The Modern Process of Lithography

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Offset Printing

With the invention of the image setter, print shops can now print directly from their computer to film...

The plates that are used today are usually plastic or aluminum. The plates are created with a brushed, or rough, texture and then brushed with a smooth emulsion which is photosensitive. A photographic negative is exposed to light on the plate, creating a positive image on the emulsion. Chemicals are then used to remove any unexposed emulsion. This plate is then placed on a drum and treated with water, which only adheres to the areas without emulsion. A roller then applies ink which only sticks to the positive parts of the image which have been fixed by the emulsion. A rubber coated drum is then rolled over the plate to both remove the water and pick up the ink. This ink is then transferred only the printing medium. This method is called offset printing because the image is transferred (“offset”) onto the rubber drum prior to being place on the printing medium.

One of the innovations that has occurred in offset printing is the ability to use multiple plates to allow several colors to be transferred in one pass through the press. Another is called the Dahlgren inking system which combines the moistening and inking stages of the process into one, thus eliminating the early moistening stage.

Professional quality layouts can now be produced by almost anyone due to advances in desktop publishing. With the invention of the image setter, print shops can now print directly from their computer to film, eliminating the need to photograph the layout. In the 21st century the development of the place setter allows print shops to print directly onto plates, eliminating film altogether.


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