Substrate Holdout and Pinhole Formation in Inline Flexographic Coating
Substrate holdout and pinhole control require matching anilox coat weight delivery to sheet absorbency while regulating dynamic wetting and surface skinning.
Reverse Doctor Blade
Coating Control
Positioning a flexible steel or plastic strip against a rotating roller in a direction that opposes the surface movement acts to shear away excess fluid. This mechanical arrangement, known as a reverse doctor blade, is commonly used in gravure and flexographic coating systems to meter a precise film of liquid. The setup ensures that only the fluid contained within the cells of the roller is transferred to the substrate.
Shearing Action
The working edge of the steel strip faces into the oncoming liquid stream to wipe the surface clean. This orientation utilizes the pressure of the fluid to help hold the tip against the roller, which creates a very uniform metering action across the width of the machine. It operates with lower mechanical pressure than a forward-angled system, which extends the life of both the blade and the ceramic roller.
Proper alignment and correct contact angle are necessary to prevent issues such as weeping or hydrodynamic lift. If the blade lifts, too much fluid passes through, which leads to heavy streaks or uneven coating distribution on the final paperboard.
Wear Resistance
Modern coating heads often use ceramic-coated steel strips to withstand the abrasive action of pigmented clays. This material choice reduces the need for frequent blade changes during continuous runs.