How to choose the right doctor blades

How to choose the right doctor blades for water-based inks ?

Over the years interest has grown in using water-based inks and varnishes for flexo- and gravure printing, as they offer advantages in health, safety and environmental management. When printing with these media, a carefully-chosen doctor blade will enhance printing results and total economy. So what is there to think about?

Material resistant to corrosion and wear

A greater presence of water in the printing facilities will increase the risk of corrosion, especially of carbon steel parts. Besides the water contained in water-based inks and varnishes, a relatively high air humidity and long periods of blades sitting in blade holders can increase the corrosion rate of carbon steel blades. That is why stainless steel blades and coated blades are often recommended for water-based inks and varnishes.

A stainless steel is generally defined as a steel alloyed with at least twelve weight-percent chromium. This allows a high enough content of chromium in the matrix to form its own protective passivation layer for corrosion resistance. However, steel is an alloy of iron, carbon and other elements which can also affect its properties. Steelmakers strive to make the best possible compromise between corrosion resistance and wear resistance when producing a stainless doctor blade steel. For example, a low carbon content

(e.g. <0,3%) will increase the corrosion resistance, but reduce the wear resistance due to lower hardenability and the formation of fewer carbides (carbides are compounds of carbon and metals that form wear-resistant particles in the steel, in a similar way as stone aggregates increase the wear resistance of paved roads). A higher carbon content (e.g. >0,6%) combined with an optimum level of chromium will allow both sufficient corrosion resistance and an increased wear resistance, due to the presence of small wear-resistant carbides distributed in a chromium-rich matrix.

An addition of molybdenum to the stainless steel increases the corrosion resistance even more, suitable for more extreme pH levels. These properties exist throughout the blade.

In some cases a coated carbon steel blade, either coated along the edge or over the entire blade, can combine sufficient corrosion resistance with lower friction at the tip. However, once the coating is worn through at the tip, the carbon steel will be exposed and can corrode during production stops. When the press is standing still for longer periods, a change of blades before start-up will help to avoid corrosion problems. Coated stainless steel blades give the ultimate in corrosion resistance and print quality, combining low friction at the tip with corrosion-resistant material through-and-through. When choosing a coated blade, the hardness and homogeneity of the coating in comparison to the printing cylinder should be considered to avoid risks of cylinder damage. Plastic blades are sometimes used as containing blades and offer excellent corrosion resistance but generally low wear resistance and low print quality.

In cases where wear resistance is the dominating requirement for long runs, such as for white and metallic inks, an alloyed carbon steel will perform well, alloyed with extra carbide-builders for higher hardness and a refined distribution of very small carbides. Here as well, a blade change at longer production stops is recommended to avoid corrosion issues.

source : Flexible Packaging Technology magazine

website: goo.gl/S3qvcJ

written by Debbie Ågren, Swedev