In lithographic printing, presensitized printing plates are used which are comprised of an aluminum sheet, which has been mechanically grained on at least one surface, an interlayer, if required, on top of the grained surface, and a light-sensitive (usually diazo-based) coating on top of the interlayer. To expose and develop the plate for lithographic printing, one usually places a negative film of the desired image in contact with the light-sensitive coating on the plate, inside a vacuum frame; the frame is evacuated so that the negative is flattened against the glass by the plate and intimate contact is achieved; an ultraviolet light source is turned on to expose the plate through the clear areas of the negative; and the plate is removed from the frame and developed so that the non-exposed, non-image areas of the light-sensitive coating are dissolved from the plate by application of an appropriate solvent. The bare, grained aluminum plate attracts the aqueous fountain solution when the plate is mounted on the cylinder of an offset printing press, and the exposed and consequently insolubilized image areas of the light-sensitive coating repel the water. When the press is started, the plate on the cylinder rotates and the image area will attract the greasy ink and in turn transfer it to a rotating blanket (rubber sheet) on another cylinder, thereby offsetting the greasy ink image and the aqueous background onto the blanket, which in turn rotates against and prints on a sheet of paper moving through the press.
USPTO Backgrounds
USPTO Backgrounds