A CDE Definition
A printing technology that dates back to 1798 when Alois Senenfelder developed a method of imaging limestone from which a print was produced. Based on the principle that oil and water do not mix, an aluminum or plastic plate is coated with a photopolymer film that is exposed to light through a photographic mask. The exposed areas are chemically "hardened," and the unexposed areas are dissolved when the plate is put through a chemical process, which is the next stage. When printing a page, the plate is dampened, and the water adheres only to the unexposed, non-image areas, which repell the greasy ink that is applied to the plate immediately thereafter.
The most common lithographic printing uses the offset method, in which the ink is "offset" onto a rubber-coated cylinder that is pressed against the paper. See offset press.
A printer that uses an intermediate rubber-coated cylinder known as a "blanket" to transfer the image onto the paper. Instead of transferring the image from a metallic drum onto paper as is done with most digital printers, the image is "offset" onto the blanket and then to paper. The blanket creates a smoother image on most types of paper and can print on rough or heavy stock as well as other media. Most offset presses use this lithographic method.
Sheet Fed and Web Fed
Sheet-fed offset printers use cut sheets of paper, whereas web-fed printers use rolls of paper that are cut and trimmed after printing. The web-based offset method is typically used for high-speed, high-volume applications such as printing newspapers and magazines.
Right Reading to Wrong Reading
Since offset presses use the intermediate blanket as the transfer mechanism, the original negative image on the drum is right-reading. It becomes wrong-reading on the blanket and then back to the original on the paper.
Combination Offset and Laser
Indigo USA, Woburn, MA (www.indigonet.com), which was acquired by HP in 2002, combines laser printing and offset printing. It makes a "digital offset color press," which is an electrophotographic printer that offsets to a blanket instead of imaging directly from the drum. It also uses a special liquid toner instead of dry toner, enabling it to print on polyester, PVC, films and other media as well as coated and uncoated paper. See lithography, digital printing, right-reading and DI press.
Offset and Digital, Too
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