A printing press is a complex piece of high-precision industrial equipment that is designed to produce printed material at a high rate of speed and low cost per page. Printing presses are commercially available which use several different types of printing technologies, but the most common type is called offset lithography. These presses are commonly designed in either sheet-fed configurations, which print on individual sheets of paper or other material, or web-fed configurations, which print on long webs of paper or other material, supplied on large reels. A so-called 'full size' sheet fed offset press prints on sheets that are about 700 mm x 1000 mm in size (about 28 inches x 40 inches). "Half size" and "quarter size" offset presses are also common, and these machines print on sheets which are one-half or one-quarter as large.
An offset printing press has a separate printing unit, or tower, for each color of ink. Some presses have as many as 12 towers, first printing 6 colors on one side of the sheet, then flipping the sheet over, in a device called a perfector, and finally printing 6 colors on the reverse of the sheet. Normally black plus the three subtractive primary colors (cyan, magenta, and yellow) are the 4 main colors printed. Other inks added to these four main colors are typically spot colors, which are highly saturated colors outside the color gamut which can be achieved with the subtractive primaries - these are commonly employed as logo colors, or they may be colors which are being used for some striking artistic effect.
Each printing tower has three main cylinders - the plate cylinder, blanket cylinder, and impression cylinder. Each of these cylinders is designed to have a surface which is slightly larger in area than the size of sheets which are printed by that particular press. The blanket cylinder is in between the plate and impression cylinders, and its surface touches the surface of the other two cylinders. The three cylinders are rotated at the same surface velocity, so that their surfaces contact each other without sliding. As explained below, the ink image is formed first on the plate, then transferred to the blanket, and finally transferred to the paper (which is held on the surface of the impression cylinder).
There is one lithographic plate per color plane, and these plates form the image. The plates operate on the principle that oil and water do not mix. Some other printing press technologies (such as flexography), use printing plates with raised areas which hold the ink, similar to a stamp pad. There are also some other printing press technologies (such as gravure) which hold the ink in engraved recesses on the equivalent of the "plate cylinder". However, lithography uses plates which are flat to within 1 micrometer - the inked areas in the image are neither raised nor recessed by any height which is of any significance to this printing process. The plates themselves are most commonly flat pieces of anodized aluminum with a thin (about 1 micrometer thick) polymer layer on their surface. The polymer has the property that it is readily wetted by the oil-based printing ink, but not wetted by water. On the other hand, the anodized aluminum itself is readily wetted by water, but is not by the ink. So, essentially, the polymer attracts ink and the aluminum attracts water.
Before the press is ready to start printing, an image is formed on the surface of each plate by selectively removing portions of the polymer layer, in a computer-to-plate machine which scans laser beams across the surface of the plate to remove the unneeded portions of the polymer layer. Then the plates are wrapped around the plate cylinders and mechanically clamped in place. A type of offset press called a direct imaging (DI) press, available since the mid 1990s, forms images on plates in situ on the plate cylinders in the press, using laser scanning systems integrated in the press. This is less common than imaging the plates outside of the press on a computer-to-plate machine. Once the press starts printing, rollers apply a colorless water-based solution (called dampening solution or fountain solution) to each plate, and this solution wets the areas where the polymer has been removed. After the fountain solution is applied, other rollers apply printing ink to the plate, and this ink coats only the portions of the plate that still are covered by the polymer layer.
The inked printing plate presses up against the second main cylinder, the blanket cylinder. A soft rubbery sheet, about 2 millimeters thick (called an offset blanket), covers the surface of this cylinder. This blanket is a distinctive feature of offset lithography - the ink in the image transfers first from the plate to the blanket and then next from the blanket to the paper. The advantage of the blanket is that it is soft and deformable, which enables it to conform to the surface contours of the paper and transfer a thin layer of ink (typically 1 micrometer of ink thickness) to the paper both efficiently and uniformly, in spite of the roughness of the paper surface. The blanket is held in place on the blanket cylinder by clamps, similar to the manner that the plate is held by clamps onto the plate cylinder.
