Blade Contact Angle Importance for Enclosed Doctor Blade Chambers
There are two angles that are of concern in Flexo metering. The first is the "Set Angle": which is the angle machined into the holder or chamber. The second is the "Contact Angle": that is the actual blade ink shearing angle. Both angles are measured off of a tangent line that is perpendicular to a line that is between the center of the roller and the point of blade contact. The difference between the angles comes about from the difference in the actual blade stiffness compared to the amount of force loading the blade (holder or chamber) toward the roller. The Contact Angle reflects the deflection in the blade caused by the loading. The following illustrations show: The no load on the left indicating the Set Angle and the 1/16" load on the right, the Contact Angle. The worn blade represented is 1/8" shorter than the new blade. Typically, loading force needs to be kept as low as possible, 1/64" could be sufficient.
The Contact Angle will always be less than the Set Angle. A worn blade in the Set position will have a larger no load angle because of where it contacts the curvature of the roll and this same reason is why when heavily loaded the Contact Angle decreases compared to a new blade. Blade wear effects the Contact Angle, so in turn the length of time that a blade can wear and meter properly is dependant on the Set Angle, loading force, roller diameter, chamber opening (blade tip to tip) and the loading mechanism.
Reverse angle (shearing) doctoring for Flexographic printing performs the best when the Contact Angle between the blade and the Anilox roll is between 28° and 35°. For the Contact Angle to stay in the above range it is best for the Set Angle to be greater than 30° and upper limit is determined by the stiffness of the doctor blade to be used. Typically there can be a correlation between blade thickness and stiffness and this can be carried to Anilox wear. So it is best to keep this in mind when selecting the blade thickness. To keep this discussion from going beyond what is needed for an basic understanding we will go into the operational aspects.
Keep the Contact Angle in the Right Range
Basically as the Contact Angle decreases, the potential for hydroplaning of the blade increases and as the Contact Angle increases so does the potential for excessive blade wearing and chattering. The blade width, the blade material, blade thickness, the Set Angle, the loading mechanism, the actual loading force and the holder/chamber and its alignment all effect the Contact Angle.
The importance of keeping the Contact Angle in the 28° to 35° range can not be over-stated. This range will provide the best metering of the ink film on the Anilox. And we can not lose site of what we are doing. That is, to deliver pigment to the substrate in the desired density. The two conditions that negatively effect the Contact Angle are excessive loading force and misalignment between the holder/chamber and the Anilox roll.