Gravure

A printing method which utilizes engraved cylinders or, infrequently, cylinder-mounted plates as the image carriers. The image areas are etched into the surface of the cylinder as a collection of tiny cells. The cylinder rotates in an ink fountain and ink collects in the cells, the excess ink being scraped from the non-image areas by a doctor blade. The paper (or other substrate) is passed between the gravure cylinder and a rubber-coated impression roller, and ink is transferred by a combination of capillary action and the pressing of the substrate into the engraved cells of the cylinder, helped by the rubber surface of the impression roller. Most gravure printing performed today is web-fed rotogravure printing, with occasional sheetfed use. Gravure is also well-suited to the printing of packaging on a variety of non-paper substrates.

Gravure printing is a direct descendent of older intaglio printing (gravure and intaglio, commonly used synonymously, are different processes; all gravure printing is intaglio, yet all intaglio printing is not gravure—for example, copperplate printing, which is an intaglio process without being considered a gravure process), developed around the same time as Gutenberg was developing relief-based printing (the mid-fifteenth century). Intaglio, primarily an artist's medium, was essentially a wooden (and soon metal) block on which the image to be printed was etched. A thin ink was poured into these etched lines or dots, and the paper on which the design was to be printed was brought into contact with the inked image carrier in such a way as to force the paper into the cells where it could pick up the ink. A porous substrate allows capillary action to enhance this process. Around 1440 C.E., the first metal plates began to be used, commonly made from copper (hence the term copperplate engraving'). Intaglio was used primarily for illustration matter and playing cards. Around the same time, Gutenberg's letterpress-based printing press was increasing in popularity, and the use of intaglio for text was not actively pursued, as the intaglio plates were incompatible with the relief method of printing. Still, intaglio represented a more artistic rather than commercial medium, perhaps best exemplified by the woodcuts and other engravings of German artist Albrecht Dürer in the late fifteenth and early sixteenth centuries, as well as engravings by other noted artists such as Rembrandt van Rijn and Peter-Paul Rubens.

In the first half of the sixteenth century, the invention of chemical etching of intaglio plates was a great leap forward for the process. Rather than laboriously scrape away the metal itself, artists could now simply scrape away a soft coating (known as a resist), which would allow the penetration of an acid only in certain areas, which would then etch the copper beneath the coating chemically. Chemical etching made the intaglio process even more favored by artists, and intaglio printing proved to provide better-quality illustrations than did letterpress, so it was not uncommon for the text of a book to be printed using letterpress, and illustrated pages to be printed using intaglio, the separate pages being collated together after printing. Denis Diderot's great and controversial Encyclopédie, published in seventeen volumes of text from 1751 to 1755, was supplemented by several additional volumes of intaglio illustrations, which served to primarily illustrate various manufacturing processes as part of Diderot's extolling of the virtues of artisans. (This would be a contributing factor in the French Revolution of 1789.) Intaglio-based printing was also widely used for the reproduction of sheet music, as well as maps, needed more than ever once the New World was found and colonized. The invention of the mezzotint (an early means of representing shades of gray in copperplate engraving; "mezzotint" itself literally means, in Italian, "halftone") in the 1600s further refined the use of intaglio for high-quality pictorial reproduction.

Following the invention of lithography at the tail-end of the eighteenth century, and its further development in the nineteenth century, the search was on for a means of printing utilizing cylinders, rather than flat plates, stones, or locked-up bits of type. The one desperate need of any printing press is, as its name indicates, pressure. It is easier and less laborious to produce suitable and uniform printing pressure in the nip of two cylinders than over the surface of a flat plate, but the question was how to accomplish it; a litho stone couldn't be bent into a cylinder, the individual letters, or even lines, of type were impractical for rotary printing, and intaglio techniques weren't able to keep the ink from spilling out of the cells. The development of stereotype platemaking eventually solved the problem for letterpress printing, and the later use of zinc and aluminum plates eventually solved it for lithography. Interestingly, the first cylinder-based printing press was a gravure press, originally developed for printing on textiles in 1680. The quality was most likely not very high, but its primary usage was in the printing of calico patterns on cheap clothing. In 1783, British textile printer Thomas Bell patented a rotary intaglio press for use in higher-quality textile printing. His patent drawings show a system very much like that still in use in gravure printing today, but for non-textile printing, the idea of a rotary press languished.

