8.4 Percent Below 100 Mesh: What the Fines Fraction Does to a Line

We sieved one 25 kg sack from a delivery that had passed every paper check: 8.4 percent below 100 mesh, 61 percent in the 30/60 window, the rest coarser. The certificate said “30/60 mesh, 95 percent passing”. Both statements were true. They were just describing different ends of the distribution, and only one of them explained why the bag dump station looked like a fog bank at seven in the morning.

Fines are the least negotiated and most felt part of a mesh specification. Nobody buys them on purpose, everybody pays for them, and in a surprising number of processes they are the reason a grade that tested well in a beaker behaves badly in a plant.

Where fines come from, and why they are always there

Polymerisation gives a gel, the gel is dried and crushed, and crushing produces a distribution. Screening cuts the top and the bottom, but the bottom cut is where suppliers save money. Pulling the fine fraction out costs a screening step, a yield loss of three to eight percent, and a disposal route for material that cannot easily be sold. So the bottom cut drifts: 100 mesh becomes 140, then 200, and the certificate still says 30/60 because that describes the nominal fraction, not the tail.

Handling adds to it. Pneumatic conveying at high velocity grinds granules against pipe walls; every elbow adds a little more. A supplier who screens carefully at the plant can ship a product that arrives with three extra points of fines after two transfers and a container journey. The sack did not change. The distribution did.

Four things fines do that nobody wants

The first is dust. Fines below 100 mesh stay airborne for minutes, coat every surface, and make a bag dump station unpleasant to work in. That is a housekeeping cost, a respiratory protection cost, and in some cases a combustible dust question that the coarse grade never raises.

The second is gel blocking. A fine particle hydrates faster than a coarse one because it has more surface per unit mass. In a mass of powder, the fines at the wetting front swell first, seal the pores, and slow water access to the interior. You end up with a lump that is fully swollen on the outside and dry in the middle. In a beaker this shows up as a lower 5 minute number and a surprisingly normal 24 hour number; in a plant it shows up as a blocked line.

The third is feeder behaviour. Volumetric feeders are calibrated on bulk density, and fines change bulk density, flowability and the tendency to flood. A screw that ran at 42 kg/h on the last lot can run at 47 on this one, and the dose error lands in the product, not in the feeder log.

The fourth is cost. You pay per tonne for material that mostly becomes airborne or seals the surface. On a 20 tonne order, 8 percent fines is 1.6 tonnes of polymer that does less work than the coarse fraction it replaced.

The cases where fines earn their place

Not every process should reject them. In water-blocking tape, fines let you load more mass per square metre and form a continuous sealing layer. In coatings and in thin-film applications, a fine cut gives a smoother, more uniform gel. In fast-wicking products, the higher surface area is the point.

In those cases the right move is to specify the fine fraction deliberately, as a target rather than a tolerance, and to check it on arrival with the same sieve you use for the coarse cut. A tape maker who asks for minus 200 mesh and receives a nominal 30/60 has a quality problem, not a bargain.

The interesting middle ground is a bimodal blend: coarse granules to keep channels open, a controlled fine fraction to seal. Suppliers who run both cuts can make this to order, and it often outperforms either single cut in spill and waste applications where both speed and retention matter.

Running the sieve test properly

You need a stack of three sieves, a balance to 0.01 g, and a mechanical shaker. Weigh 100 g, shake for five minutes, and weigh each fraction. Two minutes is not enough for a cohesive powder; the fines cling to the coarse particles and the result flatters the supplier.

Do it on a sample taken from the middle of the sack, not the top. Fines migrate downward in transit, so the top of a bulk bag is systematically coarser than the discharge point. If you sample only the top, you will approve a lot and then process the bottom.

Record the full distribution, not just the percentage passing. Two lots with identical minus 100 figures can behave very differently if one has 3 percent below 200 and the other has 0.5. The sub-200 fraction is the one that goes airborne and the one that seals a wetting front.

What to put in the purchase order

Specify a maximum below 100 mesh, a maximum below 200 mesh, and a minimum in the working window. Then specify the test: sieve sizes, sample mass, shaking time, and whether the sample is as-received or dried first. Moisture changes the result by a point or two, enough to argue about.

Ask the supplier how the powder is conveyed to the bagging line. A plant using a dense-phase system at low velocity delivers a distribution close to the sieve result; a dilute-phase system at 20 m/s delivers something else. Our note on handling fines at the bag dump covers the receiving end, but the damage usually happens before the sack is filled.

Finally, weigh the dust you collect in a week. If the dust collector is pulling 40 kg of polymer per week out of a line that uses 2 tonnes, the fines are costing you two percent of material and a maintenance shift, and a tighter bottom cut pays for itself faster than most buyers expect.