Printing Technologies
Printing Processes
The different printing techniques are being described by the way the ink gets on the paper, or how this is prevented.
Letterpress
This technique prints with areas that are embossed. The ink is being rolled over a wood block so that there is no ink in the depression, therefor no ink gets transferred onto the paper. Examples for letterpress are the Gutenberg-Press as well as wood- and linoleum-cut, known from school time. Today letterpress is only rarely used for industrial printing.
Gravure printing
At the gravure printing, the depressed parts of the printing stock are the parts containing the ink. The embossed parts are being cleaned by a squeegee before the printing-cylinder hits the paper.
(Silk-) Screen-printing
The ink is being pressed with a squeegee through a screen. The printed material can be paper, silk, cloth, etc. The screen is closed at the parts where no ink is supposed to come through.
Flexography
A mixture from letterpress and gravure printing. The printing stock is made out of rubber or plastic and works like letterpress. The inking of the stock is made with a halftone roller with a squeegee and works like the gravure printing. Main usage of flexography is printing on wrappings or foil that can not or only with a lot of problems being printed on with the other techniques.
Offset printing
Here the printing and non-printing parts are at the same level, there are no embossed or depressed parts. Printing parts accept the ink, non-printing parts reject the ink.
The major usage of offset printing is the so called wet offset print. It explains the "trick" the best:
The printing stock is made out of very fine roughened aluminum, which is covered with a light-sensitive, water rejecting layer. By exposure to light and washing, the forme is ready to print. The printing parts with their water rejecting areas, the non-printing parts with the roughened aluminum areas. In the printing machine this forme is covered with a wafer-thin layer of water and afterwards the greasy ink is being applied. The non-printing parts are protected by the water-film and therefor can not hold any ink. The printing image is being transferred to a rubber printing-blanket and from there to the paper. Because of the transfaring by the extra blanket the offset printing is also called indirect printing.
Depending on the material being printed on, the offset printing is separated into sheet- and roll-printing. Another way of making a distinction is the way of drying (Coldset, Heatset, UV-Offset).
Today offset printing is the most common and important way of printing i.e. magazines, news-papers, wrappings, inserts, catalogues, etc.
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