Dr. Siegfried Beisswenger, Jan Breiholdt

The engraving system under control Design, function and features of an engraving system

Figure 1. Engraving head production at HELL in Kiel. More than 7000 engraving systems have already been shipped.

Years before CTP (Computer to Plate) was introduced in the offset sector, this process was already routine in gravure cylinder production. This was made possible by the engraving system invented by Dr. Hell and continually developed by HELL Gravure Systems. The following report provides the reader with all the information he or she needs about the design, function and features of an

engraving system as well as tips on the care and maintenance of the heart of every engraver.

Core expertise from Kiel Engraving systems have been manufactured in Kiel since the invention of electromagnetic engraving by Dr.-Ing. Rudolf Hell in the 1960s. More than 7000 have

been shipped worldwide since then. 300 new systems are added to this figure every year. More than 20 HELL specialists are currently involved in developing and manufacturing engraving systems. This represents a bundling of expertise and resources which is unrivalled worldwide.

gravers, they simply copied HELL’s engraving head. HELL’s response to such imitations has always been to redouble its development efforts. This has ensured that HELL’s engraving systems have remained well ahead of the competition in terms of speed, reliability and engraving quality.

When other suppliers also began to manufacture en-

In addition to manufacturing engraving heads, HELL also

Special Print August 2003 · E · 1

chanical engraving to compensate unwanted mechanical effects. In simple terms, the DSP could be thought of as an intelligent unit which analyzes and corrects image signals before they are engraved. This correction, which can be parameterized using software, prevents bounce, drag and thermal drifts being seen in the engraving image.

Figure 2. Original HELL engraving head HELL (left) and a copy (right).

attaches particular importance to maintaining them. The rising number of cases in which maintenance is carried out reflects the growing quality awareness of cylinder manufacturers. Around 2000 engraving heads were overhauled at the Kiel site in the last 3 years.

Team work A comprehensive approach is essential when dealing with an engraving system.

Signal processing, the engraving amplifier and the engraving head must be finely tuned to each other. This is the only way that outstanding engraving quality can be achieved (Figure 3).

An intelligent little guy A digital signal processor (DSP) is a specialized microprocessor which can perform several computing operations in a single step. HELL uses the DSP in electrome-

In the case of the HelioSprint system, the correction is individually tuned to each engraving head. HELL achieves this by individually measuring each engraving head. The specific mechanical properties are stored in a microchip integrated in the engraving head. The HelioKlischograph is then able to extract this data prior to engraving in order to use it for optimum parameterization of the DSP. After analysis and correction, the DSP provides the digital image data to the engraving amplifier in analog form.

Show of force The purpose of the engraving amplifier is to amplify the analog image signals so that the engraving head drive can develop the exact force required to cut a precisely defined cell volume. The greatest demands are made by the large and small cells located on the contour edges. The engraving amplifier must provide dynamic forces of the highest magnitude for these.

Precision tool Figure 4 shows the principle of an electromagnetic engraving head. The engraving diamond is fixed to a lever and is driven into the copper surface by the force of a solenoid. The magnetic force does not primarily work against the cutting forces of the copper but against a spring force which is several times stronger than the pure cutting resistance of the copper would be. The engraving is thus largely inde-

Figure 3. The “team work” behind a modern engraving system involves intelligent signal processing, high amplification and high-precision mechanical parts.

Special Print August 2003 · E · 2

Figure 4. Principle of an electromagnetic engraving head.

pendent of different copper hardnesses because the size of the cell primarily depends on the equilibrium of forces between the magnetic force, i.e. the engraving current, and the spring force. On one hand, the spring force is required in order to be able to cut precisely defined cell sizes, while on the other a spring-mass system, which can oscillate with a characteristic frequency and has a pronounced resonance pattern at this frequency, is created by the spring and other mass-bearing elements of the engraving head. This property of the engraving head would lead to a situation in which changes in contrast in the images to be engraved which happen to match the natural resonance of the engraving head would be reproduced in a completely distorted form. For this reason, the engraving head must have me-

chanical damping which, together with the filter function of the DSP, ensures that all of the image and text components are engraved with the same efficiency. In the actual engraving head, the coil spring of Figure 4 is a torsion spring. It oscillates with a rotational deflection of +/- 0.5 degrees. The lever of the stylus holder translates this rotational movement into linear movements of the diamond amounting to +/- 70 µm (HelioSprint).

