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 than letterpress in that it can be used for printing on almost any type of substrate including plastic, metallic films, cellophane, and paper. It 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.
Flexography continues to be one of the fastest growing print processes and is no longer reserved just for printing specialty items. The ability of flexography to print on a variety of substrates allows the process to be used for a wide range of printed products. Food packaging is an important market because of the ability of flexography to print on non-porous substrates. 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.
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 anddigital 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.
The name “anilox” is derived from the ink that was used for the process until the 1950’s. Anilox ink was manufactured with “aniline” dyes which, in the 1950’s, were discovered to be health hazards, so pigment based inks were developed and have been used ever since. The ink carrying roller has continued to be called the “anilox roller” even though the aniline dye inks are no longer used for flexography. The current inks are very fluid and dry rapidly and are most often water based.
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: The stack press has separate printing units stacked vertically and each printing unit has its own impression cylinder. This was the first type of press developed for flexography. 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. Because of the distance between the printing units, problems with print registration can occur. 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 presses. This type of press provides for better registration of multiple colors. Some CIC presses have impression cylinders that are as much as 8 feet in diameter which allows for as many as 8 stations to be placed around the cylinder. The only drawback of the CIC press is that they can only print on one side of the substrate.