Similar to that of letterpress printing, flexography uses raised objects on a printing plate. The raised objects catch the ink and as they contact the substrate they compose the printed impression. The printing plates are made from a flexible material, usually a flexible rubber or photopolymer, that allows it conform to the irregular surfaces. Spot colors, fast-drying inks, and high-speed printing are the norm for this process.
Since it uses water-based inks, flexography has become more popular as a printing alternative. Since other printing processes typically depend on oil-based inks, flexography is used to print on items like corrugated containers, folding cartons, multi-wall sacks, paper sacks, plastic bags, milk and beverage cartons, disposable cups and containers, labels, adhesive tapes, envelopes, newspapers, and wrappers. The process has seen great improvements and is now used on more mainstream materials.
Flexographic Printing Press Types
Stack Type – The stack press is characterized by one or more stacks of printing stations arranged vertically on either side of the press frame. Each stack has its own plate cylinder which prints one color of a multicolor impression. All stations are driven from a common gear train. A drawback of stack presses is their poor registration; the image position on every printed sheet is not as consistent as in many other printing processes.
Central impression cylinder (CIC) – Like the common impression rotary letterpress, use a single impression cylinder mounted in the press frame. Two to eight color printing stations surround the central impression cylinder. Each station consists of an ink pan, fountain roller, anilox roll, doctor blade, and plate cylinder. As the web enters the press it comes into contact with the impression cylinder and remains in contact until it leaves the press. The result is precise registration which allows CIC presses to produce very good color impressions. CIC presses are used extensively for printing flexible films.
In Line Flexographic Printing – Similar to a unit type rotary press or the stacked press except the printing stations are arranged in a horizontal line. They are all driven by a common line shaft and may be coupled to folders, cutters, and other postpress equipment. These presses are used for printing bags, corrugated board, folding boxes, and similar products.
Newspaper Flexographic Presses – A newspaper flexographic press consists of multiple printing units, each unit consisting of two printing stations arranged back-to-back in a common frame. The use of paired stations allows both sides of the web to be printed in one pass. Multiple printing stations are required to print the many pages that make up a typical newspaper. Single and double color decks, stacked units, or 4-, 5-, or 6-color units are sometimes positioned above those units where the publisher wants to provide single or multiple spot color, spot color for both sides of the web, or process color, respectively (Buonicore).
Commercial Publication Flexographic Presses – Compact high-speed presses with wide web capability that utilize dedicated 4-, 5-, or 6-color units. Typically, two four-color units are paired in one press to allow printing on both sides of the web. Publication flexographic presses generally incorporate infrared dryers to ensure drying of the waterborne ink after each side of the web is printed (Buonicore).
These inks consist of colorants, which may be pigments and soluble dyes along with a binder and various solvents. Both Solvent based and water based inks commonly contain various types alcohol as the primary solvent or drier. Alcohol rapidly dries through evaporation and contributes to VOC emissions. The inks may also contain glycol ether and/or ammonia which facilitates drying.
Water based flexographic inks – dry through evaporation and absorption on paper. This evaporation requires a greater amount of fuel or energy to dry the ink. Coated papers may be used to control the absorption through the paper. Due to the speed of the presses and volume of inks consumed daily a pollution control system may be necessary, especially if the printer is using solvent based inks. If the product allows, the printer may avoid pollution control equipment if they convert to water based inks or UV curable inks. The cost of Pollution control equipment for a small Flexo or gravure printer will cost approximately $400,000 (1998 estimate) for the equipment and approximately $50,000 for testing and certification. The price increases as the size and/or volume of the operation increases.
UV flexographic inks – are commonly used for top coats and lacquers and are responsible for many improvements in image quality of flexographic printing. The use of UV curable colored inks is rising within the flexographic printing industry, but product concerns and equipment investment are obstacles. Note, water based or UV curable inks may not be an option for some printers due to the substrate being printed or design of the product.