Flexography is a roll-feed web printing process. Flexographic printing is commonly used to print high volumes of labels and packaging. Presses can print on a variety of films, foils, papers, corrugated board, and paperboard and achieve speeds of 500 to 2000 feet per minute.
Products printed on flexo presses include adhesive and shrink-sleeve labels (bottle wraps); ice cream cartons; folding cartons; shopping bags; plastic bags; pouches; packets; gift wrap; tissue; envelopes; wallpaper; and disposable plates and cups.
How It Works
A flexible, photopolymer plate is wrapped around a rotating cylinder for each color. Like the letterpress printing process, the graphics and text for each color are raised from the surface of the plate. Only the raised areas of the plate are inked.
The typical flexographic press includes four types of rollers; a fountain roller, an anilox roller, the plate cylinder, and the impression cylinder.
The fountain roller delivers ink from an ink pan to a steel or ceramic ink-metering “anilox” roller and then to the plate cylinder. The ink is applied to the substrate as the paper passes between the plate cylinder and a polished metal impression cylinder. The impression cylinder applies the pressure required to transfer the ink from the plate to the substrate.
In flexography, the anilox roller is used to transfer a uniform thickness of ink to the flexible printing plate. Each anilox roller has finely engraved cells that have a specific ink capacity. An optional doctor blade scrapes the anilox roller surface to ensure that the only ink delivered to the printing plate comes from the ink in the engraved cells.
As the substrate moves through the press, a different color or coating is applied at each printing deck. Drying units between the printing decks help ensure that each color of ink is fully dried. Dryers use hot air or infrared or ultraviolet light, depending on the application.
In a stack press, the printing decks are stacked vertically. The press can print on both sides of the materials as it travels through the press.
In a central Impression (CI) press the printing decks are located around a single, large impression cylinder. CI presses provide excellent registration.
In an in-line press, the printing decks are positioned in a straight line for easier handling of heavier substrates such as corrugated boards.
Different models of wide-web presses are built to handle rolls of material from 21 inches up to 80 inches wide.
Narrow-web presses are typically 10 inches wide or smaller because they were originally developed to print labels. Today, narrow-web presses up to 20 inches wide are being used to print shorter runs of flexible packaging including bottle wraps and the smaller packets and pouches used for teas, drink mixes, and single-serve packs.
Flexographic printing for packaging water-based, solvent-based, and energy-curable inks. Energy-curable inks are quickly cured either by ultraviolet (UV) light or electron beam (EB) systems. EB curing systems deliver energy directly to the inks in the form of energetic electrons and don’t require photoinitiators as UV-curable inks do.
Water-based inks are popular for printing corrugated packaging Solvent-based inks are used for plastic shopping bags and industrial films. UV and EB inks are used to print outer wraps for food packaging.
Flexographic presses can print on uncoated or coated papers, carton board, liner board, aluminum foil, and different types of plastic films, including low-density polypropylene (LDPE), polypropylene (PP), bio-oriented polypropylene (BOPP), and polyester (PET). They can also print on non-woven fabrics and different types of laminates.
In the packaging world, laminates are multi-layered materials made from different combinations of substrates, such as aluminum foil, film, and paper. Laminates provide the barrier layers that flexible packages may need to keep the contents fresh or protected from air, moisture or chemical contamination.
Prepress, Platemaking, and Press Set Up
Because flexography is such a high-speed process, a lot of ink and materials can be wasted if defects in print quality are discovered during a press run.
Print quality can be affected by a number of factors, including the types of plates used and how they were created. Because there are so many variables, preparing the artwork, images, and plates for flexographic printing requires expertise and experience.
For example, the image elongates as the flexible plate is attached to the cylinder. This means images to be reproduced should be created slightly shorter than their final specified size.
The dots in a screened halftone image can grow in diameter when printed due to the fluidity of the inks, the nature of the substrate, and the pressure of the impression cylinder. This dot gain can cause colors to look dark or muddy. Adjustments for the predicted amount of dot gain can be made during design and prepress.
The first flexographic printing plates were made of rubber. Today, they are typically made from photopolymers. These UV-light sensitive materials are either viscous liquids of solid sheets of a designated thickness.
