Monday 11 June 2012

Know what is Offset Printing

Source - Wikipedia


Offset printing is a commonly used printing technique in which 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.


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.
  • A further advantage of offset printing is the possibility to adjust the amount of ink on the fountain roller with screw keys. Most commonly a metal blade controls the amount of ink transferred from the ink trough to the fountain roller. By adjusting the screws the gap between the blade and the fountain roller is altered, leading to the amount of ink applied to the roller to be increased or decreased in certain areas. Consequently the density of the colour in the respective area of the image is modified. On older machines the screws are adjusted manually, but on modern machines the screw keys are operated electronically by the printer controlling the machine, enabling a much more precise result

Disadvantages

Disadvantages of offset printing compared to other printing methods include:
  • Slightly inferior image quality compared to rotogravure or photogravure printing.
  • Propensity for anodized aluminum printing plates to become sensitive (due to chemical oxidation) and print in non-image/background areas when developed plates are not cared for properly.
  • Time and cost associated with producing plates and printing press setup. As a result, very small quantity printing jobs are now moving to digital offset machines.

The offset printing process

Side view of the offset printing process. Multiple ink rollers are used to distribute and homogenize the ink.
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 lithographic 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. The ink duct roller delivers ink from the ink duct to the ink pyramid, also called the Ink Train.
  2. The ductor roller, sometimes called a vibrator roller due to its rapid back and forth motion, transfers ink from the duct roller to the first 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 the 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.

3 comments:

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  2. This article gives the light in which we can observe the reality. This is very nice one and gives in depth information.

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  3. well, this article helped me in knowing about offset printing and its process. But as they compared with letterpress printing to offset printing then the author would explain about it a little bit then it will be more clear in understanding.

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