|Offset printing is a technique that's based on the old adage, "oil and water don't mix." It is used for almost all printing on paper, including newspapers, magazines, brochures, etc. The paper products that are inserted into CD jewel cases are printed using the offset technique, as are almost all alternative cardboard packaging materials.
Step 1 - Platemaking
A flexible flat plate is covered with a photosensitive chemical. Light is projected through the negative film onto the plate, producing a positive "image area" once the plate is developed. The image area of the plate is chemically treated so that it attracts ink, but repels water. The non-image area is treated so that the reverse happens: ink is repelled, but water is attracted.
Step 2 - Wetting
The flexible plate is wrapped around a cylinder. "Water rollers" come into contact with the plate cylinder, thoroughly drenching it. The image area on the plate cylinder repels the water, however.
Step 3 - Inking
A thin, even coating of oil-based ink is transferred onto the plate cylinder via the "ink rollers." The image area, which had repelled the water earlier, accepts the ink. No ink is retained outside the image area.
Step 4 - Offsetting
The actual plate cylinder does not come into direct contact with the paper; there is one last intermediate step in which a "rubber blanket" receives the ink from the plate cylinder. This is the stage that gives "offset" printing its name.
Step 5 - Printing
Paper is fed between the rubber blanket and an "impression cylinder." The ink that was stuck to the rubber blanket is transferred onto the paper.
Step 6 - Repeat steps 1-5 for each additional color
A full complement of rollers, rubber blankets, etc. is necessary for each ink color. Typical offset presses are 1-color, 2-color, 4-color, or 6-color. 1-color presses are most commonly used to print black only, although they are not limited to black. 2-color presses often handle black and one additional spot color - perfect for printing simple brochures and newsletters. 4-color presses are usually set up for CMYK. 6-color presses are usually set up to handle CMYK inks and two additional spot inks. If you need 5 inks, and you only have a 4 color press, it's necessary to break down and set up again so that you can re-feed the paper into the press for the last color.