the material is unusual — heavily degraded post-consumer film, mixed multilayer packaging, agricultural film carrying field soil at double-digit percentages — the table should be treated as a starting hypothesis rather than an answer. In those cases a sample screening trial on the customer’s own material produces a defensible configuration in a way that no catalog can.
Integration with Downstream Pelletizing
Screening quality is measured most honestly not at the flake silo but at the extruder screen changer. Every gram of fines, every oversize survivor and every foreign fragment that leaves the screening station arrives at the melt filter, and the filter change frequency is a direct, quantitative readout of screening performance. Plants that track this number find it to be the most sensitive process indicator they have.
The relationships are consistent across installations. Fines content drives filter blinding rate more strongly than any other variable, because fines carry the inorganic load; halving fines from 1.0 percent to 0.5 percent commonly doubles the interval between screen changes. Oversize flake drives feed-throat bridging and inconsistent melt pressure, which shows up as pellet size variation and, in severe cases, surging. Residual metal drives filter element damage rather than blinding, and a single fragment can force an unscheduled change. Residual moisture, which screening influences through fines removal, drives hydrolytic degradation in PET and bubble formation in every polymer.
Screening also determines what degree of melt filtration is even feasible. A stream carrying 1.5 percent fines cannot economically be run through a 60 micron filter, because the filter will blind faster than a continuous changer can index. Cleaning that stream to 0.3 percent fines makes fine filtration practical, and fine filtration is what allows the pellet to meet gel-count specifications for film and fiber. In other words, the screening station upstream sets the ceiling on the pellet quality achievable downstream, no matter how good the extrusion equipment is.
For customers who need to move from washed flake to finished pellet, Wanplas supplies matched twin-screw pelletizing systems that integrate directly with Polyretec washing lines, including the feeding, degassing and melt filtration arrangements appropriate to recycled feedstock. Because both the washing line and the pelletizing system are engineered within the same group, interface parameters — flake bulk density, moisture at handover, fines specification, conveying method and buffer volume — are agreed at the design stage rather than negotiated between suppliers after installation. Where a customer also requires compounding, filler addition or property modification of the recycled material, the same route applies: the specification is written once, across the whole chain, with the screening cut points chosen to suit the pelletizing route rather than in isolation.
Common Problems and Fixes
The following table collects the screening faults reported most often from operating washing lines, with the causes that actually explain them and the corrections that resolve them. In nearly every case the fault is a symptom of a mismatch between the screen configuration and the material condition, not of a defective machine.
| Problem | Probable cause | Recommended fix |
|---|---|---|
| Screen blinding within minutes of start-up | Surface moisture above the deck’s tolerance; amplitude too low for the material; near-size particle wedging | Increase amplitude toward the upper range and lower frequency; add bouncing ball deck; add spray bars if in the wet section; consider polyurethane panels |
| Good flake carried over into the fines stream | Bottom aperture too large; slotted holes passing elongated flake lengthwise; deck inclination too steep so particles skip apertures | Reduce bottom aperture by one step; change slotted plate to round-hole plate; reduce inclination toward 10–15 degrees |
| Oversize material reaching the product stream | Torn or enlarged screen panel; loose tension allowing gaps at the clamp rail; top aperture set too large relative to crusher screen | Inspect and replace the panel; re-tension mesh; re-match top aperture to 1.4–1.8 times crusher rotor screen aperture |
| Screen tearing repeatedly at the feed end | Impact from tramp objects; unsupported span too long; under-tensioned mesh flexing against support bars | Fit an impact-resistant perforated plate section at the feed end; add support bars; verify tension; check upstream scalping |
| Uneven feed distribution across the deck width | Feed chute discharging to one side; no spreader plate; surging conveyor upstream | Install a spreader plate or distribution box; add a buffer hopper with variable-speed feeder; center the feed chute |
| Excessive dust in the building | Missing or under-sized extraction; hood gaps at feed and discharge; air-to-cloth ratio too high on the filter | Enclose feed and discharge points; increase extraction volume; verify air-to-cloth ratio below 2.0 cubic meters per minute per square meter |
| Low separation efficiency despite an open screen | Bed depth too great; throughput above rated capacity; residence time too short | Reduce feed rate to 70–85 percent of rating; widen deck or split feed; steepen vibration direction angle to increase residence time |
| Flake sticking to the mesh on a dry deck | Electrostatic charge from pneumatic conveying; very dry ambient conditions | Install a static eliminator bar at the feed point; consider a light humidity addition; switch to polyurethane panels |
| Deck frame cracking at weld toes | Amplitude imbalance between corners; resonance near the operating frequency; loose motor mounting | Re-synchronize motors and equalize weights; verify torque on all mounting bolts; check spring condition and replace as a set |
| Fines content rising with no process change | Screen media wear enlarging apertures; centrifugal sieve generating fines from brittle flake | Run a sieve analysis and replace worn media; reduce centrifugal paddle tip speed; verify the sieve is net-removing fines |
| Rising extruder screen changer frequency | Fines breakthrough; upstream grit removal degraded; magnet not cleaned on schedule | Verify fines cut with a sieve analysis; inspect wet screening deck and stone trap; reinstate magnet cleaning routine |
| Vibration motor bearing running hot | Grease interval exceeded; excessive eccentric weight setting; misalignment of the two motors | Regrease to schedule; reduce eccentric weight if amplitude exceeds specification; verify motor mounting flatness |
A diagnostic habit worth building: before changing anything mechanical, run a sieve analysis on the feed, the product and the fines. Three size distributions tell you immediately whether the problem is aperture selection, media condition, feed rate or upstream size reduction, and they prevent the common cycle in which a plant changes amplitude, then aperture, then media, without ever establishing what the machine was actually doing.