Champion Chip Which features make an engraving system stand out? The most important requirements on an engraving system are high engraving depth accompanied by high engraving speed. However, it is difficult to meet both these requirements. This can be understood by doing a

simple test: If you swing your arm backwards and forwards, you will see that the faster you swing your arm, the smaller the deflection automatically becomes. Fast oscillations with a large degree of deflection require very high forces of acceleration. Therefore, magnetic fields have to be generated in the engraving head which, with the exception of supraconductive coils, are on the boundary of what can be achieved technically and physically. Other features of an engraving system are its dynamic response to changes in signals and its temperature and longterm stability. For engraving, the engraving system must be adapted to the specific requirements of the respective application. Technical rollers place great demands on the ink volume to be transferred. Engraving is thus performed with coarse screens and large depths. Similarly, a high ink volume and high contour definition are required for decorative and packaging printing. Magazine and illustration gravure printing demands optimum ribbon balance, i.e. all the engraving systems of a multi-channel HelioKlischograph should have transfer properties that are as identical as possible. In security printing, on the other hand, ultrafine screens are used. This requires particular consistency in engraving.

HELL offers an optimized engraving head for each application. The A system has been optimized for engraving depth and in a 32 l/cm screen achieves cell depths of up to 110 µm, as is necessary for technical rollers. HelioSprint B has been optimized for engraving depth, high dynamic response and engraving speed and is mainly used in the packaging and decorative sector. HelioSprint C has been optimized for high dynamic response and fast engraving speed and is widely used in the illustration and packaging gravure sectors. The HelioSprint D system is optimized for top precision and consistency and is intended for security printers. “Broadband” engraving heads are not optimized for either speed or depth and are designed to cover as broad a range of application as possible. This naturally involves some element of compromise. Broadband en-graving heads are shown in their true colors if the manufacturer merely quotes a maximum engraving depth and a maximum engraving speed and leads users to believe that these values can be maintained over the whole range of application. The customer should insist on being provided with clear technical data that reflects the real link between depth, speed, range of application and screen resolution. Dynamic response,

Special Print August 2003 · E · 3

Figure 5. Static characteristic curve of an engraving head.

consistency of engraving and perfect engraving image can best be evaluated on a real engraving. In addition to engraving speed (the “screen coarseness” at which this speed can still be achieved must be indicated), other criteria for evaluating engraving systems include qualitydetermining properties such as bounce, drag, hysteresis and thermal drift. Some “high performance engraving systems” have failed to meet even the simplest expectations in terms of engraving quality in practical use.

Having the right curves is critical he engraving head is an electromagnetic oscillation drive characterized by a variety of parameters. The static characteristic curve of the engraving head (see Fig. 5)

Special Print August 2003 · E · 4

gives the deflection of the diamond stylus as a function of the engraving current. The following properties can be derived from this characteristic curve: Hysteresis width, linearity, static sensitivity and maximum deflection. The dynamic measurement (see Figure 6) provides information on how uniformly and exactly the engraving head reproduces changing image motifs, jumps in density, and contours. Experts talk about the image contents having different “spatial frequencies”. To this end, the deflection of the stylus is measured while the frequency of the control current is changing. The resonance stepup is generated partly as a result of the natural frequency of the engraving head described above. The dynamic measurement can be used, for example, to derive the dynamic damping value as a parameter of an engraving

Figure 6. Dynamic characteristic curve of an engraving head with linearization which is achieved by the interaction of mechanical damping and DSP control..

head. The dynamic damping value provides information, for example, about the engraving head's tendency to ringing. This value is much more informative than the static damping value which other manufacturers use to calibrate engraving heads because it provides a lot more information about the real behavior of the engraving head and thus about the engraving quality. As mentioned above, the parameterizing of the electronic DSP stage at HELL is fine tuned to the dynamic curve of the engraving head.

