Learn more about Gravure

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 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 . Rotogravure presses use the gravure process to print continuously on long rolls rather than sheets of paper. Unlike and , 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 , 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. Cylinders (only one if a single color) are then mounted on the press and matched with the correct size and hardness of impression rollers. When all of the cylinders have been mounted in the press, each printing unit is set with the correct inks and rollers. A proof is then pulled by the press crew (sometimes on a proof-press). Press proofs can be done on non-virgin substrates or obsolete paper and end rolls to reduce waste and pollution. Color adjustments and registration corrections are made. Once customer approval is obtained, the press run begins. When the press run is completed the cylinders are removed from the press, cleaned, wrapped and placed in protective boxes (normally constructed of aluminum or heavy pine) and then moved to a designated storage area. Cylinders are stored for future press runs or placed back into the process to be dechromed, copper plated, and re-etched with new designs.

PrePress

Cylinder Preparation

Gravure cylinders are made by engraving or etching a design on a steel/copper and aluminum/copper base that is chromed after the design is proofed. The chromed cylinder surface is hard enough to resist image breakdown on long press runs, which would occur with softer material. Solvent-based ink cylinders vary in the depth and style of engraving or etching compared to water-based ink cylinders. Generally, the engraving or etching has a shallower cell micron depth for water-based inks. In each process a design is mechanically or laser engraved into the surface of the cylinder. The circumference of the cylinder depends on the type of press and the repeat of the design, if any.

Depending on the press, cylinders can be made of copper-plated steel or aluminum. In preparation for plating, cylinders are heated in warm water and then put into a muriatic acid bath that strips the chrome plating and rust from the cylinder. They are then rinsed clean. Steel cylinders are nickel-plated to promote the bonding of the copper, and the aluminum cylinders are zinc-plated for the same reason. Aluminum cylinders are also treated with cyanide prior to copper plating. The final process after engraving is to chrome plate each cylinder and ready it for proofing (EPA/600/S- 93/009). If the printer does not make its own cylinders, they are proofed at the manufacturer before shipment to the printing company. During proofing, design, engraving and color separation approvals are given.

Press

Process Modification

There are several methods available to address pollution prevention and waste reduction in gravure printing environments. Each method should be evaluated for its practical application, both in cost and resource consumption. Caution should be used to ensure that a prevention program or waste reduction program is not discarded based solely on cost.

In conjunction with shallower ink pans, improved doctor blade technology results in reduced ink usage. Vapor recovery systems can be one of the largest contributors to reducing pollution in solvent-based operations. These systems need to be matched to the specific environment and have been successfully carried out in many printing companies. In addition, alternative chemical solutions can significantly reduce pollution. Test runs and trial projects should be considered when searching for safe alternatives.

Printers need to develop partnerships with each of their major raw material suppliers. Within these partnerships there needs to be a clear understanding of the printers needs matched with materials so a quality finished product is achieved. Suppliers have technical assistance available to their customers so an acceptable product can be manufactured with as little pollution and waste as possible. Development of partnerships with ink vendors is essential so that use of the technical assistance that the ink and solvent suppliers can provide is used. Partnerships with ink vendors allows printers to take advantage of an ink technician's assistance with "ink kitchens" that automatically mix inks to the correct color and quantity thereby reducing the opportunity for waste and pollution.

Post Press

Equipment Modifications

There are many types of equipment modifications that printers can use to help reduce pollution. The type and degree of modification depends upon the company goals, financial health, commitment to improvement, and availability of new technology. Printing press makers have taken advantage of new technology and installed several types of process improvement controls on their equipment. After market items that improve the printing process are also available to modify existing equipment. High temperature ovens, solvent and vapor recovery systems (afterburner) can be improved or modified to reduce pollution. In many cases the improvements used to reduce pollution result in increased manufacturing output that justifies the capital expenditure for these projects. Any reduction in wasted resources will improve the overall pollution prevention program.

Process Modification

The degree to which vegetable oils can replace petroleum oils in inks to reduce VOCs depends on several things, including the type of press, the type of substrate, and the type and color of the inks. Gravure presses generally use heatset inks, which are inks that are set by going through an oven or dryer. These inks generate the most VOCs because they tolerate only the smallest amount of vegetable oil content. The drying temperature needed to set vegetable oil inks will normally scorch the substrate and ruin the product. Vegetable inks dry slower than conventional inks - especially on coated papers.

The absorbency of the substrate will determine the amount of vegetable oil content that can be used in the ink. Absorbent papers hold the ink in the substrate so less VOCs are released as compared to coated papers which normally need heat to dry the inks - thereby releasing VOCs. Soy and vegetable based inks provide beneficial printing properties - but dry slower than petroleum based inks.

