Under a magnifier the web shows a fine scratch along the machine direction, in no fixed place. Move to another stretch and the scratch has turned into a small pit with material turned up at the rim. A three kilometre roll carries a few dozen of these marks.

Same batch, same settings, pump and gap where they were. Take the roll to a coating machine manufacturer and the first reaction is to look at the slurry. Look at what, exactly. At the particles.

Do particles in the slurry actually affect the coating?

Oversize Particles Have Nowhere to Hide

A particle that gets into the wet film has one place to go.

In a system with a thirty micron wet film, the D50 on the formulation sheet reads five microns, and nothing looks wrong. The D99 column is what matters. A forty micron particle that slips in cannot be covered by the film. The blade presses it down and drags it along the web, and the trail it leaves is a scratch.

This holds for every metering method. A comma blade runs a gap sized to the wet film, tens to over a hundred microns. The particle passes through and leaves its mark in the film. In a transfer head it jams in a cell, costing coat weight first and film quality next. The channel inside a die runs only a few hundred microns. Large particles collect at the lip and come out as a line across the web.

The thinner the wet film, the narrower this passage. A shop coating fifty micron dry film and one coating five micron film have tolerances for particles an order apart.

One Defect, a Billion Particles Behind It

Behind one defect sits a very large number.

A ten micron silica particle weighs on the order of a nanogram. One gram of powder carries around a billion of them. A fifty kilogram drum of slurry holds particles in the trillions.

Coat weight is counted in grams per square metre, and the particles buried in one square metre of film are likewise an astronomical figure. The good news is that most of them cause no trouble. The troublemakers are the oversize few and the large agglomerates. However good the D50 on the test sheet looks, it cannot say how many oversize particles there are.

That is where the defect rate comes from. Pick a few hundred large ones out of a billion and every square metre shows several spots. What you see on the web is the fate of those few hundred, not the average size.

Most Particles Come From Outside the Formula

Blame the formula alone and half the sources stay out of sight.

The filler in the recipe is one part. Poor dispersion lets fine particles clump, and the clump is long past the size of a single particle. Such agglomerates are soft. They pass the pump and fail at the blade.

Skin is the second route. Drum mouths, pipe elbows, dead corners of the circulation pump. Solvent leaves, the skin stays, and one squeeze of the pump puts it back into the slurry. Wear from a gear pump running hard filler is the third route. Metal fines hide in the slurry colour. Whoever chooses the pump, a machine builder above all, should raise this point. If the pipes were not flushed clean after assembly, weld beads and seal fragments reach the film first. Substrate dust and shop dust join the list as well.

A test sheet from a supplier carries no particle column, and the oversize count has to be asked for. Where the sheet does not carry it, plan for the worst case and leave margin for that figure in the proposal.

Where a Filter Stops Working

A filter is one gate, not the whole fence.

The filter bag and cartridge on the feed side are chosen by absolute rating. A thirty micron wet film takes a ten micron cartridge, and most hard particles stay outside. Push the rating finer and the pressure drop climbs, so the change interval shortens. The initial cartridge model and the pressure drop baseline are worth recording during commissioning. A coating line supplier who notes them down gives you something to compare against later.

Circulation is the step that gets skipped. Standing slurry lets particles settle, and the first stretch after restart is full of them. Keep the line moving and the particles stay suspended. Filtration and circulation each cover a section. The circulation pump setup belongs in the proposal a coating line manufacturer writes. Whether a flushing port was left in the piping is a question for the machine builder.

What the filter cannot do is just as clear. Skin is soft and squeezes through. Soluble material that has come out of solution cannot be caught. Wear fines are born upstream of the cartridge, and whatever has already passed will not be stopped.

Match the Defect Back to the Particle

The marks on the web can talk. A fine scratch along the web direction points at a hard particle pushed through. A round pit with a turned-up rim suggests agglomerates or bubbles, and the bubble route has its own earlier article. A line across the web sends you to the die lip. A mark in a fixed position points at the substrate or the roll. A mark at random points back to the slurry.

The checks are simple. Lift a cup of slurry, swirl it against the light, and look at the bottom after half an hour. Cut an spent cartridge open, and what it caught is there to see.

All of this belongs in the paperwork. In the proposal from a coating machine manufacturer, filter rating, cartridge model and circulation sit in the feed section. Check whether they are written at all. The handover file of a machine builder should carry one page on pipe flushing. A coating line manufacturer puts the cartridge change interval into the maintenance plan. On the trial report of an equipment supplier, check whether the filter rating column is filled in. The slurry in the trial and the slurry on the line are rarely in the same state. Where the column is empty, have the equipment supplier run the test again before signing. Where an OEM coordinates the project, the responsibility boundary for filtration is written down, and the acceptance day checks cartridge models one by one. Where an OEM runs the project, cartridge consumption goes into the spares list. Cartridges are consumables, so ask a supplier for consumption figures and change intervals. A manufacturer who has run your kind of powder knows that market. When the pressure drop climbs fast on site, have a coating line supplier judge whether the slurry changed or the cartridge blocked.

When the proposal from a coating machine manufacturer arrives, turn to the feed section first. Particles are always there. Whether they hurt the coating comes down to four things. How far the largest one sits from the wet film, how many oversize ones there are, and which way they get in. The fourth is whether the filter holds. With those four answered, the particles stay managed. Miss one and the spots on the web keep coming.

That set of requirements comes down to how particle data, filter rating and circulation are matched. Problems like this come up often in custom work. Innater works precision, thickness and uniformity out against your slurry and your target thickness, machine by machine.

Write the particle size distribution and the filter rating into the technical agreement together. With only a viscosity on the page, nobody has signed for the particle gate.

doctor blade and micro gravure coater-Innate

multifunctional coater-Innate

Published by Innater Intelligent Equipment Co., Ltd, a maker of non-standard coating machines and automated lines. Email 13316631159@163.com | Website https://www.getinnater.com/ | Tel +8613316631159