Methods and devices for control of inking and dampening in web-offset presses: Dissertation

The printability problems in web-fed lithographic presses attacked in the work. New types of on-press densitometers for print quality monitoring have been designed and implemented. The optical densitometer transducers have incandescent lamps as light sources and silicon photodiodes as detectors. For print on metal foils, the concept of specular reflection density has been introduced. For studies of ink transfer in a multi-roller inking train, a non-contacting, capacitive ink-film thickness monitor has been constructed. The data obtained with the instrument have been used in the development of smoothening algorithms for presetting procedures. Some typical hazards of careless inker presetting with a duct blade and inking screws are shown. An optical instrument, the plate scanner, has been designed and constructed for evaluation of the relative printing area distribution on offset plates. A measuring method using polarized light can be applied to presensitized and bi-metal plates, whereas the conventional, diffuse measuring method can only be used with presensitized plates. The plate scanner is able to plot the resulted ink consumption profile on a plastic foil with a marking pen. For actuation of the various process control operations, a remote control system has been constructed. Closed-loop control algorithms have been implemented for a laboratory web-offset press to regulate the print density within a chosen target range. The control efforts are based on density measurements anal the ink feed rate is used as a control variable. After presetting, the screw positions are controlled to maintain an even density profile. In case of press speed changes, a master-acting duct roller is used as a control device. The effect of dampening on print contrast has also been studied, and the proposal of an adaptive dampening control method has been suggested.