The anilox roll is synonymous with the flexographic printing process. With this piece of equipment being specific to flexo – the predominant PS label printing process in most parts of the world – its importance to the industry is paramount. While substrates and inks are what an end user sees and feels, the anilox roll is the vehicle for how these elements come together.
Usually made from steel or an aluminum core, an anilox roll is a hard cylinder whose surface contains millions of very fine dimples, which are known as cells. The anilox roll comes into contact with the ink via a metering roll, and after a doctor blade scrapes off the excess ink from the roll’s surface, the roll then contacts the plate, thus transferring the ink. The inked plate then comes into contact with the substrate. In a nutshell, that’s flexo, and the function of an anilox roll is to control the wet ink film thickness delivered to the printing plate.
The key to this process can’t be seen with the naked eye, as it lies in those dimples – the cells. In essence, the anilox roll’s job is to pick up the proper amount of ink to be transferred to the plate. Cell volume, angle of the cells, and line screen all factor into determining the amount of ink to be transferred to the plate.
Many aspects of the label industry have evolved, as customer demands for high quality print have never been greater. Press, plate and ink technology have improved, and all three work directly with the anilox roll. Today’s anilox rolls have evolved to keep up with their counterparts, and to meet the demands of the ever more discerning print buyer.
The method behind how anilox rolls are engraved has played a key role in their advancement. “Over the years, anilox rolls have evolved, thanks in large part to laser engraving technology, which has met the need for higher quality print,” says John Batistatos, North American sales director, Apex North America. “Before the 1980s, anilox rolls were created mechanically, by use of a knurling tool. Carbon dioxide lasers came along and allowed low line screens to be engraved onto a coated roll. Yttrium Aluminum Garnet (YAG) lasers followed, which had finer beams and could produce the finer line screen anilox. Now, fiber optic lasers are allowing for more durable, better-defined cells with smoother interiors. These advancements have allowed printers to meet the increasing need for higher quality print.”
While the engravings get the attention, anilox technology starts with its base coating – chrome oxide. “This simple idea is often overlooked. Fancy engravings and shiny finishes will wear too quickly or stain too easily on a soft porous coating,” says Fred Paonessa, national sales manager for Impreglon Cellramic, adding that the company has renewed its focus on the application of chrome oxide to improve the material properties of the coating.
With Impreglon Cellramic’s technology, the chrome oxide is applied through a plasma spray device on to a roll or sleeve’s metal substrate, forming a mechanical bond. This device relies more heavily on thermal energy than kinetic, unlike other thermal spray devices. “It is therefore paramount to optimize the melt temperature of the powder chemistry to increase hardness and to lower porosity. Our chrome oxide coat has an average hardness of 1,400 + Vickers. Abrasive wear caused by doctor blade scratching will be greatly decreased with a harder coating. Also, general abrasion rates will drop due to the increased wear resistance which comes from a harder coating. This guarantees an engraving will remain the same for a longer period of time,” Paonessa says.
Lower the volume, up the line count
Of the three differentiating elements of an anilox roll – cell volume, line count and cell angle – experts maintain that cell volume plays the most critical role in determining print quality.
“Cell volume is by far the most important consideration when specifying an engraving,” states John Bingham, VP sales, Midwestern Region, Pamarco Global Graphics. “It controls the ink film, which determines density, and has a major role in print cleanliness. Volume that is too high has a much bigger part to play in ‘dirty’ print than a line screen that is too low.”
Jean Jackson, sales manager NA Printing for Praxair, echoes the sentiment of cell volume’s crucial role. “The volume of the anilox cell controls the ink film thickness and thus density and dot gain. It is the most critical element in defining the engraving specifications as it controls the wet ink film thickness to the plate,” she says.
The line count of an anilox roll controls the resolution or smoothness of the ink laydown. “You generally use the highest line count that can stay clean through a run and is easily cleaned either on press or in a cleaning system,” Jackson explains. “The higher the line count, the smaller the cells and the higher number of cells per square inch.”
John Bingham notes that the line screen determines how much support the anilox gives to the plate. “At a given volume, the higher the line screen the more cell walls supporting the plate, and the cleaner the print should be. Be careful not to go too high in line screen for the specified volume, otherwise you’ll get plugged cells, and scored anilox rolls can be the result.”
With the improvement in inks, plate materials, and anilox engraving techniques, line screens have drastically increased and cell volumes decreased, Bingham says. “Printers can get desired ink densities with much thinner ink films which allows for the decreased volumes. The end result is smoother solids and cleaner type and half-tones. UV inks, and occasional related “spitting” problems have made channeled engravings necessary. This can be done with several different engraving techniques, but the end result are channels that allow ink to flow around the surface of the anilox instead of being confined in 60 degree cells.”
The line screen is directly related to the plate height, notes Alex James, director, Harper GraphicSolutions. He says, “Over the years, the label industry continues to improve the graphic quality of their end product. Plate technology has improved moving the graphic reproduction from 133 lines per inch (lpi) to 175 lpi and 200 lpi. This change has required adjustments in the anilox engraving specification required to produce the high-end graphics. For high-end process printing, anilox specifications have evolved from 800s to 900s to 1000s and now as high as 1400s to 1600s.”
James points out that there is a continuous trend to higher anilox specifications and lower cell volumes, thus correlating to tighter color tolerances. This, he says, exposes a challenge. “What is lacking is the adoption of new internal protocols to assure that the anilox rolls with high specifications are well maintained. Harper’s GraphicSolutions technicians are available to assist and train customers on proper care and maintenance practices.
Matching color to press is also an important part of the ongoing trend and also an opportune way to improve efficiencies. HarperScientific has a variety of QD proofing devices that can close to loop of matching color proofing, prior to going to press,” James says.
