- January 01, 2002, Dan Foy, Harper Corp. of America
Despite the amount of technology that goes into the manufacture of a laser-engraved, ceramic anilox roll, the inspection and care given this precision metering instrument can be very simple and relatively inexpensive.
Or, it can cost nothing and be very expensive. The expense comes from unnecessary anilox roll purchases, long set-up times, wasted press time, wasted ink, wasted stock, and lost business.
If you have access to the entire engraved surface, you can use simple inspection with the naked eye to determine the serviceability of an anilox roll. New anilox rolls are properly engraved and have thin cell walls and large openings. This roll surface has a dull, velvety sheen since the roll surface absorbs light.
Anilox rolls with significant wear or polishing have wide walls and appear shiny because wide walls reflect light.
Any blemish on the engraved surface of an anilox roll — cosmetic or structural in nature — will manifest itself in the printed substrate. In other words, the presence of cosmetic blemishes such as a drip mark from rinsing the roll with water will affect how the anilox receives and/or releases the ink, and it will show in the printed substrate.
An anilox roll not cleaned properly after its last use often will show the pigment color of the last ink used and obviously must be cleaned.
Score lines, caused by contamination in the ink system, are easily detected without magnification. They should be identified, documented, and addressed before the roll returns to the press and lines show up on the substrate and, more importantly, other anilox rolls are damaged.
The end or corner of the ceramic surface tends to be the first area on the roll to be damaged. This damage may not seem important since it is out of the print area, but what about doctor blades and end seals that wear against these areas? This rough surface will remove large chunks of doctor blade material, and the blade may pick off loose pieces of ceramic. End seals wearing against a rough surface cannot seal efficiently and are worn prematurely. This scenario will contribute to the presence of score lines.
A well-lit area where the anilox roll is supported by precision bearing blocks is all that is needed to inspect the engraved surface visually. These blocks also are necessary to evaluate taper and indicate runout — two critical dimensions you can measure with micrometers and dial indicators, respectively.
When you sign for a new/reconditioned anilox roll, you assume responsibility for the roll, which may have been damaged in transit. Open the container and inspect the engraved surface for damage before you sign for it.
As soon as possible, measure the diameter for correct size and taper tolerance, then rotate the roll on bearing blocks with a dial indicator to measure runout of the engraved surface compared to the bearing surfaces. This is very important, since some new/reconditioned rolls sit in their shipping containers until needed. Any significant passage of time can complicate the warranty process if and when damage is found. The time to discover damage or a nonconforming dimension is not when the roll is needed for a press run.
So far, you have invested in bearing blocks, micrometers, and a dial-indicator for an amount of money that typically is less than the cost of one new anilox roll. (Your anilox roll supplier can provide specific prices for equipment mentioned in this article for the purposes of comparison.)
On to Hand-Held Magnification
After visual evaluation, the next logical step is to inspect the engraved surface closer with the assistance of a magnifying device. You need to be able to inspect the engraved surface close enough to determine if the surface is plugged, worn, or damaged.
The line screen counts of the engravings in a printer's inventory will dictate what magnification is required for a simple evaluation. Loupes used to inspect print samples typically will not magnify more than 20x and are of minimal or no value for inspecting a laser-engraved, ceramic surface.
The lowest magnification practical to inspect this surface is a 30x, battery-operated, hand-held scope. With this it is possible to inspect screen counts to 300 cells/in. with minimal experience. With more training and experience, inspection to 440 cells/in. may be possible. Even with its limitations, the 30x scope is a valuable supplement to higher-power magnifications.
With basic hand-held microscopes, printers have the ability to inspect anilox rolls before putting them in the press or the off-press cleaning system. (1) engraving with thin cell walls, smooth surface and clean bottom; (2) engraving worn beyond use for most applications, this roll would have a very shiny surface; (3) this engraving is severely plugged except for one cell; and (4) score line on the anilox roll under magnifcation--this line also will be visible to the naked eye and in the print.
As in other areas of flexography, more is not always better. Higher magnifications limit the field of view, and if the inspector is looking for localized damage such as score lines or damage from impact, he or she may never find it with a higher magnification, due to a limited field of view. With a bigger field of view, it is easier to determine the difference between damaged and undamaged or plugged and clean cells.
For line screens to 600 cells/in., the battery-operated scopes also come in 100x magnification. Both the 30x and the 100x scopes have limitations for anilox roll inspection, but they are also the least expensive way to put a valuable tool in a printer's hands to determine if a roll is serviceable for a press run before it goes in the press.
The cost of the visual evaluation equipment and battery-operated microscopes will still be lower than that of one anilox roll.
More Ability and Cost
The next step up in this category is the light-assisted loupe with magnification capability to 200x. Significantly more expensive than battery-operated, hand-held microscopes, it offers increased power for easy inspection of screen counts to 800 cells/in. The ability to measure screen count and wall thickness is possible with a linear scale reticle in the eyepiece of the loupe.
We are now above the cost of some narrow web anilox rolls, but if just one press setup is significantly shortened, the equipment pays for itself.
For printers with larger anilox rolls and larger inventories, the cost of a gravure microscope can be justified.
In its simplest form, the gravure microscope is a heavy, portable platform allowing inspection of the engraved surface with objectives from 50-1,000x magnification.
Measurement of line screen, wall thickness, and opening dimensions is possible with a linear scale reticle in the eye-piece.
An analog or digital depth gauge affords the opportunity to measure cell depth, a critical dimension to the volume of the engraved cells.
With additional equipment, the basic gravure microscope can be transformed into a video inspection system. A video camera, video monitor, and video overlay system combine to measure screen count, wall thickness, and opening dimensions that will provide theoretical or calculated cell volume measurement.
Cell Volume Measurement
The purpose of the anilox roll is to transfer ink to the printing plate. The means by which it accomplishes this is the capacity of the engraved cells, referred to as volume.
Cell volume is the primary controlling factor in flexo. If the printer's process is optimized through scientific testing, the anilox rolls have been optimized and standardized.
That testing should include optimizing the printing press; conducting Graphic Team Meetings with suppliers; running banded roll tests; and running process characterizations.
When optimized for various elements required of the printer, including solid coverage, line work, screens, process work, and combinations, the anilox roll's performance is standardized to reproduce line colors, solid ink density standards, and acceptable dot gain values.
Part of the incoming inspection pro- cess should be the comparison of solid ink density and/or line color transferred by the new roll to established standards.
This comparison of roll performance to established standards also can be used throughout the life of the anilox roll by utilizing running targets on live jobs.
The ultimate method now available to measure cell volume is the Interferometric Measuring System, commonly called WYKO (mfr.'s name). Based on the principle of white light interferometry, this system takes the human element out of the measurement process, making it accurate, repeatable, and statistically verifiable. Volume tolerances can be as low as ±4% (0.10 BCM).
Your anilox roll supplier can and should provide support for inspections, as well as ongoing pressroom training. However, since suppliers are not typically in a printer's facility on a daily basis, be prepared with appropriate tools and knowledge to diagnose and address questions.
You may be asking if you can afford to meet these challenges internally. Can you afford not to?
Dan Foy is a technical product specialist at Harper Corp. of America, Charlotte, NC. He spends much of his time at converting facilities performing roll audits, making anilox and coating roll recommendations, performing banded roll trials, providing in-plant seminars, and assisting customers with print troubleshooting. He can be reached at 704/588-3371; 800/438-3111.