The Gravure Doctor Blade
Paul D. Fleming III, Paper Engineering, Chemical Engineering and Imaging
The doctor blade is the device that removes the excess ink from the smooth non engraved portions of the image carrier. The name doctor blade is derived from the wiping blades used on “ductor” rolls on flatbed letterpresses. In common usage ductor became doctor. The doctor blade must operate durably and efficiently to ensure the quality that is expected of gravure printing. Doctor blades are also used extensively in the paper coating processes.
The doctor blade must efficiently wipe the ink from the cylinder without excess wear to itself or the cylinder. The blade must be rigidly mounted at a controlled adjustable angle. (right) The blade wiping position should be the correct distance from the nip.
This distance is chosen to balance the press speed and ink drying speed. The doctor blade must have a straight, carefully prepared edge that “seats” against the cylinder and wipes it cleanly with a minimum of pressure and wear. Blades may be “pre-honed” by suppliers or prepared in-house. The blade must be mounted in a rigid mechanism so that it is straight and ripple-free. Most doctor blades oscillate, they move back and forth a short distance across the cylinder.
This reduces local wear and helps remove debris which can collect under the blade. The force applied to the cylinder should be measurable and controllable. Some blade deflection occurs at the cylinder contact Press and holder design must take this into account. Deflection is directly related to pressure and doctor blade thickness.
Much of the wear on a gravure printing cylinder is caused by doctor blade wiping action. This wear may be abrasive wear, fatigue wear or corrosion wear. Abrasive wear occurs whenever hard foreign particles are present between the blade and cylinder as the rub against one another. Some pigments are more abrasive than others. Other abrasive particles may be insoluble ink vehicle particles, dried ink, rust, paper dust, particles of clay coating, doctor blade particles or chromium chips from the print cylinder.
In-line filtering is essential to reduce potential abrasive materials that may be introduced. Fatigue wear results from a cracking of the doctor blade from the cyclic pressure applied during use. This is less of a problem with the high grade steel now used for doctor blades. Corrosion wear is chemical damage to the doctor blade or frictional damage from a print cylinder damaged by oxidation or other chemical reaction. It can occur when corrosive materials get into the ink.
The blade itself may be corroded by rust formation when water based inks are used. Doctor blades are generally manufactured from a special strip steel, cold rolled, hardened and tempered. For corrosion resistant applications, stainless steel blades are used. There are some plastic doctor blades in use. These are made from polypropylene, Teflon or nylon in thicknesses up to .06in. They are used on coating rolls or to extend the life of a worn cylinder.
Doctor blade holder configurations vary with press designs. Presses with a wide variety of cylinder sizes require a wide range of adjustments to the doctor blade holder mechanism. Holders may be curved or straight. Curved blades reduce the “ripple” effect of uneven bolting torque on the straightness of the wiping edge. Ripple effects are also reduced when a blade is connected with an aluminum extrusion. Publication presses have “jaws”, cams that lock up blades without the use of bolts.
Optimum angles of wipe are those that minimize cylinder wear, give the cleanest printing and allow the highest speeds. The figure below illustrates the set angle and the contact angle (X).
The set angle is the angle between the radial line through the contact point and the back end of the blade. The contact angle is the actual angle between the blade and the tangent to the cylinder.
Set angles typically vary between 10° and 40°. Contact angles should be between 55° and 65°. These may be through gear leverage activated by hand cranks, pneumatic or hydraulic systems or by spring loading. Blade pressure can be controlled manually or pneumatically. Many presses are equipped with calibrated scales or gauges for control of blade angle in order to achieve repeatable results.
Gravure inks possess lubricating qualities to minimize blade and cylinder wear and to improve printability. Lubricants such as plasticizing oils, fatty acids and waxy material are commonly used in gravure inks. If the drying speed of the ink is too fast, dry ink can accumulate on the nonimage areas and contribute to blade and cylinder wear. Gravure inks are normally made from non-abrasive pigments except for opaque white, in which titanium dioxide is used for its high opacity.