The anilox roller has long been referred to as the “heart of the flexographic press,” as it provides the precise amount of ink, laminate or coating to the printing plate or substrate.
One of the most innovative products that is utilized in the world of flexographic printing today is the lightweight, quick-change anilox sleeve.
Instead of traditional anilox rollers with heavy steel tubing, integral headers and journals that take much longer to install or remove, the anilox sleeve is mounted on a fixed air mandrel in the print station. Forcing compressed air through holes in the air mandrel enables the anilox sleeve to be mounted or removed in a matter of seconds. This allows for quicker setups of new jobs or color changes and especially provides a safer press environment when handling due to the sleeve’s lightweight construction.
Anilox Sleeve Materials & Composition
The most common anilox sleeves are 0.375-in. to 0.500-in. thick and constructed with an inner layer of fiberglass. This is a conventional type of fiber-reinforced plastic using silicon glass fibers that are woven together to form a layering material.
Fiberglass has a lower tensile modulus than some other materials, which allows it to bend or expand for the purpose of mounting on, or dismounting from, the air mandrel. This is achieved by having an expansion layer applied to the fiberglass, then a lightweight resin to reduce weight. An aluminum shell then gives the sleeve its stability and provides a suitable surface for applying a corrosion-resistant undercoat and the subsequent chromium oxide ceramic for grinding, honing and laser engraving.
Many suppliers apply a protective end ring to the sleeve. This minimizes the penetration of inks and cleaning agents into the various layers which can cause swelling, resulting in distortion, added TIR (total indicated runout) and inconsistent print quality.
For precision high graphics with exceedingly tight tolerances, many converters prefer anilox sleeves with carbon fiber construction instead of fiberglass. Carbon fiber consists of long strings of molecules held together by carbon atoms. They are combined into strands woven together to form an incredibly durable and lightweight material. Carbon fiber is about three times stiffer than steel and aluminum for a given weight, thus its high tensile modulus minimizes any potential vibration and deflection issues more so than fiberglass. In addition, its lowered rotational inertia allows for higher production speeds and easier start-ups and shutdowns with reduced web damage.
On the durability front, another major advantage of carbon fiber sleeves is that of reconditioning—once the laser engraved surface is worn, scored or damaged. Because the glass fibers in the fiberglass sleeves are not as tightly woven or rigid as carbon fiber, with continued mounting and dismounting as well as excessive air pressure and/or poor handling, the inner layer often cracks and is no longer suitable for reconditioning. It is not uncommon for a sleeve manufacturer to receive five fiberglass constructed anilox sleeves for rework only to discover that two may need to be replaced due to inner layer damage. Thus, the carbon fiber construction allows for a longer life and greater ability to be reconditioned.
This type of carbon fiber anilox sleeve construction is especially beneficial for larger flexographic preprint presses, such as those from F&K, Bobst and Allstein, as these sleeves are mounted and removed with sophisticated robotic arms. While the addition of an aluminum sleeve is optional in this type of construction, it is the carbon fiber itself that provides the sleeve’s overall rigidity, as compared to the aluminum sleeve in the fiberglass construction.
Just as important as the anilox sleeve itself, the fixed air mandrel onto which the sleeve is mounted and eventually removed must be manufactured to precision specifications as well. While most are made of ground steel with a chrome-plated surface, a carbon fiber option is often utilized for these same rigidity and durability advantages.
Anilox Sleeve Surface Technologies
Once the converter has established which type of anilox sleeve construction to utilize, it is time to determine the anilox surface and engraving specifications. The anilox sleeve is mounted on a production mandrel for surface treatment. Whether the anilox sleeve blank has an outside aluminum shell or a metalized outside layer, a corrosion-resistant barrier must be applied to the surface to protect the subsequent chromium oxide laser surface from any potential pitting or blistering.
Now, the chromium oxide can be plasma flame sprayed to the surface at approximately 0.025-in. on the diameter (0.0125-in. per side). While the quality and density of chromium oxide has certainly improved over time with smaller spherical size particles, there is still a degree of porosity in the surface, thus the need for the corrosion-resistant undercoat.
Once the chromium oxide surface has cooled, the coated sleeve and mandrel move to the cylindrical grinder to achieve critical TIR and taper tolerances. This process is followed by a honing step or superfinishing to gain the necessary RA finish for laser engraving. Prior to processing the order, the converter and the sleeve supplier will discuss the best engraving specifications for each sleeve. Will the sleeve be used for flood coating, line and solid work, process printing, etc.? What is more important to the converter—engraving quality/accuracy or anilox sleeve longevity? These answers will dictate how the anilox sleeve is laser engraved.
Sixty-degree hexagonal laser engraving is the most common choice and a standard for high graphics process work. However, the converter may want to print heavier solids with a longer 70-degree to 75-degree extended or channeled configured pattern. These engravings can achieve more volume and release more efficiently. If longer anilox sleeve life is a major requirement, the engraver can provide more thermal melting of the ceramic which results in an even harder surface that will hold up to premature blade wear and abrasive inks.
For the most pristine engravings which require the highest-quality printing, a multi-hit fiber optic laser process is the best avenue. In a nutshell, the communication that takes place between the converter and its multiple vendors (anilox sleeves, inks, plates, substrates, etc.) plays a critical role in the decision-making process of the anilox sleeve specifications.
After laser engraving, the ceramic surface is super-finished to ensure cell walls are consistent, which allows for the most efficient doctor blade metering. After a thorough cleaning of any surface residue, a quality control inspection is performed with vertical scanning interferometry equipment to confirm the accuracy of engraving and volume specifications, along with a mechanical analysis of total indicated runout (TIR) and taper tolerances. The anilox sleeve is then properly wrapped and crated with inspection paperwork and ready for shipment.
Anilox Sleeve Handling & Care
The biggest factor affecting the life of an anilox sleeve is how it is handled, cared for and maintained. Because these sleeves are so much lighter than steel rollers and have no journal ends, it is not uncommon for damage to occur on the ends from bumping into press-side frames or, even worse, the concrete floor! Thus, how sleeves are treated when not in use plays a major role.
Some anilox sleeves are stored on horizontal racks, suspended from their inside layer. Some converters feel this could potentially damage or distort the inside layer. Thus, others choose to store horizontally on the roller’s outer surface in padded cradles. Vertical racks with padded bottoms are often utilized, and in many cases the unused sleeves are covered with a protective plastic or padded vinyl wrap to avoid surface damage.
No anilox sleeve will deliver its proper performance if ink has dried in the cells. Many converters use cleaning agents on press that have a pH stronger than the inks, but not too strong as to cause corrosion and blisters beneath the ceramic surface. Outside of the print station, soda blasting, plastic bead particles and dry ice equipment are other options for cleaning. Keep in mind that these can degrade cell walls over time, if used too often. Better options include:
- Flexo washup units that minimize any potential damage
- Ultrasonic and laser cleaning equipment (but if used improperly, this can lead to imagery in the roller surface that will show up in the print)
Regardless of a converter’s graphic requirements, skill level or product mix, good housekeeping plays an important role in plant efficiency and consistency. The development and integration of anilox sleeves has provided flexo operators with a much safer and efficient pressroom environment, for sure. With the correct specifications, proper care, handling and maintenance, these laser-engraved ceramic anilox sleeves will provide top-notch performance at the highest press speeds and continue to be recognized as the “heart” of the flexographic press!