The doctor blade appears to be a simple component of the flexographic printing process but, as often is the case, appearances can be deceiving. The casual user might wonder what is there to think about, a blade is only a piece of steel or plastic. To better understand the need for all of the doctor blade options, consider the role of the doctor blade.
Flexographic printing requires the Dr blade to provide a constant wipe throughout the pressrun, so that the ink volume carried by the anilox roll to the plate is determined only by the anilox volume. If the doctor blade is not working correctly, the ink volume carried to the plate will include the anilox roll volume, plus some amount of surface ink Any sur-face ink remaining on the anilox roll will be variable and lead to variation in the printed product. To achieve constant wipe, different materials and edge profiles are available so that you can better match a doctor blade to the application.
Years ago, there were only a few choices for doctor blade materials and profiles. Today, the offering of materials, edge profiles, and added coatings has become so extensive that the converter often needs help from a doctor blade supplier to determine the best blade for the application. This article will take some of the mystery out of choosing a doctor blade by providing an overview of the features of the various doctor blade materials and configurations along with generalized application guidance.
Long before doctor blades were used in flexographic printing, blades made from various types of steel had been used in other printing processes. Today, steel is still the material of choice for high quality and repeatable doctoring results in any printing process. There are bright and blue carbon steels, stainless steel, long life steel, coated steel, and ceramics. But which steel is right for you?
First of all, bright and blue carbon steels are identical materials that share the same metallurgical properties and features. The only difference between the two is the cosmetic blue oxide process that is applied to the steel. It has been rumored that blue steel was originally chosen for doctor blades, so that when a converter was making a blade by hand shaping a bevel, he could easily see the bevel he was making as the blue color was removed. Today, most converters are not making their own blades, so the advantage of blue steel doesn’t apply anymore.
Carbon steel blades are economical choices when used with short-run process jobs on non-porous substrates and inks that aren’t very abrasive. They can be used on all anilox screens along with solvent, water, and some UV inks. If corrosion is an issue, a stainless steel blade may be a better choice, but use caution when using stainless steel blades with ceramic anilox rolls as some stainless steel materials have been associated with plugged anilox cells through adhesive wear.
Long-life steel blades are excellent for use with abrasive inks, such as white inks or other inks with high percentages of titanium dioxide, or solids and/or rough anilox rolls. Long-life steels are typically made from tool steel alloys that offer good resistance to adhesive wear. Adhesive wear is a welding like effect that quickly causes blades to fail and is also a contributor to anilox roll scoring. Long-life blades are more expensive than carbon or stainless steel blades, but the benefits they provide easily justify their added costs when compared to press down time for blade changes during a run and scored anilox roll repair costs.
Coatings can be applied to steel blades to further extend their life and the life of the anilox roll. A coating will lower the coefficient of friction between the blade and anilox roll, resulting in a clean wipe at lower pressures. However, the metal used in a coated blade has to be the same high quality steel as an uncoated blade, or the blade will not function properly. Another advantage with coated blades is that they typically offer enhanced corrosion resistance. Try a coated blade in your application if you are looking for a little more life, less corrosion, or a cleaner wipe than you are currently getting from your uncoated blade.
Ceramic blade technology will yield the longest life and comes at the highest cost of all blade materials. Ceramic blades are typically used with very abrasive inks or where you are running four-color process work every day with standardized setups. Other applications may include varnish or coating applications and corrugated applications, where it could take hours to change a blade.
All of the metal blades discussed can be used for doctoring applications as well as containment in dual blade flexographic chambers. Metal blades can vary in thickness from 0.004 in. to 0.020 in. and even thicker in some cases. Typical blade thicknesses are either 0.006 in. or 0.008 in. with more demanding applications requiring the use of 0.010 in. or 0.012 in. thick blades. blades are typically used with very abrasive inks or where you
are running four-color process work every day with standardized setups. Other applications may include varnish or coating applications and corrugated applications, where it could take hours to change a blade. All of the metal blades discussed can be used for doctoring applications as well as containment in dual blade flexographic chambers. Metal blades can vary in thickness from 0.004 in. to 0.020 in. and even thicker in some cases. Typical blade thicknesses are either 0.006 in. or 0.008 in. with more demanding applications requiring the use of 0.010 in. or 0.012 in. thick blades.