Offset Printing 101
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:
- A printing plate (with a positive image) is mounted to the plate cylinder, and a rubber blanket is mounted to the offset cylinder.
- Paper passes between the offset cylinder and the impression cylinder.
- Ink works its way through a series of ink rollers.
- Water is picked up from a tray beneath water rollers.
- 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).
- 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.
- 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.
- 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.