Water-based inks, while environmentally friendly, pose their own special kinds of concerns in gravure printing. As a rule, water-based inks dry slower than solvent- based inks resulting in initial obstacles when making a switch to water-based. They are more abrasive and cause increased cylinder wear and they require somewhat different engraving and etching processes. Water-based inks tend to have surface adhesion and lay-down problems that solvent-based inks do not have. Printing process adjustments are needed to maintain the quality of finished product.

Some of the more common solvents used in solvent-based gravure printing are toluene, xylene, methyl ethyl ketone (MEK), methyl isobutyl ketone, acetone, methylene chloride, isopropyl and normal-propyl alcohol. All pose risks that are inherent in a solvent-based system. Alternative materials with less risk associated to their use should be considered. -How does gravure work?

Of the four traditional printing techniques of offset lithography, flexography, screen-printing and gravure, which are practised worldwide, gravure bears the greatest resemblance to photography. Gravure applies the ink in controlled doses from different sizes of cell, which have been either chemically etched or electronic engraved into the surface of the printing image carrier, a commonly copper coated cylinder. These cells can vary in their size, shape and depth depending on the way in which individual printing houses work. They can have the same area and vary in depth, or both the area and the depth can vary. The smaller the volume of the printing cells, the lighter the tone, which is reproduced by them. Concentrating on gravure means obtaining the highest picture quality and maintaining it over a long print run. Through fast,

rationalised production methods costs can be kept, comparatively speaking, low. It is for this reason that gravure is ideal for the production of high circulation magazines, mail order catalogues, newspaper weekend colour supplements, and packaging and speciality products.

-Gravure production steps:

-Reproduction

All colours in a printed image arise through a mixture of the basic colours yellow, magenta, and cyan, which are present in different intensities. In order to print coloured pictures, yellow, magenta, cyan and black printing inks have to be printed exactly in register over each other. In order to do this, the amounts of the basic colours in the original have to be separated. These colour separations are made today from originals digitised in scanners or supplied from digital cameras. Each of the basic colour portions is presented in the form of higher or lower values, representing the amounts of ink needed in the subsequent printing operation. Any required colour corrections to these colour separations, re-touching and the composition of complete pages including the text are also carried out in prepress computer systems. The digital data is then supplied for the engraving of printing formes. Alternatively, photographically produced positives, negatives, - so-called opalines or bromides, can be still used as the input for the engraving of the printing cylinder.

-Cylinder preparation

Gravure cylinder preparation is based on either chemical etching or electronic engraving. For typical publication gravure, the engraving process is used. Traditionally, engraving takes place in a copper surface for mechanical engraving. Most operations work completely digitally while some of the older type engraving equipment (HelioKlischographs) still uses synchronised scanning and engraving units. Mechanical engraving uses tiny diamond stylines oscillating up to 9 000 times in one second. One mechanical engraving machine can have a number of engraving heads, which work simultaneously. Laser engraving reaches an engraving frequency of 70 000 cells per second but is still limited to one engraving head. Both systems work with the utmost precision and depending on the strength of the electronic signal, engrav cells in the cylinder surface, which are correspondingly of larger or smaller volume. During the printing process these cells take up and then transfer onto the paper larger or smaller amounts of ink. In order to achieve the hardness necessary for the production printing operation and the resilience to wear, the cylinders are finally plated with a thin chromium layer.

-Printing and finishing

Depending on the product, different production methods are applied in gravure pressrooms. Publication gravure uses typical press designs of eight printing units (one per colour for each paper side) and one folder for the on-line production of a complete set of folded pages, so-called signatures. The latest presses run at speeds higher than 55 000 cylinder revolutions per hour, printing up to 256 pages per revolution in full colour. Paper webs up to 3,6 metres in width race at a speed of 15 metres per second through the presses. The result is each hour 55 000 individual products are produced, which will be either stitched and finished I a single operation or become parts of subsequent bindery products. Bindery finishing means collecting several signatures, a combination of inserts and covers, addressing, binding and trimming.

Presses for packaging gravure printing have to fulfil different requirements, as the variety of products require different substrates colours and finishing processes. Presses run at lower speeds to enable the processing of difficult materials and drying of special inks. Packaging presses can combine in-line finishing processes including laminating, cutting, creasing, embossing, etc.

Gravure Process

Gravure transfers ink from small wells or cells that are engraved into the surface of the cylinder. This is illustrated in the figure below. The cylinder rotates through a fountain of ink. The ink is wiped from the surface by a doctor blade. The cup-like shape of each cell holds the ink in place as the cylinder turns past the doctor blade.

The gravure engraver accomplishes the formation of nearly perfect cells or wells. The gravure cell is characterized by 4 variables; depth, bottom, opening and bridge. The depth of the cell is measured from the bottom of the cell to the cylinder surface.

