Jochen Richter B.Eng., Filderstadt, Germany The enormous improvements in the quality of flexo printing in recent years have many causes and are the result of continuing developments. Direct drive in printing presses, finer screen rollers with better cell geometry, better inks and improved ink transfer have all contributed, as has laser beam engraving of the flexo printing plate. drupa 2004, world market for printing, publishing and converting, will not only provide information on the state-of-the-art in quality but will naturally also drive the developments in flexo printing.
In Germany, flexo printing means packaging printing with all its specific requirements. Although the flexo printing process is used to print newspapers in other parts of the world, with over 40 printing plants in the US alone, this article focuses on the packaging printing. The steady upward trend in market share for packaging flexo printing in comparison to competing printing processes has not remained unnoticed (image 1)
Flexo printing presses with direct drive
Within the realm of printing presses technology, the direct drive technique should naturally be mentioned. Used initially with narrow web presses in in-line configuration, the first central cylinder press for printing flexible packaging, from Windmöller & Hölscher, was installed in 1998. Today, according to information from the manufacturers, new installations in this sector in Europe are made almost exclusively in this technology. But even in countries were for economical reasons the gear-driven printing presses hold on firmly, a clear shift towards direct drive printing presses is noticeable. What advantages does this technology offer? With the so-called "electronic wave", which replaces the fixed gearwheel-based connection without any vibration, all desired finishing lengths can be obtained in a step-less manner. This means that even without a store room full of different gear wheels, repeat lengths in any gear divisions are possible. In addition, substrates with much wider range of thickness can be used. This aspect substantially simplifies the changing over of printing materials when changing jobs.
Where is then the advantage in relation to the attainable printing quality? According to information from manufacturers, direct drive machines permit targeted control on the printing length in each individual ink unit. This means that the register precision in the circumference direction is clearly improved, as for example the individual deformation of a flat imaged plate during the imposition, which is different according to the motif, can be accurately adjusted so that the required quantity of overfilling can be reduced. This aspect is not only interesting for flexible packaging, but among others also for directly-printed corrugated board. Many different concepts have been meanwhile implemented here. While in the Bobst version only individual components of the ink unit are directly driven and the others are powered as in the past by a shared motor to exploit the advantages of both processes, the Göpfert and Cuir philosophy is to use more individual motors. A direct comparison of the systems will certainly be a "must" at the upcoming drupa 2004.
Better screen rollers
Another important and ongoing innovation is the use of screen rollers for inking the plates. Six years ago the motto was still: "the finer the better", but this trend has now ended. Manufacturers are concentrating on developing new cell geometries as well as on improving the surface characteristics of the screen rollers by coating them or by further developing the ceramics used. Especially in the premium flexible packaging sector, where flexo is competing with intaglio for market share, this is a deciding factor and hardly a month goes by without a specialist publication reporting on improved screen rollers or coatings.
Optimised printing inks
The developments described above influence also the printing inks. Ever lower ink volumes have to cope with ever growing demands for detail reproduction, adherence and colourfulness. Enhancements in the bonding agent system and an increase in pigment concentration have led to attaining the required ink density with ever lower volumes. Note here also the admittedly slow but continuously growing usage of UV hardening printing inks. While there is still a great deal of reservation in Germany about printing folding boxes in flexo with UV colours, international competitions for the best printed product prove an evidently enormous potential for this printing process. In particular, the further reduction in odour and the clearly reduced fumes are a main argument for their use in narrow web printing, because further refining stages can often be left out due to the excellent gloss characteristics and adherence.
Further developments in drier technology are leading to faster and more energy-saving hardening of the ink film. The air’s oxygen that impedes the polymerisation process is largely pushed out of the drier and replaced by nitrogen. Radiator’s power can be saved and consequently the heat absorption of the substrates can be reduced. A further argument for flexo printing in this area of application is the clearly better quality when printing special inks, for example metal pigments (image 2) compared with offset printing. In recent years it has become possible to reduce the pigment size significantly and consequently to use screen rollers with higher ruling, which again permits the reproduction of the finest details.
Flexo engraving or direct engraving?