The invention of photography in the 1820s and 1830s resulted in the search for a means of transferring a photographic image to an intaglio plate. William Henry Fox Talbot devoted himself to the search for photoengraving materials and techniques. Using gelatin-based coatings for metal plates, he was able to achieve photographic etching initially for only line art, but eventually he devised formulations that would enable the selective variation of image density, which would print at varying shades. Fox Talbot soon hit upon the halftone screen, which broke up continuous images into very small, discrete dots which could be varied in size and shade of gray. This was the breakthrough photoengravers (and printers everywhere) needed. Letterpress and lithographic platemaking were the direct beneficiaries of this process, however. The intaglio process was desired by most people for little more than fine art reproductions and illustration material.

The problem for gravure still remained: how to produce a photographic coating for a cylinder that could be used for etching. The English engraver J.W. Swan solved the problem in the early 1860s with a carbon tissue, which was a gelatin resist coating on a light-sensitive material applied to the surface of paper. After exposure, the paper could be removed, and the exposed coating applied to another surface, such as a metal plate—or a cylinder.

Thus, all the disparate elements needed for modern gravure printing existed, and it remained for someone to put them all together. That someone was Karel Klic (in German spelled Karl Klietsch), from Bohemia (now the Czech Republic). In 1841, combining Bell's rotary intaglio textile press, Fox Talbot's halftone screen process, and Swan's carbon tissue coating, Klic developed the first gravure printing press. Still used exclusively in the printing of textiles, however, Klic made his way to England and teamed up with Samuel Fawcett, an engraver at Story Brothers and Company, a textile printing company. In the early 1890s, they developed new techniques for photoengraving, and began commercial printing of intaglio art prints, conducted with such secrecy that company employees were not allowed to venture into rooms other than those they were assigned to, lest they become exposed to all the various parts of the process. A bit paranoid, perhaps, but the company—under the name of Rembrandt Intaglio Printing Company—held a monopoly on the process for over a decade. In 1903, an employee of Klic's came to the United States and revealed Klic's process. The jig was up.

Meanwhile, in 1860, A French publisher named Auguste Godchaux developed a rotogravure press that printed on rolls (or webs) of paper, a design very similar to modern rotogravure press designs. In the early 1900s, gravure presses began turning up in the United States, and the New York Times in 1913 was the first to print rotogravure newspaper supplements. Other newspapers began to take notice of the high-quality reproduction of photographs the new system afforded. (Today, most Sunday newspaper supplements—such as the New York Times Magazine, Parade, USA Weekend, and other color supplements across the country—are printed on rotogravure presses.) In the 1930s, gravure presses began to be used in the printing of packaging; a single-color gravure press in 1933 was set up to print Tootsie Roll wrappers. In 1938, multi-color gravure presses were used for the printing of Jell-O boxes. These so-called "Jell-O presses" were the largest and fastest yet designed; together, they were capable of printing up to 36,000 cartons an hour, and were in use until 1987.

Modern advances in engraving technology have made gravure printing a high-quality printing operation. The expense of producing and imaging the gravure cylinders, however, still continues to make gravure printing an expensive process, and gravure is rarely used economically for print-runs of under 200,000 or so. An advantage of gravure printing, though, is the relative simplicity of the press, which doesn't require the intricate series of ink and dampening rollers that a lithographic press requires.

The gravure printing press has several basic elements:

Gravure Cylinder. A gravure press most often prints from a gravure cylinder, which comprises a steel base, which can either be a sleeve cylinder or a shaft cylinder. A sleeve cylinder requires a shaft to be attached when it is mounted on the press, or when it is mounted in the engraving mechanism. The inaccuracies inherent in the fitting of a separate shaft have brought about the development of a shaft cylinder, which comes with shafts already mounted, and they are the dominant gravure cylinder bases currently utilized. Aluminum bases have be devised to hopefully replace steel, especially in presses used in the printing of packaging, but although they are lighter they are also harder to electroplate. Newer plastic cylinder bases are being developed that are much lighter than metal bases, and contain special surface coatings (most of which are proprietary) that facilitate electroplating.

To the cylinder base is electroplated a layer of copper, which has historically been—and continues to be—the dominant surface material for gravure cylinders—and is commonly electroplated to the base utilizing a sulfuric-acid eletrolyte. On top of the copper, after engraving, is plated a thin layer of chrome, which is applied to protect the etched copper surface from the abrasion of the doctor blade during printing. After print runs, the cylinder needs to be resurfaced. (See Electroplating.)

