If you were to look at the portfolio of blades offered by any doctor blade supplier, you would likely find a vast array of materials and bevel profiles.
The doctor blade has the seemingly simple task of removing excess ink from the surface of the anilox roll, so you may wonder why there are so many options for doctor blades. Doctor blade options exist so that the blade can be matched to the application. Choosing a doctor blade is based on two deciding factors which are wipe quality and wear resistance or blade life.
The two factors are not mutually exclusive, and one can influence the other. For example, you may choose a 15-degree beveled blade for its excellent initial wipe quality but, because of the way the bevel changes as it wears, the life of the blade may be shortened, unless the bevel profile is paired with a long-life material.
So, what causes doctor blade wear? The most obvious answer is the contact between the anilox roll and the doctor blade. The anilox roll acts like a grinding wheel and as its speed increases, so does the wear on the doctor blade. In addition to speed, the anilox engraving specification creates doctor blade wear.
A 60-degree hex engraving at 280 lpi will be more abrasive to the doctor blade than a 1000 lpi engraving. Other types of engravings, like channeled or elongated cells, will also create different levels of abrasive wear on the doctor blade. Even the condition of the anilox roll can affect doctor blade wear, with new rolls generally being more abrasive than used rolls.
Inks and coatings being run will also vary in level of abrasivity. White ink with the highly abrasive titanium dioxide pigment, along with pearlescent and metallic inks, will generate much more blade wear than typical process color inks.
Photo courtesy of Allison Systems
If you combine the 280 lpi anilox engraving with white inks for a full coverage coating, be prepared for extreme doctor blade wear. The blade shown in the accompanying image resulted from an operator not wanting to stop a run before it was completed—a run in which he did not use a long-life blade in the white ink deck. This blade should have been changed much sooner and could have created other print issues due to the extreme wear.
As previously stated, the doctor blade must provide a clean wipe of the surface of the anilox roll and, to maintain press efficiency and maximize profits, the blade needs to maintain that clean wipe for a long time, so that the press does not need to be shut down during a run to change the blade. Choosing the wrong doctor blade in an extreme wear application can lead to increased operating costs due to press downtime and excessive blade usage.
Edge Shape
To optimize the doctor blade for extreme wear conditions, you must determine the appropriate edge shape and material that will deliver the wipe quality and life you need for the print deck being considered. Remember, you do not have to run the same blade in every print deck that is being used for a job. The blade being used in the highly abrasive white ink deck can be different than the blades running in less abrasive, process color decks. Ideally, this will result in all the blades lasting through the entire run while minimizing blade costs.
Wipe quality is determined mostly by the edge shape of the blade and to a lesser degree, the thickness of the blade. There are four common edge shapes used in flexo printing, a rounded edge for steel materials, square edge for plastic materials, beveled profile, and lamella or stepped profile as it is sometimes called.
- Rounded or square edge blades work well with lower line count anilox engravings, typically up to 500 lpi. Rounded or square edge blades are the most wear resistant edge shape of all since the full thickness of the material remains
- Beveled and lamella profiles work best with higher line count anilox rolls–above 500 lpi–and will, in general, provide a cleaner wipe than square or rounded profiles
Typical thicknesses for a flexo steel doctor blade range from 0.006-in to 0.012-in, with 0.008-in being the most widely used. For high wear anilox engravings or high viscosity or abrasive inks, the blade will often perform better at 0.010-in or 0.012-in thicknesses. Plastic materials have a thickness that typically ranges from 0.020-in to 0.080-in, depending on the material stiffness. Plastic blades need to be thicker than steel to provide the stiffness required for a clean wipe.
Steel vs. Plastic
Blade material is the other consideration when dealing with extreme wear situations. Doctor blade materials can be loosely grouped into steel and plastic categories. Within the steel category there are many options to consider: carbon, stainless, and tool steel. Within each of the three subcategories, there can be different grades of steel. Historically, carbon steel doctor blade materials were the most predominant materials used for doctor blades. This is still true today, however, trying to use a carbon steel doctor blade for long runs and or abrasive inks can result in press downtime during the run to change the doctor blade. In general:
- Carbon steels work well for low wear applications
- Stainless steel will provide slightly longer life
- Tool steels perform best in extreme wear situations
Long life coatings that are typically ceramic based can be added to any of the three categories of steel to further extend their life. Depending on the coating, a carbon steel with a coating may provide longer life than a tool steel material without a coating.
Alternatively, treatments can be applied to the steel that will change the molecular structure to provide longer life without needing to apply coatings. If you can add life to the steel without adding coating, a source of potentially damaging hard particles from the coating is eliminated and the steel remains environmentally friendly, since it is easier to recycle.
There are many plastic materials used for doctor blades and some of them will provide long life. In general, plastic materials will provide longer life than basic carbon steel blades, but I don’t know of any plastics that will last longer than some of the coated or treated steel blades. If your application is causing extreme blade wear, a plastic blade might not be the best option.
Best Practices
After choosing the blade you are going to use for your extreme wear application, the last thing you want to do is create more potential for wear by improperly using the blade. With any blade application;
- Always be sure to install the blade with the bevel or lamella edge shape away from the anilox
- Always run the blade with the least amount of pressure possible, still obtaining a clean wipe
- Be sure the blade or, if running a chambered inker–both blades–are properly aligned with the anilox
Failure to adhere to any of these basic principles will contribute to poor blade performance and reduced blade life. Given the wide array of press configurations and blade products available, I suggest working with your blade supplier to select the appropriate combination of blades for your applications.
There can be many reasons why a blade performs differently than expected and you shouldn’t always assume the blade is the culprit. Your blade supplier will have the knowledge and tools necessary to optimize blade selection for your extreme wear application.
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