During lithographic printing, a measure of the amount of water present on the water receptive areas of the printing plate is obtained by determining the amount of water present on the surface of a monitoring zone. The monitoring zone comprises electrodes spaced by electrically insulating material and the amount of water present on the surface of the zone is determined by measuring the electrical resistance between the electrodes. The electrical resistance may be measured using a phase sensitive detector and the amount of water subsequently applied to the printing plate may be controlled in dependence on the amount determined as being present.
A web roller is provided with at least two electrodes that are positioned parallel to each other on the roller surface end parallel to the longitudinal axis of the roller. Slip rings provide individual electrical connections to the electrodes. The electrodes are spaced apart a distance which permits both electrodes to be in contact with a web of sheet material of the type having a conductive center sandwiched between layers of insulating sheet. A position detector, positioned to detect the rotation of the roller, provides a pulse signal of a known magnitude to one of the electrodes, through a slip ring, when the electrodes are in contact with the web of sheet material. A sensor coupled to the other electrode, through a slip ring, receives the pulse signal after it has passed through the sheet material located between the two electrodes. The magnitude of the received pulse is a function of the resistivity of the sheet material.
To sense the ink density, in zones, transversely of a plate cylinder while avoiding possible erroneous determination due to transport of contaminants such paper dust or fluff, from the blanket cylinder to the plate cylinder, the attachment groove for the blanket (4) of the blanket cylinder (2) is made wider than the attachment groove (10) for the plate (3) of the plate cylinder (1), to leave a transverse strip (A) extending across the plate, on which sensing fields (12) are applied, for example by printing-on; the sensing fields, located in adjacent zones (Z) will be inked by the inking system (25) of the printing machine and, since out of contact with the blanket, will remain uncontaminated. A densitometer (13) sensing the inking density in the respective fields is translated across the printing cylinder, and sensed values are associated with the measuring fields (12), for example by determining rotation (n(sync)), permitting control of adjustment screws of a doctor blade (24) by suitable servo motors (23).
Method for regulating a supply of dampening solution in an offset printing press having a printing plate includes supplying at least one selected region of the printing plate with a well-defined amount of energy; determining and measuring scumming occurring in the selected region; comparing the measured scumming in the selected region with a prescribed scumming matched to the respective supplied amount of energy; and adjusting the supply of dampening solution, if the measured scumming and the prescribed scumming deviate from one another; and a device for performing the method.
A lithographic ink and moisture control system is provided for maintaining copy quality over a wide range of operating and environmental conditions without special operator assistance. The control system, among other things, momentarily increases the moisture feed rate each time ink is added to the system to maintain a substantially constant ink/moisture balance at the master, particularly under high coverage conditions.
A lithographic ink and moisture control system as provided for maintaining copy quality over a wide range of operating and environmental conditions without special operator assistance. The control system, among other things, adjusts the ink and moisture feed rates to compensate for predicted high moisture conditions which occur at several short copy runs, wherein significant moisture is introduced with each new master.