Service and Support
Screening equipment is bought once and lived with for a decade, so the support arrangement around it matters as much as the specification sheet. Polyretec, a Wanplas factory, has manufactured plastic recycling equipment since 2010 under a brand formalized in 2017, with more than one hundred project references, service coverage across more than fifty countries and a team of more than twenty-four engineers available for project support. The service package described below applies to screening stations supplied both as part of a complete washing line and as standalone modules retrofitted into existing plants.
Sample screening trial. Customers are encouraged to send a representative sample of their actual waste stream before the configuration is finalized. The material is run across test decks at several apertures and vibration settings, a sieve analysis report is produced for each configuration, and the resulting data becomes the technical basis of the quotation. This is the single most valuable pre-purchase step available, because it replaces assumption with measurement on exactly the material the machine will process. Representative sampling matters more than sample quantity — material from a single clean bale will produce an optimistic answer that the line cannot reproduce.
Factory testing before shipment. Every screening station is assembled, run and verified in the factory before packing. Amplitude is measured at all deck corners, motor synchronization is confirmed, bolt torque is recorded, and no-load running is performed to establish the vibration and bearing temperature baseline that the plant will later use as its maintenance reference. Where the customer has supplied material, a loaded trial is run and the results are documented in the test report that ships with the machine.
Installation and commissioning. Engineers support installation, alignment and start-up, either on site or through guided remote commissioning. Commissioning covers deck leveling, spring installation, amplitude verification, feed distribution adjustment and the first sieve analysis of production material, with parameters recorded in the machine log so the plant has a documented baseline for future troubleshooting.
Spare parts policy. A spare parts allowance of USD 500 free parts/year is provided as a Wanplas group commitment, covering routine consumables such as screen panels, bouncing balls, spray nozzles and fasteners. Beyond the allowance, screen media and wear components are held in standard panel sizes so replacements ship quickly and a plant can stock a single panel inventory across multiple machines.
Training. Operator and maintenance training covers screen media changing, tensioning technique, amplitude measurement, sieve analysis interpretation, blinding diagnosis and the inspection schedule set out earlier in this guide. Training the maintenance team to read a sieve analysis is consistently the highest-return element of the package, because it converts a reactive maintenance culture into a predictive one.
Remote support and factory visits. Remote diagnostic support is available for control, drive and process questions, with parameter review based on the machine log and photographs or video of the running deck. Customers are welcome to visit the factory to inspect equipment under construction, witness testing of their own machines and review the workmanship of welded frames, stainless fabrication and screen mounting before shipment.
Frequently Asked Questions
Can a flake sieve separate PVC from PET?
No. A sieve separates by size and, in the case of air classification, by aerodynamic behavior. PVC fragments in a PET stream are usually the same size as the product, so no aperture distinguishes them. Separation requires density-based methods such as float-sink or hydrocyclone treatment, electrostatic separation, or optical sorting. Screening is nevertheless valuable ahead of those processes, because a narrow, uniform size distribution substantially improves their efficiency.
How many screening stages does a PET bottle washing line need?
Three stages are the practical minimum for post-consumer bottle flake: a coarse scalping stage after the wet crusher, a wet fines and grit stage after friction washing, and a dry classification stage after the squeezing dryer. Fiber-grade and food-grade routes typically add a fourth stage as a final safety screen before packing or extruder feeding, together with air classification. Clean industrial scrap with consistent geometry can sometimes be handled in two stages.
What aperture should I use for PET bottle flake?
For general-purpose hot-washed PET flake, a top deck of 18 to 22 mm and a bottom deck of 2.0 to 2.5 mm suit a stream produced with a 12 mm crusher rotor screen. For a food-grade route requiring a narrow size distribution, tighten to a 14 to 16 mm top deck and a 3.0 to 4.0 mm bottom deck, accepting a small yield loss in exchange for the uniformity that decontamination processes require. Always match the top deck aperture to the crusher rotor screen, at roughly 1.4 to 1.8 times its opening.
Why does my screen blind even though the flake looks dry?
Two causes dominate when visible moisture is absent. The first is electrostatic charge, which builds during pneumatic conveying and makes film flake cling to grounded steel mesh; a static eliminator bar at the feed point usually resolves it. The second is near-size particle wedging, where a peak in the size distribution sits close to the aperture dimension; shifting the aperture away from that peak, or switching to polyurethane panels whose aperture walls flex, is the fix.
