Flexography (often abbreviated to flexo) is a form of printing process which utilizes a flexible relief plate. It is essentially a modern version of letterpress which 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 colour).
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 anilineoil-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", or rubber printing. In modern-day Germany, they continue to call the process "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 Food and Drug Administration approved the aniline process in 1949 using new, safe inks, sales continued to decline as some food manufacturers still refused to consider aniline printing. Worried about the image of the industry, packaging representatives decided the process needed to be renamed.
In 1951 Franklin Moss, then the president of the Mosstype Corporation, conducted a poll among the readers of his journal The Mosstyper to submit new names for the printing process. Over 200 names were submitted, and a subcommittee of the Packaging Institute's Printed Packaging Committee narrowed the selection to three possibilities: "permatone process", "rotopake process", and "flexographic process". Postal ballots from readers of The Mosstyper overwhelmingly chose the last of these, and "flexographic process" was chosen.
Originally, flexographic printing was rudimentary in quality. Labels requiring high quality have generally been printed using the offsetprocess until recently. Since 1990, 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 Asahi Photoproducts, AV Flexologic, Dupont, PlateCrafters, 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.
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 plate to be washed out is fixed in the orbital washout unit on a sticky base plate. The plate is washed out in a mixture of water and 1% dishwasher soap, at a temperature of approximately 40 °C. The unit is equipped with a dual membrane filter. With this the environmental burdening is kept to an absolute minimum. The membrane unit separates photopolymer from the washout water. After addition of absorb gelatine for example, the photopolymer residue can be disposed of as standard solid waste together with household refuse. The recycled water is re-used without adding any detergent.
Flexographic printing press
The second method uses a computer-guided laser to etch the image onto the printing plate. Such a direct laser engraving process is called digital platemaking. Companies such as AV Flexologic, Glunz & Jensen, Xeikon, Esko, Kodak, Polymount and Screen from The Netherlands are market leaders in manufacturing this type of equipment.
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). In the early days the metal used was zinc, leading to the name 'zincos'. Later magnesium was used.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.
For every colour to be printed, a plate is made and eventually put on a cylinder which is placed in the printing press. To make a complete picture, regardless of printing on flexible film or corrugated paper, the image transferred from each plate has to register exactly with the images transferred from the other colors. To ensure an accurate picture is made, mounting marks are made on the flexographic plates. These mounting marks can be microdots (down to 0.3 mm) and/or crosses. Special machinery is made for mounting these plates on the printing cylinders to maintain registration. Earle L. Harley invented and patent the Opti-Chek Mounting and Proofing machine enabling the operator to check the registration before going to the press.
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 immersed in the ink tank. Then it transfers to the anilox or ceramic roll (or meter roll) whose texture holds a specific amount of ink since it is 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). 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. The sheet is then fed through a dryer, which allows the inks to dry before the surface is touched again. If a UV-curing ink is used, the sheet does not have to be dried, but the ink is cured by UV rays instead.
Unwind and infeed section – The roll of stock must be held under control so the web can unwind as needed.
Printing section – Single color station including the fountain, anilox, plate and impression rolls.
Drying station – High velocity heated air, specially formulated inks and an after-dryer can be used.
Outfeed and rewind section – Similar to the unwind segment, keeps web tension controlled.
1. Fountain roller
The fountain roller transfers the ink that is located in the ink pan to the second roller, which is the anilox roller. In Modern Flexo printing this is called a Meter or "metering" roller.
2. Anilox roler
This is what makes flexography unique. The anilox roller meters the predetermined ink that is transferred for uniform thickness. It has engraved cells that carry a certain capacity of inks that can only be seen with a microscope. These rollers are responsible to transfer the inks to the flexible-plates that are already mounted on the Plate Cylinders.
3. Doctor Blade (optional)
The doctor blade scrapes the anilox roll to ensure that the predetermined ink amount delivered is only what is contained within the engraved cells. Doctor blades have predominantly been made of steel but advanced doctor blades are now made of polymer materials, with several different types of beveled edges.
4. Plate cylinder
The plate cylinder holds the printing plate, which is soft flexible rubber-like material. Tape, magnets, tension straps and/or ratchets hold the printing plate against the cylinder.
5. Impression Cylinder
The impression cylinder applies pressure to the plate cylinder, where the image is transferred to the substrate. This impression cylinder or "print Anvil" is required to apply pressure to the Plate Cylinder.
Color stations stack up vertically, which makes it easy to access. This press is able to print on both sides of the substrate.
Central Impression press
All color stations are located in a circle around the impression cylinder. This press can only print on one side. Advantage: excellent registry.
Color stations are placed horizontally. This press prints on both sides, via a turnbar. Advantage: can print on heavier substrates, such as corrugated boards.
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