The inking system of an offset press moves ink from a reservoir (ink fountain) to the printing plate by metering a thick charge of ink which has to be thinned into an ink film that produces acceptable print quality by upward of ten rollers because of the high viscosity of offset inks. Moreover, the amounts of ink that has to be dispensed from the ink fountain varies in every ink zone or in the axial direction of a printing cylinder, in accordance with image area content, which can vary from no image areas at all to solid areas and every conceivable tonal area in between.
Because the rollers (form rollers) at the end of the ink train, which apply the ink to the printing plate, have diameters that are much smaller than that of the printing cylinder, they must necessarily make several revolution to cover the distance from the leading to the trailing edge of an image area, which is problematic, as the initial thick charge of ink must be metered in such a way that all image areas in the circumferential direction of the print form receive uniform amounts of ink. We know from practical experience that this is not totally possible and must therefore accept a somewhat unequal ink film thickness in the circumferential direction of the printing cylinder as an inherent limitation of the offset lithographic inking system.
The control of ink film thickness is effected in one of two ways, 1, varying the thickness of the ink film in individual zones by means of so-called ink keys, and 2, increasing the overall charge of ink by varying the sweep of the fountain roller.
Because there is no practical method of controlling ink film thickness in the circumferential direction of the printing cylinder print quality is unpredictable in the areas that are located in the path of an ink film thickness adjustment zone.
The hypothesis of the study is that ink film adjustments in given zones affect tonal areas from shadows to highlights differently and that the inherent weaknesses of offset lithographic inking systems results in a somewhat unpredictable print quality.
A test form was designed to make spectral measurements of varying tonal areas in identical ink zones possible. This was done for the three primary colors and their secondary color composites.
An experimental press run was conducted, by means of which the ink film thickness was varied from a normal ink film thickness to three more incrementally decreased levels ink film thickness.
The results of the study will show DEab color variations caused by ink film thickness changes in ten tonal areas.