The inking system of an offset press serves five important functions: (1) ink milling, (2) varying ink input across the system, (3) storing ink for consistent ink film thickness, (4) dampening control, and (5) keeping the plate clean.
Ink milling. The many rollers in the ink train mill the ink into a thin, controlled ink film. This assures that a consistent ink film is deposited on the paper, with no variation in density. It delivers an even, controlled ink film to the plate. The thick film entering the system on each ductor stroke splits among many rollers, changing the intermittent ink feed from the ductor into the continuous feed required by the plate.
Varying ink input across the system. The inking system functions to deposit the ink film as it is needed on the plate. For example, the image areas of the plate many be very heavy on the left side and very light on the right. In this case, more ink would be needed in the left side of the inking system to replenish the higher level of ink consumption on the left side of the plate. To accomplish this, the system must allow the control of ink flowing into the system, both in total volume and side-to-side variation.
Storing ink for consistent ink film thickness. The inking system acts as a reservoir to maintain consistent ink film thickness from impression to impression. Any variability in ink film thickness affects print quality. For example, one revolution of the plate cylinder will draw a certain amount of ink from the ink system. If the ink were not stored sufficiently in the many rollers of the system, the next rotation of the plate cylinder would pick up less ink and the resulting image would be lighter than the preceding image. This condition is referred to as ink starvation, but it is rarely a problem in inking systems that have many rollers, which store large quantities of ink.
Dampening control. The inking system helps to control dampening on the plate by picking up some water as the press runs. Some of the water mixes with the ink, forming an emulsion. Too much emulsification prevents image areas from accepting ink; however, controlled emulsification is essential to the lithographic process. Some water is also picked up by the blanket and transferred to the sheet. The rest of the water evaporates.
Keeping the plate clean. The inking system helps to clean the plate by picking up foreign matter that may collect on the plate. A roller specifically designed for this purpose is often used in one of the form roller positions.