Flexo Printing Process Characteristics
Flexo, or flexography, is a form of letterpress. Like all letterpress printing, it characteristically uses a raised, inked surface to directly impress an image upon the paper or other material to be printed. (Lithography, or offset, doesn't print directly. Gravure, a third major method of printing, uses a recessed surface, and the paper is impressed upon the gravure plate.) Widely used to print on film, on gift wrap, and even on paper cups and plates, flexo is flexible, just one of the characteristics that sets it apart from other printing methods.
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The Plate's the Thing
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Flexo plates were originally made from rubber, which is molded or carved. Nowadays a photopolymer, or light-sensitive plastic, is used; the plate is exposed to light and then developed, like a photographic print. The flexo plate can print flat, or t can be flexibly wrapped around the cylinder of a rotary printing press. Because the plate stretches slightly when curved, flexo printers must adjust the image proportions slightly in the photographic process to eliminate distortion -- a characteristic (and characteristic headache) of flexography. However, plastic or rubber plates are much better than traditional metal plates for printing on rough, uneven or nonpaper surfaces, such as corrugated cardboard, film and other plastics.
The Right Ink for the Right Job
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Flexo inks must be quick-drying, because flexo presses often print on materials that are waterproof or poor at absorbing ink. Originally, flexo was called "aniline printing" because it used dyes made from aniline oil, which first was made from plants and then synthesized from coal tar. Dissolved in alcohol, which rapidly evaporates, aniline made a fine flexo ink, but it was a public-relations headache. Food manufacturers feared that aniline was toxic and refused to use flexo presses, well-suited though they were to printing the food wrap that was just then coming into use. Flexo printers abadoned aniline for water- and oil-based inks, and changed the term for the process to "flexography."
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Start the Presses
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Because flexography is direct printing, fewer cylinders are needed to move the ink from press to print job. Fewer cylinders means fewer breakdowns and less print-blurring vibration. That relative simplicity of operation is another characteristic distinguishing flexo from offset, which needs intermediary rollers between plate and paper to transfer the image; and from gravure, whose plates must be exactingly (and expensively) etched and, once on the press, constantly scraped down to remove excess ink.
Starting up a flexo press run wastes much less paper than offset or gravure, especially important when printing multiple colors on multiple press units at once. With flexo's lightweight plates, it's also easier to line up or register different color plates. Printers can fudge their way to a smooth press job just by building up the sticky tape under the flexo plate. Gravure and offset plates need to be precisely locked down and adjusted on press cylinders.
Other Flexo Characteristics
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The flexo press run usually starts at the unwind section, where the paper, film or cardboard you're printing, called the substrate, is spooled up and under tension; the customer won't want his cheese wrap, cellphone box or paper cup to be wrinkled before it even goes through the press. After printing comes drying (hot-air blowers to speed production) and sometimes suction (flexo presses can kick up an ink mist, called "flying"). Finally, there's the rewind section, not usually characteristic of other kinds of presses A spool is kept under tension to prevent the final printed product from wrinkling, so the product can be wound up and taken away to wrap that cheese, form that cellphone box or assemble and stack those paper cups.
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