Advantages of Superfinishing Your Anilox Rolls
Submitted By: David Lanska, Stork Cellramic
When a ceramic anilox cell is created by laser, some of the ceramic is vaporized, some is blown away by air jets, and some is redeposited on the surface of the roll. This redeposited slag (recast) results in a rough roll surface that can quickly wear down doctor blades and cause ink metering difficulties. Microscopic slag peaks can also cause a doctor blade to vibrate, creating an audible whining sound. That sound indicates that the blade is actually bouncing, losing close contact with the anilox surface.
The premise behind the use of a doctor blade is that the volume of ink delivered to a plate is controlled through anilox cell size and geometry. The doctor blade cleanly shears the ink from the surface of the anilox roll, leaving ink only in the cells. If the blade is bouncing, or vibrating, due to recast, volume control is lost.
Anilox rolls rough with recast tend to deliver higher ink volume initially, because the vibrating blades are not shearing ink cleanly from the surface of the anilox roll. Over a relatively short period of time, the peaks are worn or broken away, allowing the blade to come more fully in contact with the cell wall surfaces. As a result, the volume is reduced from its “artificially” high initial delivered volume, and falls more in line with the long-term volume expectations for the roll. Therefore, the smoother a roll is when it comes from the manufacturer, the more smoothly a doctor blade will ride on its surface, and, the more fully the blade contacts the surface the more consistent the ink volume delivered by the roll will be, over time.
The engraved surfaces on some (particularly lower line count) anilox rolls can be so rough that they can literally rip chunks of metal from doctor blades, leaving beads of un-doctored ink in the web direction. The loose chunks of blade material can become wedged under the sealing blade in closed-chambered blade systems. When this happens, the metal is drawn past the wall structure, breaking down cell walls in the process. The trough created in this fashion looks like a scratch on the roll surface and is referred to as a score line. Anilox rolls scored badly enough generally must be sent back to the manufacturer for re-covering, as the printing results from using them tend to show unacceptable lines in the print.
Many of the difficulties resulting from ceramic recast can be helped by superfinishing. Superfinishing is the process of polishing the surface of the anilox roll after engraving. Interestingly, as ceramic is second in hardness only to diamond, it takes a diamond-imbedded grinder wheel to grind it, and diamond-imbedded paper to polish it by hand. The diamond particles used are smaller than 3 microns across.
Superfinishing removes the rough peaks from the cell walls resulting in a smooth, even surface for doctor blades to ride on. It all but eliminates doctor blade vibration, and greatly reduces doctor blade wear. Because the blades glide across a more consistent surface from the start, the inconsistencies in delivered ink volume over time caused by anilox roll surfaces rough with ceramic recast are reduced.
To a much lesser extent, the problems with recast also occur with YAG laser-engraved rolls. Though the YAG laser more completely vaporizes ceramic and has a much shorter melt phase, a small amount of ceramic is still redeposited on the cell wall surfaces. A very light superfinish again provides a smooth doctoring surface and more consistent delivered volume over time.
While it is possible to introduce ink density variations across the roll width by superfinishing, when done properly, this rarely occurs, as very little material is removed in the process. Superfinishing is not meant to drastically change cell volume. Instead, it is meant to shave off the rough peaks. By doing so, the inherent inconsistencies in wall height are evened out and the surface is brought down to the point that most of the cell wall is touched (polished) by the diamond-imbedded paper. Superfinishing removes mostly the erratic slag peaks, leaving the cell wall structure intact, only lightly touched by the finishing process. It actually improves consistency. We have always believed in superfinishing and its benefits for printing performance.
Superfinishing can be a touchy subject among anilox roll manufacturers. One reason is that superfinishing tends to highlight minute variations in cell wall peak heights, making them appear as “imperfections”. To the human eye, the tops of the cell walls of an anilox roll that has not been superfinished all look like they are “in focus”. However, superfinishing provides a means for the human eye to differentiate peak heights on the magnitude of only a micron. From a manufacturer’s standpoint, superfinishing means admitting that laser engraved cells are not perfect. It means looking through a gravure scope and seeing some white polished areas against dark areas that have not been polished.
For most, it is much more palatable (and sellable) to look through the scope and see no evidence of superfinishing, since there is then no significant contrast in the reflected light to give away the ever so subtle variations in peak height. These inconsistencies in peak height are present on every laser engraved anilox roll provided by every anilox roll manufacturer.
Although superfinishing can highlight these inconsistencies, it offers so many important benefits that it should not be quickly dismissed. Superfinishing provides a consistent ink-doctoring surface, reduces doctor blade vibration and ink pooling, and insures consistent ink delivery over time. By providing a smooth and polished surface, superfinishing also helps reduce the incidence of anilox scoring and significantly slows doctor blade wear. It reduces the amount of blade material going into the ink and reduces losses in productivity resulting from frequent doctor blade replacement.
E-Mail the Author: David Lanska, Stork Cellramic
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