Bartek Kaplan, Head of R&D
To understand the effect of the properties of a material in a given doctor blade, it is necessary to consider the application, i.e., the environment that the material will experience.
In flexographic printing, the doctor blade is in contact with two other main components: the anilox roll and the printing ink. The former provides an abrasive surface, often considerably harder than the doctor blade, while the latter provides abrasive particles (the ink pigment) in a liquid suspension. This liquid may in turn interact chemically with the doctor blade if the two are matched poorly.
One must further realize that there is no such thing as a perfectly flat surface; the contact between the doctor blade, the anilox roll and the ink pigment particles occurs only at
superficial asperities due to the surface roughness, see Figure 1 . In principle, the respective surface roughness of the doctor blade and the anilox roll will in combination contribute to the print quality since the accuracy of the doctoring will depend on the contact between the two.
Figure 1. Schematic cross-section view of the contact points between the anilox roll surface and the doctor blade edge. The dimensions of the surface asperities of the doctor blade are exaggerated for clarity and are not drawn to scale with the anilox roll cells.
In general, there are four main categories of metallic doctor blade materials: carbon steel, stainless steel, tool steel and surface treated steel. Each of these is tailored to tackle specific challenges and environments. In some flexographic printing applications, it is also common to use polymer doctor blades. However, polymer blades usually exhibit considerably shorter service life and inferior print quality as compared to steel blades.
The first and perhaps most important distinction must be made between carbon steel and stainless steel. Carbon steel blades are not suitable for use with water-based inks due to an extensive corrosive reaction between water and unprotected steel – commonly known as rust. Stainless steels were developed to tackle this specific issue, where the protective effect is attained by alloying with chromium in excess of 11 mass-%. The chromium content is most commonly kept to around 13 mass-% in blade steel materials, in order to limit detrimental effects to hardenability and thus wear resistance.
Alloying with chromium will enable the formation of a very thin, very dense, and chemically very stable surface oxide consisting mainly of chromium. The oxide will rapidly heal if
removed or damaged by, e.g., abrasion. This phenomenon is known as surface passivation. Considering other ink liquids than water, for example various alcohols, it is still important to note that one is dealing with an unprotected blade. Thus, it is important to maintain the pH of the ink at least neutral or preferably slightly basic. If these conditions are met, carbon steels can offer a good versatility and adequate price versus performance ratio, owing to the considerably lower price of iron as compared to, e.g., chromium. Wear resistance can be
tailored by carbon content and appropriate heat treatment. Note that it indeed is possible to achieve a carbon steel with a hardness comparable to the hardness of the anilox roll. However, such a steel is not desirable since this will mean one of two things: 1) the anilox roll will be worn more rapidly, or 2) the doctor blade will be exhibit brittleness and may break in small or large fragments, which will destroy the print job and the ink batch. For the highest levels of wear resistance in steels, it is normally recommended to use tool steel grades. Tool steels were initially developed for metal cutting operations such as turning or drilling, by alloying with tungsten or molybdenum to produce a highly heat resistant steel that would retain its hardness even at elevated temperatures. The wear resistance of tool steels relies on very hard carbide particles, achieved by alloying with tungsten or molybdenum and commonly also with vanadium and other elements. In some cases, it is desirable to combine wear resistance and print quality in a single doctor blade. For such applications, a post-treated steel doctor blade is the weapon of choice. In the printing industry, this may refer to a blade that has been coated to reduce friction and increase hardness. At PrimeBlade, the Nano treated doctor blade is one very successful example of a post-treated steel blade, where both print quality, due to reduced friction, and increased service life is observed in flexographic applications. The benefit of a post-treatment lies in its versatility. Many steel grades, carbon, stainless and tool steels, can successfully be treated to have their properties improved. A further benefit often comes from the reduced tendency for corrosion of the surface, owing to the protective properties of the treatment.
The plethora of doctor blade products available on the market is of course much more extensive than detailed here. However, the bespoke four categories form the basis of most product portfolios for flexographic printing applications.