What are flexographic printing plates ?
Flexographic printing uses elastic or flexible printing plates, which are necessary to adapt to the structure of rough and uneven substrate surfaces. Their task is to transfer the ink from the anilox roller to the substrate. Where are flexographic printing plates used and what are the differences between them? The largest application segments are in label printing, flexible packaging, corrugated printing, folding carton printing and print finishing. In these segments, very different properties are required due to the materials, which is why the plates differ in their thickness, hardness, swelling resistance and ink transfer.
In label printing:
the flexographic printing plates usually have a thickness of 1.14 or 1.70mm. If used with UV inks, the plate must be as swell-resistant as possible. Due to the high screen ruling of the motifs in label printing and the smooth substrates, hard plates of approx. 60° ShA (DIN) are mainly used.
The usual flexographic plate thickness in flexible packaging is 1.14mm thick, but the thickness of 1.70 mm is also used in many countries of the world. Besides these thin plates, conventional plate thicknesses of 2.54 and 2.84mm are also used. In printing, they must be resistant to alcoholic solvents as well as to acetates to the greatest possible extent. For flexographic printing on packaging films, mainly hard plate types are used, similar to label printing. For more porous or rough substrates such as paper, medium-hard flexo plates of 40 – 50° ShA (DIN) are common.
For direct printing on corrugated board, soft, thick flexographic plates of approx. 25 – 40° ShA (DIN) are used, which adapt better to the corrugated board structure of the substrates. They must have good ink transfer with water-based inks in order to print solid areas in a closed manner. Their flexo plate thickness is usually between 3.94 and 6.35mm. Here, too, the trend is toward harder, thin plates 2.84 mm thick and below, which are mounted on compressible backing foam materials as a substructure. They significantly reduce the washboard effect that occurs in corrugated printing.
Folding box printing:
Depending on the quality of the cardboard and its coating, medium-hard to hard flexographic plates are used in folding box printing, which have good ink transfer with water-based inks.
The plates in this area of application are also medium-hard to hard, have a thickness of 1.16mm and must be resistant to print runs with UV printing varnishes.
So how does the print motif get onto the printing plates? Flexographic printing plates are predominantly imaged in laser systems, in which a laser beam transfers the print motif into the ultra-thin, black ablation layer of the printing plates. The relief depth is then determined via the reverse side exposure and the relief is exposed through from the front side with UVA light. During washout, the unexposed parts are removed and the relief, which was previously only latent, is exposed. After drying, the plates are ready for mounting and arrive at the flexographic presses on printing cylinders, sleeves or mounting carrier sheets ready for printing.
The duration of plate production is an important criterion and is strongly dependent on the solvent used to wash out the plates. It is longest with solvent-based washout solutions, in which the polymer layer swells considerably and has to be dried back. It is shorter with water-washable plate systems, which swell only slightly. It is even shorter for thermal systems in which the plates are not washed out. In this process, the non-crosslinked polymer is removed by heating with a non-woven material, which means that no swelling occurs.