The copper surface of the cylinder, prior to printing, is etched or engraved. A particular image printed in gravure is essentially a collection of many tiny cells that are etched with varying depths (darker regions of a print utilize deeper cells which can hold more ink, while lighter regions utilize shallower cells which hold less ink). This is why gravure-printed type can look fuzzy when examined under magnification. But due to this printing mechanism, gravure can print halftones extremely well. Before the development of electromechanical engraving in the 1960s, most gravure cylinder etching was performed photochemically, using carbon tissue resist coatings and ferric chloride etchants to chemically etch the image areas. Now, the artwork to be engraved is often placed before an optical scanning device, which uses photodiode to receive the image, and the image is transformed into digital data, which is then used to drive an engraving head (typically a diamond stylus), which can produce as many as 5,000 cells per second. New developments in direct computer-to-engraving-head imaging are removing the need for a film positive from which to obtain the information to drive the engraving head.

One particular consideration with the gravure cylinder is ensuring that it is as close to perfectly round as possible (and that the circumference of the cylinder is large enough to carry the image to be printed). The term total indicated runout is used to measure the roundness of the cylinder, and gravure cylinders are manufactured—and need to be kept—within strict tolerances. (See Gravure Cylinder and Gravure Engraving.)

Ink Fountain. The inking system for a gravure press is far less complex than that used for offset lithography. The gravure cylinder is partially submerged in a large pan of thin, highly fluid ink. (Ink is pumped into the pan as needed from a sump, typically located below the fountain pan.) As the cylinder rotates in the ink, its surface becomes covered with ink, and the cells fill. A thin, flexible steel doctor blade, either alone or in tandem with other pre-wiping devices, scrapes the excess ink from the surface of the cylinder before the inked cells contact the substrate. Some gravure inking fountains utilize a fountain roller, a cloth-covered roller that is partially submerged in the ink fountain and which contacts the surface of the gravure cylinder. In some configurations, ink is sprayed onto the surface of the cylinder by a nozzle. (See Inking System: Gravure.)

Impression Roller. The gravure impression cylinder, or impression roller, is a hard cylinder covered with a synthetic rubber lying directly above the gravure cylinder. The purpose of the impression roller is to exert pressure on the substrate passing through the nip between the impression roller and gravure cylinder. This forces the substrate partially into the cells on the gravure cylinder, where capillary action transfers the ink to the substrate. The pressure exerted on the substrate as it passes though the nip can be adjusted. The impression roller typically has a smaller diameter than the gravure cylinder and consequently rotates at a faster rate. However, in the nip between the two cylinders, the rubber is deformed slightly by the pressure of impression roller against the gravure cylinder. Faster press speeds in recent years, however, have resulted in excessive heat buildup in smaller impression rollers. Consequently, many presses now utilize larger-diameter rollers, which also have the added advantage of reducing stress on the web, as the increased size of the nip results in the same total amount of pressure being applied but over a larger surface area. Too large an impression roller, however, can cause printing defects, as the substrate remains in the nip for a longer period of time. As with the gravure cylinder itself, the TIR of an impression roller should be carefully monitored. The excessive friction caused during web gravure printing can also result in high static charge buildup. These charges can exceed 25,000 volts and can cause such printing problems as whiskering, or health hazards such as severe electrocution. A related phenomenon, but one which is induced deliberately and which has positive effects, is known as electrostatic assist, in which the impression roller is given a static charge that attracts the droplets of ink from the gravure cells to the substrate, and helps to more completely transfer ink and reduce the occurrence and severity of such problems as snowflaking. (See Impression Roller.)

Substrate Control. The feeding systems used to control the movement of the web of paper (or other material) through the press vary by press. Since gravure is used for a wide variety of different types of substrates, all of which contribute various feeding problems, web handling equipment comes in a number of different configurations. Plastics, films, and other non-paper substances are often heat-sensitive, non-absorbent, and easily stretched beyond their ability to return to their original dimensions. Paper, on the other hand, is more resistant to stretching, is less heat-sensitive, and is more absorbent. But it is also bulkier, and more often than not needs to be printed on both sides simultaneously. Consequently, web handling units for packaging films requires a more tension-controlled path, less heat for drying and, consequently, a faster-drying ink. Immediately after the printing unit, it is not uncommon for the web's drying path to be a vertical one; the web travels vertically up to a fixed distance, allowing it time to dry—expedited by hot-air dryers—and either out to the finishing section of the press, or back down again, depending upon how much drying time is specifically required. It has become more common for drying paths to be varied according to the job by add-on modules that provide more or less drying space. This has become increasingly necessary on higher-speed presses; modern packaging presses print at speeds of up to 1,000 feet per minute, while publication presses can print speeds exceeding 3,000 feet per minute.

