There are a variety of plates used in lithography. Each type of plates may have characteristics that make it suitable for a given type of press or type of production workflow. Even so, any plate used in lithography falls into one of two general categories that describe how the plate is imaged: (1) contact or (2) digitally. Contact plates are made by exposing the plate to imaged film in a contact frame. This type of plate assumes that the printer is using a film-based workflow. Pre-sensitized surface plates are the most common type of contact plates, but some use of multi-metal plates also exists. Digitally imaged plates are quickly gaining a share of production work in lithography. These plates are made in some type of platesetting device, which is an output device within a complete digital workflow, requiring no film. There are several types of digitally imaged plates including silver halide and photopolymer. Lithographic plates are made of various base materials, but most of them are made of grained aluminum, usually anodized and then silicated to create a durable water-receptive surface.
Lithography demonstrates the principle that oil and water, generally, do not mix. A conventional lithographic plate consists of image areas and nonimage areas that are distinguished by differences in surface chemistry. Image areas -- the parts that are to print are made oil-receptive (oleophilic) and water-repellent (hydrophobic). Nonimage areas -- the parts that are not to print -- are made water-receptive (hydrophilic) and oil-repellent (oleophobic). Therefore, when the printing plate is contacted by the dampening rollers, only the nonimage areas accept the water-based dampening solution and become wet. When the dampened plate is contacted by the inking form rollers, only the image areas accept the oily lithographic ink. These image and nonimage areas exist on essentially the same plane; thus, lithography is a planographic process. The objective of the plate manufacturer is to make the image areas as ink-receptive and water-repellent as possible and to make the nonimage areas as water-receptive and ink-receptive and ink-repellent as possible. The press operator must try to keep them that way.
The lithographic press operator is concerned with plates in two ways. One is the physical action of mounting, adjusting, and removing the plates. The other is assuring that the delicate chemical separation of image and nonimage areas is completely maintained during the pressrun. This requires more careful attention in lithography than in any other printing process. In rotogravure, the image areas are cut into a metal surface. In letterpress, they stand out solidly above the surface. But the level surface of the lithographic plate requires a chemical distinction between image and nonimage areas. When a properly made plate is run on the press, the dampening rollers keep the nonimage areas of the plate moist so that these do not accept ink. The ink rollers then transer ink only to the dry image areas, on which there is no water. The press operator does not need to know all the techniques required to make a plate, but understanding the nature of the lithographic plate and how it carries its image is helpful in maintaining the image satisfactorily.