Since the first presentation worldwide of digitally imaged flexo printing plates at drupa 1995 and the presentation of directly engraved polymer printing plates at drupa 2000, a fierce controversy has continued and an ongoing debate has commenced as to which process is qualitatively and economically the best. In direct engraving the printing relief is ablated from a full surface using a laser. This is by far the older method, as rubber printing formes have been laser-engraved for about 30 years. CtP flexo engraving uses flexo polymer plates with a black masking layer (LAMS layer), which is ablated by the laser beam at the printing image portions, after which the plates are imaged with UV light and finally washed out and dried. Where is then the difference?
Until very recently polymer plates indisputably had a higher resolution and a shorter imaging time. Modern laser imagers for flexo plates use up to eight parallel-positioned laser beams (Nd: YAG laser), which significantly shorten the imaging time. Nonetheless the plates must then be imaged, washed out and dried, which represents a substantial loss of time. New developments such as the Cyrel FAST system promise help, with the non-integrated plate material being removed thermally and the printing forme is immediately ready for use. At present this system is still limited in size, but it can only be a matter of time until it is used also for large-format printing jobs.
Thin-sleeve technology
A further development which promises to enhance the quality attained with flexo is thin-sleeve technology, as offered by Cyrel or Akl for example. It combines the advantages of a polymer plate with those of a round imaged printing formes that do not experience any motif-dependent deformations during the imposition The photopolymer material is placed on a thin and therefore inexpensive sleeve, round imaged and washed out. The change-over in the press follows with the help of the adapter sleeve. Unfortunately due to the limited market penetration of this system it is still expensive to use.
The advantages of direct engraving
In principle a distinction must be made as to which material is being used. While low resolution engraved rubber plates are nothing new on the market, the directly engraved BASF polymer plate nonetheless drew considerable attention at drupa 2000 – even though it tended to be laughed at. Statements such as "uneconomical, because it is much too slow", "high resolutions are unattainable, because the diameter of the CO2 laser used is much too big!" etc. could be heard. Lately these voices were silenced or made more reflective, as the first installation at GRS GmbH in Uder by BASF and STK Schablonentechnik was recently announced. This was made possible through the clever use of overlapping laser spots, that allows for imaging much smaller elements, as well as the smaller spot itself. (Image 3)
In addition the systems of the various providers have been modified so that a transition has been made from the original one-beam technique to as many as three beams that use different energy levels to ablate the vaporising material at different depths and thus guarantee a substantially better construction of the walls of the screen dots. A further innovation in this area is the combination of a CO2 laser for the rough forming of the relief, and particularly the relief depth, with Nd:YAG laser that can build different walls due to its much small spot diameter. Restrictions arise due to the plate material itself, as Nd:YAG laser is not absorbed by all materials cf. digital printing plate imaging. (Image 5)
The clear improvement in engraved printing formes described here is also presented in two projects carried out by the DFTA-TZ. While the comparison beteen the most varied forme production processes, presented at the DFTA spring professional conference 2001, still turned out unequivocally in favour of polymer printing formes, the 2003 results for directly engraved printing formes showed a major advance in quality and have prompted heated discussions. (Image 4)
A research report on the results appeared in summer 2003. These are only some changes that plate production has experienced in the recent past. In the domain of the still ubiquitous analogue printing plate production, introductions such as the QualiyDOT moving light imager - developed by Theimer GmbH in co-operation with Flexo-Technik Handels GmbH and InnoFlex - lead to a more stable printing plate imaging: Also software developments to obtain smaller area coverage are holding their own against the digital competition.
The development goes on
The development of flexo printing has by no means reached an end and the latest on the market when this article is being written may soon be overtaken again. The most current information on the latest developments, whether in printing presses, screen rollers, printing inks and naturally printing formes, or the many factors that are not examined here but which immensely influence quality (e.g. automatic register controls, optimised chamber racle systems) can be directly compared only at the same point in time. We are thrilled to see what drupa 2004 will offer us.
(Jochen Richter: The author has worked at the DFTA technology centre for around six years and works on the entire range of domains of the technology centre. This covers activities from the planning and implementation of seminars, planning of printing trials and co-operation with industrial suppliers and research on new areas of application for flexo printing as well as projects on topics specific to flexo.)
