Achieving Ideal Doctor Blade Alignment
Ideal Doctor Blade alignment is achieved when the Doctor Blade wipes ink off of the top of the cell walls of the Anilox Roll consistently and effectively. This requires that the blade holding device be properly aligned to the Anilox roll. Misalignment can effect the blade Contact Angle and the amount of loading force needed to make the doctor blade meter adequately. Since most doctor blades are made from thin flexible materials, they will conform to the roll surface aligned or not, given enough loading force. In the unaligned case, the blade pressure is steadily increased to wipe the remainder of the Anilox roll clean. Excessive blade pressure resulting from misalignment, can shorten the blade life, cause blade burring and even score the Anilox roll. If the blade holder/chamber is properly aligned this excessive pressure would not be necessary.
The blade needs to be held parallel to the axis of the roller (skewing) and parallel to the roller surface. It is also important to note that the T.I.R. (Total Indicator Runout) of the roller to be metered as mounted in its bearings, measured in the press is very important. Also, roll balance is another important issue. Any condition that causes the roll to move in an elliptical path can cause metering problems.
In addition to parallel and skew, when dealing with a chamber, it is important to keep both blades aligned. If the skew and parallel alignment is proper but the two blades do not contact the Anilox surface simultaneously, the blade that contacts first will become over loaded. This condition will drastically change the blade Contact Angle with only a small tilting of the chamber. This is less of a problem if the chambers containment blade hits first, but needless to say it is an undesirable condition.
Tilted only 2° could change the contact angle out of the acceptable range. 5° would cause the side of the blade to meter instead of its tip. Holder/Chamber alignment should be routinely checked.