Flexography takes letterpress to the next level.
Flexography is a newer version of letterpress, the oldest printing method. Unlike letterpress, with its bulky and heavy cast-metal printing plates, flexography uses the lightweight, flexible printing plates that give the process its name. Its advantage over traditional letterpress is that flexography can be used to print on a bewildering variety of print media that would otherwise be chewed up in ordinary rotary presses -- such as wax paper, plastic film, gift wrapping, even milk cartons. The special characteristics of flexography indeed make it a flexible solution to printing problems.
The Beginnings of Flexography
Traditional letterpress dominated publishing for at least four centuries after Gutenberg invented printing from movable type in the middle of the 15th century. However, improvements in rubber manufacturing in the 19th century led to rubber stamps poured from plaster molds formed over metal type. It was only a short step from that hand stamp to a full-sized, flexible rubber plate, which printers found much better suited to printing on nontraditional materials, such as corrugated cardboard. The system was named "aniline printing," after the coal-tar-based, quick-drying inks used on the new flexible plates. "Flexography" later became the name still used today for the process, once printers abandoned the highly toxic aniline inks.
How a Flexography Press Works
Like all letterpress, or relief, printing, flexography uses a raised printing surface to apply ink to paper, cardboard, film or whatever is being printed. Ink stored in a pan, known as a fountain, is picked up by a fountain roller, which transfers ink to tiny cells in a metering roller. The metering roller precisely controls the amount of ink applied to the flexography plate, which is mounted on the printing cylinder. As the printing cylinder turns, the inked flexography plate applies the plate's image to the printed surface, known as the substrate. Supporting the substrate as it rolls past the printing cylinder is an impression cylinder. This cylinder turns at the same rate as the printing plate and the other rollers, maintaining steady production. An adjustable reel stand controls the tension of the substrate to ensure a proper register -- that is, a consistent image on the printed surface throughout the press run.
It's All in the Ink
Flexographic inks, which must be fast drying to avoid smearing the substrate, typically contain alcohol as the primary solvent that allows the ink pigment to flow. (The oil-based inks used in traditional letterpress can cause rubber to swell, distorting the printing plate.) The flexographic ink also can contain ammonia as a drying agent. Alcohol certainly evaporates rapidly, but those alcohol and ammonia emissions add to the so-called volatile organic compounds that can be thrown off the press and into the air during high-speed printing. This raises concerns about air pollution, and even a small flexo press can require hundreds of thousands of dollars worth of pollution control equipment, says the Printers' National Environmental Assistance Center. Thanks to advances in ink chemistry, an alternative has been pioneered -- water-based inks. These inks are more environmentally friendly, and it's cheaper to attach a dryer than complicated pollution equipment to a press.
Progress in Platemaking
Photopolymer plates have gradually been replacing rubber plates as the printing plate of choice in flexography. Photopolymer is as flexible as rubber, and produces a sharper image, allowing flexo printers to compete better with their principal rivals, the lithography or offset printers. Photopolymer, as the name suggests, uses a light process to create the plate -- in this case, ultraviolet (UV) light. The UV light, shined through a negative carrying the image to be printed, causes parts of the printing plate to harden. The unexposed parts of the plate wash away with a solvent and the plate is dried, ready to be mounted on the press.
