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A flexographic printing plate.

Flexography (often abbreviated to flexo) is a form of printing process which utilizes a flexible relief plate. It is basically an updated version of letterpress that can be used for printing on almost any type of substrate including plastic, metallic films, cellophane, and paper. It is widely used for printing on the non-porous substrates required for various types of food packaging (it is also well suited for printing large areas of solid color).

History

In 1890, the first such patented press was built in Liverpool, England by Bibby, Baron and Sons. The water-based ink smeared easily, leading the device to be known as "Bibby's Folly". In the early 1900s, other European presses using rubber printing plates and aniline oil-based ink were developed. This led to the process being called "aniline printing". By the 1920s, most presses were made in Germany, where the process was called "gummidruck".

During the early part of the 20th century, the technique was used extensively in food packaging in the United States. However, in the 1940s, the Food and Drug Administration classified aniline dyes as unsuitable for food packaging. Printing sales plummeted. Individual firms tried using new names for the process, such as "Lustro Printing" and "Transglo Printing," but met with limited success. Even after the government approved the aniline process using new, safe inks, sales continued to decline. Intent on re-popularizing aniline printing by changing its name, Franklin Moss, president of Mosstype Corporation, surveyed the industry in 1951 and received over 200 different name suggestions. In October 1952, the new name, "flexography", was announced.^[1]

Evolution

Originally, flexographic printing was rudimentary in quality. Labels requiring high quality have generally been printed using the offset process until recently. Since 1990^[2] great advances have been made to the quality of flexographic printing presses, printing plates and printing inks.

The greatest advances in flexographic printing have been in the area of photopolymer printing plates, including improvements to the plate material and the method of plate creation.

Digital direct to plate systems have been a good improvement in the industry recently. Companies like Dupont, MacDermid, Kodak and Esko have pioneered the latest technologies, with advances in fast washout and the latest screening technology.

Laser-etched ceramic anilox rolls also play a part in the improvement of print quality. Full color picture printing is now possible, and some of the finer presses available today, in combination with a skilled operator, allow quality that rivals the lithographic process. One ongoing improvement has been the increasing ability to reproduce highlight tonal values, thereby providing a workaround for the very high dot gain associated with flexographic printing.

Process overview

1. Platemaking^[3]
The first method of plate development uses light-sensitive polymer. A film negative is placed over the plate, which is exposed to ultra-violet light. The polymer hardens where light passes through the film. The remaining polymer has the consistency of chewed gum. It is washed away in a tank of either water or solvent. Brushes scrub the plate to facilitate the "washout" process. The process can differ depending on whether solid sheets of photopolymer or liquid photopolymer are used, but the principle is still the same. The second method used a computer-guided laser to etch the image onto the printing plate. Such a direct laser engraving process is called digital platemaking. The third method is to go through a molding process. The first step is to create a metal plate out of the negative of our initial image through an exposition process (followed by an acid bath). This metal plate in relief is then used in the second step to create the mold that could be in bakelite board or even glass or plastic, through a first molding process. Once cooled, this master mold will press the rubber or plastic compound (under both controlled temperature and pressure) through a second molding process to create the printing plate.

2. Printing
A flexographic print is made by creating a positive mirrored master of the required image as a 3D relief in a rubber or polymer material. Flexographic plates can be created with analog and digital platemaking processes. The image areas are raised above the non image areas on the rubber or polymer plate. The ink is transferred from the ink roll which is partially immerged in the ink tank. Then it transfers to the anilox roll (or meter roll) whose texture holds a specific amount of ink since it's covered with thousands of small wells or cups that enable it to meter ink to the printing plate in a uniform thickness evenly and quickly (the number of cells per linear inch can vary according to the type of print job and the quality required).^[4] To avoid getting a final product with a smudgy or lumpy look, it must be ensured that the amount of ink on the printing plate is not excessive. This is achieved by using a scraper, called a doctor blade. The doctor blade removes excess ink from the anilox roller before inking the printing plate. The substrate is finally sandwiched between the plate and the impression cylinder to transfer the image.^[5]

Flexographic printing inks

The nature and demands of the printing process and the application of the printed product determine the fundamental properties required of flexographic inks. Measuring the physical properties of inks and understanding how these are affected by the choice of ingredients is a large part of ink technology. Formulation of inks requires a detailed knowledge of the physical and chemical properties of the raw materials composing the inks, and how these ingredients affect or react with each other as well as with the environment. Flexographic printing inks are primarily formulated to remain compatible with the wide variety of substrates used in the process. Each formulation component individually fulfills a special function and the proportion and composition will vary according to the substrate.

There are five types of inks that can be used in flexography: Solvent-based Inks, Water-based Inks, EB (Electron Beam) curing inks, UV(ultraviolet) Curing Inks and two part chemically curing inks (usually based on polyurethane isocyanate reactions), although these are uncommon at the moment.^[6] Water based flexo inks with particle sizes below 5 µm may cause problems when deinking recycled paper.

Ink control

The ink is controlled in the flexographic printing process by the inking unit. The inking unit can be either of Fountain Roll system or Doctor Blade System. The Fountain roll system is a simple old system yet if there is too much or too less ink this system would likely not control in a good way. The doctor blade inside the Anilox roller uses cell geometry and distribution. These blades insure that the cells are filled with enough ink.^[2]

Applications

Flexo has an advantage over lithography in that it can use a wider range of inks, water based rather than oil based inks, and is good at printing on a variety of different materials like plastic, foil, acetate film, brown paper, and other materials used in packaging. Typical products printed using flexography include brown corrugated boxes, flexible packaging including retail and shopping bags, food and hygiene bags and sacks, milk and beverage cartons, flexible plastics, self adhesive labels, disposable cups and containers, envelopes and wallpaper. A number of newspapers now eschew the more common offset lithography process in favour of flexo. Flexographic inks, like those used in gravure and unlike those used in lithography, generally have a low viscosity. This enables faster drying and, as a result, faster production, which results in lower costs.

Printing press speeds of up to 600 meters per minute (2000 feet per minute) are achieveable now with modern technology high-end printers, like Flexotecnica [1], which introduced the world's first 12-color central impression (CI) drum press at Drupa 2008. This groundbreaking technology won the prestigious FlexoTech (UK) Innovation Award in 2008 [2].

Other press formats, such as in-line and stack presses, are available from Tresu and other suppliers.

References

External links

FlexoGlobal http://www.flexoglobal.com

Flexographic Technical Association http://www.flexography.org