Lift a stirring rod out of the drum and the slurry clings to it, letting go in a slow clump. Poured into a tank, it sits in a heap and spreads for a long time. Does that slurry belong on a coating line?

The pilot sample was drawn down by hand and looked flat enough. On the production line the problems arrive one after another. The gauge at the pump outlet never settles. Thickness is a step off from one edge to the other, and an uncoated stripe shows up now and then in the middle. The web leaves the oven with a skin on top and a soft layer underneath. Slow the line and it improves, along with the output.

Some buyers are told to dilute the slurry until it flows. Dilution drops the solids, and the dry film weight will not get there.

What stops it is not the viscosity number.

Where High Viscosity Jams First

Pressure drop in the pipe shows up first.

In laminar flow, pressure drop rises with viscosity and falls with the fourth power of the pipe diameter. Take viscosity from 3,000 to 30,000 cps and the drop along the same pipe rises ten times. Halve the bore and it rises sixteen times.

A gear pump slips less on a thick slurry, so its volumetric efficiency holds up. The trouble sits on the suction side. A narrow bore, a long run and too many elbows pull the inlet into a deeper vacuum. The slurry cannot fill the tooth spaces fast enough, and the flow starts to pulse. On the same drum, a full tank and a low tank will differ by about a tenth in output.

Motor torque needs checking as well. Ten times the viscosity raises the shaft power at the same speed. The gear ratio chosen earlier may no longer fit. A machine builder states the pump calibration conditions in the electrical package. That is the figure to check here.

There is more room to work in this section than at the head. Shorten the suction run. Open up the bore and move the pump closer to the head. A positive-displacement pump is often the better fit for thick material. What a coating line supplier recommends here often decides whether the system runs at all.

Transfer Heads Run Short of Slurry

A gravure roll carries slurry up in its cells.

The cells have to fill first, then release the slurry onto the substrate under pressure. Raise the viscosity and they fill short and transfer less. The coat weight lands below the theoretical figure. Micro-gravure cells are shallower and more sensitive still. On these heads the coat weight moves with viscosity and line speed together. The setting has to be found by trial rather than calculated.

If the cells cannot fill, what makes you think the coat weight will hold?

Gap-metering heads do not have that problem. A comma blade leaves a fixed gap between the blade edge and the backing roll. The slurry is pressed through it, so the coat weight follows the gap and the line speed. A slot die is fed under pressure and meters inside the cavity ahead of the lips. On one drum of slurry, a gravure roll may not run while a comma blade will.

An equipment supplier asks about viscosity and solids in the first round of questions. A coating machine manufacturer names the metering method in the proposal. On a thick slurry, that one line decides most of what follows.

Why the Pattern Will Not Level Out

Slurry that will not flow leaves the pattern where it is.

Levelling is surface tension pulling peaks into troughs, and viscosity is the resistance. Levelling time rises with viscosity and with the fourth power of the wavelength. On the same drum, ten times the viscosity turns a two-second levelling into twenty seconds. Move the wavelength from 0.1 mm to 0.2 mm and the time goes up sixteen times.

The first oven zone helps a little. Heat brings viscosity down and levelling speeds up, while the surface starts to skin at the same time. The window is narrow.

The table handed over by a manufacturer usually carries one viscosity figure. A thick slurry is the least sensitive to it. Two or three shear rates are what show the trend. High viscosity usually comes with high solids and less solvent. Buying time with solvent is not open either.

A Thick Wet Film Skins Over

Drying in a thick film works from the inside out.

Solvent has to diffuse to the surface before the air can carry it away. The characteristic time rises with the square of the thickness. Go from a 100 micron wet film to 200 microns and drying takes four times as long. Thick slurry usually means a thicker wet film with high solids and little solvent. A hot first zone skins the surface. Solvent underneath has nowhere to go and breaks through, leaving pinholes and blisters.

Whether the profile moves forward or back depends on the solvent blend. An equipment supplier can give you flow against pressure drop for the new blend. That saves several rounds of trial on site.

Which Readings Come First

Read the actual viscosity first.

Take two or three spindle speeds and watch how far the reading falls. A single point tells you least on a thick slurry.

Then draw the slurry down on the substrate you intend to use and watch how it spreads. If it creeps and pulls back, wetting has to be solved first.

Then watch the gauge on the feed section. A needle that will not settle points at starved suction or pump slip. The head is not involved. The machine builder leaves the calibration curve for that pump in the handover package. Compare against it and you can see how much margin is left.

Finally, read the piping drawing. The sheets in a coating line manufacturer's handover package show which section has the smallest bore and which elbows collect material.

The data sits with three different parties, the formulation, the machine and the feed system. Where an OEM coordinates the project, one contact can gather it. A supplier who lists what was measured at handover gives you something to compare against.

Matching the Machine to the Slurry

Choose the head first.

Metering heads let you keep the head when the viscosity moves, because the coat weight follows the gap and the line speed. Set the gap from the new wet film rather than nudging the old one. Blade pressure and die cavity pressure both have to be reset for the new viscosity. A coating machine manufacturer sets the adjustment range here, and that range is your starting point on site.

Recalculate the feed section against the measured viscosity. Short runs, wide bores, the pump close to the head, and insulation along the pipe. Viscosity follows temperature, and ten degrees in the slurry can halve it. A coating line supplier who works out those dimensions once saves several rounds of trial.

Leave the stretch between the blade and the oven for levelling. Distance, substrate temperature and air movement across the web all count. Run the first zone cool and give levelling its time.

Push the drying profile back and let the solvent leave slowly. Do not open the air volume fully at the first zone. Where a machine builder has listed zone temperatures and air volumes, you can read off which zone to move. A supplier who puts those figures in the proposal hands you a starting point.

Conclusion

Whether a thick slurry can be coated does not come down to the viscosity figure. A metering head, a feed section that delivers steadily, and a drying section with time to spare. Get all three and 50,000 cps runs continuously. Miss one and diluting the slurry still leaves you with a problem.

That set of requirements comes down to how the head, the feed and the drying section are matched. Problems like this come up often in custom work. Innater has built coating machines to order across a range of products and processes. Precision, thickness and uniformity are worked out against your slurry and your target thickness.

So where does high viscosity actually jam?

Write the measured viscosity and solids into the technical agreement. Say which page carries the metering method, the pipe bore and the zone temperatures. A coating machine manufacturer and a coating line manufacturer each own part of that list. An OEM coordinating the project should have the acceptance method fixed in writing.

automatic roll slitter-Innate

waterproofing coating machine-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