Offset lithography is based on a very simple principle: ink and water don’t mix. Text and pictures are imaged onto printing plates, which are dampened first by water, then ink. Ink adheres to the image area, the water to the non-image area.Next, the image is transferred to a rubber blanket, and from the rubber blanket onto the paper. We call this process “offset” because the image does not go directly to the paper from the plate.
Step 1: Platemaking:
Before any project can go on press, digital files must be imaged onto printing plates. A flexible flat plate is covered with a photosensitive chemical. Light is projected through the negative film onto the plate, producing a positive “image area” once the plate is developed. When exposed to light, a chemical reaction occurs that activates an ink-receptive coating on the printing plate. This results in the transfer of the digital image onto the plate. we have a small process before beginning with the platemaking, the files are arranged on the computer in such a way as to maximize the use of the sheet of paper being used.this is the imposition. we have different materials for printing plates. The best plate material is aluminum which is more costly but provides the quality and durability necessary to produce high-quality offset printing. Moving to colors now, each of the primary process colors — black, cyan (blue), magenta (red), and yellow — has a separate printing plate, also called by CMYK printing process .
Step 2: The Press Run
As a sheet of paper is fed into the printing press, it will move through a series of rollers and blankets where the ink is transferred onto the paper. A four-color press will have 4 or more “units” which are separate sets of rollers and blankets for each color to be oriented. As the press sheet moves from one unit to the next, another color will be transferred to the sheet of paper.
The Inking Process the most important step to have a good quality printing. as said before, ink and water could not be mixed. The ink is transferred to the printing plates through a series of rollers. On the press, the printing plates are dampened, first by the water rollers, then by the ink rollers. These rollers move the ink from the ink fountain onto the printing plates.The image area of the printing plate accepts ink from the ink rollers. The water rollers help to keep ink off of the non-image areas of the printing plate. Each printing plate then transfers the image to a rubber blanket that in turn transfers the image to the paper. The plate itself never actually touches the paper. This is what is meant by offset printing. All of these processes occur at very high speed.
A second important process is the color and registration Control, Color and registration control are processes that are controlled by computers. Registration is the alignment of the printing plates as they transfer their respective color images to the press sheet. If the plates do not line up perfectly, the image will appear out of focus and the color will be blurry. As the press runs, a computer scans a set of registration marks that have been placed on the press sheet. Each printing plate has its own individual set of registration marks. All of this occurs many times per second while the press is running at speed.
Color control is a process that controls the placement of ink on the press sheet, and is related to plate registration. The amount of ink that is released into the color units depends on how much ink is required to achieve a certain ink density on the press sheet.
Step 3: Bindery
The bindery is where the printing manufacturing process is completed. Press sheets have been run through the press, allowed to dry, and transported to the bindery for finishing. then sheets are cut into the appropriate configuration for the desired product.