A press consisting of a single printing unit, with its integral inking and dampening systems, a feeder, a sheet transfer system, and a delivery is called a single-color press (Figure 3-1). Normally, it can print only a single color in any one pass through the press. On some presses, the inking system can be modified-split-with ink fountain and ink roller dividers so that two or more colors can be printed at one time. On these presses, the same printing plate is used, and the colors are widely separated.
A single-color press can also be used for true multicolor printing, the printing of two or more colors, often one over another. Multicolor printing on a single-color press requires that the sheet be fed through the press as many times as there are colors to be printed. After each printing, the just-used plate is removed and the inking system is thoroughly cleaned. A new plate is mounted on the plate cylinder, and the inking system is filled with the next color. After the just-printed ink dries, the sheet of paper is run through the press again and printed with this new color. (multicolor printing on a single-color press is dependent upon dry trapping -- the ability of dry, printed ink film to accept a wet ink film over it. The wet ink dries try oxidation polymerization.)
The printing unit of the single-color sheetfed press is sometimes described as an "open unit." The plate cylinder, blanket cylinder, and impression cylinder are usually arranged in a near right-angle relationship (Figure 3-1). This arrangement is common for three reasons:
. To reduce the overall height of the press.
. To make it easier to feed the paper into the impression cylinder grippers.
. To make ite possible, by movement of only the blanket cylinder, to throw all three cylinders out of contact with each other.