What Is Gravure Printing?
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Learn more about Gravure Printing Gravure history Gravure printing is a very old process, the principles of which started in China in 100 AD. For the next 1400 year’s gravure progressed very slowly and all images were produced by hand using an engraving tool. Then, in the 16th century chemical etching was invented whereby the image could be scratched into a resistant rcoating on the metal surface of the plate and then engraved using an acid. This was a major step forward for the gravure process and high quality printing in general. By 1838 the principles of photography had been fully tested and proven and this gave a great challenge to the printing industry to find ways to reproduce these direct images of the real world in a fast and accurate manner. In 1875 gravure was able to duplicate the photographic films by using gelatine which hardened when exposed to light, the un-hardened areas were then washed away and etched using acid. This development of photoresist technology progressed to the first use of carbon tissue, which is basically a thin layer of gelatine with a paper backing. In 1880, the first laboratory rotogravure press was tested in England by Karel Klitsch, and the first rotogravure presses were used at a plant called Rembrandt, from where the process quickly expanded throughout the world. Carbon tissue continued to be the main method of producing gravure cylinders, using acid to etch away the cells, and the process at this time required considerable skill to achieve the fantastic results demanded. Those involved in the gravure process were committed to producing a work of art. Electronic engraving started in the late 1960's and has become the main method for producing cylinders for the gravure publication industry. Also in the packaging and product gravure markets, electronic engraving is the main method in the western world. However, worldwide, chemical etching is still the most common method for producing gravure cylinders, particularly in Asia, and there have been remarkable developments in this technology, including filmless laser exposure techniques. As soon as gravure moved to electronic engraving the process no longer needed film, all it required was digital signals to drive the engraving heads. Therefore it was natural that gravure would become the first mass printing process to produce the printing forme from digital data only (computer-to-cylinder). In 1981 the industry started engraving directly from digital data and has progressed considerably since this early beginning. The majority of the industry in Europe and the USA has moved to filmless engraving: the advantages are so substantial that it has become the normal way of production. Gravure has long since moved from an art form and craft skill to being a computer controlled manufacturing process, ready to compete in the next century. Outstanding print quality and high output consistency makes gravure the ideal printing process for a wide range of high circulation and high quality publications and products. What is gravure printing? Gravure is an industrial printing process mainly used for the high-speed production of large print runs at constant and top quality. The gravure process prints millions of magazines each week: this is because advertisers want their products promoted in the best possible way. A large number of mail order catalogues are printed in gravure because the products must look attractive and must also demonstrate exactly what is on offer: therefore the catalogues must all look the same, which requires the constant print quality of the gravure process. People buy packages and are greatly influenced by the printed image: for this reason gravure is used for a vast range of packaging on all types of materials in order to offer superb printed results whilst maintaining consistency. Other uses for the gravure process are in wallpaper and laminates for furniture: again for reasons of high quality and consistency. Gravure is the quality printing process producing fantastic and constant reproductions throughout the print run because the secret of the gravure process lies in the cylinder. More robotics and total plant computer control systems will bring many further advantages for gravure. It is a very simple printing process and can produce millions of perfect copies at enormous speed. Another very useful advantage of gravure is that it produces superb colours and good gloss on relatively low quality paper, which can bring big cost savings. Today's rotogravure presses for publication gravure run at 15 metres per second and more with paper reel widths of over 3,5 metres. This means that an eight-unit press can print about seven million four-colour pages per hour. Surely the only way to produce consistent quality for the mass publication market. Very quick job changeover methods have been developed by the press manufacturers for publication gravure, and off-line press make- ready systems using trolleys have greatly assisted packaging printers to compete well for the shorter run work that is becoming more common in today's competitive market. Gravure printing process is used for long runs of multi-colored, high quality jobs at high press speeds. Examples of gravure printed products include art books, greeting cards, advertising, currency, stamps, wall paper, wrapping paper, magazines, wood laminates and some packaging. 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 .Often corrections and touch- ups are still done using the old process. 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 of the design is flooded with ink. A system called a "doctor blade" is angled against the cylinder to wipe away the 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 done so ink in the cells has less time to dry out before it meets the substrate via 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). Figure 1 Figure 1. Principle of Gravure Printing Gravure printers usually use solvent-based inks, although use of water-based is increasing due to regulatory issues. Processes that continue to use solvent inks can run considerably faster than processes that have changed to water-based inks. The nature of solvent evaporation allows the inks to dry much quicker and allows for faster press runs. This is especially true on multi-color jobs where the basic process color scheme, CMYK (cyan-magenta-yellow-black [or key]) is used to produce many different hues, shades, and colors. This is commonly called process color printing. Rotogravure presses use the gravure process to print continuously on long rolls rather than sheets of paper. Unlike lithography and flexography, gravure printing does not break solid, colored areas into minute dots (half tones) to print the areas, which makes it ideal for reproducing high-quality continuous tone pictures, especially when using glossy inks. Many state-of-the-art printing presses are now able to run 8 to 10-color jobs at high speeds. The basic raw materials used in most gravure printing techniques are those of a substrate, either in sheet or web(roll) form; a direct transfer or mechanically engraved etched cylinder; impression cylinders; ink systems; ink viscosity control; solvent recovery system; drying ovens; in-line cutting and stripping to remove excess margin waste; quality control systems or procedures to control the quality of the product, and a finished product that ends in sheet form or roll form. Substrates have an impact on several parts of the printing process. Substrates can affect how the ink is transferred to the surface, how the ink lies on the surface, how well the ink dries and is absorbed by the surface, and how well the press operator can control the register of the finished product. Common substrates include coated and non-coated papers, coated and non-coated board, release papers for the food industry, foils, and metallized papers. Less common substrates are cellophane, polyurethanes and tissues. Coated papers and board probably make up the bulk of the more common printing substrates. One of the more popular coatings used is a clay coating. This coating is generally applied when the paper or board is manufactured. There are single, double, one-sided, and two-sided coated papers. The end use is generally decided by end product/customer specification and the manufacturing process. Engraved cylinders are stored by the printer until the job is scheduled on the press.