Doctor blade

In rotogravure printing the doctor blade (from ductor blade) removes the excess ink from the smooth non-engraved portions of the image carrier and the land areas of the cell walls. Doctor blades are also used in other printing and coating processes, such as flexo and pad printing for the same function. It is believed that the name derives from the blades used in flatbed letterpress equipment for blades used to wipe ductor rolls, and "ductor" became doctor.

Given that a blade is necessary to remove the excess ink from a cylinder which is continuously running, the blade/cylinder combination is designed such that it is (chiefly) the blade which wears away, not the cylinder. The doctor blade is cheap and disposable and the gravure cylinder or flexo anilox is relatively expensive. Typical materials for the blade are steel or plastic: gravure cylinders are normally hard-chrome plated and aniloxes may be chromed or ceramic.

Important process variables in gravure coating & printing are the rotational position at which the blade wipes the cylinder; the angle that the blade makes with the tangent to the cylinder at the contact point; and the pressure applied to the blade. Fine adjustment of in-and-out position, independently on left and right, must also be possible, in order to compensate for any misalignment between blade geometry and cylinder axis. These requirements are fulfilled by a doctor blade assembly which includes a doctor blade holder.

In order to keep the wear pattern as even as possible, and to help prevent any particles in the angle between blade and cylinder, all except the smallest machines use oscillating doctor blades. The entire doctor blade assembly oscillates from side to side. The oscillation, flexibility of the blade, and the various possible misalignments and problems due to a lack of stiffness may result in the failure to achieve the desired angle of contact between the blade and the cylinder, or at least a failure to consistently achieve this desired angle.

Of necessity, the doctor blade must be easy and quick to replace. On many machines, the doctor blade holder is easily removable from the machine and the replacement of a doctor blade is done by removing the blade holder from the machine; discarding the old blade; inserting the new blade; and then replacing the blade holder + new blade in the machine.

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Wikipedia