The gravure printing process is a printing technique in which the elements to be imaged are present as depressions in the printing form. The entire printing form is colored before printing and the excess ink is then removed with a squeegee or wiper so that the printing ink is only in the depressions. A high pressure and the adhesive forces between paper and ink cause the ink to be transferred. The process is used both in commercial gravure printing and in the artistic field.
The gravure form is mainly used in packaging, label, decor and magazine printing. Laser engraving in combination with the etching process is used in packaging printing. Standard PCs and application-specific programs are used. Data servers with the appropriate capacity are available for data backup.
In the industrial sector, only the gravure doctor blade is used. The printing forms in doctor blade gravure do not consist of printing plates but of steel cylinders. With four-color printing , four separate printing cylinders are used per job. Due to the format, these gravure cylinders have a considerable weight and have to be replaced with special transport systems.
Printing form production
The production of printing forme in doctor blade gravure has been subject to considerable changes in the course of technical development. There are mainly two types of cylinder structure. The steel cylinder wrapped around a 2 mm thick copper layer, the basic copper is mentioned. On this inner copper layer, either engravable copper is galvanized in a layer thickness of about 100 μm or a so-called peelable Ballard skin is galvanically applied to the base copper. There is a separating layer between this 100 μm thin Ballard skin and the base copper. This makes it easy to remove the Ballard skin after printing and replace it with a new one.
The screening in gravure printing split the print image in tiny print form elements that wells are called, while the non-printing elements webs hot. The bars are necessary so that the squeegee can wipe off the excess ink from the cylinder. The doctor blade consists of a steel ruler that corresponds to the length of the printing cylinder.
Conventional etching or heliogravure
The Austrian Karl Klietsch , who in 1879 combined the use of pigment paper with the photographic transfer process, is considered to be the inventor of heliogravure . Initially, copper plates were used for printing, until the use of high-speed presses in the commercial sector made the manufacture of printing forme cylinders necessary.
The conventional etching of the printing cylinder, which was practiced until the end of the 1970s, was done using the pigment paper . A uniform grid is first copied onto the paper, which is covered by a light-sensitive chrome gelatin layer. In a second copying process, a photographic halftone positive film is then exposed over the screen. The pigment paper with the gelatin layer is then glued to the printing cylinder. The paper is carefully loosened using warm water and the unexposed gelatine is then removed until a negative, screened gelatine relief remains on the cylinder.
The printing cylinder is then etched one after the other in several acid baths made of ferric chloride and water, starting with the bath with the highest concentration. In the outer copper layer, cells of different depths but of the same size are created. The deeper the cell, the more color it can absorb and the printing results in a correspondingly dark tone. One speaks here of "depth-variable low pressure". Finally, the remaining gelatin layer is washed off the cylinder and the pressure can be applied.
Electromechanical engraving is one of the conventional processes and has been used in isolated cases from around 1960 until today. In 1966, the Hell company in Kiel brought the first electronically controlled engraving machine onto the market, the " Helio-Klischograph ". The scanning cylinder and the printing forme cylinder to be engraved are connected to one another via a shaft . On the scanning cylinder there is a so-called opal film with all image information, texts and line drawings. According to the brightness of the scanned image point on the opal film, an adequate amount of light is reflected and converted into an electrical signal. This output signal from the scanning head of the engraving machine is processed in the computer and is used to control the engraving head, which receives two pieces of information: firstly, the actual image signal and position, and secondly, the signal for defining the raster resolution and the raster angle. A diamond stylus then physically cuts a correspondingly large and deep cup into the copper layer of the printing cylinder.
Instead of a halftone slide as opal film, screened opal films are also used as engraving templates. An economical method is to take over offset films, i.e. print templates that were previously used in offset printing for cost reasons . This process is called offset gravure conversion or OT conversion . The scanning head is adjusted so that the screened offset films are integrally scanned and converted into corresponding halftone values. The most important prerequisite for the OT conversion is the descreening of the originals, as otherwise a moiré would arise on the engraved gravure cylinder .
From 1995 the gravure cylinder is mainly engraved directly, without going through an opal film. The signals for the engraving head come from the digital database for the printing form description. Today, laser engraving is increasingly used , which enables higher engraving speeds. Depending on the process, a zinc surface or a thicker chrome layer is engraved instead of the copper. Attempts with a plastic coating have not been successful.
Basically, even with laser engraving, cells are burned into the cylinder surface, which have a correspondingly larger or smaller volume depending on the strength of the electronic signal. The high writing resolution of up to 2540 dpi (= 100 points per millimeter) enables very high contour definition. This is why direct laser engraving is particularly suitable for printing labels and packaging, typically for cigarettes, for example.
Another electronic process for printing form production is the modern etching process. Here the cylinder is first coated with a lacquer or acid-proof layer, which is then burned away with a laser. This removes the layer in the places where the cells should appear later. The subsequent computer-monitored etching or electrolytic copper removal generates the final cells.
Engraved gravure forms tend to have so-called missing dots , which means missing raster points , as some cells transfer too little ink to the paper due to their shape. Based on the number of missing dots , experts judge the quality of the gravure products. This problem is addressed with electrostatic printing aids.
Gravure printing machines
The printing unit of a gravure printing machine consisting essentially of the printing cylinder, the ink tray, the blade , the impression roller and the drying system and operates as follows: The printing cylinder is immersed in the ink tray in the low-viscosity printing ink. The cells in the printing forme cylinder are completely flooded with color. The squeegee removes the excess ink so that there is only printing ink in the cells, but the webs between the cells are color-free. The excess paint flows back into the paint tray. Then the printing process takes place. The impression roller ensures high contact pressure in order to transfer almost all of the ink from the cells onto the substrate. The impression roller is used in gravure printing in order to generate the necessary mechanical pressure for sucking the printing ink out of the cells of the printing plate cylinder through the paper. To do this, the impression roller presses the substrate web against the forme cylinder and ensures that the paper is transported through the entire printing unit. In multi-color rotogravure printing, drying must be carried out after each printing process, because wet-on-wet printing is not possible with low-viscosity or low- viscosity rotogravure printing inks.
