Gravure printing without proofing – does this work?
von Gabriele Kirchmeier,
Over the past 20 years, the production of gravure cylinders has been considerably improved by using faster engraving machines, more precise engraving processes and fully automatic production lines. However, one process step has not yet been taken into account within all these optimizations and is still carried out manually: The proofing of the cylinder.
Gravure printing without proofing – does this work? The finished gravure cylinder is proofed after engraving and chrome plating. The proof has always served as a means for quality control, which is determined by many factors. The basic conditions and the full manual work steps involved in creating cylinder proofs are very complex. But is this procedure still up-to-date as it is still necessary to comply with the requirements of Industry 4.0 and the increasing use digitalization?
The process of proof printing
The proof is created after the cylinder has passed through the manufacturing steps “Copper plating – engraving – chrome plating”. For this purpose, a special proofing machine (for example from Heaford) is used. The proofer carries out all the required work steps manually on the machine: The feeding of the respective substrate, clamping, adjusting and fitting of the cylinder down to the exact mixing of even the smallest ink quantities. (For detailed information about this procedure see box). Before shipping the cylinders to the customer, the proof is meticulously checked at the final inspection stage. Trained personnel visually inspect the proof for certain quality features, e.g. missing areas in the print, uniformity of the multiple-ups, completeness, colour or engraving misfire.
What can be achieved with the proof? The proof is an instrument to enable quality control of the gravure cylinder to be completed. This includes looking for missing areas and also to prove the reproducibility of the printing cylinder. In most cases, the proof matches the master copy provided by the customer and is used as a colour copy on the press.
The proof is a sort of insurance for the cylinder manufacturer and the print shop alike. Because in this way it provides confirmation that the cylinder set is faultless and the signed off colour result can be achieved in print production. However, if despite this insurance, the cylinder at a later time show flaws or the colour targets cannot be achieved, the cylinder manufacturer must expect a complaint.
Why can the proof be dispensed with? The discussion about the efforts and benefits of cylinder proofing has been going on for a long time and is fuelled by the market demand for shorter lead times. In this respect, gravure printing is in direct competition with flexo and offset and increasingly also with digital processes. For other processes, the printing formes are produced much faster than in gravure printing. This is partly due to the following features:
1 .Time required to produce the proof For complete proofing the whole cylinder set is printed out. In package printing, such a set consists mainly of the process colours (cyan, magenta, yellow and black) and in addition also special colours and varnishes. Depending on the design, a colour set includes between six and ten cylinders.
Often all the gravure cylinders in a set are produced except for black or the fonts. The finished cylinders are held in stock until the final one is chrome-plated and ready for printing. If faults are discovered after the complex combination printing on the proofing machine, the corresponding cylinder must be re-manufactured and the other cylinders returned to storage.
However, frequent moving of the cylinders increases the risk of damage. It should be emphasized that the printing department must provide a comprehensive stock of inks and consumables. This is because proof printing is usually carried out with original colours on original substrates.
There are a variety of different films, papers and cardboards as well as different colour systems for solvent- and water-based colours available. The storage of consumables and the accurate colour mixing in small quantities requires a lot of effort. Most other printing processes do not require proof printing at all. In flexo printing for instance, a simple test print is made with each printing plate.
2. Digitization / Industry 4.0 Compared to proofing, almost all upstream cylinder production processes are partially or fully digitized. Except for the proof, the work in the repro department is carried out digitally and the cylinders are imaged digitally using appropriate engraving data. Even the production of the printing cylinders is mostly automated.
In the context of digitization and Industry 4.0, replacing the manual proof printing process and manual cylinder control with a digital solution is mandatory. This would also result in a competitive advantage over other printing processes.
3. Colour management – framework conditions for the changeover
The discussions on proof printing are supported by the use of reliable proofing processes, which can also represent the gravure-relevant spot colours in combination with the colour set.
In a stable colour management process, the printing press, the inks and the proofing device (proofer, software, measuring instruments) are precisely coordinated. The proof serves as a mandatory colour template on the press, which the customer has signed off as suitable to print.
However, considering the overall process “Design-to-Press”, proof printing of the cylinder only represents an intermediate result, which is often used as a template for setting the gravure printing machine. If there are colour deviations between the proof and the original of the customer, this results in greater colour tolerances on the production machine if the machine operator takes the proof as a colour reference. However, if the proof is used on the press as a colour template, the tolerances are smaller.