Traditionally, anilox cells have been based on a hexagonal pattern. Recently, however, suppliers have developed alternative cell structures in order to and push the print quality standard even further.
Praxair’s Proline ART engravings feature a distinctive appearance due to their non-traditional cell configuration. The raised “post” between the cells supports the doctor blade while the smooth finished engraved surface is designed to provide a superior surface for ink or coating laydown. “These two features work in tandem to provide exceptional uniformity of coverage, virtually eliminating pinholing and possibly the need for a double bump,” Jackson says.
Proline REV engravings from Praxair have a distinct slotted cell shape around the circumference of the roller. “This engraving technology offers an outstanding blend of higher densities, better ink transfer, and excellent solids coverage,” Jackson says. “REV engravings also provide greater latitude and superior results when metering ink for combination print jobs, allowing you greater on-press flexibility.”
Pamarco features its EFlo cell technology, which allows for shallower, more open engravings so ink will transfer and re-wet more readily. This can help with improved, more consistent ink density. John Bingham explains: “EFlo is an elongated cell where the cell width is half of the vertical length of the cell. It has a very flat, shallow bottom that transfers ink exceptionally well. In addition, the vertical line screen is typically 50% higher than a 60 degree engraving at the same volume. The end result is typically cleaner print at higher densities,” he says.
John Batistatos of Apex, says the company’s patented Genetic Transfer Technology (GTT) is something new altogether as it replaces conventional anilox technology. “GTT is a completely new way of engraving,” he explains. “Instead of standard hex or elongated cells, GTT features S-groove channels that run perpendicular to the face of the roll. It changes the dynamic of how ink flows on the roll, how ink is released off the roll, and how dots are supported.”
GTT channels are shallower than conventional hex cells thus allowing a printer to achieve better densities while using less ink, Batistatos says. “The S-pattern allows for more dot support, thus keeping screens and vignettes open, while still achieving better density. Being able to combine more screens and solids reduces the amount of rolls needed. In addition, we are seeing tremendous advantages in solids, whites, coatings and specialty inks. The improved release eliminates pinholing and provides a much smoother laydown.”
Decreased makeready time due to less print stations, less anilox change-outs and quicker color matching are all advantages GTT provides, Batistatos adds. “Some customers are telling us they have been able to reduce makeready times by two to three hours because of these factors,” he says.
Take good care
A damaged anilox roll can be detrimental to a print run. Regardless of cell structure or how they’re engraved, anilox rolls require a strong care and maintenance program.
“Unfortunately, approximately 80% of anilox rolls are damaged before exhausting their use,” says Harper’s Alex James. “When environments have good cleaning practices and rolls are well maintained, some rolls can last three to five years.
“As the anilox engraving specification transitions to smaller cells, it is absolutely important to adopt daily anilox cleaning practices.Investment in chemical bath anilox cleaners is a must, as is investment in an onsite microscope for visual inspections,” James adds.
“A combination of hand cleaning and mechanical cleaning are required,” Batistatos says. “By mechanical cleaning, I mean equipment that is designed for cleaning rolls. It’s critical that all instructions are followed with the mechanical cleaners or rolls can get damaged. Along with cleaning, proper storage is a must, which includes racks and roll covers.”
John Bingham points out that there are many variables involved in the lifespan of the anilox, including line screen, type of doctor blade used, blade pressure, press speed and ink type. He says, “As a rule of thumb, a process anilox should last about two years and as the line screen drops, the life should improve. Quality doctor blades, low blade pressure, ink magnets, roll covers and regular cleaning are care items that will extend the life of the roller.”
“Rolls should be cleaned right after use to eliminate ink drying in the cells or linking with other ink systems,” Praxair’s Jean Jackson says. “Automatic off press cleaning systems help greatly, especially when the cleaner is optimized for the ink system and safe for the roll base. Score lines or pickup lines (material built up on the engraving lands) are common occurrences that can be helped by optimizing blade setting and using filters and magnets to remove metal fines, paper, dust, etc.”
Technological advancements in flexography, while improving process and print quality, have also led to a new set of anilox roll-related challenges.
“UV ink spitting or misting is still a challenge that can be helped by some doctor blade configurations and specialty anilox engravings that have more open structures that help the ink flow. At Praxair, we have our Novaline UV Gold and IST engravings that reduce or eliminate ink spitting,” Jackson says.
Pamarco’s Bingham sees scoring as the biggest challenge. “The fiber optic lasers that are now being used can cause the cell ‘corners’ to stick up. Doctor blades and foreign particles in the ink can knock these corners off causing small score lines. Converters should use magnets in their ink system, low carbon or composite doctor blades and the lowest possible blade pressure. Anilox suppliers should machine polish their engravings, after engraving, to create a smooth consistent surface. Though this can limit the cell volume availability at the line screen chosen,” he says.
Anilox sleeves are a relatively new technology that has become quite popular in wide web formats and is gaining traction in narrow web. The technology has coincided with the emergence of quick-change, short run environments.
Harper’s Alex James describes sleeves as multi-layered, lightweight ceramic coated products. He explains how they work: “Sleeves are loaded onto presses that are outfitted with air-supplied mandrels. Air is powered to the mandrel for loading and unloading the anilox sleeve on each station. Sleeves are much lighter compared to the traditional, journaled aniloxes. A great advantage of the sleeve technology is that they are ideal for quick-change presses, thus making them the preference for most press operators. The unfortunate consequence of the light weight is that sleeves are easy to move and handle and, as a result, get damaged much quicker compared to a traditional anilox.”
Like the methods of engraving and new cell structures, sleeves are another anilox component that demonstrate an evolving segment of the industry. James concludes, “Anilox technology continues to evolve as the needs of label converters continue to diversify.”