Cleaning sieve — the quality control point in granulate transloading

Cleaning sieve in the granulate transloading line: it separates contaminants and agglomerates and acts as a quality control point. Types of sieves, magnetic separators, mesh selection and maintenance — a practitioner's guide.

Autor: Aleksy Pasternak 19 March 2026 Aktualizacja: 5 June 2026 11 min czytania

Cleaning sieve in the granulate transloading line — SMIALA terminal in Chorula

Definition

A cleaning sieve is a sieving screen placed in the gravity transloading line, which separates mechanical contaminants, agglomerates and foreign bodies from the granulate, while at the same time serving as a quality control point for the incoming batch.

It sounds like a simple piece of equipment — a bit of mesh in the loading hopper. In reality it is a device that distinguishes conscious transloading from the mere transfer of material from one package to another. Over more than thirty years working at the terminal I have learned that it is precisely on the sieve that the truth about the quality of the incoming batch most often comes to light. Granulate of the right size passes freely through it; what should not be in the material is held back. And what remains tells the operator more about the delivery than many a document.

In our method of transloading without pneumatic conveying the sieve is not an add-on — it is the heart of the process. It is through it that granulate from a big bag flows by gravity into a silo tanker, cleaned and checked in a single movement.

The sieve as a quality control point

The most important thing to understand about a cleaning sieve is that it performs two functions at once, and the second is often more important than the first.

The obvious function is the mechanical separation of contaminants. The sieve lets through the grain of the target granulate while holding back everything larger or non-matching: lumps, agglomerates formed by moisture, fragments of tape, film or the liner from packaging, splinters from pallets, particles of foreign plastic. The material that comes down from the sieve into the tank is uniform and free of foreign bodies — exactly what the processor expects.

The less obvious function, but for me the key one, is feedback about the batch quality. What remains on the sieve is a signal. An excess of lumps says the material was damp — perhaps the big bag stood in the open air or leaky packaging let in water. Fragments of film or tape mean damaged, breached packaging. Foreign pellets — coloured grains in a transparent batch, a different shape — are a red flag: a possible material identification error at an earlier stage or a mixing of grades. An operator who habitually looks at the reject on the sieve catches these signals before the material flows into the silo tanker and travels to the recipient.

That is why the sieve is a quality control point and not just a filter. A filter works quietly; a control point generates knowledge. Combined with transloading documentation and batch tracking it gives a full picture of what passed through the terminal and in what condition.

Types of sieves and separators

In terminal practice several solutions are encountered, chosen to suit the material, the required accuracy and the line throughput.

Static (gravity) sieves are the simplest form: a mesh or perforated plate in the chute line, through which the material passes under its own weight. They work well with materials of good, free flow, where the aim is mainly to catch foreign bodies and agglomerates larger than the mesh. They have no drive, so they are failure-free and quiet, but they sift out fine dust less well and clog faster with damp material.

Vibrating sieves (screens) have a vibratory or centrifugal drive that sets the mesh into vibration. The vibrations keep the openings clear, intensify the sieving and allow particles and dust (fines) to be separated, not just large foreign bodies. This is a more precise solution, used wherever fraction purity matters, but it requires a power supply, drive maintenance and control of the mesh condition.

Magnetic separators are a separate, indispensable category. A mechanical sieve alone will not hold back metal contaminants smaller than the mesh — swarf, fine rivets, fragments of wire or metal worn off tools. A permanent or neodymium magnet placed in the chute line attracts and holds ferromagnetic particles. For granulates of polyethylene (PE) or polypropylene directed to injection moulding machines this is critical protection: a particle of metal that passes through can damage the processor’s screw, nozzle or die and bring the production line to a standstill. The magnetic separator and the mechanical sieve complement each other — one catches what the other will not.

Solution	What it holds back	Strengths	Limitations
Static sieve	foreign bodies, large agglomerates	simple, quiet, failure-free	sifts dust less well, clogs with moisture
Vibrating sieve	agglomerates, particles, dust (fines)	precise, clear openings	requires a drive and maintenance
Magnetic separator	ferromagnetic metals	catches metal smaller than the mesh	only magnetic contaminants

In practice, at a professional terminal these solutions are combined: a sieve for the mechanics, a magnet for the metal. The choice of a specific configuration depends on what material and for which recipient we are handling.

