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FLEXO Magazine : June 2011
Technologies & Techniques Cell Geometry: Why It Matters and How It Affects Print Results ultimate Control of ink Film Thickness is Yours By Dan Foy The anilox roll is a precision metering device designed to transfer a measured amount of ink or coating to the printing plate, tint screen or substrate. Almost univer- sally, whether English or metric units, it is typically specified by line count and cell volume. That is, if the engraved surface has 800 cells per linear inch and a volume of 3.0 , then it is a 800 LPI @ 3.0 BCM. Key specifications for this precision metering device are: • Volume--described as “the most controlling factor in flexo printing.” Volume is critical to every aspect of a flexo application; dot gain, print contrast, solid ink densities, combinations, line work, solid coverage and coating weights or thicknesses. Unit of measure: English = billion cubic microns (BCM) /square inch. • Line Count--unit of measure = cells per linear inch (@ angle of engraving). • Cell Angle--unit of measure = degrees (°). In relation to longitude axis of the roll. • Cell Shape--geometric pattern of the engraving. Though it is listed last, overlooked and taken for granted, the cell shape of an anilox roll can mean the difference between success or failure, acceptable product and entrance into new markets; or lost time, wasted materials, rejected product and lost opportunities. What then, are your choices for engraved cell shape and what affect can they have on; print quality, process efficiency, solid coverage, material lay down and clean up at the end of a pressrun? Under the best conditions, the anilox roll delivers a deposit efficiency of less than 25 percent for flexo applications, and less than 50 percent for gravure applications. Hydraulics, pneumatics, surface tension and ink rheology all combine to seemingly work against us. Cell shape may not affect these numbers dramatically, however, the resulting ink or coating film transferred could measure out more accurately, as well as be more visually appealing. In almost all ink applications (Pantone / process colors) the cell shape is presumed to be 60-degree hexagonal. Why? Because these cells are created by a round laser beam allow- ing precise placement of the laser blasts to facilitate thin cell walls with large open areas to carry ink. What are the other options for cell shape and what do they have to offer in terms of print quality, solid coverage, material lay down and clean-up at the end of use? 45° QUAD / DIAMOND Earliest laser-engraved cell shapes tried to duplicate existing mechanical cell shapes. Here we are trying to make a square or diamond shaped cell with a round laser beam which leads to wider walls and large post areas compared to other cell shapes. Unfortunately, as process printing grew in capability and popularity, a phenomenon called a moiré pattern was caused by the conflict of separated screen angles on the printing plate with the continuous walls of the quad shaped cells and showed as a flaw in the print. Historically, quad shaped cells offered an option for high volumes from early laser-engraving technology--up to 75.0 BCM or higher for extreme coating applications. Now, with few exceptions, the quad cells are replaced by more efficient shapes being produced by current engraving technology. New engraving technology can sculpt-out better geometry than in the past, but to what end? TRIHELICAL Similar to the creation of laser-engraved quad shaped cells, the trihelical cell pattern was developed to emulate a mechanical cell shape. The trihelical is best described as a 50 FLeXO June 2011 www.flexography.org Sizing Up the Shapes... • New, diamond engraving technology can sculpt-out better geometry than in the past. • Trihelicals transfer materials with large particle sizes, like varnishes or coatings. • The 60-degree hex offers the ultimate control of ink film thickness with all six walls being the same height, when made properly. • The 30-degree hex, when specified and made prop- erly, incorporates a channeling effect, allowing the flow of inks and or coatings underneath the doctor blade.
Sustainable Spring 2011