The printer must be sure that the delivered cylinders can actually reproduce the proof. To ensure this is so, a reliable colour management process has to be established. With the current software and proofing solutions, sustainable colour management can be set up for almost all applications, which are required for the production of customer proofs. With a high-quality colour management system, the colour-reliable proof would be obsolete.
4. Single proof printing Missing areas or incorrect engravings are very difficult to detect in the manufacturing process, which speaks in favour of the cylinder proof printing. However, for this test, the colour-reliable proofing and the combination printing of the complete cylinder set is no longer necessary, which means time savings in the overall process. In addition, there is no need to wait for the preparation of all cylinders, as they are proof printed independently of each other. This offers the advantage of being able to resolve faults that have already been discovered, while the other cylinders of the relevant set are still being processed. One objection against the proof printing of individual cylinders is the need to constantly change the width of the proof press. However, such adjustment work can be largely avoided by reliable detailed planning. In addition, standardized processes ensure the merging and joint shipping of all cylinders belonging to a set after the quality control measures.
Single-cylinder printing eliminates colour mixing because standard colours are used and all cylinders are printed with the same high-contrast colour. The availability of different consumables can also be significantly reduced, since in this area it is also possible to work with standard materials.
Digital quality control
Various digital quality control systems are available in the market for proof printing. For example, EyeC offers a control software that checks the previously scanned print against defined comparison data and displays any deviations. Further approaches are available from cylinder manufacturers Saueressig and 4Packaging, both of which work with self-developed scan controls for the already engraved cylinder. However, mature and commercially available scanner and software technology for the quality control of finished printing cylinders is not yet available.
Framework conditions for changing the proofing process The framework conditions for the scenarios shown above include reliable colour management, well-functioning detailed pressroom planning and digitally-supported quality control.
Print shops are accustomed to using the cylinder proof printing as a colour-reliable template for the print run. When changing over to the monochrome proof or the complete avoidance of the proof, it must be ensured that the proof is perfectly colour reproducible. In addition, the cylinder may have no other defects. These parameters have to be considered urgently during any procedural changings, so that the press operator gains confidence in the new system.
Where can proof printing not (yet) be replaced? For all of the above-mentioned possibilities of procedural changes in the proofing process, attention must be paid to areas in which proof printing is still an absolute must: Packaging provided with various varnishes and special colours such as metallic inks still require proof printing, because such inks and varnishes used in gravure printing cannot be simulated with current proofing systems.
Colour profiles which are to be read and measured in the colour management systems are still handled by means of proof printing.
Summary As mentioned above, proofing process are very complex which makes gravure more sluggish compared to other printing processes. However, it must be clearly stated that eliminating proof printing would significantly reduce the lead-times, which certainly would mean a gain in attractiveness for gravure printing.
Therefore, freely available digital test systems for printing cylinders must be developed. Digital in-line inspection after each manufacturing step in the cylinder production (copper plating, engraving, chrome plating) would mean further optimization and digitization of the processes.
The process of proof printing The engraved and chrome-plated gravure cylinder is printed on a special machine and the operator performs all the steps for each individual cylinder manually:
Mounting the respective substrate on the drum of the proof printing machine
Setting the colour in a cup:
Blending cyan, magenta, yellow or black
Mixing and blending special colours in the cup
Keep the viscosity stable for all colours
Lift the provided cylinder out of the trolley with a crane
Insertion, clamping and adjusting of the cylinder in the proof printing machine 5. Move the cylinder to the drum along with the substrate
Fill the resulting space between cylinder and doctor blade with the ink from the cup along the whole width.
Start the proof printing process: The cylinder rotates, the cells are filled with ink and the superfluous ink is removed from the surface before the ink in the cells is transferred onto the substrate. At the same time, the drum rotates with the substrate.
After printing: cylinder and doctor blade are cleaned and excess ink is removed
Lift the cylinder into the transport rack and pack it if necessary
For multi-colour jobs, repeat the process according to the number of colours (up to 10 times)
Taking the substrate off the drum and check
In case the proof printed colour does not meet the requirements of the customer, steps 1-11 must be repeated for all cylinders