In traditional platemaking, a digital imagesetter makes film negatives of content that will be printed at each print deck. Photographs and illustrations with tonal gradations are processed with halftone screening methods that re-create tones with different sizes of dots, dot placements, and ink combinations.
The film negative is placed onto the photopolymer plate material and loaded into an exposure unit and exposed to controlled amounts of UV light. The exposed photopolymer plate material is then developed with water or solvents that remove the unexposed image areas from the plate. (Dry thermal polymer plate processing is an alternative platemaking method that eliminates the need to wash the plates after exposure.)
Digital photopolymer plates eliminate the need to create film negatives. A black, light-sensitive layer covers the surface of the plate material. A digital imaging device then uses a high-powered infrared laser to remove the black layer where the image will be produced on the plate. This process is called laser ablation.
When the digital photopolymer plate is exposed to UV light, the light activates the photopolymer plate material where the black layer has been removed. The plate is washed, dried, and cut to the size that will be mounted on the cylinder.
Some users of flexographic presses send digital files to flexo-printing experts at platemaking service bureaus.
As expectations for fast turnaround times grow, makers of prepress software and equipment have been automating the flexographic prepress and platemaking process. Automation will reduce the need to outsource platemaking and minimize the risk of errors in designing files for full-color flexo prints that will be converted into different types and sizes of packages.
For example, Esko packaging design software helps designers create and preview three-dimensional designs for folding cartons, shrink sleeves, corrugated boxes, and flexible packaging. The Print Control Wizard for Esko’s digital platemaking systems takes into account variables such as imaging resolution, exposure settings, plate materials, substrate, anilox, press type, and inks to generate the proper screens and dot-gain compensation curves for each application.
Registration (the precise alignment of ink combinations on the substrate) is also critical to color accuracy and print quality. Registration errors can be caused by prepress or plate mounting mistakes, defects in the impression cylinder, dirty cylinders or plates, lateral movements of the webbed substrate, the improper surface tension of the substrate and other factors.
Finishing processes such as die-cutting, cold foil stamping, and varnishing can be integrated into in-line presses to improve the efficiency of producing ready-to-use rolls of labels.
Slitting machines are used to cut large rolls of printed materials to their specified width.
Specialized folders, gluers, die-cutters, and other converting equipment transforms the printed rolls of materials into folding cartons, corrugated boxes, sealable pouches, and other forms of packaging.
Flexographic technology is advancing rapidly to meet the growing demands for higher-quality prints, shorter run lengths, and faster turnaround times.
Technologies are being developed to (1) reduce the time and expertise required for prepress, platemaking, and plate changeovers; (2) improve the recyclability of printed labels and packages; and (3) minimize the high levels of waste caused by prepress or press set-up errors.
In 2013, the flexo-printer manufacturer BOBST launched the REVO DigiFlex project in conjunction with manufacturers of quality-control systems, inks, plates, color-measurement tools, and other pressroom supplies. This consortium of companies has been collaborating to build an automated, extended color-gamut flexographic printing system that will require minimal operator intervention. The goal is to provide global color consistency and repeatability in labels, flexible packaging, folding cartons, and corrugated boards. The project will also help ensure color matching between flexo-printed materials and digitally printed materials.
Extended Color Gamut (ECG) flexo printing uses orange, green, and violet inks, in addition, the cyan, magenta, yellow, and black inks to match a higher percentage of Pantone colors. Digitized systems will control the mixing and delivery of these seven process colors of UV-curable inks. Systems to automatically engage the plate cylinder set the proper impression, and register each color have also been developed.
Hybrid flexo/inkjet printing systems are another avenue of technology development. Some flexographic presses have been retrofitted with inkjet-printing units that can add full-color variable data to flexo-printed materials. This allows flexo print shops to better serve customers who want to add codes for tracking or authenticating packaging or enabling consumers to connect to online content.
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This post is part of a series we’re producing to explain the basics of different analog and digital printing processes. At the end of this series, we’ll review the strengths and limitations of each process, provide helpful glossaries, and talk about pricing models.
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