There are both rotary and sheet-fed presses in gravure printing. In rotary printing , mainly high-circulation products are produced. Their printing performance is currently 60,000 prints per hour, which corresponds to a paper web speed of around 16 m / s. The less common sheet-fed gravure printing is mainly used for printing fluorescent inks, metal inks and varnishes (e.g. metallic, UV-activated, coarse-pigmented inks) as a special color on finished printing sheets from offset printing.
In combination with offset printing, sheet-fed gravure printing enables high-quality and visually appealing printing results. Applications can be found primarily in the cigarette industry, but also in perfume, cosmetic packaging, displays and in security and security printing. In rotogravure printing, excellent results are achieved in commercial printing. An image reproduced in rotogravure comes very close to the original.
Features of gravure printing
The most important feature in gravure printing is the jagged edge or sawtooth effect on letters and line drawings, since not only images but also texts and line drawings are rasterized. In the case of depth-variable gravure printing, the square halftone dots of the same size are visible under the magnifying glass, which often look hollowed out in the lighter tonal values. In the case of intaglio printing with variable depth and area after an engraved printing form, the screen dots have different sizes and color saturation.
All manually produced artistic gravure prints - usually using the sheet-fed printing process - have a plate edge created by printing into the moistened paper, the so-called facet , which is imprinted on the paper and also causes an increase on the back of the printed paper. Another feature is that different prints from one plate have small differences in dimensions. The reason for this is the moisture in the finished prints, which causes the sheets to shrink slightly differently when they dry out.
The gravure printing process is now used economically for print jobs with high print runs of more than 300,000 copies. The choice of the printing process depends on various criteria, such as the format or the number of printing forme changes. In illustration gravure, magazines such as stern , TV magazines, newspaper and magazine inserts and catalogs (for example from IKEA ) are produced in this printing process. Another big market is the packaging gravure printing, for example, the production of folding cartons , Tetra Paks or foil packages . Further areas of application are decor, textile, banknote and security printing . Most of the printing is done on absorbent paper with a smooth, satin finish. Illustration gravure competes with web offset printing , while packaging can more often be produced using the somewhat cheaper flexographic printing process.
As an artistic medium, gravure printing gives the artist great sovereignty with which his ingenuity can be realized unhindered. Etching in particular has repeatedly been a preferred field of work for visual artists since Rembrandt .
The artistic gravure printing processes are divided into two groups: on the one hand, the manual, dry processes , which are based on physical cold deformation of the printing plate, such as drypoint etching , copper engraving , mezzotint (scraping technique), stone etching , and on the other hand, the wet etching processes , in which chemical or galvanic methods are used Material is removed, such as etching , aquatint , vernis mou (soft ground etching) and sometimes also with steel engraving .
In addition, many mixing techniques are used, so that the strict separation between dry and wet processes can no longer be so narrowly limited. With embossing , also called relief printing and blind printing without color , metal plates are cut into shape, in some cases sheets or wires are soldered on or parts of the plate are punched out or sawed out.
Some erasers emboss their artist's mark as a colorless blind print in the handmade paper beneath the actual colored representation . The carborundum is a combination of etching and embossing, in which (in addition to the etching) a mixture of carborundum (abrasive) or marble powder with acrylic resin is applied to parts of the printing plate . The printing ink adheres to the spaces between the grains and produces colored areas of high intensity when printed. Joan Miró created such works.
The heliogravure originated at the end of the 19th century when light-sensitive coatings appeared with the high-quality printing process . It allows photographic reproductions. At the end of the 20th century, new processes were added that are produced with a (mostly light-sensitive) polymer film laminated on a smooth surface . Cerataryt is a print using wax plates or waxed fabrics.
- Wolfgang Autenrieth: New and old techniques of etching and fine printing processes - An alchemistic workshop book for erasers: From 'witch's meal and dragon's blood' to the photopolymer layer. Tips, tricks, instructions and recipes from five centuries . Selbstverlag, Krauchenwies 2010, 230 pages, ISBN 978-3-00-035619-3 , ( table of contents , (→ excerpts online) )
- Eckhard Bremenfeld, Ralf Kapalla, Holger Knapp: Expertise in newspaper and magazine publishers . Guide for publishing professions and career changers. 4th edition. Springer-VDI, Düsseldorf 2001, ISBN 3-935065-03-5 .
- Walter Koschatzky : The art of graphics . DTV, Munich 1985, ISBN 3-423-02868-8 .
- Lothar Lang : The graphics collector . Henschelverlag Art and Society, Berlin 1979.
- Bernd Ollech: Gravure, basics and process steps of modern gravure technology . Polygraph, Bielefeld 1999.
- Hans Jürgen Scheper: Examination knowledge printing technology . Verlag Beruf und Schule, Itzehoe 2005, ISBN 3-88013-623-8 .
- Extensive collection of links on gravure printing / etching techniques
- Online workshop book on gravure / etching
- Artistic gravure printing techniques
- Literature on gravure printing in the catalog of the German National Library
- Helmut Kipphan (Ed.): Handbuch der Printmedien , page 49ff. Springer-Verlag , November 2000. ISBN 3-540-66941-8
- Photogravure. Retrieved August 24, 2009 .
- Electronic cylinder engraving. (No longer available online.) Archived from the original on November 7, 2009 ; Retrieved August 25, 2009 . Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice.
- Gravure printing in art. Retrieved August 28, 2009 .