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.
Our particular platemaking process is self-contained within our state-of-the-art Computer-to-Plate platemaker. It receives the printing data, much like a laser printer, and burns the image onto a length of polyester-based plate material, which is cut to size and then developed within the platemaker itself.
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.
Our particular 2-color presses are unusual in that they use a single large blanket to receive the image from both plates, rather than a separate blanket for each ink.
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.
With our blankets, both inks are transferred to the paper at the same time, making printing time a little shorter, and allowing the press operator to more easily maintain the position of the image on the paper.