How Does Offset Printing Work?
Brief History of Offset Lithography
Offset printing provides sharper, cleaner reproductions than the letterpress (because the process reproduces at higher resolutions) and is less expensive than rotogravure printing (an even higher resolution but a much more costly printing process).
A German named Alois Senefelder first demonstrated lithographic printing on a smooth-faced, flat and porous rock in the late 1700s. Aware that oil and water do not mix, he sketched a design on polished stone with a greasy substance the rock absorbed. When he dampened the stone's surface, the design area repelled the water and when he rolled ink over the entire surface of the stone, the ink stuck to his design, but not to the dampened blank area. Pressing a sheet of paper to the stone created a clean image of the design.
Fifty years later, the French-developed the steam lithographic press and by the 1950s, lithographic offset printing had become the printer's process of choice for both high-volume web and lower-volume sheet-fed printing.
Offset Printing Plates
Modern lithography combines photography with Senefelder's original use of oil to repel water. Various manufacturers offer printers offset printing plates made of "photo sensitive" thin paper, plastic (photo polymer) or thin metal. They sensitize their plates by coating them with photo polymer or a diazo compound, making them ready for the printer to expose to light--like photographic film. Most printers use pre-sensitized aluminum sheets with a grain or granular finish that imparts water-carrying properties to the aluminum.
The Pre-Press Process
The pre-press process begins with the preparation of camera-ready copy. Camera-ready copy can consist of "paste ups" (also called "mechanicals"), created in-house or provided by the client or as digital data (files created in a graphic design program), and again created by the printer or the client.
The printer creates film negatives (or positives) of the mechanicals or data file. The printer then makes the printing plate by exposing it to light filtered by the film. Light passes through the clear areas of the negative, where it hits the polymer, which hardens as a result, and bounces off the negative's dark areas, leaving the underlying polymer unhardened. That unhardened polymer washes away during processing, an automated process not unlike developing a photograph, to expose the water-receptive aluminum plate. After processing, the printing plate is ready for the printing press.
Implementing the Offset in "Offset" Printing
The pressman mounts the printing plate on a cylinder. As the press runs, the mounted plate first rolls under a water roller, which wets it. The damp plate then moves under the ink rollers where the image area accepts the ink. Still rotating, the inked plate leaves its image on the "offset" blanket and heads back to the water to repeat the process. The "offset" blanket, in turn, transfers its image to the paper, completing the offset portion of the process. After the paper leaves the rollers the printer lets--or forces via a "heat" section built into the press--the ink dry. The printer cuts, trims and binds, if required, before packaging and delivering the finished job to the client.
