A key concept to flexographic printing involves consistently delivering an exact amount of ink to the substrate, which simply cannot be done when anilox rolls become clogged with dirt, ink, coatings or other contaminants. Normal cleanings, often performed daily or upon completion of a press run, remove most of the residual ink or coating. But gradual build-up of dried deposits requires more effective methods to maintaining accurate ink volume transfer.
Wash your contaminants awayChemical washes use caustic liquids to dissolve ink and are followed by a high-pressure water rinse. This method is not abrasive and is not size dependent-that is, the combination of chemicals and water can penetrate even the tiny cells of the highest screen count rolls, preserving the life of an anilox roll with with thorough cleanings while protecting a roll's fragile cell structure.
On the other hand, chemical washes present potential effluent disposal problems and employee safety issues. Further, improperly trained operators may not dilute a chemical to the proper ratio and, in turn, may corrode the cells of an anilox roll.
While exact procedures vary with roll size, characteristics of the roll surface and cleaning equipment, ultrasonic cleaning involves submersing a roll in a solution then bombarding the tub with intense sound waves. When introduced to the liquid, pressure differentials create microscopic air bubbles that implode and agitate the roll surface, dislodging ink and contaminants. To maximize cleaning efficiency, printers use corrosive solutions during the ultrasonic process to tag-team contaminants. However, ultrasonic cleaning is a precise process and requires that cleaning units be tuned to the specific frequency required for cleaning. Further, roll exposure time must be limited to prevent damage through stress cracking in the roll coatings or to the cell walls.
Taking a blast at contaminantsMedia blast cleaning methods use compressed air to shoot abrasive material-which must be softer than the roll's surface-perpendicular to the roll surface at a pressure specific enough to thoroughly clean the surface but without causing damage to the roll. The media itself is size dependent, meaning the media must have particles small enough to penetrate the anilox cells. Further, the nozzle must be controlled to ensure the blast does not linger over a specific area for an excessive amount of time, otherwise the process would cause more damage than it set out to protect against. Over time, repeated media blast cleanings may cause deterioration to cell walls. Therefore this method is intended to complement routine cleaning or restore engraving to original cell volume as needed.
Baking soda blasting is a specific and environmentally friendly media blast method that shoots easy-to-handle, specially formulated baking soda powder at the anilox roll surface. Larger crystals break into smaller crystals upon impact in a process known as refracture. Depending on the resolution and composition of the anilox roll, baking soda blasts involve air pressures between 20 and 35 pounds per square inch (psi). While the majority of baking soda processes take place in enclosed cabinets, in some applications-usually on wide web machines-baking soda roll cleaning can be done in-press with media collection systems.
Another non-abrasive cleaning method, plastic pellet blasting, uses compressed air to shoot polyethylene pellets against the anilox roll. Similar to refracture, the sharp edges of the plastic soften on the rebound, lifting inks and other deposits out of anilox cells. The media can be recycled and reused after it is separated from the contaminant particles. Plastic pellet cleaning is performed off-press in units featuring enclosed cabinets.
Hot (and ice cold) new methodsCryogenic cleaning, a relatively new process, uses carbon dioxide pellets (dry ice) blasted at the surface of the anilox roll to dislodge deposits of inks, coatings and contaminants from cells. While the method is useful in removing dried ink and is environmentally friendly, the dry ice pellets can't penetrate the smallest anilox cells and have both cracked the ceramic of anilox rolls and damaged cell walls, according to one anilox roll supplier.
Laser cleaning, another up-and-coming technology, permits anilox rolls with high screen counts to be thoroughly cleaned without causing damage to the delicate engraving. While other cleaning processes remove most inks, coatings and contaminants from an anilox roll, laser cleaning obiliterates-or technically, thermally decomposes-more resilient substances like Teflon, silicones or waxes embedded in a roll’s engraving, especially in cases of ceramic surfaces with screen counts exceeding 1,000 lines. This method requires lasers tuned to evaporate only foreign material, leaving the anilox roll’s ceramic surface intact.
No matter what method you choose, proper and regular cleaning of anilox rolls will help you maintain the high-quality print standards your customers expect.