Continual improvement in productivity in gravure cylinder engraving using HelioKlischographs

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1 Continual improvement in productivity in gravure cylinder engraving using HelioKlischographs Dr. Siegfried Beißwenger, HELL Gravure Systems GmbH, Kiel, Germany Over the past 30 years, we have witnessed a continuous increase in productivity in all sectors of manufacturing. This growth, which has accelerated even further in the last few years, is based on three factors: Firstly, the speeding up of individual production processes, Secondly, the reduction in the complexity of production processes and Thirdly, the automation of production flows. Numerous examples of this trend are to be found in all sectors of industry. In particular, the metal machining sector has achieved particularly dramatic increases in cutting speeds in recent times. One long-known way to lower production complexity by reducing the number of process steps is the introduction of multi-axled, numerically controlled metal working machines which perform several different steps in a single operation that involves clamping the work piece only once. In the past, these steps would have had to be performed one after the other on traditional drilling, milling and grinding machines and lathes. The microelectronics industry has led the way in reducing complexity. Today, a single chip integrates the power of millions of transistors, resistors and capacitors. And these days, electronic I&C technology allows the majority of production operations to be automated. The result is a further increase in productivity, reduction of rejects and improvement of quality. That last point may sound surprising, but it is due to the reduced error rate found in automated processes compared with manual operation. Immense productivity advances in electromechanical engraving The development of electromechanical engraving technology for gravure printing cylinders is a prime example of what we have just discussed. We have chosen to look at the development of productivity in the various processing stages of magazine and catalog printing and, for purposes of comparison, have decided to focus on the parameter number of color separations engraved per man and per shift. In a large German gravure printshop, this figure has risen from 1.4 in 1965, when all work was still done by etching, to 110 today. The milestones in this development were the complete changeover to electromechanical engraving with simultaneous bromide scanning in 1972, the introduction of OT conversion in 1982, the changeover to digital engraving in 1990 and the introduction of automation technology in conjunction with doublespeed engraving heads in 2000/2001. The number of engraved color separations is an average taken over a year, the figure for 2001 being a mixture of statistics from older, non-automatic machines with 4 khz engraving heads and new, automatic, high-speed machines. Since the average number of ribbons per cylinder has risen from 3 to 9 during the period in question, the changeover from etching to electromechanical engraving with the various development stages along the way actually represents an increase in productivity of a factor of 30. 1

2 The simplification of the actual impression cylinder imaging process is particularly important here. Today, this process basically consists of three steps: Firstly, digital imposition work on the basis of a job ticket workflow, secondly, the double-speed, automatic engraving process, and thirdly, chromium-plating. Back in the time of etching, these three steps used to require 10 process stages. And even in the early days of engraving in conjunction with bromide scanning (regardless of the time-consuming production and mounting of the bromide), it still took 6 process steps, since cylinder correction was usually unavoidable. The high levels of precision achieved by the latest HelioKlischographs (K406) mean that print proofing and cylinder correction are often unnecessary. Thus, added to the productivity increases enjoyed by engraving departments (which now boast 110 engraved color separations per man and per shift) are other productivity enhancements not quantified here, namely a much-reduced outlay in terms of proofing and cylinder correction work. This graphic shows the number of engraved color separations per man and per shift in a large publication printshop over the last 35 years etching, 1972 bromide scanning and engraving, 1982 OT conversion and engraving, 1990 digital engraving, 2001 some automatic engraving with double-speed engraving heads. We will take a closer look at the improvements in quality and the reduction in cylinder remakes achieved by progress in engraving technology below. At this point, it is important to remember that the importance of shortening the entire production chain in terms of quality and costs cannot be stressed enough. 2

3 After all, a production step which does not exist cannot go wrong, does not take any time or require any operators, does not consume any resources and does not tie up any capital. The HelioSprint engraving head The double-speed HelioSprint engraving head, introduced by HELL in Over 700 units have since been sold. One of the major milestones since the introduction of digital engraving in the early 1990s has been the development of the HelioSprint engraving head, which was introduced in While engraving performance had been stable at between 3,000 and 4,000 cells per second for over 30 years following the invention of the HelioKlischograph in 1967, HELL managed to increase output to 7,500 cells per second with the Sprint head. The increased cell volume and exactly symmetrical shape of the cells demonstrate the improved stability of the head and allow increased printing density. With its features (high engraving frequency and stability, with minimal bounce, drag and responsiveness to temperature change), the Sprint head is unique, and bears witness to HELL s core expertise. The head is constantly undergoing development work by a large team of people, whose objectives are to achieve even higher engraving frequency and 100% engraving consistency. Automatic calibration of engraving heads Alongside the actual engraving, the task of calibrating the engraving heads used to be the most complicated, time-consuming and error-prone step in the process. With 14 engraving channels, calibration could easily take an operator 3