The opening is described by shape and cross sectional area. The bridge is the surface of the cylinder between cells. The doctor blade rides along the cell bridges or ridges (also called walls).

-Environmental protection

Gravure is an environmentally friendly printing process. Special care is taken to ensure the application of the most ecological production methods for the printing process, as well as for the production of materials. Currently, publication gravure printers and suppliers are active in the following areas:

-Increasing use of papers with higher recycled fibre content

-Use of chlorine free bleached paper

-Use of highly effective solvent recovery installations

-Application of latest methods to save paper, ink and energy

-Further reduction of residual ink solvent content in publication gravure products

-Processing of gravure inks with even more environmental friendly formulations

Benefits of gravure, unavailable with other methods

Due to the precise ability of the gravure cell to lay down a specific amount of ink, gravure is able to print the widest variety of inks, UV, water based, solvent, metallics, flourescents - from the lightest continuous vignette to heavy laydowns resembling screen printing Flexographic and Gravure Printing

Flexography

Flexography, also known as aniline printing, is a form of relief printing; the image is slightly raised, inked and then transferred directly to the subtrate. This printing method utilizes a flexible plate, usually made from soft rubber or plastic and a quick-drying ink. This system can be well suited for a wide variety of materials including acetate film, polyethylene, brown paper and newsprint. It is a high-speed process used for extra large print runs.

Gravure/Rotogravure

Gravure is the opposite of flexography; the printing area is actually etched into the surface of a plate or metal cylinder. The etched out sections are "filled" with ink, the excess ink in the non-image area is removed with a thin stainless steel blade (doctor blade). The size and depth of the etched out areas determine how much ink is deposited on the substrate. Web fed and high- speed decorating systems are better known as rotogravure. This is a high- speed process used for large print runs.

Advantages and Disadvantages

Flexography was the standard years ago but its place in the decorating market is not as profound as it was. It is more suited to printing paper bags and plastic labels and films. It is a high speed process using quick drying inks. Gravure, also a high speed process, produces high quality images and is just the thing for large jobs. These two processes have their place in the industry but have their limitations. Of course, the major drawback would be the set-up costs. Flexo is not nearly as expensive as gravure but the cost for shorter runs would be far too expensive. Gravure printing utilizes an etched cylinder. This printing cylinder can cost thousands of dollars. For a gravure run to be cost effective, a run in the millions would be required. Si-Cal offers an alternatve to these processes that may be just the process for your decorating project. Computer to Gravure and Computer to Flexo

OFFSET PRINTING is the dominant form of printing for most types of products and for the most common run lengths—in spite of offset’s well- known liabilities. For example, it requires more craft knowledge and training on the pressman’s part than gravure or flexography; the press itself is more complicated than presses for alternative printing technologies; and water- based inks cannot be used.

But offset, with its flat-surface, light-sensitive plates, is the ideal companion to photographic prepress processes. Producing flexo plates and gravure cylinders from film is fraught with time and quality limitations. As long as film dominates prepress, offset will dominate printing.

Completely digital prepress changes the equation, however. With the advent of highly automated, computer-controlled cylinder engraving and flexo plate making, most of the prepress advantages of offset disappear (or are at least greatly reduced). Flexo printing can now begin to compete in quality for many offset jobs, and gravure (which has always offered high-quality colour reproduction) can begin to compete for shorter run lengths. Both technologies are likely to take work away gradually from offset. At Drupa, there were a number of new developments that emphasized this trend.

A Few Facts:

• Most of the large pressure sensitive label producers employ gravure as their premium process. • Gravure was the first process to become all digital. • Almost every gravure printer in Northeastern United States uses waterborne inks. • While flexo and rotary letterpress pre-press costs have risen dramatically over the last decade, gravure pre-press has seen only a modest increase. In many situations, gravure printing does compete favourably with flexo production costs.

Relevance of printing and packaging sector

The printing and packaging industries in India have assumed growing significance during the last decade. The printing industry is one of the biggest and fastest growing industries in India. More than 1,20,000 printing presses are in operation all over the country, with a capital investment of over Rs. 80,000 million. This industry provides direct employment to over 6,00,000 people and indirect employment to another 2,00,000. It is natural that along-side the growth of literacy, there is a commensurate rise in demand for various inputs for the printing industry.

Packaging has become a dynamic and key area for manufacturers and trading companies all over the country. The elements of aesthetics, hygiene and cost- effectiveness on packaging receiving greater importance in commercial operations. The exterior look and presentability of marketable goods leave a lasting impression on the minds of consumers and in the context, packaging occupies centre-stage. New packaging machines and technologies have been introduced in the country to meet the challenges. Today, the Indian packaging industry is growing at a rate of 15 per cent per annum.