The flexographic printing process comprises a multitude of correlating parameters that determine the final print result and influence cost efficiency.
One key performance indicator—press downtime—must be minimized because it has a negative impact on productivity. Package printers have turned to competitively positioning themselves to make the plant as efficient and productive as possible, setting sights on controlling and improving performance whenever and wherever possible. Every contributing cause gets proper attention.
A recurring cause of press downtime is setup time for changing doctor blades and seals in the chamber doctor blade system. Both doctor blades and seals are wearing parts and by their very nature have a limited lifespan. It is not uncommon for an unplanned replacement to occur as a result of leaks and/or excessive or irregular wear before the job is finished.
A chamber doctor blade system usually takes half an hour to pump out the ink, clean the chamber, install new doctor blades and seals, and put the machine back into operation. At a press hourly rate of $500, the cost of setup time is therefore around $250. In most cases, unscheduled changes due to leaks also result in production waste and ink loss, which are accompanied by additional expenses. In addition, the cleaning effort required for ink changes increases, so that the setup times for job changes are longer and more expensive, to the detriment of productivity and cost efficiency.
How It Happens
Simply put: too high pressure against the anilox roller is the cause. When ink drips out of the chamber, the chamber is often pressed even tighter against the anilox roller. This may stop the leakage in the short term, but it is at the expense of the life and function of seals and doctor blades.
As the pressure increases, the doctor blade is bent and there is a larger contact zone between the doctor blade and the roller, so the doctor blade scrapes flat, rather than with the tip. This can lead to the formation of burrs or, in extreme cases, to the material breaking out.
Seals are compressed at higher pressure, increasing friction between the anilox roller and the sealing surface. Heat generated causes carrier fluid to evaporate and pigments accumulate on the seal. Seal contour is distorted and can no longer seal reliably.
Avoid It!
The mechanical adjustment of the chamber should be checked to ensure the chamber rests evenly against the roller along its length. The machine manufacturer’s recommendations should be considered when setting the contact pressure. Optimum contact pressure is achieved when a 0.006-in. probe on the active doctor blade can be pushed between the anilox roller and the doctor blade with slight pressure. With the doctor blade at the bottom, the probe should stay in place and not fall out on its own. Contact pressure with the containment blade should only be so high that the chamber is tight. This requires a much lower pressure than with the working doctor blade.
Back doctoring is a phenomenon where ink from unemptied anilox cells gets on the outside of the doctor blade when it is returned to the chamber. It manifests itself as drops or splashes of ink.
When the containment blade at the bottom—usually on the outside printing units, i.e., unit 5 to unit 8 on an 8-color central impression (CI) press—the ink drips down along the bottom of the chamber. Depending on the design of the chamber, this problem is more or less pronounced. The steeper the angle between the doctor blade and the roller, the more likely it is the doctor blade will scrape the ink off the anilox roller, which, due to the surface tension, protrudes above the height of the roller cells, instead of letting it pass back into the chamber via the suction effect.
With the containment blade at the top (on the internal printing units of CI presses), the ink collects on the doctor blade, runs to the chamber ends and from there reaches the end faces of the anilox roller, whose rotation then catapults ink splashes into the press. The effects increase at higher printing speeds.
Beat It!
The best way to avoid drop formulation on the bottom containment blade is to adjust the blade material. If a steel doctor blade is used as the containment blade, reducing the thickness could help; for example, by using a doctor blade with a material thickness of 0.006-in. instead of 0.008-in. This makes the doctor blade more flexible and able to run within the direction of the roller and ink return.
With the top containment blade, it is recommended to make sure containment blades do not extend laterally beyond the seal, so the ink does not pass over the containment blade outside the ink carrying area of the roller.
As a low-cost and very effective alternative for both containment blade positions, plastic blades made of polyester are available. They offer better sealing properties than steel due to their material flexibility. Furthermore, they are extremely wear and corrosion resistant, thus enabling a longer lifespan.
The seals and doctor blades used are not optimally suitable. In view of the increasing specialization of printers and the associated individual printing parameters, it is not always possible to achieve the desired result in terms of function, durability and price/performance ratio using standard materials, such as those available from press manufacturers or the relevant distributors. It is then advisable to turn to a specialist in the field of printing unit optimization. Agergaard Graphic Supplies GmbH develops and produces application-specific chamber doctor blade end seals and blades that are optimally matched to the respective printing application.
Choose Wisely!
To identify the appropriate sealing and doctoring solution, aspects such as chamber structure, ink system, printing speed, substrate and structure of the print job are examined. For the printer, this involves minimal effort and is limited to answering a few questions. Clarifying the conditions of use of the doctor blades and seals is immensely important. In shortrun label printing with UV-based inks and frequent ink changes, the requirements are considerably different than in long-run printing of film packaging with solvent-based ink and printing speeds beyond 1,500 fpm.
Based on many years of cross-disciplinary know-how about the various printing processes in the areas of flexible packaging, labels, corrugated board, coatings and varnishes, an individual solution concept is worked out, drawing on a pool of high-quality raw materials and state-of-the-art production technologies. Suppliers, like Agergaard, offer printers the opportunity to test the recommended doctor blades and seals in a free sample delivery.
In a recent customer project, a special seal for water-based inks was developed that has achieved a lifespan of more than 4.5 million production meters and is now going into series production.
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