Is a centrifugal sieve better than a vibrating screen for fines removal?
It depends on the flake condition. A centrifugal sieve delivers higher throughput in a much smaller footprint and copes with slightly damp material, which makes it attractive for post-dryer duty on tough flake. However, its rotating paddles impact the flake and can generate fines from brittle material such as heavily degraded post-consumer PET. On brittle streams a gentler linear vibrating deck combined with air classification often achieves lower net fines content despite a lower nameplate capacity.
How much screening area do I need for 1000 kg/h?
Apply A = Q / (q × k1 × k2 × k3). For 1000 kg/h of PET flake at 7 percent surface moisture with a 3 mm fines cut, base capacity of about 280 kg per hour per square meter is corrected by an aperture factor of 0.85, a moisture factor of 0.7 and a shape factor of 0.8, giving roughly 7.5 square meters. Dry, equidimensional rigid flake at the same tonnage needs far less. Always size for the most difficult material the line will realistically see, and add 15 to 20 percent margin.
Should I screen wet or dry?
Both, at different points. Wet screening eliminates dust and static and delivers a sharper fines cut, so it suits coarse scalping and grit removal in the wash section where water is already present. Dry screening gives higher capacity per square meter and is the only option immediately before extruder feeding. The standard layout assigns bulky and abrasive contamination to wet stages and precision classification to dry stages after the squeezing dryer.
How often should screen media be replaced?
Replace on condition, not on calendar. As a planning guide, 304 stainless woven mesh lasts roughly 1800 to 2400 hours on clean washed flake and as little as 300 to 600 hours on abrasive material, while perforated plate lasts 3500 to 8000 hours and polyurethane panels 5000 to 15000 hours depending on abrasivity. Inspect weekly for wire thinning, enlarged apertures at the feed end and any tearing, and run a monthly sieve analysis to detect cut-point drift before it affects product grade.
What purity can I realistically reach with screening alone?
Screening alone controls size-related contamination and fines, typically bringing fines below 0.3 to 0.6 percent and removing 95 to 99 percent of out-of-window material. It cannot lower foreign-polymer content below what the density and optical stages deliver. Reaching fiber grade at below 50 parts per million of foreign polymer needs screening plus float separation plus air classification; reaching food-grade levels below 10 parts per million additionally requires an approved decontamination route and, usually, an electrostatic or optical inspection stage.
Can screening equipment be retrofitted into an existing washing line?
Yes, and it is one of the most cost-effective upgrades available to an underperforming line. The usual retrofit points are a two-deck classification screen after the squeezing dryer and a zigzag air classifier before packing, since these two additions address the fines and label-residue problems that most often cap product grade. The main site constraints are headroom, conveyor routing and dust extraction capacity, all of which should be surveyed before the configuration is fixed.
Conclusion
Choosing a flake sieve for a plastic washing line is an engineering decision with four linked parts, and skipping any one of them undermines the others. First, define what purification actually means for the target market — which contaminant classes matter, at what parts-per-million level, and which of them a screen can influence at all. Second, place screening at every point in the line where it earns its keep, rather than treating it as a single machine near the discharge. Third, match the screen family, aperture, media and vibration parameters to the real flake geometry and moisture condition, using calculation rather than analogy. Fourth, design against blinding from the outset, because blinding is the failure mode that quietly converts a correctly specified machine into an underperforming one.
The economics reinforce the engineering. Screening is a small share of washing line capital cost and a decisive share of product value. A line that delivers flake at fiber grade rather than strapping grade earns a materially different return on the same feedstock, and the difference is usually determined at the screening stations rather than in the wash tanks. Downstream, the same screening quality shows up as longer melt filter intervals, lower screw and barrel wear, more stable extrusion and fewer rejected lots.
Polyretec, a Wanplas factory, builds washing lines from 500 kg/h to 6000 kg/h for PET bottle streams and from 500 kg/h to 1500 kg/h for PP and PE soft plastics, with screening stations engineered to the specific waste stream rather than selected from a catalog. The PLS linear vibrating screens, PCS centrifugal sieves and PTS trommel screens described in this guide are configured together with the crusher, friction washer, float tank, dryer and air classification package, so the interface conditions that determine screening performance are fixed at the design stage. Behind that equipment stand more than a decade of recycling plant experience, over one hundred project references, service in more than fifty countries and a factory that welcomes inspection.
If you are specifying a new washing line or trying to lift the grade of an existing one, the most useful next step is a conversation grounded in your actual material. Send your throughput target, polymer type, contamination profile and the purity specification your customers demand, and our engineers will return a screening configuration with calculated screening area, aperture combination, media selection and dust handling package. Send a representative sample and we will run a screening trial and give you a measured sieve analysis report rather than assumptions. You are also welcome to visit the factory, see the equipment under construction and witness the testing of your own machine before it ships.
