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In order to compete in the ever-growing packaging segment and the accompanying market and price pressure, many packaging printers are taking the path of specialisation so that they can position themselves clearly and offer their customers a USP. In terms of profitability, this goes hand in hand with the endeavour to make the printing operation as efficient and productive as possible.
The printing process comprises a multitude of correlating parameters that not only determine the final print result, but also influence cost efficiency. One key performance indicator is press downtime - this must be minimised because it has a negative impact on productivity. A recurring cause of press downtime is set-up time for changing doctor blades and seals in the chamber doctor blade system of the flexo printing unit. Both doctor blades and seals are wearing parts and by their very nature have a limited service life. However, 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 or printing unit 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 euros, the cost of set-up time is therefore around 250 euros. In most cases, however, unscheduled changes due to leaks also result in production waste and ink loss, which are accompanied by additional costs. In addition, the cleaning effort required for ink changes increases, so that the set-up times for job changes are longer and more expensive, to the detriment of productivity and cost efficiency.
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 more and there is a larger contact zone between the doctor blade and the roller, so that 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 also compressed correspondingly more at higher pressure, which increases the friction between the anilox roller and the sealing surface. Consequently, more heat is generated in the sealing area, which causes carrier fluid in the ink to evaporate and poses the risk of ink pigments accumulating on the seal. Due to the accumulation of ink, the seal contour is distorted and can no longer seal reliably.
The mechanical adjustment of the chamber should be checked to ensure that 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 0.10 mm 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. The 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 roller cells gets onto the outside of the doctor blade when it is returned to the chamber, which manifests itself as drops or splashes of ink.
When the containment blade is at the bottom (usually on the outside printing units, i.e. unit 5 to unit 8 on an 8-colour CI press), the ink drips 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 that 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.
The best way to avoid drop formation on the bottom containment blade is to make adjustments to the blade material. If a steel doctor blade is used as the containment blade, reducing the thickness could help, e.g. by using a doctor blade with a material thickness of 0.10 mm instead of 0.20 mm. This makes the doctor blade more flexible and easier to run with in the direction of the roller and ink return.
With the top containment blade, it is recommended to make sure that the doctor blades do not extend laterally beyond the seal so that the ink does not pass over the doctor 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, which offer better sealing properties than steel due to their material flexibility and are also extremely wear- and corrosion-resistant, thus enabling a longer service life.
In view of the increasing specialisation 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 optimisation. 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.
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 - for example, in short-run label printing with UV-based inks and frequent colour changes, the requirements are considerably different than in long-run printing of film packaging with solvent-based ink and printing speeds beyond 400 m/min.
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. Agergaard offers 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 which has achieved a service life of more than 4.5 million production metres and is now going into series production.