Offset Printing 101

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Offset printing is a printing technique where an inked image is transferred (or “offset”) from a plate to a blanket, then to the paper. The lithographic process is based on the repulsion of oil and water; the image to be printed obtains ink from ink rollers, while the non-printing area attracts a water-based film (“fountain solution”), keeping the non-printing areas ink-free.

In very basic terms:

 

  1. A printing plate (with a positive image) is mounted to the plate cylinder, and a rubber blanket is mounted to the offset cylinder.
  2. Paper passes between the offset cylinder and the impression cylinder.
  3. Ink works its way through a series of ink rollers.
  4. Water is picked up from a tray beneath water rollers.
  5. It is the delicate balance of ink and water that allows the image areas of the plate to accept ink and the non-image areas to accept water (and, thus repel ink).
  6. The ink from the image areas on the plate is transferred to the rubber blanket. At this point, the image is now a negative, reverse image.
  7. The negative image from the offset cylinder (blanket) is transferred to the paper, which passes in between the offset cylinder and the impression cylinder, and is again a positive image.
  8. This process occurs—one printing unit at a time—for each of the process colors (in this order: black, cyan, magenta and yellow). And, depending on the press for up to three additional spot colors.
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Author: Nicole Hannover

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