Flexography
Flexography is a direct high-pressure process.
It is a web-fed printing process in which flexible printing plates made of rubber or photopolymer, and low viscosity inks can be used. As a high-pressure process, the raised areas of the printing plate are image- supporting, while the printing unit construction is simple and the intaglio printing process is similar. At the beginning of aniline dyes were used by the flexographic printing was one of the high-quality printing process. Today, it is distinguished by its wide range of application areas from, for flexographic printing can print on many materials that are to be printed or only limited interconnection with other printing method.
- 5.1 Solvent and water-based paints 5.1.1 Two - component paints
Area of application
Main area of application is the printing of plastic packing goods ( such as PE, PET, PVC, PS, PP, PC, metalized film ), paper and cardboard. The printability of the plastic films is by pretreatment of the printing material (usually corona treated to increase its surface tension) allows, and the wetting properties of the low viscosity flexographic ink. Other possible uses include adhesive films, insulation paper, beverage containers, napkins, carbonless sets, wallpaper and latex balloons. The newspaper printing is another area of application dar. In the United States, Italy and the UK newspapers are produced in flexographic printing. However, the dominant on the world market newspaper printing process is the web offset.
The printing unit construction
In general, the flexographic printing contains a screen roller, over which the printing plate is inked, called a pressure cylinder, and plate cylinder on which the printing plate is mounted and an impression cylinder which carries the printing. Variations in the design produced by various methods of coloring the anilox roller. Differences dipping roller printing unit here (obsolete) and chambered doctor blade printing unit.
The dive roll printing unit
When dipping roller printing unit is delivered via the immersion roller paint on the screen roller. The excess paint can be with the help of a doctor blade, which is steeply against the anilox roller, stripped. The screen roller has an open or closed doctor blade system.
The chambered blade system
When chambered doctor blade system or open blade system, the excess ink is also stripped with a doctor blade of the anilox roll surface. However, the ink is pumped into a chamber of this process is that air pressure pushes tightly against the anilox roller and the doctor blade contains. Doctor blades are usually made of steel, variants are plastic squeegee and ceramic coated steel blade.
We distinguish between two types of printing forms: made by laser engraving rubber printing plates and photopolymer plates made of UV- sensitive plastic. The latter are produced by exposure of the printing and subsequent washing out the non-printing elements. The printing form, with the next line and text semitones can be simulated by rasterization is tensioned after their production to the printing cylinder. For mounting adhesive films are used which significantly influence by their compressibility ink transfer. While incompressible films are suitable for the printing of solid, compressible films are beneficial for halftone printing. Not only the properties of the adhesive films, but also the thicknesses even affect the printing process. In general, thin plate thicknesses for screen printing and thick are used for surface pressure. For thick substrates (eg, corrugated cardboard ) and thick plates are used, as they can compensate for changes in thickness of the printed material. Both rubber and now from photopolymer gaps and seamless print forms can be made for continuous printing in specific manufacturing processes.
The anilox roller
The anilox roller is used for ink metering. Its surface consists of either chrome or ceramic ( the most common ) and is engraved by Moulette, YAG or CO2 laser. Accessible depending on the application are engravings of 60 to 500 cells per centimeter. If the Rasterwalzenrasterung too coarse, this may lead to unwanted tapering ( fouling ) of the smallest grid points. In the wells created by the engraving, the color is stored in the printing process. The wells are then partially emptied on the printing form. It remains always at least half of the color as the remainder in the pan. Impact on the Transferable ink volume have well profile and web content. The wells of the anilox roller are arranged orthogonal or hexagonal generally. Alternatively, there are also line- engraved anilox rolls. However, these are not in flexographic printing, but used in gluing units or short inking units of offset printing.
Ink
Solvent and water-based paints
These colors are made up of
- Colored pigments,
- Binder (soluble resins or acrylates to fix the pigments),
- And additives (waxes for abrasion resistance, flexibility and sliding behavior of the ink film or blend for correcting hue and color strength ).
Such inks dry physically by driving off the solvent or evaporation of water shares. This evaporation process is promoted in drying boxes behind the color works by fans. Supportive can IR radiation aggregates are used. Since solvents and their vapors are flammable, they must be extracted in a closed air circuit. When using water-based inks, however, the viscosity and the pH must be controlled and regulated during long print jobs.
Two - component paints
UV inks
UV inks are made in addition to a high proportion of binder with pigments, additives and photoinitiators. Dry in a photochemical process, by means of photo-initiators, curing of the binder contained is raised under the action of UV radiation. Immediately behind the inking unit and the object passes through the radiator, the color is almost completely fixed. During the handling of 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 because direct contact with the UV radiation changes in the skin cells (particularly the eyes) caused.
The machine construction
Today's flexographic presses generally have between four and ten printing units. These are arranged in-line, satellite design (central cylinder) or multi-cylinder compact design. In contrast to the two alternatives, the satellite design has only one central impression cylinder around which are arranged the individual works. Since the web is held on the impression cylinder during the entire printing process achieves this type of machine the maximum register accuracy and high production speed, there are up to 800 m / min are possible. An additional advantage over the series design is the low space requirements. A disadvantage is the limited accessibility of the individual printing units. In all constructions are located after each printing unit drying facilities. While the web of material of the construction range of the drying path thus can be extended and deflected, this is not possible in the satellite configuration. The short path and the high rate of production do not allow for complete drying between printing units. Other necessary equipment components are off and winder, ink pumps and web edge control. In addition, there are ways to integrate additional components such as cutting or punching, coating, corona treatment plant as well as measuring and monitoring systems.
The largest flexographic presses are Zeitungsflexodruckmaschinen. In Italy are central cylinder machines, double wide with a capacity of up to 96 pages 4/4farbig. The currently largest Zeitungsflexodruckmaschine is in London. It consists of eight rotations with a total of 320 pressure points.
Features
As well as prints that are produced in the high-pressure process, Prints flexography may have pinched edges. These occur at too high a pressure to the pressure zone, using photopolymer plates. Most of this disturbing factor is accompanied by a dot gain. If the pressure between the impression cylinder and the impression cylinder, however, too low, the risk of bubble formation between the printing form and substrate, which can lead to uneven wetting of the material to be printed with color. Unlike other high-pressure process no shades can be seen in flexographic printing.
In newspaper printing water-based colors are used. The result is an abrasive print. The newspaper reader after reading any ink on your fingers, as in the offset.