As the plate rotates into contact with the blanket and then separates from it, about half of the ink on the plate transfers to the blanket. This ink image travels on the surface of the blanket as the blanket cylinder rotates and then comes into contact with the paper. As the blanket separates from the paper, about half of the ink transfers onto the paper. Printing presses are designed to print the same image page after page on a large number of pages, so it is not a problem that only half of the ink transfers from the plate to the blanket, and then half from the blanket to the paper. More ink is added to the image areas on the plate during each revolution, as the plate passes under the inking rollers. And during each revolution, half of the ink is transferred to the blanket. After some number of initial "startup" rotations of the cylinders, the ink flow off of the plate becomes equalized to the ink flow onto the plate, and similarly the ink flow off of the blanket (onto the paper) becomes equalized to the ink from onto the blanket.
The paper which passes through a sheetfed offset press is held by grippers. There are many sets of grippers in the press, and the sheet is relayed from one set of grippers to the next as it travels through the press. Each set of grippers consists of numerous individual pairs of jaws spaced across the entire width of the leading edge of the sheet (each pair of jaws is somewhat like a miniature pair of pliers which grabs part of the leading edge of the sheet). Each impression cylinder has a set of grippers which hold the leading edge of the sheet as the ink is applied to it. Other sets of grippers transfer the paper from one impression cylinder to the next. The paper in a sheetfed offset press is like a baton in a relay race - one of the sets of grippers is always holding it securely, and at each handoff the new set of grippers engages before the prior set lets go of it.
Offset presses use specially designed inks which are very viscous (with a viscosity of about 100 Pascal - seconds, i.e. about 100,000 times more viscous than water and 100 times more viscous than honey). A complex set of rollers feeds the ink to each plate cylinder - typically about 20 individual rollers for the ink supply to each plate. This set of rollers splits the ink film numerous times, creating a thinner and thinner ink layer as the ink moves from the ink supply toward the plate cylinder. At the very beginning of this set of rollers is a set of ink keys: mechanical adjustments which control the thickness of ink flowing out of the ink supply. Each key controls the volume of ink which is printed on about 2 centimeters or so of the width across the printed page. These keys are adjusted so the volume of ink flowing for each 2 centimeters or so of width across the page is matched to the image area on that printing plate (for that color) in that same portion of the page.
To start a print run, the plates are first imaged and clamped onto the plate cylinders on the press, one plate for every color of ink. In some recent model presses, motorized plate changing mechanisms un-mount the plates from the previous run and mount the new ones (otherwise this is a manual step). Next the ink keys are adjusted across the width of each ink supply system. In some recent model presses, these are motorized as well, and the settings are taken directly from the image density across of the width of each plate (which can be measured using a scanner, or taken directly from the computer-to-plate file). Then the press is started running at a low speed. The initial sheets which come off of the press are carefully checked by the press operator for registration (i.e. alignment) of each color plane, using printed crosses and other alignment marks. The operator can adjust the position of each plate in both dimensions as needed with precision adjustment gears while the press is running at low speed. The operator also waits until enough sheets have been printed to achieve ink/water balance and to achieve steady-state ink flow on each of the sets of inking rollers, and then checks the optical density of test patches to verify that the proper amount of ink is being printed. All in all, installing and adjusting a new set of plates takes several minutes to perhaps a couple of tens of minutes, depending of the degree of automation of the particular press and the skill of the operator.
Once the registration and color are good, the operator increases the speed of the press to its full production speed. Modern offset presses operate at speeds up to 18,000 sheets per hour, or 300 sheets per minute. Because of the large sheet size (about 700 mm x 1000 mm for a "full size" sheet fed press), each sheet will commonly have 8 pages of printed material on it, and if it is a perfecting press, it prints both sides of the page in a single pass (i.e. the equivalent of 16 pages of information). So the press speed is equivalent to (300 sheets/minute) x (16 pages / sheet) = 4800 pages / min. However, each time a new set of pages is needed, new plates must be mounted and the press must be adjusted, which stops the production for up to a couple of tens of minutes. So an offset press is best suited to long print runs in which many thousand of identical copies of the same printed material are needed. For shorter print runs, digital printing technologies are preferable.