What exactly happens in the flexographic press? How is ink transferred to the substrate and what ink systems are available for flexographic printing?
In flexographic printing, everything in the printing unit revolves around the interaction of several rollers and cylinders on their counter-rotating axes: anilox roller and, if necessary, metering roller, as well as impression cylinder. As a result of the counter-rotations in each case, the printing ink finally reaches the impression cylinder, where – still mirror-inverted – it first takes shape in the form of areas and typography, which in turn attains its correct representation there after contact with the substrate web.
Printing units in the flexographic printing process
There are essentially two different methods of ink transfer to the anilox roller in flexographic printing: via a metering roller and with the aid of a chambered doctor blade system.
Ink transfer by metering roller
If an metering roller transfers the ink to the anilox roller, this is referred to as an metering roller printing unit. This method also uses the simple principle of transfer by counter-rotation.
The disadvantage of this method of ink transfer is the limited possibility of ink metering. To remove excess ink from the anilox roller, a doctor blade is required. The metering roller printing unit corresponds to traditional ink transfer and is considered obsolete compared to chambered doctor blade technology.
Ink transfer by chambered doctor blade system
Printing units with a chambered doctor blade system supply the anilox roller via a compressed, modern ink transfer system. With this method, flexographic printing works without an ink tray and metering roller.
(Please reproduce the motif example if required, unfortunately I have only found pictures of printing companies)
Ink application and smearing of excess ink is done in a closed system in which two doctor blades are used to seal the doctor blade chamber as tightly as possible. In this way, the anilox roller only picks up the amount of ink that is actually required. In this way, ink delivery can be precisely metered and controlled.
How does the motif get onto the printing substrate?
Modern flexographic presses have up to 10 printing units. The centerpoint in each of these printing units is the printing cylinder or sleeve on which the elastic printing form (printing plate) made of photopolymer or elastomer is mounted – the so-called flexo cliché. The term “cliché” comes from the French “cliché” and means stencil, replica. In technical jargon, the cliché is also referred to as a printing block. The simplest way to explain how a cliché works is to compare it to a stamp: it only prints the raised areas of the printing form.
As with ink transfer to the anilox roller, there are two fundamentally different alternatives for cliché production in flexographic printing:
– In the case of the rubber cliché, the print image/printing screen is defined by laser engraving,
– in the case of the photopolymer plate cliché, the more common method, the print motif is washed out of a UV light-sensitive surface layer after exposure.
Tipping the scales: the impression roller or the central impression drum
The supposedly “easiest job” is performed by the impression roller or central impression drum (CI), which must ensure uniform ink transfer from the printing cylinder to the substrate web by means of constant contact pressure. Incidentally, the contact pressure is regulated by the printing roller; the impression cylinder or CI is static. Problems arise when there are fluctuations or variations in the contact pressure:
– If the counterpressure is too little, the print image becomes incomplete or uneven, with gaps. Surfaces appear blotchy.
– If the counterpressure is too high, unsightly squeeze marks can appear in the print area, along with an increase in tonal value.
Flexographic printing produces particularly good results when reproducing solids (fonts, areas), e.g. in label printing. First-class print quality is also standard today in halftone printing (picture motifs).
What should be considered when choosing an ink system?
Parallel to the wide range of substrates, many ink types are available in flexographic printing. Low-viscosity, low-viscosity ink systems with readily evaporable solvents are usually used as printing inks. Polyamides and synthetic resins, for example, are used as binders for printing on films and plastics in flexographic printing.
Common ink systems in flexographic printing
– Solvent- and water-based inks
– Solvent-free UV inks
– Two-component inks (2k)
Solvent-based and water-based inks dry by evaporation of the solvent/water components. Usually drying systems (blowers) or IR systems (infrared rays) accelerate this process.
However, solvent-free UV ink is cured even faster in flexographic printing, using UV irradiation.
Two-component ink systems usually contain a special hardener in addition to the solvent-based ink, which in the end ensures high resistance of the ink on the substrate. Advantages include better ink adhesion or higher abrasion resistance compared to water-based inks in flexographic printing.
The choice of ink is mainly determined by the substrate, i.e. the printing stock. Absorbent materials such as papers tend to be printed with water-based inks, non-absorbent ones with solvent-based inks, and UV inks are often used in label printing.
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