Risks and side effects It is clear from what is said above that it is relatively easy to manufacture an engraving system which produces an even tone with low engraving depth using an engraving frequency of 20 kHz. The skill is being able to

engrave changing engraving densities with large cell depths and high speed over a long period of time with reproducible results. The total input from the DSP stage, amplifier stage,

Figure 7. Following jumps in density, bouncing leads to incorrect cell sizes, as can be seen in the example shown through the narrower walls at the start of the engraving edge.

the measurement curves and electronic parameterization helps HELL to come as close as possible to this ideal. If one element of this systematic approach is missing, is unsatisfactory or is not carried out properly, visible engraving errors will occur such as bouncing, for example. Bouncing (see Figure 7) occurs directly after a change in density in the direction of engraving and causes visible dark edges.

Drag is a memory effect of the engraving head which gradually fades away with time. For example, if the engraving density 128 is engraved after an engraving signal 1 (full depth), a larger cell is engraved than if the previous value had been 255 (super white). Because the effect gradually fades away, it is most prominent directly after the change in density and continually reduces thereafter. The main causes of this effect are less than ideal damping

properties and magnetic properties. Like drag, hysteresis is also a memory effect but it does not fade away. It remains equally prominent until the next change in density. The main cause for this is the magnetic hysteresis of the magnet structure used. Drift is a gradual process in which engraved cell geometries “migrate” over seconds, minutes and hours. The main causes are thermal

effects such as material expansion or thermal dependencies relating to the damping. If a tolerance of 1 µm cross diagonal is required, then in the case of a 130° stylus this corresponds to a tolerance of 0.233 µm in the stylus stroke. For comparison purposes, the thermal expansion of a 20 mm long steel part heated by 1°C is approximately 0.26 µm. This highlights the high demands that have to be placed on the consistency of an engraving head in practice. In this

Figure 8. Storing specific mechanical properties in the integrated microchip represents the final quality assurance step for the engraving head.

Special Print August 2003 · E · 5

regard, consistent cooling of the engraving head is particularly important.

Precision work A range of parameters affect the quality of the finished product when manufacturing engraving systems. The torsion spring has to be manufactured with great precision, the moving masses (including the stylus) are subject to very small tolerances, the mechanical damping unit must be very finely tuned, eddycurrent losses in the ferromagnetic circuit must be avoided as far as possible, and pole shoes, air gaps, permanent magnets, mounts, slide spindles and the whole casing must be manufactured with great precision.

in the production process even if all possible efforts are made to do so, maximum precision is achieved in a final fine-tuning process (see Figure 8). To this end, the static and dynamic characteristic curves of the engraving head are recorded and analyzed. If the parameters lie outside the specification, the engraving head must be mechanically overhauled. If the values lie within the specification, the “fingerprinting” method is applied in the case of a modern engraving system such as the Heliosprint. To this end, the customized parameters are stored in the “Touchmemory” of the engraving head as

described above. Prior to engraving, the values are retrieved and transferred to the DSP. The DSP then calculates correction values which are applied to the image signal. This means that even the smallest component or production tolerances can be compensated. Every HelioSprint engraving head is also fitted with temperature sensors to compensate thermal effects. The temperature values measured are also continuously analyzed by the DSP to correct the image signals.

Care and maintenance The fact that an engraving head’s engraving accuracy is greater than the thermal expansions which the head itself experiences in the course of a working day highlights how unbelievably precise this instrument is. Therefore, it has to be treated with great care. In particular, the specified ambient temperature must be adhered to. From time to time, the cooling air should be checked to ensure it is working properly. With some types of engraving head, the air filter has to be changed at regular intervals. Correct slide shoe adjustment is particularly useful in achieving

As the company that invented electromechanical engraving, HELL has comprehensive expertise in the whole field of engraving technology. In-depth knowledge about the physical relationships involved paired with many years of experience have led to the creation of advanced and reliable engraving systems. Through close cooperation between development and manufacturing, HELL ensures that technical improvements can rapidly be integrated into series production. Since component and manufacturing tolerances cannot be completely ruled out

Special Print August 2003 · E · 6

Figure 9. This spoke bearing was damaged when changing a stylus with an unsuitable tool.

ribbon balance. HELL has developed the SprintEasy setting device for this process. It frees the HelioKlischograph from expensive setting time and increases the setting precision and reproducibility of the slide shoe setting. Figure 10. SprintEasy increases setting accuracy and reproducibility of the slide shoe setting.