Sieve mesh selection

Mesh selection is part of the know-how that cannot be reduced to a single table. The principle, however, is simple: the mesh must freely let through the grain of the target granulate while holding back what is larger than it — agglomerates and foreign bodies.

Too fine a mesh is a more common mistake than it seems. It slows the chute, lowers throughput, and with a material of wider size spread it begins to hold back correct grain — generating a false reject and losses. Too coarse a mesh, in turn, lets through what should be held back, so the sieve loses its point. The optimum lies in between and is specific to each material.

Standard polymer granulates have a grain on the order of a few millimetres — typical pellets are lenses or cylinders 2–5 mm in diameter. The mesh is chosen so that these grains pass with a margin, but so as to hold back lumps two or three times larger and any foreign bodies. With microgranulates, powders or recyclates of non-uniform size, the selection is harder and requires experience. That is why with an atypical material I always encourage you to ask for a quote and the parameters of transloading — we match the mesh to the specific batch, not the other way round.

The place of the sieve in the gravity process

The sieve makes sense only at a particular place in the chain. In gravity transloading the material from a big bag falls under its own weight through the loading hopper — and it is precisely on this path, before the inlet to the silo tanker, that the sieve is placed. The sequence looks like this:

Big bag above the hopper. The packaging is lifted and positioned above the loading hopper, the spout untied or cut open.
Gravity chute. The granulate flows down under its own weight, without compressed air and without acceleration in pipes.
Passing through the sieve and separator. The material passes through the cleaning sieve and the magnetic separator. The cleaned grain flows on, the reject stays on the sieve.
Loading the silo tanker. Clean, checked granulate reaches the tank — ready for bulk delivery to the recipient’s silo.

The key thing is that the sieve works in gravity flow, not under pressure. The material is not pushed through it by air — it passes calmly, without additional grain abrasion. Thanks to this the sieve cleans the material without worsening its parameters, which cannot be said of intense sieving in a pneumatic installation.

The link with contamination and transloading without pneumatic conveying

The cleaning sieve is best understood in the wider context of the fight against granulate contamination. Polyolefin producers place an absolute requirement on their logistics partners: the material must reach the processor intact and free of contaminants. Every foreign grain, every fragment of film, every particle of metal is a potential complaint and a stoppage of the production line.

The sieve is one of two barriers that fulfil this requirement. The first barrier is the gravity method itself: giving up pneumatics eliminates grain abrasion, the formation of angel hair and the introduction of moisture, and the short, controlled path of the material minimises the risk of grade mixing. The second barrier is the sieve: it catches what, despite everything, ended up in the batch — contaminants from packaging, transport or an earlier stage.

These two barriers work together. Pneumatics not only carries the risk of abrasion — it sometimes is itself a source of contamination, because residue of a previous material lingers in the installation. A gravity chute through a sieve reverses this logic: it does not add contaminants, it subtracts them. That is why for granulates of PE, PP, PVC and PET, where purity is sacred, the combination of the non-pneumatic method with a proper sieve and magnetic separator is the standard, not a luxury. A functioning sieve also has an environmental dimension: by holding back the reject instead of scattering it around the workstation, it limits dusting and the escape of pellets beyond the line.

Sieve selection and maintenance

A sieve is worth as much as its technical condition. The best mesh, torn or clogged, stops protecting the material, and a false sense of security is sometimes more dangerous than no sieve at all. That is why the maintenance routine is as important as the choice of the device itself.

In daily practice we check:

Cleanliness of the mesh and clearness of the openings. Accumulated reject and clogged openings slow the chute and reduce sieving effectiveness. The sieve is cleaned regularly, and with intense work — daily.
Condition of the mesh. Tears, wear and abraded wires mean that foreign bodies pass through. Damaged mesh is replaced without delay, because a hole in the sieve is a hole in quality control.
Strength of the separator magnets. Permanent magnets lose part of their strength over time, and the metal collected on them must be periodically removed so that the separator remains effective.
Cleaning at material changeover. The most important moment. When switching to a different grade or colour, the sieve, hopper and separator are cleaned thoroughly to avoid batch mixing. It is the same discipline that stands behind batch traceability tracking and the line cleanliness document.

The choice of a sieve is a one-off decision, maintenance — a daily one. At the intensity of work of our terminal in Chorula, control of the condition of the sieves and separators is a permanent element of the procedure, not an exception — the sieve is checked just as routinely as the cleanliness of the hopper or tank. Because gentle transloading without a functioning sieve is only half the method — and half-hearted quality control in working with granulates is not enough. The full bulk material transloading offer is described by the PHS Magnum portal.