4 over half an hour, and required him to perform microscopic measurements of several test cut cells in each ribbon. With 14 ribbons, this meant measuring almost 50 cells, moving, positioning and focussing the microscope for each separate measurement. As well as the time consumed, another problem was measuring errors. There was also a risk that different operators (and shifts) would proceed differently in large printshops. The CellGuard system takes less than 10 minutes to automatically perform these measurements and set the engraving electronics accordingly. It uses a single rapid-adjustment camera to measure all the ribbons. This principle avoids the systematic errors inherent in measuring systems using more than one camera. The slide used to move the CellGuard camera microscope is also fitted with a precise linear scale. When measuring the test cut cells, the system can also use this scale to determine the axial position of the engraving styli on all the ribbons. The CellGuard system automatically measures cells using a precision-positioned camera operated by a linear motor. This means that all the engraving heads are calibrated with speed and repeat precision to ensure ribbon uniformity. This means that the engraving heads need only rough axial prepositioning, because absolute register accuracy can be assured electronically by controlling the engraving data in such a way that it is engraved at the right time and in exactly the right place. The AutoSpacer function thereby cuts makeready times and avoids register errors. Intelligent software and HELL s own image processing system ensure that measurements are correct to 1 µm and eliminate false measurements. In this context, it is particularly important to note that only stylus-engraved cells and not laser-engraved cells support volume measurement using optical distance measurement. This is because of the fixed relationship between the diameter and the depth of the pyramidshaped cell. The relevance of this fact becomes clear if we remember that only processes which can be measured can be kept constant in the short and long term. 4

5 The hardware and software in the K406 machine are carefully tuned The HelioKlischographs from HELL are extremely precise engraving machines. Their mechanical precision makes it possible to produce regular engraving screens which are free from problems caused by vibration and the like, and to achieve exact register accuracy to composite prints from different cylinders. It is therefore possible to engrave the four cylinders in a color set on different machines. In fact, in order to delay the start of the engraving process for as long as possible, this is common practice in many printshops. HELL has also made sure that even HelioKlischographs of different generations are compatible with one another. Digital engraving machines: The HelioKlischograph K 406 The software modules of the server for engraving data, the cylinder layout station (HelioForm), the proof station (HelioProof) and the conversion station (Gipsy) run on standard platforms with Windows NT operating systems. Imposition work and the many possible engraving-related functions and parameters (which include customized definition parameters and test cut settings) are handled using HelioForm, or are adapted automatically via interfaces from upstream systems and passed to the engraving machine as an approved form in the shape of a job ticket. Engraving is then started on the engraving machine by simply selecting the job to be engraved. The test cut and engraving are then performed automatically. The Gipsy system retrieves the engraving data from its various storage locations and feeds the data flow for image and line, plus associated marks, to the individual engraving channels. Gipsy also integrates print-related adjustments and corrections such as web shrinkage compensation, shingling and the like, and converts the data format of the pages (TIFF/IT or PDF) into the relevant engraving screen onthe-fly. 5

6 Further shortening of engraving cycle times possible Let s assume that a printshop has an engraving department with four HelioKlischograph K406 machines. Thanks to the advanced automation technology, these machines can be operated by 2 people. With a TR10 cylinder with 12 ribbons, a total of 48 pages and a screen ruling of 70 l/cm, the cycle time would break down as follows: Loading the cylinder: 3 min; automatic test cut: 10 min; engraving: 32 min; unloading the cylinder: 3 min; average waiting time for new cylinder 20 min, average total cycle time: 68 min. The waiting time between engraving two cylinders is often caused by the fact that the crane has to take away the finished cylinder before a fresh cylinder can be brought and placed in the machine. In order to eliminate this waiting period, HELL has developed the Autoloader. This device allows the next cylinder to be prepared while the current cylinder is being engraved. Once engraving is completed, the in-house crane is used to unload the engraving machine. At the touch of a button, the next cylinder is then automatically placed in the K406, clamped into position, and engraving can begin. In this manner, the Autoloader cuts the theoretical cycle time to 50 minutes. The K406 engraving machine can be combined with an Autoloader. This eliminates waiting periods caused by the crane by allowing the next cylinder to be prepared in the Autoloader whilst the current cylinder is being engraved. Assuming a waiting period of 20 minutes between two engraving operations, an engraving department with four K406 Klischographs and a two-man team has a theoretical top capacity of 28 cylinders or 672 engraved color 6

7 separations per man and per shift. If the waiting time is minimized using the Autoloader, this top capacity increases to 36 cylinders or 864 engraved color separations per man and per shift. The state of the art in digital packaging engraving Like publication gravure, packaging gravure has also enjoyed immense growth in productivity. Here too, the engraving machine has developed into a job ticket-controlled output device, the copper recorder. This means that functions like impositioning and assignment of production parameters (screen, angle, gradation) have been transferred to front-end devices (e.g. HelioCom). Carefully tailored proofing solutions for cylinder layout (e.g. HelioFormproof) now ensure that the required level of production reliability is achieved before engraving. The introduction of the HelioSprint engraving head was a great boost to packaging engraving. Since packaging engraving usually uses only one engraving head, the engraving time accounts for a relatively large proportion of the total production time for the impression cylinder. Thanks to HelioSprint and other functions (automatic fast crossfeed, helical line engraving, sequential engraving), engraving time has been cut by more than half. The HelioKlischograph K500 fully-automated packaging engraving from the data Automated calibration of the engraving head using a suitable camera system such as CellGuard is a precondition for automated cylinder production. Operating an engraving machine now simply involves fitting the cylinder and selecting a job ticket. The installation of a cylinder loading system, for example an automatic ceiling-mounted crane, also allows fully-automated cylinder production. Obviously, this requires specialist software to handle process control and quality assurance. Conclusion With its Klischographs, HELL aims to provide the printing industry with the simplest, most reliable and most cost-effective engraving systems possible. The modular technology in these systems allows every user to optimally 7

8 configure his engraving machines to suit his own production conditions. With this in mind, many customers are particularly interested in being able to upgrade and retrofit their older equipment. On the basis of its great successes in recent years, HELL is stepping up its development efforts even further. The primary objective is to enhance cost-effectiveness to an even greater extent by further simplifying operation and increasing the degree of automation whilst improving engraving speed and consistency even more. 8