Offset Printing
 Offset printing is a widely used printing technique where the inked image is transferred (or "offset") from a plate first to a rubber blanket, then to the printing surface. When used in combination with the lithographic process, which is based on the repulsion of oil and water, the offset technique employs a flat (planographic) image carrier on which the image to be printed obtains ink from ink rollers, while the non-printing area attracts a film of water, keeping the nonprinting areas ink-free.
The advantages of offset printing include:
- Consistent high image quality — sharper and cleaner than letterpress printing because the rubber blanket conforms to the texture of the printing surface
- Usability on a wide range of printing surfaces in addition to smooth paper (e.g., wood, cloth, metal, leather, rough paper)
- Quick and easy production of printing plates
- Longer plate life than on direct litho presses — because there is no direct contact between the plate and the printing surface.
History
The first lithographic offset 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. 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).
During the 1950s, 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.
Photo offset
The most common kind of offset printing is derived from photo offset process. In such cases, the documents to be printed are first recorded on film negatives. Images from such negatives are then transferred to photomechanical printing plates much the same way as photographs are developed. A measured amount of light is allowed to pass through the negatives and exposed the printing plate. A chemical reaction then occurs that allows an ink-receptive coating to be activated, thus transferring of the image from the negative to the plate.
Present day
Offset printing is the most common form of high volume commercial printing, due to advantages in quality and efficiency in high volume jobs. However, modern digital "presses" (inkjet based) are getting closer to the cost/benefit of offset for high quality work. However, 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 dominantly become the main form of printing in packaging due to a lower quality expectations and the significantly lower costs in comparison to other forms of printing. |