What the reject on the sieve tells you about delivery quality

From the perspective of over thirty years of work I can say that the habit of looking at the reject on the sieve has saved us from complaints more times than I can count. The sieve is the most honest witness to the state of a delivery — it shows what no transport document will reveal. That is why it is worth knowing how to read what remains on it.

A clean reject, in small quantities is the picture of a batch in good condition: single splinters from a pallet, trace lumps, an occasional fragment of tape. This is the norm — the sieve catches what would not significantly affect quality anyway, and the operator notes that the delivery is fine.

An excess of lumps and agglomerates is a warning of dampening. Hygroscopic or poorly stored granulates absorb moisture and cake. If the sieve holds back a lot of lumps, it is a sign that the material was exposed to water — and a signal to verify the storage conditions at the supplier before the same problem recurs with the next batch.

Fragments of film, tape, string speak of breached packaging. A damaged big bag is not only a risk of spillage but also an open door for moisture and contaminants from the surroundings. Such a batch requires a more careful assessment before it is admitted to the chute.

Foreign grains — a different colour, shape, gloss — are the most serious signal: a possible mixing of grades or a material identification error. Detecting them on the sieve, and not in the processor’s silo, is the difference between a minor correction and a costly complaint with a production stoppage. That is precisely why the sieve and the batch documentation must work together — one detects, the other allows reconstruction of where the problem came from.

Each of these signals is information that feeds back to the supplier and to the process. The sieve does not only clean the current batch — it helps improve the whole chain, because it shows where quality is breaking down before the breakage becomes entrenched.

Sources

Industry guidelines on granulate cleanliness (Operation Clean Sweep).
Technical materials on sieving and magnetic separation in plastics transloading lines.
Operational practice of the SMIALA terminal, Chorula — Aleksy Pasternak.

Najczęstsze pytania (FAQ)

What is a cleaning sieve in granulate transloading?

It is a sieving screen placed in the line of the gravity chute of material from a big bag to a silo tanker. Granulate of the right size passes through the mesh, while the sieve holds back what should not be in the material: lumps, agglomerates, fragments of packaging and foreign bodies. At the same time it serves as a quality control point for the incoming batch.

What contaminants does a cleaning sieve catch?

Above all foreign bodies (pieces of plastic of another grade, particles, splinters, fragments of tape or film from packaging), lumps and agglomerates formed by moisture, and any elements larger than the sieve mesh. Combined with a magnetic separator, it also catches metal contaminants such as swarf, rivets or fragments of tools.

Why is the sieve a quality control point and not just a filter?

Because what remains on the sieve is feedback about the state of the batch. An excess of lumps signals dampening, fragments of film — damaged packaging, foreign pellets — a material identification error. An operator who looks at the held-back reject detects a problem before the material reaches the silo tanker and the recipient. The sieve is therefore a mechanical barrier and a quality sensor at the same time.

What are the types of cleaning sieves?

Most often vibrating sieves are used (screens with a vibratory drive, intensively sifting out particles and dust) and static sieves in the gravity line. They are complemented by magnetic separators — permanent or neodymium magnets removing ferromagnetic contaminants. The choice depends on the material, the required accuracy and the line throughput.

How is the sieve mesh matched to the material?

The mesh must freely let through the grain of the target granulate while holding back agglomerates and foreign bodies. Too fine a mesh slows the chute and may hold back correct material, too coarse — lets through what should be rejected. The selection is part of the terminal’s know-how and depends on the size of a specific batch. Ask for a quote and the parameters of transloading your material.

Is a magnetic separator needed when transloading plastics?

With high-purity granulates — yes, it is the standard. Fine metal contaminants are invisible to the naked eye, and in processing they can damage the injection moulding screw or the die. A magnetic separator catches swarf and fine ferromagnetic elements that a mechanical sieve alone will not hold back, because they are smaller than the mesh.

How often is a cleaning sieve maintained?

The sieve requires regular cleaning of the accumulated reject, control of the mesh condition (tears, wear, clogged openings) and periodic checking of the magnet strength in the separator. When changing material in the line, the sieve and hopper are cleaned to avoid contamination. The frequency depends on the intensity of work — at 200 tonnes per day the control is daily.