A press consisting of several printing units (each with its own inking and dampening system), a feeder, a sheet transfer system, and a delivery is called a multicolor, or multiunit press. A multicolor press can have two, four, five, six, or more printing units, and two or more colors are printed in a single pass through the printing press. High-quality printing on a multicolor press depends on wet trapping, which is the ability of a wet, printed ink film to accept another wet ink film printed over it. A six-color Roland 900 sheetfed press is shown in Figure 3-4.
In the larger press sizes, the printing units are almost identical and are arranged in tandem. With some of the other two-oand four-color presses, one printing unit may be higher than the other to obtain better accessibility.
When placed in tandem, the open-unit type of single-color sheetfed press becomes a multicolor press, capable of printing a different color on each unit. One or more transter cylinders are placed between units to transport the sheet from one printing unit to the next. Some presses have three transfer cylinders between units, while other presses have a single, double-size transfer cylinder. An odd number of transfer cylinders is needed betwen units so that the side of the sheet to be printed faces away from the impression cylinder.
In another multicolor sheetfed press design, sometimes called the "semiopen design", a single impression cylinder serves two pairs of plate and blanket cylinders. The printed sheet is held by the common impression cylinder and successively brought into contact with each blanket. A press consisting of two semiopen units would then be capable of printing four colors on one side of the press sheet in single pass.