Flexography exists because of the anilox roll. This engraved cylinder, which carries the ink to the plate, gave the process its original name: aniline printing. In the early days it was a rough, and much maligned, process. That’s all different today, of course. In its short life flexography has matured in leaps, and every component and technology surrounding it has matured exponentially. Improvements in anilox rolls must be counted among the foremost.
Over the years, chrome rolls gave way to ceramic-coated rolls, and mechanical cell engraving yielded to the precision of the laser beam. A couple of years ago, the common CO2 laser began sharing space with the yag laser (official name: Nd:Yag, or Neodymium:Yttrium/aluminum/garnet). The result of these changes are anilox rolls with more cells, better cells, and improved surfaces.
Time was when an upgrade to a 400 line count (cells per inch) anilox was a big step from a 300. Common today are anilox rolls with counts of 800, 900 and 1,000. Some converters are employing rolls of 1,200 count. Anilox manufacturers have developed their equipment, materials and skills to the point where they now produce rolls with 2,000 lines of cells per inch. Few are employing these, but their existence is a measure of progress.
“We see a lot more demand for rolls in the 1,000 to 1,200 count range,” says David Lanska, regional anilox sales manager for Stork Cellramic, Milwaukee. “There’s a lot of call for more exotic applications. Most people have settled down a bit, and are not pursuing the ultra-high line counts of 1,500 and above. We get very few requests for 1,500 counts, and nobody’s asking for anything higher than that. Right now the emphasis is on plate technologies, in getting that part of the process to where it will benefit from the higher line count anilox rolls.”
A change in one printing component pushes others along. Significant improvements in ink quality, for example, inspires printers to utilize higher count anilox rolls. “Instead of running a coarse line count for solid coverage, converters are going to higher line counts because the inks have increased strength and density,” observes Cameron Wright, vice president of sales for Printech Anilox Roll Service, Charlotte. “The printer who was using a 360 count anilox roll a couple of years ago is now using a 600, and can get the exact same coverage and maintain the density.”
The “average workhorse” among anilox rolls, Wright says, is now a 440 count cylinder.
Technologies on the move
Recent months have seen the emergence of new coatings and engraving processes. Harper Corporation of America, based in Charlotte, has introduced a ceramic plasma thermal spraying technology that has increased the quality of the rolls. “We are now able to spray ceramic material at five times the normal rate, bringing down the cost of production,” says Art Ehrenberg, vice president of manufacturing operations. “Not only have we maintained the quality, but we were able to increase the quality. We’re changing some phases in the material that gives us better cohesion, and we’ve increased the hardness slightly.”
Harper teamed up with Lucian Delcea, a plasma physicist, and with Progressive Technologies of Grand Rapids, MI, to develop the new process, which is called Echoplasma II. The new system, Ehrenberg says, “achieves a ceramic deposit efficiency of 65 percent, compared with the industry standard of 40 percent.”
Harper also has announced that it is “closing in” on an technology it calls “WaveCell engraving.” The new cell “cleans easier than the standard 60-degree angle cell,” says Mary Keeney, market segment manager for narrow web. “With this innovative cell pattern, narrow web flexographers now need just one anilox roll to print line, solid and vignette work with plate screens between 133 and 200 lines per inch and using a high strength UV or water based ink system to deliver the necessary densities.”
Research and testing of the new rolls, called Z-One, was undertaken in large part at the Flexographic Trade School in Charlotte. “The Z-One anilox is extremely user friendly, giving even inexperienced flexo press operators the ability to hit density on spot colors and keep dots as small as 1 percent open,” says Art Fields, the school’s founder.
Apex Europe, the anilox manufacturer based in Hapert, Netherlands, recently introduced Ultra Melt technology, which it says decreases chances of score lines, increases the roll’s life span, and improves cleaning characteristics.
In the Ultra Melt process, engraving starts with an extreme power shot that determines the main shape of the cell. During a secondary low powered after-treatment, the laser beam closes and hardens the ceramic surface, making it smooth and shining. During the primary phase the laser pulse vaporizes 100 percent of the ceramic, ensuring a geometric and open cell shape. The post-treatment melts the surface of the ceramic and makes the cells internally smoother, and the walls harder and more resilient.
According to the company, the high surface tension of the roll gives it ink-repulsive self-cleaning characteristics; after each rotation, ink that is not released to the plate is replaced with fresh ink.
