The sizer is one of the hardest-working machines in any processing plant, and one of the least understood. Plenty of operators run one every day without quite knowing why it holds its cut when ordinary screens blind over, or how a machine so short does the work of a deck twice its length. This article explains what a sizer is, the principle that makes it different, and where it fits among the screening options.
The short definition
A sizer is a vibratory screening machine that grades material using several short, steeply inclined decks stacked in a cascade, each fitted with apertures deliberately larger than the size being cut. Material meets each deck at speed and at a steep angle, and it is that geometry, not the hole itself, that sets the cut.
The principle: apertures larger than the cut
On a conventional screen the aperture equals the cut: to separate at 10mm you fit a 10mm mesh, and every particle close to 10mm becomes a pegging candidate while every damp fine works to blind the opening. The sizer breaks that link. Because material crosses the deck fast and at a steep incline, each particle sees the aperture foreshortened, at an angle, for an instant. A hole physically larger than the cut size behaves, at that speed and angle, like a smaller one.
Run the numbers on what that removes. Near-size particles meet an opening comfortably larger than themselves, so pegging virtually disappears. Damp fines have a larger opening to bridge, so blinding pressure drops away. And because open area is generous, each square metre of deck passes more material. The cut is then refined by repetition: the cascade presents the stream to deck after deck, each one correcting the errors of the last.
Anatomy of the machine
A typical Mogensen sizer stacks between two and six decks in one compact body, each deck short and steep, driven by unbalance motors. Feed enters at the top, and the cascade does the rest: coarse fractions leave early, finer fractions sieve out deck by deck, and the machine delivers up to several graded products simultaneously from a footprint that looks implausibly small beside the conventional alternative. Media can be woven wire, polyurethane or combinations, chosen deck by deck against the duty.
The compactness is not a party trick; it is the direct consequence of the principle. Decks that barely blind and rarely peg can be short, because they do not need length to compensate for blocked apertures, and short decks stack.
Following material through the cascade
Picture a four-deck machine grading a 0 to 30mm crushed feed into five products. The stream lands on the top deck moving quickly, and the coarsest fraction runs straight over the end into its chute. Everything smaller drops through the generous apertures onto deck two, which repeats the interrogation at its own cut, and so on down the stack, each deck shorter-lived in the material’s journey than a conventional deck would be, and each one sharpening the work of the deck above.
By the bottom of the cascade the feed has been questioned four times and has sorted itself into five streams, all from a machine a fraction of the size of the screen train it replaces. Watching one run is the quickest way to understand why the geometry works; the material visibly never settles long enough to cause the trouble it causes elsewhere.
What duties suit a sizer
Three situations call for one loudly. The first is difficult material: damp, fine, cohesive or otherwise blinding-prone feeds, from quarry fines and crushed sand through recycling soils to chemical and mineral powders. The second is multiple accurate cuts from one machine, where a five-deck cascade replaces a train of conventional screens. The third is floor space, or the lack of it: re-equipping an existing building, adding a grading stage to a crowded plant, or squeezing capacity into a structure that cannot take another large machine.
Sizers run across aggregates, minerals, glass recycling, chemicals and food duties, with the construction adapted to each: wear protection at the heavy end, stainless and hygienic detailing at the clean end.
Glass: the signature application
Few duties show the machine off like cullet. Crushed glass is sharp, abrasive, damp from washing and required in tight grades for remelt, a combination that punishes flat decks. Multi-deck sizers grade cullet into several furnace-ready fractions in one pass, which is why they have become standard equipment in glass recycling plants across Europe.
Sizer or conventional screen?
Where the feed is clean, the cut generous and the floor space open, a conventional screen remains a perfectly good and usually cheaper answer, and an honest supplier says so. The sizer takes over as the feed gets finer, damper and more blinding-prone, as the number of simultaneous cuts rises, and as space tightens. It is also the machine to shortlist when an existing conventional screen spends its life being unblocked; replacing like with like buys the same problem in newer paint.
There are trade-offs. The steep, fast geometry gives each deck a specific working band rather than infinite flexibility, and a machine this compact concentrates its maintenance points, so access design matters. Neither counts for much against the duties the sizer exists for.
Proving it before buying: test work
Because the principle interacts with the material, moisture, shape, fines content, the honest route to a specification is trial screening with the customer’s own feed. A modest sample through a test machine answers the questions a datasheet cannot: which apertures give the target cuts, how the material behaves at its wettest, and what capacity each deck genuinely supports. Mogensen can arrange exactly this kind of trial, and a duty proven on real material removes the risk from what is otherwise the least familiar machine on the shortlist.
A machine with a Mogensen history
The divergent-aperture principle was developed by Fredrik Mogensen in the 1950s and has been manufactured under his name since, in the UK from our works in Grantham, Lincolnshire. Decades of application engineering sit behind the current range, which is why sizing a sizer starts with material test work rather than a catalogue page. The principle is simple; matching decks, apertures, angles and media to a specific feed is the accumulated craft.
Frequently asked questions
Is the cut as sharp as a conventional screen’s?
On easy, dry feeds a long conventional deck can edge it at a single cut. On real feeds, damp, fine, variable, the sizer’s repeated interrogation usually holds the tighter envelope over a working week, because it is still cutting cleanly while the flat deck is quietly blinding over. Judge sharpness on month-long product data, not on a commissioning-day sample.
How many products can one sizer make?
Up to one more than its deck count: a five-deck machine can deliver six graded streams at once. Most installations use fewer, wider cuts, but the capability is there and it is the reason a single sizer so often replaces several conventional machines.
What size range does the principle cover?
Broadly from coarse millimetre cuts down towards the fine end of dry screening. It is not a scalper for blasted rock, and at the ultra-fine end specialist high-frequency machines take over. Between those poles, which is where most grading work lives, the principle is at home.
Does a sizer need special maintenance?
Nothing exotic: the same media inspections, spring checks and drive care as any vibratory machine, concentrated in a smaller body. Deck access is the point to examine at purchase, since a compact cascade rewards designs that release decks quickly.
Will it really not blind on damp feed?
It blinds far later and far less than a conventional deck at the same cut; physics offers no absolute immunity. On genuinely wet, clay-heavy feeds the sizer is combined with deck heating and mesh cleaning, and that combination handles feeds most flat decks cannot start on. Where doubt remains, test work with your actual material settles it before anyone buys steel.
Can a sizer be enclosed for dust or hygiene?
Yes, and the compact body makes enclosure easier than on a sprawling flat deck. Dust-tight casings suit minerals and recycling duties; stainless construction and hygienic detailing suit food and chemical lines, where the machine’s small footprint is often what lets it into the building at all.
How does it compare on energy?
Favourably per tonne screened. The decks are small, the bodies light for their duty, and the generous open area means less re-circulated material. No vibrating machine is a major power consumer by plant standards, but replacing two or three conventional screens with one cascade shows up in the metering.
Worth knowing by name
Most screening problems are ordinary and most machines can be tuned to solve them. The stubborn residue, the damp fines, the tight footprint, the five products from one feed, is sizer territory, and has been for seventy years. If a duty on your site fits that description, talk to us about test work, and see the principle earn its keep on your own material.
