Printing Technique-III

Gravure

Gravure, also called rotogravure, is a high-volume printing process employing an ink transfer mechanism that is fundamentally different from that of relief printing. The printing surface is a polished metal cylinder covered with an array of tiny recesses, or cells (as many as 50,000 per sq in), that constitute the images to be printed. The cylinder, which can be 2.5 m (8 ft) or more in length, is partially immersed in a reservoir of solvent-based fluid ink. As the cylinder rotates, it is bathed in ink. A steel blade called a doctor blade running the entire length of the cylinder wipes the ink from the polished surface, leaving ink only in the cells. The ink is then transferred immediately to a moving web of paper forced against the cylinder under great pressure.

Gravure cylinders are constructed of steel with a thin surface layer of electroplated copper. The copper can be either chemically etched or electronically engraved to form the cells that will transfer ink. Once the cells have been created, the cylinder is electroplated with a thin layer of chromium to produce a hard surface for the doctor blade. Each cell transfers a tiny spot of ink to the paper. The cells can be made to vary in depth from one part of a cylinder to another, causing the darkness of the resulting ink spots to vary also. This enables gravure to print a wide range of gray tones and thus to render excellent reproductions of photographic originals.

Color printing is accomplished by using separate printing cylinders for the cyan, magenta, yellow, and black inks. Each cylinder is housed in a separate printing unit. The web is transported by rollers from unit to unit and can reach speeds of close to 900 m (3000 ft) per minute. After each color is printed, the web passes through a dryer, where the solvent base of the ink is evaporated. The solvent is either reclaimed or burned to produce energy. Some gravure printers have begun to use water-based inks. This trend is likely to continue because of health and environmental threats posed by the use of hydrocarbon-based solvents.

The expense of manufacturing a set of gravure cylinders has restricted its use to long-run jobs (millions of reproductions). Mass-circulation monthly magazines, mail-order catalogs, and packaging are natural markets for the process. Gravure is also used to reproduce a variety of textures and patterns on decorative materials. Most of the simulated wood grains on inexpensive furniture, for example, are printed by gravure. New methods of manufacturing gravure cylinders using computer-controlled electronic engraving machines have reduced the time required to prepare a set of cylinders, but they are still far more expensive than lithographic printing surfaces