The flexographic printing is a direct high-pressure process .
It is a web-fed rotary printing process that uses flexible printing plates made of photopolymer or rubber and low-viscosity inks . As a letterpress process, the raised areas of the printing form are image-bearing, while the printing unit structure is simple and similar to that of the gravure process. At the beginning, aniline inks were used, which made flexographic printing one of the high-quality printing processes . Today it is particularly characterized by its versatile areas of application, because many materials can be printed in flexographic printing that cannot be printed or can only be printed to a limited extent with other printing processes.
The main area of application is the printing of packaging made of plastic (such as PE , PET , PVC , PS , PP , PC , metallized film ), paper , cardboard and cardboard. The printability of plastic films is made possible by pretreatment of the printing material (usually corona treatment to increase the surface tension) and the wetting properties of the low-viscosity flexographic ink. Other possible uses are adhesive films, insulating paper, beverage packaging, serviettes, carbon copy sets, wallpapers and latex balloons. Newspaper flexographic printing is another area of application. Newspapers are produced using flexographic printing in the USA, Italy and Great Britain. However, the newspaper printing process that dominates the world market is web offset .
The printing unit structure
In general, the flexographic printing unit contains an anilox roller through which the printing form is colored, a printing cylinder, also called a format cylinder, on which the printing form is attached, and an impression cylinder that guides the printing material. Deviations in the construction are caused by different methods of coloring the anilox roller. A distinction is made here dipping roller printing unit (obsolete) and chambered doctor blade printing .
The immersion roller printing unit
In the case of the immersion roller printing unit, ink is transferred to the anilox roller via the immersion roller. The excess ink can be wiped off with the help of a doctor blade , which stands steeply against the anilox roller. The anilox roller has an open or closed squeegee system.
The chambered doctor blade system
With the chamber doctor blade system or the open doctor blade system, the excess printing ink is also wiped off the anilox roller surface with a doctor blade. In this process, however, the ink is pumped into a chamber which presses tightly against the anilox roller with air pressure and which contains the doctor blades. Doctor blades are usually made of steel, variants are plastic doctor blades and ceramic-coated steel doctor blades.
A distinction is made between two types of printing forms : rubber clichés made using laser engraving and photopolymer plates made from UV-sensitive plastic. The latter are produced by exposing the printing elements to light and then washing out the non-printing elements. The printing form, with which not only lines and text but also halftones can be simulated by screening , is clamped onto the printing cylinder after it has been produced. Adhesive foils are used for assembly, which have a significant impact on color transfer due to their compressibility properties. While incompressible foils are suitable for printing full-tone areas, compressible foils are advantageous for halftone printing. Not only the properties of the adhesive film, but also the thickness of the panels themselves influence the printing process. As a rule, thin plate thicknesses are used for halftone printing and thick ones for surface printing. Thick plates are also used for thick substrates (for example corrugated cardboard), as these can compensate for changes in the thickness of the printing material. Gap-free and seamless printing forms for continuous printing can be produced from rubber and now also from photopolymer in special manufacturing processes.
The anilox roller
The anilox roller is used to dose the ink. Their surface consists of either chrome or ceramic (the most common) and is engraved with a Moulette , YAG or CO 2 laser . Depending on the intended use, engravings with 60 to 500 cells per centimeter are common. If the anilox roller grid is too coarse, this can lead to undesirable infiltration (contamination) of the smallest grid points. The color is stored in the cells created by the engraving during the printing process. The cells are then partially emptied on the printing form. There is always at least half of the color remaining in the cup. The cell geometry and the portion of the bridge have an influence on the transferable ink volume. The cells of the anilox roller are usually arranged orthogonally or hexagonally. Alternatively, there are also line-engraved anilox rollers. However, these are not used in flexographic printing, but in glue units or short inking units in offset printing.
The printing ink
In accordance with the wide range of substrates, a variety of low-viscosity inks are used in flexographic printing. These can be subdivided into solvent-based , water-based and UV- curing inks. The two-component inks are a special form of solvent paint.
Solvent and water based paints
These colors are made up of
- Solvents ( ethanol or water in water-based paints) to regulate viscosity,
- Color pigments ,
- Binders (soluble resins or acrylates to fix the pigments),
- and additives (waxes for rub resistance, flexibility and sliding behavior of the color film or offcuts to correct color tone and color strength).
