Sheetfed Gravure Although almost all gravure production is on web presses, sheetfed gravure presses still exist. Sheetfed gravure is used for proofing; for printing jobs which must be printed on paper available only as sheet; for fine art reproduction and posters; for extremely high quality cosmetics cartons; and overall coating of a product that is printed by sheetfed offset (in order to provide high brilliancy to the printed sheet), or for metallic inks that can-not be applied by the offset method.
A schematic of a sheetfed gravure press is shown in Figure 4-4. The printing unit consists of printing cylinder, doctor blade and ink fountain (with ink pan and applicator roller). The dryer is positioned below the sheet transport and slow-down system.
The sheet fed gravure press can work with either a gravure printing cylinder or a flat gravure printing plate.
Intaglio Plate Printing Intaglio plate printing is used for printing bank notes, currency, securities, and social printing (traditional "engraving" of business cards and invitations).
Intaglio printing presses are generally sheetfed, although some intaglio work, such as postage stamps, is done by web.
The substrate is printed between an impression roller, and a printing cylinder. The ink, a thick paste, is transferred to the print cylinder by meas of separate inking groups (one for each color) which consist of ink pan, transfer rollers, milling roller and inking roller.
Intaglio printing shares the principle of engraved cylinders with gravure, but the inking system is totally different. The non-printing areas of the printing cylinder are cleaned with cleaning paper, supplied from a continuous roll. The image is then transferred from the plate to substrate under extremely high pressure. After printing, the sheets pass into a dryer. Intaglio, the forerunner of modern gravure printing, is also still used for fine art printing in small presses which consist of simply an engraved plate and an impression mechanism which presses the paper against the plate, inking and doctoring are carried out by hand, and the paper is usually moistened to enhance ink transfer.
Offset Gravure and Flexo Gravure The offset gravure system or "indirect gravure" is used mainly to print materials with irregular surfaces.
The unit is a standard gravure print unit, with the addition of a rubber-covered transfer roller. The substrate is threaded between the standard impression roller and the transfer roller. The transfer roller is then used to print the image on the product to be decorated. This transfer of image from cylinder to roller to product is identical to the image transfer method of offset lithography, hence the name "offset gravure". Typical products are wood veneer or decorated metal surfaces. Irregular shapes such as construction molding and individual parts or products may be carried to the printing nip in a recessed conveyor belt designed to hold them.
A variation of offset gravure is the use of a gravure cylinder in place of an anilox roll on a flexo press. The flexo plate cylinder is mounted with a smooth rubber blanket and the image from the gravure engraved cylinder is printed onto the rubber blanket and then transferred to the substrate. This application is usually referred to as flexo gravure, Examples of this application are clear film overwraps for towels and tissues and premium plastic handle bags used for advertising where high quality reproductions are desired.
Gravure with Flexo Units Flexographic units may be added to gravure presses to imprint prices and addresses on a product whose main designs are printed by gravure. In such cases, the flexo unit is designed as an attachment, installed on the frame of a gravure printing unit.
In some cases presses are designed so that a flexo print unit may be substituted for a gravure unit by means of a movable cart containing a plate cylinder, anilox roller and reverse angle doctor blade mechanism. In this type of combination press the gravure cart carries printing cylinder, splash guards, inking system and doctor blade mechanism. Such presses are used to print pharmaceutical packaging or paper cups, where simpler designs may be printed in flexo, while the more complex ones may be printed by gravure.
Gravure and flexo are also used simultaneously on the same press when part of the copy (such as sale prices or retail addresses) has to be changed several times during the run, and it is therefore done in flexo. The combination is also used to personalize special orders of products.
Gravure Roller coating Gravure engraved cylinders are used to apply primers, coatings and adhesives on overall or spot coating to a wide variety of products. Examples are:
• Primer coating, to facilitate adhesion of ink on foil.
• Coatings or primers to provide barrier properties.
• Cold seal adhesives.
• Adhesive coating for dry bond lamination.
• Registered spot adhesive.
• Heat seal coatings.
• Coatings for hot stamp foils.
• Release lacquers.
• Hot melt coatings.
In all the cases above, the coating unit is a standard gravure unit. For hot melt (wax) coating, a gravure system is also used but it is slightly modified because the pan, the coating cylinder and the doctor blade are generally heated. Coating materials, although supplied to the coating cylinder from the ink fountain, are generally not supplied to the ink pan through the press's conventional ink circulation. Physical properties of the coating will determine how the material is brought into contact with the engraved cylinder.
Gravure Units on Other Equipment Gravure units may be installed in line with other equipment, mainly in line with flexo or offset print units, as well as laminating equipment. Generally, the scope of the gravure unit is to apply an overall lacquer or lamination adhesive.
Gravure units are also installed in line with extrusion coating or extrusion laminating lines.