Rotogravure

Rotogravure (roto or gravure for short) is a type of intaglio printing process, that is, 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 rolls (also known as webs) 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 (4 m)-wide rolls of vinyl flooring. Additional operations may be in-line with a gravure press, such as saddle stitching facilities for magazine/brochure work. Once a staple of newspaper photo features, the rotogravure process is still used for commercial printing of magazines, postcards, and corrugated (cardboard) product packaging.

Diagram of rotogravure process

History and development

In the last 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. In the 1930s–1960s, newspapers published relatively few photographs and instead many newspapers published separate rotogravure sections in their Sunday editions. These sections were devoted to photographs and identifying captions, not news stories. Irving Berlin's song Easter Parade specifically refers to these sections in the lines "the photographers will snap us, and you'll find that you're in the rotogravure." And the song Hooray for Hollywood contains the line "...armed with photos from local rotos" referring to young actresses hoping to make it in the movie industry.

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 these special 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.

Process

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

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 varies depending on what colors are required to produce the final image. There are five basic components in each color unit: an engraved cylinder (AKA "Gravure 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.

Rotogravure portrait of Charles Darwin, c. 1880

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.

Gravure is an industrial printing process mainly used for the high-speed production of large print magazines and runs at a constant and top quality, such as in the printing of large numbers of magazines and mail order catalogues. Other uses for the gravure process are in wallpaper and laminates for furniture where quality and consistency are desired.

Rotogravure presses for publication run at 45 feet (14 m) per second and more, with paper reel widths of over 10 feet (3 m), enabling an eight-unit press to print about seven million four-colour pages per hour. Gravure printing is a direct printing process that uses a type of image carrier called intaglio. Intaglio means the printing plate, in cylinder form, is recessed and consists of cell wells that are etched or engraved to differing depths and/or sizes. These cylinders are usually made of steel and plated with copper and a light-sensitive coating. After being machined to remove imperfections in the copper, most cylinders are now laser engraved. In the past, they were either engraved using a diamond stylus or chemically etched using ferric chloride which creates pollution. If the cylinder was chemically etched, a resist (in the form of a negative image) was transferred to the cylinder before etching. The resist protects the non-image areas of the cylinder from the etchant. After etching, the resist was stripped off. The operation is analogous to the manufacture of printed circuit boards. Following engraving, the cylinder is proofed and tested, reworked if necessary, and then chrome plated.

In direct image carriers such as gravure cylinders, the ink is applied directly to the cylinder and from the cylinder it is transferred to the substrate. Modern gravure presses have the cylinders rotate in an ink bath where each cell is flooded with ink. A system called a "doctor blade" rides against the cylinder to wipe away excess ink, leaving ink only in the cell wells. The doctor blade is normally positioned as close as possible to the nip point of the substrate meeting the cylinder. This is so that ink in the cells has less time to dry before meeting the substrate at the impression rollers. The capillary action of the substrate and the pressure from impression rollers draw/force the ink out of the cell cavity and transfer it to the substrate (Figure 1).

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 is transferred to the substrate (paper or other material, such as plastic or foil) via pressure, osmosis, and electrostatic pull. (A patented process called "Electrostatic Assist" is sometimes used to enhance ink transfer.)