Water-based coating problems: foam, drying and blocking

Author: WIT

Diagnose water-based coating problems by separating foam, uneven application and blocking. Check circulation, coating transfer and drying conditions. Record the results before changing the coating or press settings.

Water-based varnishing: where to start with the diagnosis?

Write down the symptom and when you notice it. Do the sheets stick together immediately after turning the stack or only later? Does the problem affect the entire sheet or just areas with heavy ink coverage?

Change one parameter at a time. Compare sheets from the same batch of substrate. Record machine speed, viscosity, measurement temperature and drying settings. This will allow you to check what actually helped.

The coating does not dry: check airflow before adding heat

Water-based varnish dries by evaporation of water and its absorption into the substrate. Therefore, efficient circulation of warm air is of great importance. Simply increasing the power of the IR heaters may overheat the stack rather than solve the problem.

First check the drying and the amount of varnish applied. A thick layer takes more time. High ink coverage and a less absorbent substrate also make drying difficult. This applies especially to demanding materials such as foils or polyethylene-coated substrates.

Control the stack temperature and compare it to the requirements of the specific varnish and machine. Don’t use one temperature for all orders. Select drying depending on the substrate, ink coverage and production conditions.

If the ink under the varnish is drying too slowly, also check the ink layer. Maintain the amount of ink needed to achieve the correct color. More information can be found in the guide about problems with the drying of offset ink.

The sheets stick together after water-based varnishing

Sticking, or blocking of the stack, may result from under-drying or excessive heating of the sheets. Check the temperature, varnish layer and depositing conditions. Don’t judge the readiness of the entire stack just by touching the top sheet.

Choose the appropriate varnish to varnish both sides. A coating intended for single-sided high gloss may soften again during a subsequent pass. In such a situation, increasing the drying power alone will not solve the cause.

For heavy cartons, high speed and fast stack rotation, check the need for anti-set-off powder. Select the granulation and quantity for the order and further processing. The powder helps separate the sheets, but it is not a substitute for proper drying.

By selecting water-based varnish, provide the substrate, effect and printing method. Indicate whether you are varnishing one or both sides.

Water-based varnishing: foam and variable viscosity

Foam often occurs when the system introduces air into the coating. Check the tightness of the pipes and the operation of the pump. Check the return line: the return at the wall of the container, just below the surface of the liquid, limits aeration of the liquid.

Do not judge the viscosity of a highly foamed sample. Mix the varnish and measure according to the method for the given product. Record the type of measuring cup and the temperature of the sample. A result without this information makes it difficult to compare subsequent measurements.

Increased viscosity may result from water loss during operation. Close the container and check that the hoses do not get too hot. On the other hand, water residues after washing may dilute the varnish too much.

Do not add water or defoamer by eye. First determine the cause and check the allowable corrections for a specific coating. The general guide does not replace recommendations for the selected product.

Water-based varnishing: uneven coating and poor gloss

Start with a clean system. Dried varnish limits the capacity of the anilox cells and makes it difficult to apply evenly. Also check the squeegee, varnishing blanket and pressure according to the machine’s instructions.

Too much pressure between the blanket and the impression cylinder may promote ink deposition on the blanket. Then the coating starts to look uneven. More pressure does not always mean better transfer.

Match coating blanket and anilox cell volume to the required effect. Do not equate anilox cell volume with the ruling or directly with the weight of the wet layer.

If you want to increase the shine, don’t start with the maximum amount of varnish. A thicker layer may make drying more difficult and increase the risk of blocking. Water-based varnish and UV varnish create different coatings, so do not assume the same effect.

In case of poor abrasion resistance, check the drying of the ink and the compatibility of the entire set of materials. Foils, PE substrates or demanding recycled materials may require a different varnish. Also select the slip to suit the operation of the packaging line; the highest glide will not always be the best.

Coating cracking, sheet curling and color change

Fine eggshell cracks may appear when varnish dries too quickly over a thick layer of ink. Check the relationship between ink coverage and drying. Do not accidentally add drying retarder.

Curling of sheets may result from loss of moisture during drying. Reduce excessive heating while maintaining effective drying. For lightweight papers, consider a varnish suitable for this application.

Before coating spot colours, test the resistance of the mixture. Especially check out colors containing Reflex Blue, Blue 072, Warm Red or Rhodamine. Varnish ingredients may affect the color of some inks. This does not mean an automatic ban on the use of these colors.

When selecting inks according to the Pantone scale inform us about the planned coating. Evaluate the final effect on the appropriate surface, with the target ink and varnish.

What should you record when diagnosing coating defects?

Send the name of the varnish and ink, the type of substrate and a description of the symptom. Add information about single- or double-sided printing, machine speed and further processing. Stack temperature, viscosity with measurement method, and anilox volume will also be useful.

Attach photos of the defects and note when they appeared. Compare samples before and after the adjustment. Confirm improvement in a production test before re-issuing the entire print run.