New foam substructure for direct printing on corrugated cardboard in a practical test
The fleece-finished reverse side makes handling in the printing machine easier
Foam substructures with fleece finishing correspond to the latest developments in the prepress of corrugated cardboard direct printing. The fleece on the back makes it easier for the printer to position the post-print assembly on the printing cylinder. Smufit Kappa Neuburg tested the handling and printing properties of a new fleece substructure together with the prepress company Horst Gries.
Pressure-absorbing foams made from closed-cell polyethylene, more precisely from HDPE, have long proven themselves in direct printing on corrugated cardboard. As a compressible base, they allow the use of thinner and harder photopolymer flexographic printing plates. In the printing press, they reduce the contact pressure on the plate in the printing gap and thus enable a lower tone value increase, finer raster images and higher print quality. At the same time, they reduce the washboard effect. For users, in addition to the improved printing properties of the foam material, easy handling when positioning the unwieldy, large-format post-print assemblies on the printing cylinder is important. In order to achieve the best printing properties and easy handling of post-print assemblies, Innova Flexo Products developed a fleece-refined foam substructure, which guarantees stable print quality and a long service life. The laminated thin fleece layer on this substructure ensures easy positioning of the post-print assembly on the printing cylinder due to its excellent sliding properties. The substructure is offered in two versions: Either the polyethylene foam is mounted directly on a carrier film (in certain positions on request) or it is adhesive on one side for self-assembly by the repro house.
Demanding print motif
For the print test at Smurfit Kappa in Neuburg on the Göpfert Ovation printing machine, Kerstin Sudau (Operations Manager at Horst Gries) and Uwe Dittenhauser (Technical Clearing House at Smurfit Kappa Neuburg) selected a sophisticated, five-color print motif in a 42 grid. Due to the flexographic printing plates used (2.84 mm), the thickness of the substructure was specified as 1.65 mm. This resulted in a total thickness of the post-print motage of 4.59 mm - including a 0.1 mm thick adhesive tape.
The 1400 mm high assembly consisted of the 1350 mm wide substructure and a foam-free zone of 50 mm, to which a plastic strip was glued and sewn on top.
The sealing of the bar serves to protect against washing water. After completing the panel assembly, a plastic strip was installed on the lower side of the post-print assembly. For this purpose, an approximately 50 mm wide foam strip was first scratched and then peeled off.
The selected Paulaner motif was not only suitable for the print test because of its high technical printing requirements, it also enabled a comparison of handling between post-print assemblies with and without fleece coating. The assemblies without fleece coating were available as a reference from another order of the same type.
The judgment of the experts on the machine
During the test, Andreas Croce, printer at the Göpfert Ovation, took on the task of comparing the handling of post-print assemblies with foam substructures on different surfaces. To do this, he hung the bar of the post-print assembly into the rail of the printing cylinder, which is drawn in upwards by turning the cylinder. In the event that the assembly does not sit at an angle on the cylinder when it is being pulled in, it was possible to compensate for this by adjusting the cylinder at an angle. For easier positioning, air was blown under the assembly from the cylinder nozzles. In addition, Andreas Croce, who described his impressions of hanging the post-print montages in the Göpfert Ovation printing units as follows:
The use of post-print assemblies with fleece finishing on printing machines of the Bobst 1600 type is also of interest. This machine type also has an inclined adjustment of the cylinders, with which the fixed post-print assembly on the printing cylinder can be precisely aligned at an angle. Here, however, the rubber bands for fixing the assembly normally have to be loosened except for one, in order to be able to correct the assembly in its position. “With these machines, it's just as important that the post-print assembly slide smoothly on the cylinder. With a fleece-coated substructure, the fixation of the assembly no longer has to be loosened when positioning, ”says Uwe Dittenhauser, explaining the difference between the Bobst 1600 and assemblies without fleece. In the plate cleaning systems at Smurfit Kappa in Neuburg, the printing plate and substructure are completely wetted with water. The cleaning water drips off the fleece surface more slowly than on a smooth and closed polyethylene surface, so that the drying time after the cleaning process is somewhat longer than with post-print assemblies without fleece.
How does the print result look to us?
While the advantages of easier handling are in the foreground on the printing press, Kerstin Sudau asked the question of the printing properties of the substructure, in particular whether there will be deviations from the previously known printing characteristics. This would make it necessary to use new tone value curves in the reproduction. For clarification, the characteristic curves were determined using 10 sheets of corrugated cardboard that were drawn during the print production of the test run. Using the sample sheets, the highlights and the location of the individual colors were assessed.
In the case of the process colors yellow, magenta and black, the evaluation of the screen wedges showed a flat course of the tone value increases in panels with foam substructures. The edition was printed with the water-based “Euro Base” ink system from Doneck Network. Anilox rollers were used for printing the process colors, all of which had the same specification: scoop volume: 7.5 g / m2; Number of lines: 225 L / cm.
The approximately 15% higher tone value increase of the cyan characteristic curve was caused by a slight shift, so that cyan fell out of the evaluation. The closely spaced, flat dot gain curves of magenta, yellow and black made adjustments in the repro not necessary.
The higher solid tone density (DV 2.73) of the special color "Blue P282C" as well as the higher scooping volume of the anilox roller used (10 g / m2, 160 L / cm) led to a significantly higher tone value increase compared to the process inks YMK. In the grid around the Paulaner logo, the fine black grid dots printed round and pointed and ran out homogeneously, regardless of whether the S-flute was printed on the crest of the wave or the valley of the wave.
The evaluation of the solid tone densities showed constant values for all colors and printed sheets, to which the firm compression behavior of the substructure contributed essentially. The "worm structure" recognizable in the pictures showed a clean color in the full-tone areas without the white liner surface flashing through.
The foam substructure with fleece was able to convince both in the handling test in the pressroom and in the printing test. After the test production, four more print runs were made with a total of 160,000 sheets, which confirm the positive properties of the substructure. The next step for Uwe Dittenhauser is an expansion of the test series with further motifs on the Göpfert Ovation. The use of post-print assemblies with a fleece substructure on the Bobst 1600 is also planned.