Push a stirring rod into the drum and it takes some effort to pull out, with a thick layer clinging to it. Turn it over and a clump takes fifteen seconds to fall. The viscometer reads around 15,000 cps.

Take that number to three suppliers and you get three answers. One recommends a transfer head for thin-film precision. One insists on a comma blade and says nothing else will run. A third recommends a slot die because it covers thick and thin.

All three can be right, under different conditions. Choosing wrong shows up fast. On a transfer head the delivered coat weight may come in at four tenths of the theoretical figure, with a mottled surface on top. On a blade running a thin target, the gap closes to tens of microns. The blade edge and roll surface will not last a week. On a die with an undersized feed pump, the bead bulges before the lips.

So where does 15,000 cps actually belong?

Where Cell Volume Stops Metering

A transfer head meters by the volume of its cells.

The cells are small pits engraved into the roll. Each holds a fixed volume, so the theoretical coat weight is cell volume times slurry density. That arithmetic only works if the cells fill.

Viscosity takes that away. Slurry stalls at the cell opening and will not go in. At 15,000 cps on a gravure roll at room temperature, fill can drop below half. Transfer ratio follows it down, from a usual sixty or seventy percent to thirty or forty. The number you calculate and the number you weigh off the web stop matching.

Micro-gravure suffers more. Its cells are shallow, often ten to thirty microns deep. The volume is small to begin with, and the tolerance for viscosity is lower. On the same drum, a gravure roll carries some slurry up. Micro-gravure carries less.

None of this rules transfer metering out. Heat the slurry above forty degrees and the cells fill. The cost is that roll temperature and slurry temperature both have to be held. Let the slurry sit on the roll a few seconds longer and the viscosity comes back.

An equipment supplier usually stops here, because their question is whether the head can run at all. A quote that carries a viscosity range usually comes from a manufacturer who knows about this exception. Whether it runs continuously depends on the metering method and speed range a coating machine manufacturer writes into the proposal.

Gap and Speed Set the Coat Weight

A comma blade meters between the blade edge and the backing roll.

Slurry is dragged through a fixed gap, and the gap height becomes the wet film thickness. Viscosity does not appear in that sum at all. It only decides whether the layer is dragged through evenly, which is why 15,000 cps is easier here rather than harder. Thick slurry leaks past the edges of the gap in small amounts.

One conversion makes it concrete. At sixty percent solids and a density of 2.0, a dry coat weight of 200 g/m² needs 333 g/m² of wet film. That is 167 microns thick. Set the blade gap from that figure, and allow for transfer losses by running 250 to 300 microns.

Once the gap is set, coat weight follows line speed. Double the speed and the same area gets half the slurry. That relationship is the most useful thing on site. Coarse adjustment on the gap, fine adjustment on the speed.

Blade and roll accuracy move to the front at this point. Twenty or thirty microns of roll runout is more than ten percent of a 167 micron wet film. The runout figure a machine builder records in the assembly file is worth more than the tolerance printed in the catalogue.

Wide web, thick film and mid-to-high viscosity together point at gap metering as the simpler route. The adjustment range a coating line manufacturer offers here sets how many parameter sets you can try on site.

Can the Pump Hold the Pressure?

A slot die meters inside its cavity.

At high viscosity that becomes a pressure problem. Pressure drop in channel flow rises with viscosity and falls with the cube of the flow gap. The internal gap is a few hundred microns. Pushing 15,000 cps through takes a feed pressure an order of magnitude above what a thin slurry needs.

One thing works in your favour. Most high-viscosity slurries shear thin. A viscometer reads the low-shear figure, at shear rates from a few to a few hundred reciprocal seconds. Shear rates inside a die channel run above a thousand, and can reach a hundred thousand. The slurry thins on the way through, and apparent viscosity may sit at half the reading or lower. Size the pump against the viscosity at coating shear rate and the pressure requirement drops a long way.

Shear thinning brings its own trouble. The slurry flows inside the die, and once it leaves, the shear ends and viscosity springs back. The bead breaks more easily at speed, which is where edge build-up and heavy edges come from. The vacuum box has to be reset, and pulling too hard distorts the bead.

When a coating machine manufacturer works out a high-pressure feed package, the pump curve and the die pressure drop have to be checked together. Each side calculated alone leaves a gap at the joint. Ask the machine builder for one checking sheet rather than two separate ones. It makes a difference on site.

A Thin Film With Thick Slurry

A thin film is the first thing to fall out of this choice.

Drop the target wet film below twenty microns and the gap has to close inside fifty. For 15,000 cps at thirty metres a minute, that is a shear rate around ten thousand reciprocal seconds. Blade load, roll heating and wear all climb with it.

Three routes stay open. Heat the slurry to between forty and sixty degrees. Viscosity falls to a third or a fifth, and the gap does not have to close so far. Where an OEM runs the project, the three routes usually end up in one trial report. Go back to the formulation, drop the solids a little or move to a lower molecular weight resin. Or pressurise a die and squeeze the thin layer out through a small, high-pressure channel.

Each route has a price. Heating means insulation along the whole slurry path, and short piping is easier to control. Lower solids push the load onto the drying section. Die pressure puts the highest demand on the feed pump and the seals.

Before a coating line supplier walks you through what each route buys, ask what their most recent job was, in viscosity against wet film. Where the numbers a supplier sends carry a viscosity but no shear rate, you cannot use them.

Two More Things Follow the Method

Once the method is fixed, two other things move with it.

On the feed side, pump displacement has to be quoted at the viscosity seen at coating shear rate, not the viscometer reading. The same pump can differ by thirty percent in output between those two viscosities. Without the calibration conditions written down, site work starts from zero. Pump speed, back pressure and slurry temperature belong in the same document. Where an OEM coordinates the project, all three sets of figures sit with one contact, and handover gets a lot quieter. A coating line manufacturer marks a pressure drop on the piping drawing. What a coating line supplier measures on site is worth setting beside it.

On the drying side, wet film thickness decides everything. A 167 micron wet film and a 50 micron wet film do not take the same zone temperatures. Where there is little solvent to remove, the first zone should run cooler, so the solvent leaves before the surface skins. The drying curve a machine builder ships covers one formulation. Ask for the version that matches the slurry you are moving to. Whether you get one version or both tells you how far they went.

One document holds both ends. Target wet film thickness, viscosity at coating shear rate, metering method, gap or pressure range, zone temperatures. Five lines on one page. What an equipment supplier has signed against on that page is what you compare against later.

When the proposal arrives from a coating machine manufacturer, check first whether the gap and pressure ranges sit on the same page. Back to the question. Which method fits 15,000 cps is not answered by the viscosity figure. Target wet film thickness decides whether the head can take it. Viscosity at shear rate decides how the pump and the gap are set. Solids and solvent decide how much room the oven has. Match all three and transfer metering is not out of reach. Miss them and the most expensive head on the list still will not run steadily.

That choice comes down to how the metering method, the feed pressure and the zone temperatures 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 viscosity at coating shear rate into the technical agreement, with the shear rate beside it. One without the other is not a number anyone can use.

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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