Gravure printing process
Rotogravure printing system of printing based on the transfer of fluid ink from depressions in a printing plate to the paper. It is an intaglio process, so-called because the design to be printed is etched or engraved below the surface of the printing plate. At the start of the gravure printing process, the plate is covered with ink and the surface is then wiped clean. When is pressed against the inked plate, the paper penetrates the sunken parts slightly and draws out the ink.
All copy (the material to be reproduced, or printed) in rotogravure printing is prepared as positive film. A gravure printing plate made of copper is then prepared to accept photomechanical transfer by means of a gravure screen; this is a grid of closely intersecting lines that create thousands of tiny squares on the plate, which will in turn react differentially to an etching bath after the plate has been exposed to light through the positive film. The varying depths of the etched cells determine the tones of the printed image, with deeper cells holding more ink (and thus printing darker) than shallow cells, which may hold no ink at all.
In the printing process, the cylinder bearing the printing plates is usually arranged so that during its rotary movement it passes through a trough filled with a thin solution of fast-drying ink, or is sprayed with ink instead. A thin steel blade (called the doctor blade) moves across the cylinder and removes the ink from the plate’s surface but not from the tiny depressions in its cells. The plate cylinder then comes in contact with the paper, and the paper draws the ink out of the depressions in the plate. In colour rotogravure printing, a separate cylinder is prepared for each colour.
Because cells of different depths can hold varying amounts of ink, the rotogravure process can print a rich, full range of tonal values, making it ideal for reproducing photographs and other continuous-tone copy in large quantities. Rotogravure is thus widely used to print newspaper supplements (e.g., colour magazine sections), catalogues, illustrated magazines and books, advertising, calendars, and so on. It is also employed for printing labels and wrappers on cellophane and similar materials. A high-speed printing process, rotogravure is best used for printing long runs of material bearing black-and-white or colour illustrations.