Offset lithography process
Unlike sheetfed offset lithography, web offset lithography prints at high speeds on large rolls—or webs—of paper, often on both sides of the paper simultanously, using in-line printing units and in-line finishing systems. Web presses are bulky and expensive, but are widely used for many large-volume applications, most notably newspapers. The basic principle of web offset lithography is the same as that for the sheetfed variety. This article will concern itself primarily with the specific differences of web offset printing versus sheetfed printing. For a general discussion of offset lithography, see Lithography and Offset Lithography.
EVOLUTION OF WEB PRINTING
In the late 1700s, Alois Senefelder had invented the concept of lithography, and lithographic stone printing began in earnest, not long afterward. Advancements such as steam-powered presses and later the rotary press enhanced the process. In the 1850s, the newspaper industry was booming. A need arose for high-speed printing, and in 1856 the first perfecting press was invented, which allowed for the simultaneous printing on both sides of the paper. A second distinguishing feature of this press was that it printed on a continuous roll of paper. And so was born web printing. Subsequent finishing devices—such as folders—increased the capability of the process while remaining a limitation to the speeds achieved. Still lithography languished as primarily an artistic medium rather than a commercially viable means of printing. (Most presses were still letterpress presses.) In the early twentieth century, the accidental discovery that a rubber blanket transferred images to paper more efficiently and with greater quality than lithographic stones (the "offset" in offset lithography) gave the printing process the impetus it needed for wide commercial acceptance.