Flexography used to be considered a “down and dirty” process, mainly for printing text and rough graphics onto corrugated cartons and plastic bags. This perception comes from the early years when flexography used rubber plates and inks that smeared and ran. In fact, flexography was called “Bibby’s Folly” in honor of the manufacturer Bibby, Baron, and Sons (Liverpool England) which created the process in 1980. Even today, many people still remember when flexo was called the “rubber stamp” printing process.
Today, flexography, is a very fast-growing printing process, taking market share from letterpress, gravure, and even offset, especially in the narrow-web label and folding carton markets. Although flexography is used primarily in the packaging marketplace because of its ability to handle non-paper substrate it is also making in-road into commercial printing, particularly in the book, newspaper inserts, and publication markets.
An understanding of the flexo process is important for all students of the printing process because of its extremely broad range of applications. According to the Flexographic Technical Association, one-quarter of all printing was done flexographically by the end of the 1990s. In the packaging segment, this rose to sixty-five percent. Most plants that print flexography do not call themselves printers, particularly in the product and package printing market. Many flexographic printers call themselves converters rather than printers because their job is really to convert the substrate into a package. Printing is only part of what they do.
PRESS TECHNOLOGY
At its simplest, the flexographic printing press consists of four primary elements; the fountain roll, the ink metering (or anilox) roll, the plate cylinder, and the impression cylinder. The fountain cylinder rotates in an ink reservoir to pick up ink the ink is then transferred to the anilox roll (which might also be called the form roll, meter roll, knurled roll, engraved roll, ink applicator roll, and ink transfer roll).
The anilox roll accepts the ink into tiny engraved cells, which can vary from 80 to 1200 cells per linear inch. These cells can be engraved mechanically or by using lasers and can be tri-helical, pyramid-shaped, quadrangular, or hexagonal. The finer the detail desired, the more cells are engraved. Anilox rolls are often paired with a doctor blade, which shaves the excess ink from the anilox roll at a reverse angle. This leaves the surface of the anilox roll clean, with the ink contained mainly within the cells.
The depth of the anilox cells is inversely proportional to the number of cells on the roll. The coarser the anilox (the fewer cells per linear inch), the deeper the cells. The finer the anilox roll (the more cells per linear inch) the shallower the cells and the more precise the ink placement.
Once the anilox roll has been filled with ink and the doctor blade has shaved off the excess, the ink is transferred from the anilox roll to the plate. The place is held to the plate cylinder with a special double-sided tape called stickyback. The raised surface of the plate picks up the ink from the anilox roller and transfers it to the substrate. The impression cylinder holds the substrate at just the right tension against the plate to form a clean impression. The fountain roller, the anilox roller, the plate cylinder, and the impression cylinder must operate at the same speed to ensure the correct image.
Because flexographic presses print from roll to roll, the front and back of the press are designed with units called unwinds and rewinds. The main purpose of these units is to maintain the proper tension and direction for the substrate as it passes through the press. If the web is too loose, it will have slack and wrinkles. If the web is too tight the image will be stretched out of proportion and the web could break. The outfeed, or rewind, is located at the end of the press, it runs at a rate slightly faster than the infeed, pulling tension across the web. This gives the press operator the ability to fine-tune the tension of the substrate as it is running through the press.
A drying system at the end of the flexographic print station dries the ink between one color station and the next. Dryers generally consist of gas-fired burners along with both supply and exhaust fans. By raising the temperature, this forces the evaporation of moisture from the inks.
In some cases, the presses are configured with cutters and stackers at the end of the press so the press can deliver sheets instead of rolls. Other presses are configured with full die-cutting operations so the press can deliver finished folding cartons, rolls of labels, and similar products. This is why people often talk about flexography as creating finished products at the end of the press.
TYPES OF FLEXOGRAPHIC PRESSES
There are three types of flexographic presses; stack, central impression, and inline presses
INLINE PRESSES
Inline presses have separate print stations located in individual units driven by a common lineshaft. End step in the process occurs in line, one after the other these presses are common for printing pressure-sensitive and standard labels. They can be made to perfect (two-sided printing) by turning the web over a turn bar. The average flexo press is capable of printing four to six colors but turn bar. The average flexo press is capable of printing four to six colors. But flexo presses capable of printing ten or even twelve colors are increasingly common.
STACK PRESSES
The term stack refers to the location of the printing units on the press. In a stack configuration, the printing units are placed vertically, one on top of the other, rather than horizontally. Six is the most common stack configuration, but some flexo presses have as many as eight stacks.
CENTRAL IMPRESSION
Central impression presses sometimes called drum or common impression presses, fundamentally differ from inline presses in that they support all of their color stations around a single steel impression cylinder.
FLEXOGRAPHIC INK
Flexographic inks have a reputation for being very bright and vivid as such, flexo is often chosen to print packaging that requires a large area of solid color. The inks also have a reputation –if not formulated properly –of drying too quickly, which causes the plate to print dirty
TYPES OF FLEXOGRAPHIC INK
Solvent- Based Inks use liquid- usually petroleum or other organic material- as the vehicle. Solvent-based inks offer excellent printability characteristics for non-paper substrates, but they contain volatile organic compounds that have undesirable environmental characteristics.
WATER- BASED INKS
Water-based ink use water as the vehicle. They have few VOCs and therefore exhibit preferable environmental characteristics over solvent-based inks. Water-based ink relies heavily on absorption into the substrate to achieve drying.
PROCESS-COLOR INKS
These inks are the four basic colors –Cyan, Magenta, Yellow, and Black-used to create the entire color gamut of process printing. In theory, the same combinations of these colors should result in the same end colors no matter which inks are used. However, the actual color produced is dependent on the individual pigments used by the ink manufacturer.