Such inks dry physically by driving out the solvents or evaporating the water. This evaporation process is promoted by fans in drying boxes behind the inking units. IR radiation units can be used as a support. Since solvents and their vapors are flammable, they must be extracted in a closed air circuit. When using water-based printing inks, on the other hand, the viscosity and the pH value must be controlled and regulated for long-lasting print jobs .
Two component colors
When printing on foils, a special two-component ink can usually be used as a primer. In addition to the solvent-based printing ink, hardeners are added to the systems, which enter into a longer-lasting chemical reaction with the ink binder. The paint initially dries normally through evaporation of the solvents, while the full curing process can take a few days. The advantages over normal inks on these substrates are higher seal resistance, scratch resistance and better adhesion properties.
In addition to a high proportion of binders, UV inks consist of color pigments, additives and photoinitiators . They dry in a photochemical process; photoinitiators under the action of UV radiation trigger the curing of the binders they contain. The color is almost completely fixed immediately behind the inking unit and the passage through the lamp. When handling UV inks, it is important to avoid direct skin contact as they can cause skin irritation and allergies. In addition, the radiation units must be completely sealed off, as direct contact with UV radiation causes changes in the skin cells (especially in the eye).
The machine structure
Today's flexographic printing machines generally have between four and ten printing units. These are arranged in a row construction, satellite construction (central cylinder) or multi-cylinder compact construction. In contrast to the two alternatives, the satellite design has only one central impression cylinder around which the individual works are arranged. Since the material web is held on the impression cylinder during the entire printing process, this type of machine achieves the greatest possible register accuracy and the highest production speed, up to 800 m / min. An additional advantage compared to the row construction is the small space requirement. The difficulty in accessing the individual printing units is a disadvantage. In all construction methods there are drying facilities after each printing unit. While the material web of the row construction can be deflected and thus the drying path can be extended, this is not possible in the satellite construction. The short distance and the high production speed do not allow complete drying between the printing units. Other necessary machine components are unwinder and rewinder, ink pumps and web edge controls. In addition, there are options to integrate additional units such as cutting or punching units, coating units, corona treatment systems as well as measuring and monitoring systems.
The largest flexographic printing machines are newspaper flexographic printing machines. In Italy there are central cylinder machines, double-wide with a capacity of up to 96 pages in 4/4 color. The largest newspaper flexo press is currently in London . It consists of eight presses with a total of 320 printing points.
Just like prints that are produced using the letterpress process, prints from flexographic printing can have pinched edges. These occur when the pressure is too high in the pressure zone when using photopolymer plates. This disruptive factor is usually accompanied by an increase in tone value. If, on the other hand, the pressure between the impression cylinder and the impression cylinder is too low, there is a risk of bubbles forming between the printing form and the substrate, which lead to uneven wetting of the printing material with ink. In contrast to other letterpress printing processes, no shading can be seen in flexographic printing.
For newspaper flexo printing, thin, color-intensive, water-based inks are used. The result is a more abrasion-resistant and less odorous print than with coldset. After reading the newspaper, newspaper readers have no printing ink on their fingers as in offset.
History and Development
|Mid 19th century||first use of aniline inks in wallpaper printing|
|1907||First German patent for anil printing process by Carl Holweg. Anil printing units were initially used as upstream units on bag machines|
|1912||After Holweg, the companies Windmöller & Hölscher and Strachan & Henshaw will be presenting their own machine designs for anil printing units|
|1924||First distribution of anil printing units in the USA|
|1930-1940||Further development of aniline printing inks (opacity, white pigments, metal pigments), first water colors, introduction of the anilox roller, stabilization of the printing plate production|
|1952||Renaming of the aniline print to flexographic print to remove the negative image (aniline ink was wrongly considered harmful)|
|1950-1960||New substrates: pre-prepared plastics and absorbent materials|
|1960-1970||Improvement of the web run|
|1970-1980||Improvement of color formulas for reproducing halftones|
|1980-1990||Flexographic printing with watercolors becomes established in the USA, GB and IT in newspaper printing, introduction of the chambered doctor blade system|
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