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Advantages of offset printing compared to other printing methods include:
The first lithographic offset axiom printing press was created in England around 1875 and was designed for printing on metal. The offset cylinder was covered with specially treated cardboard that transferred the printed image from the litho stone to the surface of the metal. About five years later, the cardboard covering of the offset cylinder was changed to rubber, which is still the most commonly used material.
The first person to use an offset press to print on paper was most likely American Ira Washington Rubel in 1903. Roughly at the same time, a German engineer by the name of Christopher Hermann invented a similar machine. He got the idea accidentally by noticing that whenever a sheet of paper was not fed into his lithographic press during operation, the stone printed its image to the rubber-covered impression cylinder, and the next impression had an image on both sides: direct litho on the front and an image from the rubber blanket on the back. Rubel then noticed that the image on the back of the sheet was much sharper and clearer than the direct litho image because the soft rubber was able to press the image onto the paper better than the hard stone. He soon decided to build a press which printed every image from the plate to the blanket and then to the paper. Brothers Charles and Albert Harris independently observed this process at about the same time and developed an offset press for the Harris Automatic Press Company soon after.
Harris designed his offset press around a rotary letterpress machine. It used a metal plate bent around a cylinder at the top of the machine that pressed against ink and water rollers. A blanket cylinder was positioned directly below, and in contact with, the plate cylinder. The impression cylinder below pressed the paper to the blanket in order to transfer the image to the sheet (see diagram). While this basic process is still used today, refinements include two-sided printing and web feeding (using rolls of paper rather than sheets).
Offset printing became the most popular form of commercial printing as improvements were made in plates, inks and paper, maximizing the technique's superior production speed and plate durability. Today, the majority of printing, including newspapers, is done by the offset process, although digital printing has greatly increased in popularity due to demand and cost advantages for low quantity runs.
The most common kind of offset printing is derived from photo offset process, which involves using light-sensitive chemicals and photographic techniques to transfer images and type from original materials to printing plates.
In current use, original materials may be actual photographic prints and typeset text. However, it's more common -- with the prevalence of computers and digital images -- that the source material exists only as data in a digital publishing system.
If actual source materials are used, they are assembled in a page layout by "pasting up" the type (text) and images (such as photographs) on heavy paper sheets or boards, referred to as "mechanicals" or "paste-ups." These are exact representations of the printed page.
To produce a printing plate, the layout is photographed using a "copy" or "process" camera, from which a full-size negative film (flexible plastic sheet) is produced.
This film, or "page negative" (in which black areas such as text are clear, and white areas such as the background are opaque black) is then placed in contact with an emulsion-coated printing plate. A high-intensity light exposes the printing plate through the negative. After chemical development, the plate is coated with a liquid that sticks to the exposed areas of the plate and dries into an ink-receptive surface. This forms a positive (and "right-reading") image of the page to be printed.
With digital publishing systems, physical paste-ups are replaced by virtual layouts created on a computer, which outputs page negatives or printing plates. If negative film is output, it is used to expose printing plates. In "Direct-to-plate" systems, the printing plate is exposed directly by a computer-driven device.
Offset printing is the most common form of high volume commercial printing, due to advantages in quality and efficiency in high volume jobs. While modern digital "presses" (inkjet based) are getting closer to the cost/benefit of offset for high quality work, they have not yet been able to compete with the sheer volume of product that an offset press can produce. Furthermore, many modern offset presses are using computer to plate systems as opposed to the older computer to film workflows, which further increases their quality.
Private or hobby presses, engaged in patient production of limited editions of fine quality books, often use letterpress as well as offset methods, some "purists" preferring the slightly embossed look resulting from the direct impression of inked type upon fine paper. These books are sometimes printed from hand-set foundry type (individual pieces of movable, lead-alloy type). Flexography, a form of letterpress, is still used in the printing of high-quality premium labels, in ticket printing, and in envelope manufacturing/printing, though is now no longer the dominant technology. In Europe, however, in the last two decades flexography has become the dominant form of printing in packaging due to lower quality expectations and the significantly lower costs in comparison to other forms of printing.
