What Is a Lithographic Printer?
There are many ways of printing on paper. With etching, for example, grooves are made into a copper plate to form an image. Ink is then poured into the grooves. Paper is pressed on top of the copper plate and the image forms on the paper. The opposite occurs with letterpress when the printing surface is raised. Lithographic printers do not etch or raise the printing surface. Instead, they use a chemical process that makes the image remain on one plane of the surface.
Process
Lithography, which was invented in Germany at the end of the 1700s, is based on the scientific fact that oil and water do not mix but instead repel each other. With the first lithographs, a grease crayon was used to draw an image on a large piece of limestone or marble. The lithographic printer then wet the stone with water. The oil from the crayon repelled the liquid. Therefore, when grease-based ink was rolled over the stone, it only stuck to the inked parts of the drawing.
Metal vs. Stone
Lithography is done essentially the same way now, but the work is done on a metal plate instead of a stone slab. Printers use a high-quality lithographic process for a wide range of documents on smooth paper, such as maps, posters and book covers. The process is used for both text and illustrations.
Graphic Design
Lithographer printers work closely with graphic designers who use the computer to complete each page of the document. When the complete layout and design is edited and proofread, the graphic designer sends all the pages to the printer in a digital format. It is then converted to negatives that are printed on the plates.
Offset Printing
Today's lithographic process is called offset printing. When the image area on the plate goes through the press, it picks up ink from the ink rollers. Water rollers force the ink off those areas of the plate without a drawing. The image is then transferred to paper. The term "offset" is used, because the plate never touches the paper. The entire process takes place at high speeds.
Drying process
When the paper comes off the press, it is still wet and can smudge. The paper is thus placed in an oven to dry. After it is dry, the paper is then run over with large metal rollers that are cooled with ice-cold water. This keeps the paper from smearing.
