Enclosed Ink Delivery
While the anilox was evolving, so was the ink metering system. Totally enclosed ink metering systems consisting of a reversed angle doctor blade, a containment blade—in the positive or wiping position—and end seals, acting as dams at each end, that were pressed against the anilox roll. Ink was pumped into the chamber while gravity returned it to an ink pail. The amount of ink required to charge the system was 50 percent less than traditional systems. At the same time, an enclosed system reduced evaporation and contamination of ink.
Doctor Blade Material
The fundamental job of a doctor blade is to meter or remove excess ink from the anilox surface. After all, excess anilox ink film is at the heart of dirty print and dot gain.
In the beginning, different plastic and steel materials at varying dimensions, making contact at different angles, were evaluated to determine which one could meter best, last longest and be least damaging to the anilox surface. We learned the best metering occurs when the contact angle between the blade and anilox is set at ±35 degrees and the contact area is minimized.
Wide contact areas, even when in a reversed position, are subject to hydraulic lift, resulting in a thicker anilox surface ink film. Contact angles less than 35 degrees result in a wider contact area. Contact angles greater than 35 degrees are prone to chatter and become more abrasive and more wearing to the anilox surface.
At a 35-degree contact angle, it was determined that only steel had the rigidity to make anilox contact with the least-wide contact area. Other materials, like plastics, were not rigid enough at 0.006-in. and 0.008-in. thick. Plastics had to be thicker to achieve rigidity, resulting in a thicker, wider, contact area; hence plastic was never a real contender for use in high-quality process print, even with a stepped down tip. This is especially important when blade-to-anilox pressure is high, as so often is the case. In addition, plastic polymers would become soft and embed into the ceramic’s porosity, changing the anilox surface wettability.
Doctor Blade Strip Steel Micro-Structure Advancements
2000s
Custom Strip Steel
- Improved Chemistry
- Smaller carbide sizes: ≤1µ
- High occurrence density carbides
- Even distribution of carbides
- Customized treatments
Flexo Application
- Extended life
- Multi-use blade
- Wear resistant
- Even edge wear
- Micro-refined wear debris
- Low friction
2010s
Custom Strip Steel w/ Coating
- Multi-layer applied coating
- Applied to HD steel carbide sizes: ≤3µ
- Corrosion Resistance
Flexo Application
- Extended life metering white ink
- Multi-use blade
- Wear resistant
- Even edge wear
- Micro-refined wear debris
- Low friction
2020s
Custom Strip Steel w/ Enhanced Coating
- Multi-layer applied coating
- Enhanced custom coatings
- Applied to HD steel carbide sizes: ≤1µ
- Even distribution of carbides
- Customized treatments
- Enhanced corrosion resistance
Flexo Application
- Extended life metering white ink
- Multi-use blade
- Wear resistant
- Even edge wear
- Micro-refined wear debris
- Low friction
Swedish Doctor Blade Steel
In 1980, a typical OEM doctor blade spec described using “the best Swedish Blue Strip Steel.” Swedish steel evolved from its long history of high-quality iron production dating back to the 16th and 17th centuries. It turned out Swedish iron ore contains the proper percentages of trace elements to make very high-quality alloy steel. In the later part of the 19th century, Swedish steel took a leadership position in the development, production and fabrication of the metal. Today, Sweden is recognized in the world for producing among the highest-quality strip steels used for production of scientific instruments, defect-free razor blades and, more recently, doctor blades.
In the beginning, when steel was selected as the best material to precisely meter ink, it was a general-use strip steel. It performed well enough against coarse line anilox at slow press speeds. However, as anilox line counts and press speeds increased, the industry realized a more specialized steel was needed.
Customized Doctor Blades
In recent years, new specialty strip steels have been developed. Changes to chemical composition combined with modified hot rolling, cold rolling and tempering has resulted in much slower and even wearing doctor blades while making them less abrasive against an anilox.
As these new customized blades slowly wear, they yield a fine micro powder rather than anilox-damaging slivers and chunks. In most cases, extended blade life is a result of a higher occurrence of ever smaller, harder and evenly distributed micro particles in the steel’s composition. These materials are far less hard than the ceramic on an anilox.
A pressroom producing high-definition flexography at high speeds can now use one top performing blade for multiple days, on multiple jobs, in place of 10 or more single-use blades—even on white inks. Their higher cost is easily justified, as waste and downtime are reduced and annual blade spends drop. They also require a lot less handling, thereby reducing safety concerns, while achieving a 10-fold reduction in the disposal of used blades.
Developments leading to new types of doctor blade steel did not happen overnight. It is the result of an evolutionary response over many years to changes made to flexography’s ink transfer process—changes to anilox, ink metering systems, inks and new, faster and better controlled press technology.
Today, a majority of flexo pressrooms recognize doctor blades as a truly key process control mechanism. Few still buy blades on price. Instead, buyers do the math. The right blade matched to your press can achieve real and sustainable savings. Check with your doctor blade supplier. They can help you select the best one for your application.
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