A offset machine is a complex piece of high-precision industrial equipment designed to produce printed material at high speed and low cost per page. Printing presses are commercially available and they use several different types of printing techniques, but the most common type is called offset printing. These presses are usually designed as sheet-fed construction, printing on single sheets of paper or other materials, or web construction, printing on long rolls of paper or other materials supplied on large rolls. So-called "full-size" sheetfed offset presses print on paper that measures approximately 700 mm x 1000 mm (approximately 28 inches x 40 inches). "Half-size" and "quarter-size" offset presses are also common.
Offset presses have a separate printing unit, or tower, for each color of ink. Some presses have as many as 12 towers and print 6 colors on one side of the paper first, then flip the paper over in a device called a duplexer, and finally print 6 colors on the reverse side of the paper. Usually black plus the three subtractive primary colors (cyan, magenta, and yellow) are the 4 main colors printed. Additional inks added to these four primary colors are usually spot colors, which are highly saturated colors out of gamut and can be achieved by subtractive primary colors. These are often used as logo colors, or they may be colors used for certain colors. Eye-catching artistic effects.
Paper that passes through a sheetfed offset press is held by clamps. There are many sets of grippers in the press, and as the paper passes through the press, the paper is passed from one set of grippers to the next. Each set of grippers consists of a number of individual jaws spaced across the width of the leading edge of the sheet (each pair of jaws is a bit like a pair of miniature pincers, gripping part of the leading edge of the sheet). Each impression cylinder has a set of grippers that grab the leading edge of the paper as ink is applied to it. Other sets of grippers transport the sheet from one impression cylinder to the next. The paper in a sheetfed offset press is like the baton in a relay race - one set of grippers keeps holding it firmly.
Offset machines use specially designed, very viscous inks. A complex set of rollers delivers ink to each plate cylinder—typically around 20 individual rollers are used to ink each plate. This set of cylinders splits the ink film multiple times, creating thinner and thinner ink layers as the ink moves from the ink supply to the plate cylinders. At the very beginning of this set of rollers is a set of ink keys: mechanical adjustments that control the thickness of the ink flowing out of the ink supply. Each key controls the amount of ink that prints approximately 2 cm across the width of the printed page.