The term gravure refers to a printing process. It is an intaglio printing process where the image is engraved onto the surface of an image carrier below its nominal surface. When the process is carried out on a continuous web or substrate, we need to use a rotary cylindrical printing plate or roller that is called a printing cylinder. This process is termed rotogravure when it uses such a cylinder for continuous roll-to-roll printing. It is sometimes referred to as photogravure because it uses a photochromatic principle for engraving the cylinder but this term has now come to represent only the original process of etching a flat copper plate that was developed to produce artistic prints (the precursor to the rotogravure process).
The Rotogravure Process
To understand the principles behind production or engraving of rotogravure cylinders, it is necessary to understand the basic rotogravure printing process itself. The cylinder has the printing design etched into its nominal surface broken up into tiny cells. Each cell is like a tiny well that holds a small quantity of ink. The cylinder rotates in a bath of ink and the cells get filled up with the ink. A flat blade called a doctor blade is then used to “wipe” the cylinder surface. It removes all excess ink on the cylinder surface leaving only the ink contained inside the cells. Now, when the cylinder is pressed against the surface to be printed, the ink in the cells is transferred to the substrate surface thus creating the print impression. The depth of the colour tone is dependent on the depth of the cell and, by varying the cell depth to the required extent, one can create a whole range of tonal intensities. In fact, the ability to very precisely control the cell depth makes rotogravure the best printing reproduction process for the printing of print designs that require high tonal qualities. What is more, being a precisely engineered product, the cylinder continues to deliver a very consistent print quality both during a printing run and during repeat print runs provided the same ink is used. The quality is not dependent on press settings as such, as is the case with other printing processes.
The gravure cylinder is actually a steel roller that has an electro-deposition of copper on its surface. The design is engraved or etched on the copper surface which is then chromium-plated to reduce cylinder wear and to enhance ink transfer. Under good operating conditions, a chrome-faced cylinder is good for several million print impressions over many repeat print runs. Quite often, the Ballard process is used where the engraving is actually done on a copper “skin” deposited on top of a copper-plated steel base. The advantage of this is that, when the cylinder needs to be re-engraved either with another design or the same design (if wear is excessive), the outer skin is just torn and peeled off and replaced by another skin which can be engraved or etched, instead of the expensive and time-consuming process of making a new cylinder by building up a copper surface on a steel base.
Photo Chemical etching
The engraving of rotogravure cylinders was initially developed in the last quarter of the nineteenth century and the method of image photo transfer consisted of first exposing a carbon tissue covered with light-sensitive gelatin to UV light passed through a diapositive of the image. Depending on the amount of light passing through the diapositive, the gelatin would get differentially hardened. The non-image areas would get totally hardened while the print areas would be hardened depending on the amount of light incident on them; the lighter tone areas would be harder than the denser tones because of more UV light passing through the diapositive. Before the image exposure, the gelatine is exposed to a screen pattern so that a basic cell structure is obtained and the whole image is broken up into a network of tiny cells. The exposed gelatin surface is then transferred to the surface of a highly polished copper-plated cylinder and adhered to it. When the carbon tissue is peeled off, it leaves behind the differentially hardened gelatin coating on the copper. The next step is to chemically etch the copper surface by using a corrosive liquid (typically ferric chloride). This liquid eats through the hardened gelatin and etches the copper surface differentially depending on the hardness of the exposed gelatin – the harder areas will have a more shallow etch as compared to the softer areas, which will have a deeper etch. Thus, the cells created will have varying depth depending on the colour intensity required. The deeper cells will hold more ink and, so, will transfer deeper colour tones to the surface to be printed than shallower cells. Although the conventional photochemical gravure engraving process uses cells of fixed size and variable depth, other variations like the lateral hard dot method (which uses cells of variable size and depth) or the direct transfer method (which uses cells of uniform depth but variable size) are also used.
The cell itself could, again, have various shapes e.g. circular dots, diamond shapes or hexagonal shapes and most conventional gravure processes that required high quality print reproduction typically used a ruling of 150 lines per linear inch with a 3:1 ratio of cell width to cell wall thickness.
The conventional gravure process had a problem in that, because of the horizontal butt joint that was created while transferring the carbon tissue to the cylindrical copper surface, it was necessary to ensure that this joint did not affect the etching of the print design. Also, the process could not be used for etching all kinds of continuously repeating designs or for producing anilox rolls as the joint would show up.
The direct transfer process
The conventional gravure process gave way to the halftone gravure or direct transfer process where, instead of exposing a carbon tissue and then transferring the gelatin to a copper surface, the copper surface was directly coated with a light-sensitive photopolymer material and exposed via the diapositive to UV light while the cylinder rotated very slowly. The diapositive was rolled over the cylinder surface while it was placed on a highly transparent and dimensionally stable film. This substantially simplified and speeded up the cylinder engraving process as compared to conventional gravure although some sacrifices had to be made in reproduction quality.
The first major technological development in gravure cylinder engraving came via converting the photochemical etching process to a mechanical engraving system or, more accurately, an electromechanical engraving process. Here, the engraving is done by an oscillating diamond stylus that is shaped in such a way that, as it digs into a rotating copper cylinder, it produces cells in the form of inverted pyramids. As it penetrates deeper into the copper, it produces cells of larger volume. Following the stylus is a diamond scraper that removes the burr produced by the gouging action. We thus get cells of variable depth and area as dictated by the print image. The advantage of the electromechanical method is that it eliminates the variability associated with light-sensitive coatings and chemical etching (that is largely dependent on operator skills). Thus, the image transfer is much more closely controlled and variable independent. The engraved cells have a more uniform shape and smoother walls which effect better ink transfer. The print reproduction is of better quality and has smooth tones.
