Offset printing is a process based on the principle that oil and water do not readily mix. In the case of offset, the “oil” is printing ink and the “water” is fountain solution (a solution of mostly water and small amounts of chemicals to dampen the printing plate during printing). Offset printing uses three cylinders (plate, blanket, and impression), a dampening system (although some newer presses use temperature instead of dampening), and an inking system to produce a printed image in one color on one side of the paper (see figure above).
A developed printing plate is mounted onto the plate cylinder and the press is started. The rollers of the dampening system first lay a thin film of fountain solution onto the plate. The ink, which is also brought to the plate by a series of rollers, then contacts the plate and adheres only to the printing areas of the plate. The amount of ink brought to different areas across the width of the plate is controlled by the inking adjustment keys. As long as the plate remains wet, the plate is inked only in the desired printing areas. The inked image is then “offset” or transferred to the rubber-covered blanket cylinder when the two cylinders are in contact, producing a mirrored image relative to the plate. When paper is fed through the press (between the blanket and impression cylinders), the image is again offset to the paper when these two cylinders are in contact, and is now right reading again. Each ink color is printed separately (from a separate plate) in the same way. On multiple color jobs, the different ink colors are “registered” (or lined up) by the printing press operator in a single pass on multicolor presses or multiple passes through the press.
Offset presses are configured to have anywhere from one to twelve printing units and some can print on both sides of the paper in one pass (called “perfecting”). Printing is done either on precut sheets of paper (sheetfed presses) or rolls of paper (web presses). Offset presses are a marvel of engineering that can produce precisely positioned and very high quality images.