Chambered Doctor Blade Coaters are the Key to Precision Offset Coatings
Flexo printing has long utilized doctor blade metering systems to precisely control the amount of ink delivered to an anilox roll. This results in the anilox roll delivering a predictable and repeatable quantity of ink to a plate and a substrate.
Because of these qualities, chambered doctor blade systems are used on Central Impression (CI) presses, most wide presses, numerous corrugated, and some narrow web presses. Chambered doctor blade systems have also become the dominant coater type used on high-speed sheet-fed offset, web offset, and digital printing presses to apply coatings.
The development of aqueous and UV coatings and a growing quest for print protection, embellishment, and added value, have provided the impetus for an accurate coating application system. Applications are found in packaging, labeling, commercially printed covers, and shrink-wrap sleeves.
Integral to this success has been the development of the laser-engraved ceramic anilox roll, which is durable and does not wear over time. This means that cells engraved into ceramic, to carry a precise determined volume of liquid, are consistently the same.
Chambered Doctor Blades: A Time-Proven System
Chambered doctor blade, laser-engraved ceramic anilox roll systems proven by years of use in flexo are effective metering inks, UV, aqueous coatings, and primers. Now proven in offset, the application is precise, uniform, and consistent. Scratches, voids, thickness variations, slinging, and ribbing, all common problems to be guarded against with multiple roll systems, are eliminated.
A chambered doctor blade system functions based on the creation of an enclosed fluid-containing chamber with two doctor blades and two end seals running against an anilox roll to effectively contain a fluid within. One blade, set to a reverse angle, meters, or scrapes fluid from the anilox. The second blade mounted in a forward angle wipes fluid from the anilox surface to contain fluid inside the chamber. Some compare the reverse angle blade to an auto windshield ice scraper with the blade scraping fluid from the anilox surface.
The forward angle blade can be compared to a squeegee wiping fluid from the anilox leaving the surface clean with only the fluid volume filling engraved cells. The chamber itself is locked in a bearing locating it correctly and accurately to the anilox. A pneumatic system maintains a constant pressure between the chamber and the anilox chamber ends which are sealed liquid-tight by gaskets.
When the anilox contacts a raised surface plate or blanket cylinder the fluid volume contained in the anilox cells is precisely released for transfer to a passing substrate. The enclosed chamber minimizes evaporation, changes in viscosity, and contamination of inks/coatings. After initial setup, contact pressure is automatically regulated compensating for doctor blade wear.
Anilox Roll and Transfer Efficiency
It’s important to recognize that not all the ink/coating in the cells of an anilox gets transferred to the substrate. The amount of ink/coating that transfers is called Transfer Efficiency. For Flexo, it typically ranges from 19% to 24% of the engraved cells’ fluid volume. Transfer efficiency must be considered in the design of an anilox.
The heart of the flexo printing/coating process is the anilox. Early rolls were covered in copper & chrome using 45° quadrangle geometry. In the 1970s, 60° hex honeycomb geometry appeared followed by 60° ceramic laser engraving, which remains the most efficient geometry to date.
Variables to be decided in anilox engraving are pattern (screen angle), screen count, and cell volume. The pattern is the engraving angle measured from the axis of the roll: 30°, 45°, and 60° are the most common. Thick film coatings use 45° tri-helical and quad patterns. Thin film coatings use a 60° hex pattern, the same as flexo inks. The screen count of an anilox is the number of engraved cells per linear inch along the engraving angle.
Generally, the higher the screen counts, the smoother the application. The fluid carrying capacity of an anilox is measured in billion cubic microns (BCM)/in² of anilox surface area. One billion microns = one micro-liter, or one-millionth of a liter. European anilox volume is measured in cm³/M² (cubic centimeters/square meter). A BCM is converted to the European unit by multiplying the BCM by 1.55.
The European unit is converted to a BCM by multiplying by 0.6455. For example, 12 BCM/in² x 1.55 = 18.6cm³/M². Volume can be chosen independently of the screen count. Application volume depends on the % solids, rheology, and the weight per gallon coating as applied since coatings are applied by volume, but are weight specified.
Ceramic anilox rolls are fabricated using plasma torch spraying to deposit ceramic onto the surface of a steel roll. Chromium oxide (ceramic), extremely hard and low in porosity, is commonly used. Diamond wheels are used to grind the ceramic roll surface to precise dimensions and tolerances. Engraving uses laser-generated heat to vaporize a predetermined pattern of microscopic holes (cells) into the ceramic coating.
Laser engraved cells are hemispherical or droplet shaped in cross-section. After engraving the roll is finished by fine polishing. Laser engraving advancements include CO2 gas fired lasers later adapted to YAG engravings that provide 1200 LPI line screens and higher. The addition of digital fiber optics now offers the most accurate and advanced laser engraving process.
The use of specialty inks, opaque whites, fluorescents, and embellishment coatings have placed higher volume, and lower line screen cell geometry demands on anilox design. Tri-helical, 30° open channel and other geometries have evolved to allow free flow and transfer of higher viscosity inks and coatings. These engravings allow heavier coat weight applications, higher opacities, and improved transfer rates.
Proper Cleanup of Anilox Roll is Important
Proper anilox cleaning is extremely important as cell volume can change radically with dry ink/coating build-up. Simply flushing an enclosed chamber system with an ammonia-water mixture will clean up aqueous ink and coatings. However, there can still be a drying-in tendency of product in the anilox cells themselves. Never allow inks, or coatings to dry on an anilox roll.
Use clean-up solutions recommended by your ink/coating supplier. Always use a stainless-steel brush when cleaning a laser engraved roll, never use a brass brush which will transfer brass to the hard ceramic surface, so that ink/coating will be repelled.
Clean by working the stainless brush in a circular pattern with heavy pressure reaching into the depth of cells. After cleaning, flush with the recommended solvent, and wipe with a lint-free cloth. If water is used, finish with an alcohol wipe to remove any moisture left in the cells.
Corks’ business is the development and formulation of Aqueous, energy-curing Ultraviolet (UV), and Electron Beam (EB) specialty coatings and adhesives. Let us know if need any assistance developing your next project. Our team of experts is here to help.
