Offset Printing

'Offset printing is a commonly used printing technique where the inked image is transferred (or "offset") from a plate 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 water-based film (called "fountain solution"), keeping the non-printing areas ink-free.

Development of the offset press came in two versions: in 1875 by Robert Barclay of England for printing on tin, and in 1903 by Ira Washington Rubel of the United States for printing on paper.

History

Lithography was initially created to be a low cost method of reproducing artwork.^[1][2] This printing process was limited to use on flat, porous surfaces because the printing plates were produced from limestone.^[1] Tin cans were popular packaging materials in the 19th century, but transfer technologies were required before the lithographic process could be used to print on the tin.^[1]

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”).

Present day

Compared to other printing methods, offset printing is best suited for cost-effectively producing large volumes of high quality prints in an economically sound manner that requires little maintenance.^[5]

Applications

Offset lithography is one of the most common ways of creating printed matter. A few of its common applications include: newspapers, magazines, brochures, stationery, and books.

Many modern offset presses use computer to plate systems as opposed to the older computer to film workflows, which further increases their quality.

Advantages

Advantages of offset printing compared to other printing methods include:

• Consistent high image quality. Offset printing produces sharp and clean images and type more easily than letterpress printing because the rubber blanket conforms to the texture of the printing surface.
• Quick and easy production of printing plates.
• Longer printing plate life than on direct litho presses because there is no direct contact between the plate and the printing surface. Properly developed plates running in conjunction with optimized inks and fountain solution may exceed run lengths of a million impressions.
• Cost. Offset printing is the cheapest method to produce high quality printing in commercial printing quantities.

Disadvantages

Disadvantages of offset printing compared to other printing methods include:

• Slightly inferior image to rotogravure or photogravure printing.
• Propensity for anodized aluminum printing plates to become sensitive (due to chemical oxidation)
• Time and cost associated with producing plates and printing press setup. As a result, very small quantity printing jobs

Types

Photo offset

Side view of the offset printing process. Multiple ink rollers are used to distribute and homogenize the ink.^[6]

The most common kind of offset printing is derived from the 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 an actual photographic print and typeset text. However, it is more common — with the prevalence of computers and digital images — that the source material exists only as data in a digital publishing system.

Offset litho printing on to a web (reel) of paper is commonly used for printing of newspapers and magazines for high speed production.

Ink is transferred from the ink duct to the paper in several steps :-

1. Ink duct roller delivers ink from the ink duct to the ink pyramid.
2. The drop roller transfers ink from duct roller to distribution roller. It is never in contact with both rollers at the same time.
3. The distribution rollers evenly distribute the ink. The first distribution roller picks up the ink from driving rollers, and the last distribution rollers transfer the ink to the form rollers.
4. The transfer rollers transfer ink between the ink-absorbing and ink-delivering driving rollers.
5. Driving rollers roll against the distribution rollers and either absorb or deliver ink, depending on their placement.
6. Ink form rollers transfer ink from the last distribution rollers on to the printing plate.
7. The printing plate transfers the ink to the offset cylinder(typically called blanket cylinder) usually covered with a rubber 'blanket'.
8. The paper is then pressed against the blanket cylinder by the impression cylinder, transferring the ink onto the paper to form the printed image.