Design For Print... Offset Lithography

Definition:
A printing method in which the image on a plate is transferred 'offset' to an immediate roller prior to final transfer to the paper.

Taken from:
Production For Print by Mark Gatter

'Paper is loaded onto a platform at the back end of the press, above which is a series of suckers. These lift the paper, one sheet at a time, and enable the grippers (grabbing pincers mounted on a chain-link drive) to grab it along one edge (called the gripper edge) and pull it into the first ink unit, where it is printed with the first colour. As it leaves, it is grabbed again and pulled forwards into the next ink unit, and so on.'

A brief history...

The first rotary offset lithographic printing press was created in England and patented in 1875 by Robert Barclay.[1] This development combined mid-19th century transfer printing technologies and Richard March Hoe’s 1843 rotary printing press—a press that used a metal cylinder instead of a flat stone.[1] The offset cylinder was covered with specially treated cardboard that transferred the printed image from the stone to the surface of the metal. Later, the cardboard covering of the offset cylinder was changed to rubber,[1] which is still the most commonly used material.
As the 19th century closed and photography captured favor, many lithographic firms went out of business.[1] Photoengraving, a process that used halftone technology instead of illustration, became the leading aesthetic of the era. Many printers, including Ira Washington Rubel of New Jersey, were using the low-cost lithograph process to produce copies of photographs and books.[3] Rubel discovered in 1901—by forgetting to load a sheet—that when printing from the rubber roller, instead of the metal, the printed page was clearer and sharper.[3] After further refinement, the Potter Press printing Company in New York produced a press in 1903.[3] By 1907 the Rubel offset press was in use in San Francisco.[4]
The Harris Automatic Press Company also created a similar press around the same time. Charles and Albert Harris modeled their press “on a rotary letter press machine,” (“Short History of Offset Printing”).

Advantages:

- The high quality of print is consistent.
- Plated are easy to produce.
- Cheap - medium to long print runs benefit from this.

Disavantages:

- Not as high quality as rotogravure.
- Colour variation can occur when the ink and water levels fail to remain consistent, this is likely to occur in longer print runs.
- Imperfections on the printed image that are caused by dirt or other alien particles cause 'hickeys' or 'bullseyes'.