One of the great challenges in corrugated postprint is the fluting effect i.e. the streaky appearance of the printed image due to the internal fluting and its resistance when printing on the corrugated surface. The influence of the surface geometry of printing plates on the print results has been debated for some time: are “flat top dots”, unlike the rounded points of fine halftone dots from digitally imaged and conventionally exposed plates, capable of reducing the fluting effect in corrugated postprint? A range of systems for producing flat top dots are available on the market, using techniques such as film lamination or exposure under inert gas to prevent oxygen inhibition, therefore rounding of relief edges during UV exposure.
Flint Group Flexographic Products, a leading supplier to the printing and packaging industry worldwide, offers the innovative nyloflex® NExT exposure technology for flat top dots and surface screening. This technology can be used universally for all digital flexographic printing plates and can be integrated smoothly into the existing digital workflow. It provides a system solution, that it does not require any additional consumables, and where exposure can simply take place outside of the laser.
Extensive printing trials were conducted in collaboration with Hans-Joachim Heinerich GmbH to investigate whether, and to what extent, flat top dot printing plates can minimise or even eliminate the fluting effect. The printing plates were produced by exposure using the nyloflex® NExT technology. Different combinations of UV LED surface exposure and through-cure with UV tubes were tested in order to examine the influence of flat top dots in conjunction with relief depth and steep shoulder angles. Two exposure settings (S4 and S5) were selected, as they produce a good combination of steep relief shoulders and flat dot surfaces with the nyloflex® FAC and ART plate types used.
Experience gained in the past has shown that although microstructures on the plate surface bring great advantages in printing of flexible packaging due to the improved print characteristics in solid areas and higher colour densities, the use for printing on paper or other absorbent materials has been limited. An exception is highly coated liner materials, in which slight increases in solid densities can be achieved with microstructures.
This series of trials, therefore, focused on the extent to which corrugated postprint processes can be optimised as a result of accurate monitoring of relief formation during exposure, e.g. by reducing or eliminating the fluting effect or through lower dot gain tolerances.
The printing trials were conducted using conventional exposed flexographic printing plates (by means of UV tubes), and plates prepared using the nyloflex® NExT exposure technology (both Flint Group plates).
With this new UV exposure method, the production of flat top dots is based on accelerated crosslinking/polymerisation using high-intensity UV sources (UV LEDs); this process minimises or even prevents the oxygen effect during UV exposure.
The flexible exposure options available with the nyloflex® NExT system make it possible to control the relief profile and to adjust it to meet the printing requirements. This is achieved by varying the UV LED pre-exposure, e.g. through slow, multiple exposure of the plate surface. The duration of the main exposure using UV tubes can also be varied. The two images below show the influence of the number of UV LED pre-exposure cycles.
Exposure 4 Exposure 5
The printing trials clearly demonstrated that these two settings can improve the quality of tonal value transfer and significantly reduce the fluting effect in corrugated postprint.
Different paper qualities, anilox rolls and two different plate carriers were used in the trials in order to take account of as many quality-related influencing factors as possible in the materials used. The nyloflex® FAC and ART printing plates, which are highly suitable for corrugated postprint, were selected in different plate thicknesses. Exposure was carried out using conventional UV tubes and nyloflex® NExT (UV LEDs) at the two different exposure settings.
The selected materials are commonly used in practice. Although they are not statistically representative, they still produce meaningful results.
Plate types: nyloflex® ART 170 D, ART 284 D, FAC 284 D
Exposure setting: nyloflex® NExT S4 and S5, standard UV tubes
Liners:Rieger Liner EC-D 140 g/m2, Chromo Duplex 2 -GD 2- (145 g/m2, 210 g/m2)
Flutes: B flute
Foam / cushion: Rogers R/bak Super Soft (black), ProShim Medium (Red)
Anilox roll: Apex Unicorr S, Classic 260 anilox roll 6.5 cm3/m2
General factors which influence the fluting effect:
Printing plate type, hardness and thickness: It is essential to choose the optimum plate for the printing parameters in order to achieve good, non-streaky print results.
Liner quality and fineness of the flute: This variable must be regarded as the most important quality-related factor where the fluting effect is concerned. The rule is: the higher the quality of the liner and/or the finer the flute (flute pitch/flute height), the lower the risk of the fluting effect.
Other possible influencing factors:
Ink and ink viscosity
In addition to these conventional factors, the trials focus on investigating the effects of the different relief geometries produced by the different exposure technologies and surfaces discussed and examining the hypothesis that flat dot surfaces will have a positive influence on the fluting effect. The images below show a selection of the print results obtained.
Comparison: nyloflex® NExT system with standard tube exposure (extract from the print analysis).
nyloflex® NExT – LED exposure: nyloflex® FAC 284 D plate, GD 2 145 g/m2, B flute, UniCorr anilox roll
Conventional UVA tube exposure: nyloflex® FAC 284 D plate, GD 2 145 g/m2, B flute, UniCorr anilox roll
The print samples shown in the images clearly demonstrate the impressive effects of UV LED pre-exposure on the print result. The fluting effect was almost completely eliminated in the printed image shown. The measurements taken also made this difference clear: the curve diagram is based on the greyscale differences in the print and is therefore a graphical representation of the visible fluting. It confirms the large difference in quality between the two print samples.
A considerable difference was observed for all combinations of cushion, printing substrate, anilox roll and print plate. The reasons for this also lie in the general factors which influence the fluting effect, such as differences in printing plate hardness. However, the use of the nyloflex® NExT exposure technology resulted in an improvement in all cases.
Overall, the results of the extensive printing trials were as follows:
When compared with conventionally exposed digital plates, all the printing plates produced with the nyloflex® NExT exposure technology shows actually in many cases significant improvement in the fluting effect during printing. The scale of the increase in quality is, naturally, also directly dependent on having the ideal combination of the various general influencing factors referred to above, which must be identified individually for each printing job.
The best results with respect to the fluting effect problem were obtained with the following variables:
It was demonstrated that targeted control of the relief formation in terms of creating flat dot surfaces and steep shoulder angles can significantly improve quality in corrugated postprint. The nyloflex® NExT technology provides excellent opportunities to achieve this result, delivering highly accurate reproduction and superb reliability. Along with the fact that, due to microstructured surfaces, UV LED exposure also contributes to improved print results in printing of flexible packaging, the nyloflex® NExT exposure from Flint Group Flexographic Products represents an established system, which will improve quality in flexographic printing.
In this article, it has only been possible to show a small number of the print results obtained. Please contact your Flint Group partner for more detailed information.
Peter Großstück, Flint Group Flexographic Products