Doctor blades for printing with white inks

Printing with white inks is challenging in today’s packaging industry, as these inks are not only abrasive, but they are often used in higher volumes to cover large areas and can have tough requirements on opacity. Find out more about Doctor blades and white inks.

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Printing with white inks is challenging in today’s packaging industry, as these inks are not only abrasive, but they are often used in higher volumes to cover large areas and can have tough requirements on opacity.  These inks give special effects to the printing results, but the process parameters and consumables such as the doctor blades need to be adapted for best overall results.

White ink is abrasive due to the hardness of its titanium dioxide pigments. In printing whites by gravure, opacity levels of over 85 percent can be achieved, while in flexo printing there are challenges in transferring the large volumes of ink needed as a smooth coverage of the substrate without defects such as pinholes. The larger ink volumes mean coarser aniloxes (lower line counts) which can wear blades faster.

A large portion of white ink on the market is solvent-based for flexo-printing on packaging film.  This often goes on wide web presses, where higher printing speeds of up to 400 – 600 meters per minute can bring on higher pressures and/or vibrations.  These conditions as well as the abrasivity of the ink raise requirements for thicker blades and tips and more wear resistant material such as tool steels and higher hardness coatings.

Planned maintenance is being implemented more and more as part of the running schedule, from multi-national companies to small privately-owned ones.  In planned maintenance, a stop is scheduled once or twice a week, where all the blades are changed and everything including blade holders, pumps, pipes etc is cleaned over a period of one or a few hours.  Since white ink wears standard blades at a faster rate than other inks do, more wear resistant blades are needed to hold out for the planned maintenance stops.

A well-proven combination is 0,20 – 0,30 mm thick SWED/CUT®H9 with radius edge for metering and SWED/CUT®MicroNox II 0,15 – 0,20 mm thick for containing. This has given total production of 850 000 meters of flexible packaging over a week’s printing (on a Uteco machine), reducing downtime with no issues of neither print quality nor blade longevity.

In another case of printing brown bags on a wide web machine (W&H Novaflex) at 500 – 600 meters per minute in white inks, a SWED/CUT®H9 blade 0,25 mm thick with radius edge doctoring and MicroNox II blade 0,15 mm thick with radius edge for containing have run between 1.7 and 2.7 million meters compared with 1.5 million meters for a high-value competitor blade.

Yet another case, this time at a large North American printer, two SWED/CUT®H9 blades (35 x 0,25 mm radius edge for metering and containing) ran 1.3 million meters at 666 meters per minute.

In label printing, UV inks are popular for their ability among other things to give vivid covering. Thicker, hence stiffer blades like 0.25 mm and 0.30 mm are often chosen depending on printing speed. The abrasivity and viscosity of white UV inks calls for more wear-resistant materials and harder coatings on blades with tips tailored to the anilox line count.

In rotogravure, printing white solvent-based inks on flexible film packaging is well-addressed by using lamella tips with fine and wear-resistant coatings of higher hardness. The following figures show contact surface photos of two different brands of lamella blade tips with “ceramic” components in the coatings, run in white solvent-based ink for the same number of meters, revealing an etched exposed base steel on one while in the second example the more durable SWED/CUT®coating remains intact.

1) Steel is exposed where the tip coating has worn off
2) The more durable SWED/CUT® coating has not yet worn off