Flexography is a printing process which utilizes a flexible relief plate that can be adhered to a printing cylinder. It is basically an updated version of letterpress. It much more versatile in that it can be used for printing on almost any type of substrate including plastic, metallic films, cellophane, and paper. The inks currently used for this process are usually water based and dry rapidly.
Applications
Flexography 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 color. This ability makes it useful for printing on plastic bags as well. Other common applications printed with flexography include gift wrap, wallcovering, magazines, newspaper inserts, paperback books, telephone directories, and business forms.
Printing Process
The relief plate used for flexography is made of molded rubber or photopolymer materials with the image areas raised above the non-image areas of the plate. Flexographic plates can be created with analog and digital platemaking processes.
Flexography is a direct printing method in that the inked plate applies the image directly to the substrate. An inked roller known as the “anilox roller”, applies ink to the raised portions of the plate which is then transferred to the substrate. The anilox roller has cells that carry a specific amount of ink to the plate. The number of cells per linear inch can vary according to the type of print job and the quality required.
Press Types
Flexographic printing is accomplished on rotary web presses. The presses can be divided into three main categories: stack press, in-line press, and common impression cylinder press.
Stack Press
This was the first type of press developed for flexography. The stack press has separate printing units stacked vertically and each printing unit has its own impression cylinder. It is difficult to hold registration between multiple colors printed on stretchable substrates using the stack press, even with web tension controls. It is best suited for heavier gauge substances, such as thick paper products, that will not stretch or for applications that do not require good registration.
In-Line Press
Like the stack press, the in-line press has separate printing units for each color and each unit has its own impression cylinder, but they are arranged in a horizontal configuration just like a rotary offset press. Tension control equipment is used on the press to hold proper registration between multiple colors. The wider in-line presses are used for printing large items such as corrugated cartons and smaller width webs are used for printing pressure sensitive labels at high speeds.
Common Impression Cylinder Press
Instead of the printing units being independent from the others, they are all grouped around a common impression cylinder. Substrates do not stretch as they move around the impression cylinder so the common impression cylinder press is a good choice for printing on substrates such as thin plastics that would normally stretch when used on other types of flexography presses. This type of press provides for better registration of multiple colors. The only drawback of the CIC press is that they can only print on one side of the substrate.
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