The original electromechanical method used an analogue input and used screened film separations as basis. It thus provided a form of offset-to-gravure conversion that vastly simplified the prepress process and enabled the use of prepress colour proofing systems like Cromalin, which produced a reasonable simulation of what the print would ultimately look like. The analogue process comprises of two synchronised rotating cylinders. One of these is the gravure cylinder to be engraved and the other is a scanning cylinder, on which is mounted a photographic copy of the respective film separation. The scanning head, which contains a light source and a photo-cell, moves cross-wise across the scanning cylinder at a constant speed as the cylinders rotate. The light beam scans the copy and light is reflected back to the photo-cell. The intensity of the reflected light beam is directly governed by the integrated density of the screened colour separations. These signals are then processed to convert them into electromagnetic impulses that directly drive the engraving stylus.
In 1972, electromechanical engraving with simultaneous bromide scanning was introduced and 1982 saw the advent of OT conversion. Over a period of time, in 1990, the analogue process was converted into a digital one, where the analyzing part of the system has been converted into directly feeding digital data to the engraving head. Also came the development of totally digital workflow (similar to CTP) which gave way to a more automated and efficient approach to gravure cylinder production. Today, most electromechanical engraving systems use the digital process. The process is now also used to engrave anilox rolls for flexographic printing and coatings although those rolls have graduated to ceramic surfaces from metallic surfaces for lower wear and smoother release.
The electromechanical engraving process has been around for over 3 decades and, while developments have taken place to digitise it, make it faster and more efficient (double-speed engraving heads and parallel processing were developed in 2000/2001) and to improve print reproduction quality, the essential technology is still the same and this process will continue to dominate rotogravure cylinder engraving for some time to come despite the advent of direct laser engraving technologies. Today, engraving heads can operate at 7,500 cells per second or more compared to the earlier generation heads which ran at 3,000 to 4,000 cells per minute and automatic calibration systems have made it possible to set processing conditions in less than 10 minutes as compared to over half an hour some years ago.
Direct laser engraving
The latest technological development is the direct engraving of cylinder surfaces using pulsating lasers. For a long time, the constraint was the availability of lasers of sufficient intensity and power to directly engrave metallic surfaces at reasonable speeds. While the “holy grail” is a laser that can engrave directly on a chromium-plated surface, this is still some years away as lasers presently available are not powerful enough to achieve this. Even now, only one company offers a laser engraving system that can directly work on a copper surface at reasonable production speeds. One other company offers a laser that can economically engrave zinc but not copper as yet. Of course, direct laser engraving offers vastly superior print quality. One can achieve up to 2540 dpi resolution making it possible to obviate the “saw-tooth” effect of gravure cells and achieve high print densities and ultra fine text sizes- especially in reverse print – that are demanded by applications like pharmaceutical packaging.
The latest development in direct laser engraving is the ability to process elastomeric plates and sleeves for flexographic printing. New generation photopolymers have been developed that can be directly engraved using lasers. This process vastly improves print quality and facilitates the latest trend in flexographic printing plates – that of what is now known as in-the-round (ITR) processing. It requires only two processing steps – engraving and rinsing. One of the major bottlenecks to rapid adoption is the extremely high cost of quality flexo sleeves at the moment but this situation is expected to ease over time.
While direct laser engraving is still a very expensive proposition in terms of up-front capital investment, more and more cutting edge convertors are seriously looking at it in view of the substantial quality and productivity benefits this technology offers. One advantage it has is that it can be easily incorporated into existing electromechanical production lines as no changes have to be made to the electroplating process.
Strong gravure growth in India
New electronic engravers to increase to increase by 30% in next two years
Imported gravure presses from Cerrutti continue to be installed by the big flexible packaging players in the country. Some of the leading Indian gravure press manufacturers such as Expert, Lapra, Pelican, Kusters Calico and Kohli are also busy with both domestic and export sales. This continued growth of gravure (almost entirely for packaging applications) has also fuelled the demand for the more efficient production of higher quality cylinders.
Gravure prepress or cylinder engraving is emerging as a volume game. Almost half the existing population of the approximately 80 modern electro-mechanical engravers are with just six or seven large flexible packaging companies and cylinder suppliers to the trade. However in the last couple of years the rate of new engravers coming into the Indian market has increased from six or seven a year into the double digits. All this adds up to at least another two-dozen modern cylinder making machines being imported in the next two years.
Essentially the local market is still dominated by hundreds of chemical etcher cylinder makers. As Arjun Kohli Vice President of KGS, the company that sells and services Hell Gravure systems in the country says, “the current phenomenon is a result of the top chemical etchers having to buy an electronic engraver just to survive. Of these, hundreds or as many as 20 per cent, can be converted to high quality and productive state-of-the-art equipment. Our experience is that within a year or eighteen months of buying their first or second electronic engraver, another engraver is ordered.”
According to industry experts, with one new electronic engraver most cylinder makers are able to run one or two shifts at most. As soon as the second or third engraver arrives they are in a stronger market position with better logistics and the operation ramps up to a 7/24 three-shift operation.
Positive Packaging’s sister company Acuprint Systems have become the first company in India, and possibly in South Asia, to go in for a direct laser engraving system for rotogravure cylinders at their Mumbai plant. We have been advised by their Director Mr. Krishnamurthy that the system has been successfully commissioned on the 15th of February 2008. We will be visiting their plant soon after Interpack to bring our readers a first-hand report on this installation in our next issue.