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Using Doctor Blades to Control Ink Film Thickness

When converting to or using doctor blades to control ink film thickness, there are several things to consider. A few factors to examine are the type of work that will be done, line count of the anilox roller, type of doctor blade material (steel, plastic, composite), design of the tip and types of steel.

First, let’s discuss the type of work. Will it be line work or process color? Normally, this will dictate the line count of the anilox roller. Most line work is done using lower line count anilox with higher cell volume therefore, a thicker doctor blade is needed to control this heavier volume of ink. Most printers use .006” thick carbon steel blade without a preground tip or a radius edge for this type of work. The blunt, 90-degree tip will hone itself in a short time and is the most cost-effective choice. A radius edge hones itself faster, however, the additional manufacturing time needed to produce a radius edge on both sides of the blade adds to the cost. The full thickness of either blade provides a wide contact area with the roller that slows down doctor blade wear. This is especially helpful with abrasive inks. You may even increase the blade thickness to .008” or even .010” to help control these inks. Keep in mind, a heavier blade of this type produces a wider contact area with the roller. The wider contact area results in a lower actual wiping pressure because the actual wiping force is spread out over a larger area. As a result, extra blade pressure is then needed to achieve a clean, consistent wipe. This additional pressure can result in unneeded wear on the doctor blade and the anilox roller.

For some applications you may also use plastic or a plastic composite blade. These materials normally wear slowly, are easy on anilox rollers, and are safer to use. Most of these materials need to be thicker to provide the needed support, therefore, the contact area is normally wider than steel and the wipe may not be as clean. Plastic and plastic composite blades are very popular with corrugated printers. Blade systems on many corrugated presses are not easily accessible and changing blades may take several hours. For this reason, the durability of plastic blades makes them the better choice.

Next, let’s discuss what type of blade to use for higher line counts on the anilox. Using higher line counts normally requires some type of preground tip. A thinner ink film thickness is needed; therefore a smaller contact area with the surface of the anilox roller is necessary to give a cleaner wipe. The smaller the contact area, the less pressure needed to obtain a clean and consistent wipe. By using different preground tips, either a beveled tip blade or a reduced tip thickness (lamella) blade, the contact area can be reduced to achieve a thinner ink film thickness. A bevel blade, a blade with a pre-ground bevel, normally between 2 and 45 degrees is most widely used on mid-range line counts (400-600). This type of tip provides a short wear-in period and a smaller contact area for a longer period of time when compared to a straight or radius edge. This slows anilox wear by allowing less material to contact the surface of the roller as well as allowing you to run with less pressure. How quickly the contact area can change depends on the degree of bevel.

When printing process color with line counts over 600, it is best to keep the contact area small and constant. A clean, consistent wipe is achieved with light pressure, which protects the thinner cell walls present on these rollers. This is done in two ways, first, by using a thinner steel thickness (.004”) and second, by using a lamella tipped blade. The thinner steel provides this constant contact area as it wears, but may not provide the stability needed to restrain more viscous inks. By using a lamella tip the contact area stays constant as the blade wears, while the thicker body of the blade provides the needed support. Different lamella thickness’ are available depending upon the desired result. Thicker lamella tips, 125 - 150 microns, yield longer life while thinner tips, 75-95 microns, yield a cleaner wipe with less pressure. Once this choice has been made, longer blade life may be achieved by using blade with a protective coating. These coatings protect the blade from abrasive or corrosive inks. Abrasive or corrosive inks can create problems with any doctor blade. To achieve the best results, the technology of the lamella tip can be combined with a coating to increase blade life, lower coefficient of friction, and lessen anilox wear.

Finally, let’s discuss steel types. There are three basic types, blue carbon steel, white carbon steel, and stainless steel, with only small variations in each. Blue and white carbon steel is very similar in chemical composition. Through an oxidation process the color is changed but this does not change the performance of the blade. Consistent, inclusion-free grain structure of the steel is important for achieving longer doctor blade life and low anilox wear. There are variations in the carbon particle size; however, the smallest particle size does not always prove to be the best. Most stainless steel is close in comparison to white carbon steel but with chromium added to protect against corrosion. Special applications require variations of the doctor blades mentioned above. Some UV printing requires a heavy doctor blade to help restrain more viscous inks. However, a thin tip is most often needed to achieve the sharp image associated with UV printing.

As you can see, there are many factors to consider when choosing doctor blades, but with the large variety of blades available, you can guarantee quality results every time!

By Marty Cansler, Max Daetwyler Corporation

Max Daetwyler Corporation is a member of the PICA Flexo Services Committee