Offset Lithographic Printing Process (continued)

As the plate cylinder rotates, the plate comes in contact with the dampening rollers first. The dampening rollers wet the plate so the non-printing areas repel ink. Then the inking rollers transfer ink to the dampened plate, where ink only adheres to the image areas. The inked image is transferred to the rubber blanket, and the substrate is printed as it passes between the blanket and impression cylinder.

There are three basic lithographic press designs: unit-design, common impression cylinder design, and blanket-to-blanket design. The unit-design press is a self-contained printing station consisting of a plate cylinder, a blanket cylinder, and an impression cylinder. Two or more stations may be joined to perform multi-color printing. A common impression cylinder press consists of two or more sets of plate and blanket cylinders sharing a common impression cylinder. This allows two or more colors to be printed at a single station. A blanket-to-blanket press consists of two sets of plate and blanket cylinders without an impression cylinder. The paper is printed on both sides simultaneously as it passes between the two blanket cylinders (Field).

The major unit operations in a lithographic printing operation include:

• Image preparation
• Processing printing plates
• Printing
• Finishing
• Image Preparation of Lithographic Printing Plates

Image preparation begins with camera-ready (mechanical) art/copy or electronically produced art supplied by the customer. Images are captured for printing by camera, scanner or computer. Components of the image are manually assembled and positioned in a printing flat when a camera is used. This process is called stripping. When art/copy is scanned or digitally captured the image is assembled by the computer with special software. A simple proof (brown print) is prepared to check for position and accuracy. When color is involved, a color proof is submitted to the customer for approval.

There are eight different types of litho plates common to the commercial printing industry: Diazo, Photopolymer, Silver Halide, Electrophotographic (Electrostatic), Bimetal, Waterless, Thermal, and Ablation. The predominant surface plate in use today is termed a "presensitized" plate. Most printers will primarily use one or two types of plates. It is highly unlikely that you would encounter a printer that could use a few of each type of plate nor is it easy for them to switch to a different type of plate due to equipment, expense and application reasons.
More Plate Information...