Doctor blades and ink filtration
Proper use of doctor blades is essential to the life span and performance of an anilox roll. “What’s important about the doctor blade is its relation to the anilox roll,” says Wright. “In the old days people used archaic pieces of metal. Today there are seven or eight configurations — stepped, beveled, reverse angle designs, and so on.”
Good doctor blades are designed to wear a bit, Wright says. “If you have a blade that is ever so slightly shredded, that’s a sign of a good blade, because it shows good steel integrity and doesn’t break off in chunks.”
A hot item in press rooms today, he adds, are ink pan filtration systems that will remove most foreign particles, including metal. They are equipped with strong magnets. “Particles could mess up an ink pump,” he says, “but dried water based ink can congeal with pieces of metal from the blade, and those can grind and score and anilox roll.”
Printers should pay close attention to alignment and pressure of doctor blades, says Lanska of Stork Cellramic. “Too much pressure can cause the metal to be ripped from the blade, leaving a gap in the doctoring surface of the blade and causing a bead of undoctored ink to remain on the anilox roll,” he says. “The excess pressure wears the blades much more quickly, and can cause a lot of friction, which can lead to some wear on the roll from unfiltered particles that get drawn across the surface of the cells. It’s important that inks be filtered.”
Care and cleaning
Everyone knows how important it is to care for anilox rolls. They are expensive, delicate, and critical to quality. Still, careless handling occurs.
“Over all, I think printers are taking better care of their anilox rolls,” says Lanska. “They’re paying attention to their press settings, and therefore not causing headaches and damage to their equipment. That’s probably the single most important change over the last couple of years.”
Anilox rolls should be inspected visually before each use. Evidence of improper cleaning is apparent if the pigment of the last ink color used is visible. Hand-held magnifiers help to determine if the surface is worn, damaged, or plugged with ink pigments. Other forms of roll inspection equipment, such as loupes with lights, are available for even closer examination.
There are three types of anilox cleaning. The first, hand cleaning and soaking, should be universally employed in the press room. The other two are ultrasonic and blasting systems. The latter are divided between soda blast and plastic media blast.
Hand cleaning with a stainless steel (for ceramics) or bristle (for chrome) brush is recommended. Brush fibers, however, will not penetrate the cells of a roll. Soaking in a cleaning solution goes a bit further.
“One of the necessities in ultrasonic cleaning is to have a really strong cleaning solution,” says Joe Walczak, president of Sonic Solutions, Yorkville, IL. “The solution is very highly alkaline, about 13 pH, because with water based inks the resins will dry rock hard and clear in the cells. When you look at them through a scope the cells look empty, but that’s not true. Those resins have to be softened up, and it takes a strong solution to do that.”
Ultrasonic cleaning involves the application of sound pressure to the cells of the anilox roll. Critical to the process is an understanding of ultrasound exposure. “We call our theory the ‘90/10 max 5’ rule,” Walczak says. “Ninety percent is soaking the roll, which rotates at a slow rate a portion at a time in the liquid bath. Ten percent is ultrasonic treatment, for a maximum of five minutes at a time.
“If the roll hasn’t soaked enough, you can apply excess pressure, and that could cause some breakage in the ceramics,” he says. “That’s sheer abuse. Some people have a tendency to turn on the ultrasound right away, and they shouldn’t do that. Make sure the roll soaks.”
A first-time cleaning, or a roll with a high level of build-up, can take up to an hour to clean, he adds. Subsequent cleanings should last no more than five to 10 minutes.
Baking soda blast systems, such as those marketed by BioBlast, Sani-Blast and Armakleen, are quite popular. In this method, rolls are placed in a closed chamber and subjected to a pressurized flow of sodium bicarbonate, between 30 and 60 psi. The crystals break down during the process, and the used media are discarded.
Plastic media blast systems are marketed by Micro*Clean International, of Torrance, CA. The method is mechanically similar to that used in soda blasting, but the media are microscopic plastic spheres that strike the cells and surface of the anilox roll. They are not abrasive, as is soda, and can be re-used many times.
Micro*Clean has several media versions: Most popular is the standard media, which in testing proved to clean an 800 count anilox roll 400 times without any damage or degradation to the roll; special blend media is for cleaning hard-to-remove stains; heavy duty extra fast is for larger rolls and for removal of metallic inks; and Ultra Fine is designed for yag rolls of high line counts. The company is at work on media for even higher count rolls.
|In the microscope photos above, supplied by Stork Cellramic, are examples of some of the conditions that can occur in anilox rolls that are damaged, aged, or not cleaned. From left: plugged cells, score line, severe wall deterioration, and cell walls scraped by a doctor blade holder.|