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Rotogravure
From Wikipedia, the free encyclopedia
- Gravure may also refer to Gravure idol.
Rotogravure (roto or gravure for short) is a type of intaglio printing process, in that it involves engraving the image onto an image carrier. In gravure printing, the image is engraved onto a copper cylinder because, like offset and flexography, it uses a rotary printing press. The vast majority of gravure presses print on reels of paper, rather than sheets of paper. (Sheetfed gravure is a small, specialty market.) Rotary gravure presses are the fastest and widest presses in operation, printing everything from narrow labels to 12-feet-wide rolls of vinyl flooring. Additional operations may be in-line with a gravure press, such as saddle stitching facilities for magazine/brochure work.
In 1932 a George Gallup "Survey of Reader Interest in Various Sections of Sunday Newspapers to Determine the Relative Value of Rotogravure as an Advertising Medium" found that rotogravures were the most widely read sections of the paper and that advertisements there were three times more likely to be seen by readers than in any other section. The rotogravure process is still used for commercial printing of magazines, postcards, and corrugated (cardboard) product packaging.
In the latter quarter of the 19th century, the method of image photo transfer onto carbon tissue covered with light-sensitive gelatin was discovered and was the beginning of rotogravure.
Three methods of photoengraving have been used for engraving of gravure cylinders, where the cell open size or the depth of cells can be uniform or variable:
| Method | cell size | cell depth |
|---|---|---|
| Conventional | uniform | variable |
| "Two positive" or "Lateral hard dot" | variable | variable |
| Direct transfer | variable | uniform |
Gravure cylinders nowadays are typically engraved digitally by a diamond tipped or laser etching machine. On the gravure cylinder, the engraved image is composed of small recessed cells (or 'dots') that act as tiny wells. Their depth and size control the amount of ink that gets transferred to the substrate (paper or other material, such as plastic or foil) via a process of pressure, osmosis, and electrostatic pull. (A patented process called "Electrostatic Assist" is sometimes used to enhance ink transfer.)
A rotogravure printing press has one printing unit for each color, typically CMYK or cyan, magenta, yellow and key (printing terminology for black). The number of units vary depending on what colors are required to produce the final image. There are five basic components in each color unit: an engraved cylinder (whose circumference can change according to the layout of the job), an ink fountain, a doctor blade, an impression roller, and a dryer. While the press is in operation, the engraved cylinder is partially immersed in the ink fountain, filling the recessed cells. As the cylinder rotates, it draws ink out of the fountain with it. Acting as a squeegee, the doctor blade scrapes the cylinder before it makes contact with the paper, removing ink from the non-printing (non-recessed) areas. Next, the paper gets sandwiched between the impression roller and the gravure cylinder. This is where the ink gets transferred from the recessed cells to the paper. The purpose of the impression roller is to apply force, pressing the paper onto the gravure cylinder, ensuring even and maximum coverage of the ink. Then the paper goes through a dryer because it must be completely dry before going through the next color unit and absorbing another coat of ink.
Because gravure is capable of transferring more ink to the paper than other printing processes, gravure is noted for its remarkable density range (light to shadow) and hence is a process of choice for fine art and photography reproduction, though not typically as clean an image as that of sheet fed litho or web offset litho. Gravure is widely used for long-run magazine printing in excess of 1 million copies. Gravure's major quality shortcoming is that all images, including type and "solids," are actually printed as dots, and the screen pattern of these dots is readily visible to the naked eye. Examples of gravure work in the United States are typically long-run magazines, mail order catalogs, consumer packaging, and Sunday newspaper ad inserts.
Other application area of gravure printing is in the flexible packaging sector. A wide range of substrates such as Polyethylene, Polypropylene, Polyester, BOPP etc.. can be printed in the gravure press.