The web roll is placed on a reel stand, which has developed over the years from simply holding one roll at a time (which required press stoppage when the roll ran out and needed to be replaced) to a two-roll stand (which required a good deal of operator skill to switch to the new roll when the first one ran out) to fully automated, two-roll unwinding systems. Most webs—either paper or packaging—tend to have three- or six-inch-diamater cores, made primarily out of cardboard, with plastic and metal cores becoming more popular, as they tend to retain their roundness more easily. (Cores that are out-of-round will result in the roll unwinding with a bump, which will cause feeding problems and perhaps web breaks.) The most common type of reel stand consists of two metal arms, one fixed, the other moveable. Attached to each is a cone which fits into the core of the roll. The roll is mounted first on the fixed arm, then the second is moved in to engage the other side and hold it firmly. The centering of the roll for travel into the press can be performed by moving the arms in or out, as may be necessary. Some reel stands also make use of an earlier configuration involving a metal bar that runs through the center of the roll. Many configurations involve two unwind stands at the end of a long central arm, the whole assembly looking rather like a see-saw. One basic problem that needs to be accounted for is, as was mentioned, the out-of-roundness of the core, which always exists to some degree. If kept within certain tolerances, it is acceptable, but the reel stand must be sufficiently sturdy to guard against any vibration caused by the non-concentric core disrupting the printing units of the press. When one roll runs out, the new one must immediately and carefully be spliced to it, the point being to avoid having to stop the press. Often, this system is automated, but it still requires careful preparation on the part of the press operator. The appropriate amount of web tension is carefully regulated by running the web around a dancing roll, a roller connected to an air cylinder that can be adjusted to apply the appropriate amount of force to the web. Newer systems carefully measure the diameter of the roll repeatedly as it is unwinding (to account of any eccentricities or out-of-roundness), either by ultrasound sensors or other means, and automatically adjust the speed of the motor driving the unwinding reel.

The final portion of the press just prior to the printing unit is known as the infeed tension unit, which is little more than two rollers, the nip of which the web passes through to reach the printing unit. This nip, regulated by a mechanism similar to a dancing roll, ensures that the web tension beyond it is consistent, regardless of what is happening to the web prior to reaching the nip. This tendency to isolate regions of web tension ensure that any anomalies are dealt with before the printing unit. (See also Web Offset Lithography: Feeding Section.)

Sheetfed Gravure. Most of the gravure presses in operation are web-fed presses, but occasional sheetfed gravure work is done, such as for printing proofs, fine art posters and prints, cartons, and other high-quality work for which sheetfed offset lithography is inappropriate (such as the printing of metallic inks that are incompatible with offset press chemistry). Sheetfed gravure presses consist of a pile table on which the sheets are stacked, and which are fed into the press, through the printing unit (a standard gravure cylinder-impression roller-doctor blade arrangement, with the cylinder typically inked by a fountain roller), transported by a series of transfer cylinders, over several drying nozzles, and finally to the delivery pile. Some configurations of sheetfed gravure presses also replace the gravure cylinder with a flat gravure plate.

A variety of intaglio plates are used for high-quality, specialty printing such as bank notes, postage stamps, money, securities, and other such documents. These can either be sheetfed or web-fed, and are more commonly known as copperplate printing. See Copperplate Printing.

Offset Gravure. Some substrates (such as those with irregular surfaces) are printed by a process called offset gravure, or indirect gravure, which comprises the standard gravure printing unit, except that the image is first transferred from the gravure cylinder to a rubber-covered transfer roller which first receives the image from the gravure cylinder, then transfers it to the substrate passing between the transfer roller and the impression roller. (This is based on essentially the same principle as offset lithography.) Products printed by this method include decorated metals and woods, and other types of irregular surfaces. The resilience of the rubber image-carrying blanket makes printing on hard surfaces such as these much easier. A variety of offset gravure takes place on a flexographic press, where the Anilox roller of the flexo press is replaced by a gravure cylinder. The gravure cylinder transfers the image to a rubber blanket, which has been mounted to the flexo plate cylinder. The blanket then transfers the image to the substrate. This is known as flexo gravure, and is used to print high-quality packaging, advertising, and other materials commonly printed by traditional flexographic means, but with the increased quality of gravure printing. Gravure units are also occasionally added to regular flexographic presses, for the overprinting of various elements, such as prices, store addresses, and other design elements that need to be changed several times over the course of a print run, on products whose other elements are printed by traditional flexography.

Gravure, like other printing processes, has specific ink requirements that produce the best results, specifically, highly fluid liquid inks with volatile solvents. (See Ink: Printing Requirements: Gravure.) Gravure presses also require paper substrates with certain characteristics to produce best results. (See Paper and Papermaking: Printing Requirements: Gravure.) Gravure is also well-suited for printing on a host of other types of substrates, such as foils, plastics, etc. When used on plastic packaging, most gravure presses require the use of fast-drying solvents.

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