To ensure consistently high engraving quality over a period of years, the engraving head should regularly be serviced by HELL. Regularly maintained engraving heads display significantly less bounce, drag and drift. The engraving of the cells is more symmetrical and ribbon balance is much improved. HELL subjects systems sent for maintenance to the same quality testing as new systems. To this end, the engraving heads are first completely dismantled, cleaned and subjected to a visual check using stereomicroscopes. The static and dynamic parameters are then recorded. If the engraving head lies outside the specification, it must be mechanically overhauled. Particular attention is paid to the damping. The damping unit suffers slight wear as a matter of course. With normal systems, the Oring damping is set using three consecutive dynamic measurements. HELL strongly advises against the practice employed by some companies of only testing the O-ring damping using static measurements.

The damping unit of the HelioSprint engraving head is based on a much more complex principle. Tiny damping discs move in a special fluid, resulting in what is known as viscous damping. This viscous damping is very effective and is far better than other methods which have much greater “side effects”. Another element of the maintenance is replacing the damping fluid in the vacuum. Furthermore, for each HelioSprint engraving head, the compensation parameters are re-calculated (fingerprinting) and the Touchmemory is rewritten on a routine basis. All the parameters are recorded in the test certificate. HELL is the only manufacturer to carry out practical dynamic measurement and temperature drift measurement in addition to static measurement during engraving head maintenance. At HELL, any technical improvements in the engraving head put into practice in the series production of new heads are automatically integrated into the maintenance

process. Engraving heads maintained by HELL are thus always state-of-the-art, just like new heads.

ving systems state-of-theart and ensures that they achieve top quality over many years.

Closeness to the customer is essential in maintaining engraving heads. Special preferences and customerspecific fine tuning are often known and can be taken into consideration. A special service offered by HELL is the provision of free replacement engraving heads while the old engraving heads are undergoing maintenance.

While other manufacturers have, in recent times, allowed their piezo-driven engraving heads to quietly and secretly disappear from their websites, HELL has remained true to the electromagnetic principle. From 1963 to 1998, the engraving frequency was approximately 4 kHz. HELL then increased this to 8 kHz. This engraving system will remain a key area of HELL’s development work in the future. In particular, the use of new materials in combination with tried and tested basic principles will enable further increases in speed and engraving quality in terms of consistency and fine resolution to be achieved.

Summary and prospects With the launch of the HelioSprint engraving system, HELL has once again demonstrated that the competence center for gravure printing is located in Kiel. HelioSprint is the only engraving system that meets all of the demands placed on a high-performance engraving system – namely fast engraving, large engraving depths and consistent quality. With advanced engraving head maintenance using unique methods, HELL helps to keep its customers’ engra-

About HELL Gravure Systems HELL Gravure Systems GmbH is the world's market leader for digital gravure systems. Since Dr.-Ing. Rudolf Hell invented electromecha-

Special Print August 2003 · E · 7

nical engraving, more than 1000 HelioKlischographs have been installed worldwide. HELL Gravure Systems is focused on the development, manufacturing, sales and support of system solutions for digital print form production in packaging, decorative and publication gravure, and now also in flexo packaging. The primary

focus is the engraving machine and laser imager itself and on the image processing systems for controlling these. HELL Gravure Systems is located in Kiel, Germany, with subsidiaries and demonstration facilities in Germany, Malaysia and China. In all other countries authorized agents represent the HELL product lines.

HELL Gravure Systems GmbH Philipp-Reis-Weg 5 24148 Kiel, Germany Fon. + 49 (0) 431-2377-0 Fax: + 49 (0) 431-2377-1235 [email protected] www.hell-gravure-systems.com

Special Print August 2003 · E · 8