Coatings
Specialized coatings have been developed to further increase the wear resistance of blades, while also offering additional benefits to protect anilox rollers, such as added lubricity. Most blade coatings are nickel-based, which increases lubricity (see Figure 3). This coating will reduce the friction of the blade, as well as provide anti-corrosive properties.
The nickel coating can be chemically altered to further increase wear resistance, as well as naturally increase the lubricity, thereby reducing friction and potential scoring (see Figure 4). Small hardened particles, such as ceramic, can be suspended in nickel coatings to further increase wear resistance.
TKM offers an additional ceramic coating that is plasma sprayed to the blade surface—a similar process to how anilox rollers are coated. This process optimizes the wear resistance and lubricity even further. This type of ceramic coating offers very low friction at lower working temperatures in comparison to alternative coatings. The result is the optimal combination of extremely long lifetime, along with the elimination of possible damage to the anilox surface by metal slivers or burrs.
A true ceramic sprayed surface offers the best solution whether you are printing with white inks containing high amounts of abrasive titanium, high-volume anilox rollers with rough surface or simply want the longest possible lifetime from a doctor blade.
Thickness & Tip
Doctor blades should be somewhat flexible to ensure they are gentle to the anilox roller. The most common thicknesses are between 0.006-in. and 0.010-in. If the doctor blade is too rigid, it cannot accommodate the flexibility that is needed to provide the thin ink film on the anilox surface for lubrication. Less lubrication can damage your anilox, and also impact the lifetime of the doctor blade.
Most applications can use a round or parabolic edge. Square edge doctor blades, with a rough or square edge from slitting the steel to width, should not be used in flexographic printing. The rough edge can damage the anilox. If you experience squeaky sounds from your doctor blade at startup, or while running the press on slow speed, the cause may be the use of a doctor blade with a rough edge. In this situation, a round or parabolic edge can help.
The surface of the blade tip should be very smooth and free from nicks or grinding marks to ensure an immediate, clean wipe, and reduce any chance for burr formation (see Figure 5).
For high linescreen and low-volume anilox rollers, such as those typically used in expanded gamut (EG) printing, doctor blades with a pre-ground edge are common. The pre-ground edge means the doctor blade thickness at the tip is reduced to only 0.003-in. to 0.005-in from the original thickness of 0.006-in. to 0.010-in. (see Figure 6). This small tip will result in less ink being transferred to the plate for better control of the tightest density or reproduction standards. In many cases, a coated blade is used to increase lifetime because of the reduced thickness at the tip.
Conclusion
Doctor blades are an integral part of an efficient, repeatable printing standard. There are many choices for materials, coatings and tip designs. Choosing a blade that provides a clean wipe along with effective wear resistance, while still being gentle to your anilox rollers, is key. Be sure to work closely with your blade supplier to determine the proper doctor blade platform for your unique operation. Some suppliers are even offering free apps for troubleshooting doctor blade and anilox issues.
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