Ghosting in Heatset Web Offset Printing

Transcription

1 Das Papier Science and Technology 6/2008 Dr. Gerd Meder, Mohn media, Carl-Bertelsmann-Str. 161 M, Gütersloh, Germany Dr.-Ing. Wilhelm Busse, Verein ZELLCHEMING e.v., Emilstr. 21, Darmstadt, Germany Dipl.-Ing. Maik Coesfeld, Mohn media, Carl-Bertelsmann-Str. 161 M, Gütersloh, Germany Dr. Erich Frank, Flint Group Germany GmbH, Sieglestr. 25, Stuttgart, Germany Anne-Sopie Gombart, Sappi Netherlands Services B.V., Biesenweg 16, 6211 AA Maasticht, Netherlands Dr. Joop van Hesteren, Solco NV, Zwaluwbeek14, 9150 Kruibeke, Belgium Dr. Dirk Lawrenz, BASF Aktiengesellschaft, ED/PD H 201, Ludwigshafen, Germany Dr. rer.nat. Christian Naydowski, Voith Paper Holding GmbH & Co. KG, St. Pöltener Str. 43, Heidenheim, Germany Peter Resch, Sappi Papier Holding GmbH, Brucker Str. 21, 8101 Gratkorn, Austria Dr. Joachim Schoelkopf, Omya Development AG, P.O. Box 32, 4665 Oftringen, Switzerland G. Meder, W. Busse, M. Coesfeld, E. Frank, A.-S. Gombart, J. van Hesteren, D. Lawrenz, C. Naydowski, P. Resch, and J. Schoelkopf Ghosting in Heatset Web Offset Printing Ghosting is certainly the most irritating of known technical problems in heatset web offset printing. Depending on the angle of the blanket cylinder, the printed image on one side of a page will appear on the other side as a printing error a ghost image in effect. Fig. 1 shows four examples of ghosting during ongoing production. The name ghosting is very appropriate, not just because of the ghost-like reduction in tonal values, but also because of the apparently random ghostly occurrence of this effect. Fig. 1: Four examples of ghosting. Example 1: Recycling matt-coated paper, 65 g/m 2, Example 2: Woodpulp paper, white, glossy coated, illustration printing, 80 g/m 2, Example 3: Wood-free paper, white, glossy coated, illustration printing, 100 g/m 2, Example 4: Newspaper printing, matt, upgraded, 52 g/m 2 Das Papier 2008 T56

2 6/2008 Science and Technology Das Papier Das Papier Zusammenfassung Summary Ghosting in Heatset Web Offset Printing The problem of ghosting during printing will be examined using a special test form. It will be shown that ghosting results from mutual changes in the conditions as the paper separates from the rubber blanket, due to different tack forces on the top and bottom sides of the In the past a number of authors have examined this phenomenon and indicated the influence of various factors, such as paper, ink, fountain solution and rubber blanket 1-3. The literature also mentions measures to reduce or even avoid ghosting. However these generally prove to be contradictory and unreliable. Fig. 2 shows a rough summary of the key parameters of the materials involved paper web, and as the dot size continuously reduces. This reduction in dot size on the ghosting side leads to a decrease in the tonal values and thus to ghosting. Results were compiled from a working group consisting of the authors. The full-length version of this article will be made available for download at in the printing process. An exhaustive scientific analysis would list a lot more parameters, however the key question is whether parameters can be named that have a major influence on the printing problem and that are significant in its resolution. In order to gain a deeper understanding of ghosting as a printing problem, it is necessary to move away from the more or less random colour combination patterns in the printed copies from ongoing production and to use a test form to examine the question of changes in tonal value as a result of colour combination patterns from the front and reverse sides (Fig. 3). Different colour combination patterns were arranged in this test form, both for the ghosting side and for the reverse. On the ghosting side, as well as the four individual colours, large areas of combined colour such as C20M20Y20 (tonal value in each case 20 % for cyan, magenta and yellow) were printed. On the reverse, the individual colours and colour combinations were arranged in a tile formation so that increasing tonal values (20 % to 100 %) occur in the various coloured areas on the ghosting side. For simplicity, the reverse with the tiles will be referred to below as the tile side. In Fig. 3 the colour combinations used on both sides are indicated on the margins. The choice of colour combination pattern on the ghosting side and on the tile Fig. 2: Factors affecting piling Das Papier 2008 T57 2

3 Das Papier Science and Technology 6/2008 Fig. 3: Ghosting test elements. Ghosting side and tile side side makes it possible to examine the question of with which combinations of colour on the front and reverse or at which level of tonal values ghosting occurs. Fig. 4 shows the print copy (mirrored) and areas of the black, cyan and magenta rubber blankets after several thousand impressions. It is evident that the ghosting areas on the rubber blankets are much clearer and stronger than on the printed copy. The time-dependent nature of ghosting becomes particularly apparent if print samples are continuously taken while printing. Fig. 5 shows the temporal change in direct ghosting for the ink combinations 20K-K, 40C-C; 20M-M (L value over tonal value of the tile with the number of print copies as a parameter). Ghosting can occur more quickly or slowly, depending on the paper, ink, fountain solution and rubber blanket. However as a rule it becomes more distinct as rollover increases. The rubber blankets show the structure not just in their own inks, but also in the previous ones. This is particularly obvious on the magenta rubber blanket (Fig. 6). Thus there is direct ghosting (change in tonal value due to the interplay of inks from the top side and underside in the same print unit) and indirect, consecutive ghosting (change in tonal value due to the interplay of inks from the top side and underside in the different print units), Fig. 7. Fig. 8 shows the M20Y20 area of the ghosting side with the increasing tonal values of the tile side ( MY20 MY100 ). As the tonal value of the MY tile increases, ghosting becomes more marked, reaching its maximum between 60 % and 80 % and decreasing slightly at 100 % (full tone). The 200 x enlargements of the relevant ghosting areas show clearly formed dots at 20 %. As the tile tonal values increase, the full dot is continuously reduced to an ever-smaller, ring-shaped marginal area. If we look at the 400 x enlarged dots of the ghosting side (in and around the tile area on the reverse) (Fig. 9 and Fig. 10), then the marked difference between the formation of the dots in the undisturbed area and in the area disturbed by the tiles is clearly visible. The mountain-like formation of the dots is remarkable. When the dots are separated, there is obviously a forced flow of water and ink, causing this effect. The dot is increasingly narrowed by deposits on areas without ink occupancy. Until now it has not been possible to analyse the deposits clearly because components found cannot be assigned Das Papier 2008 T58

4 6/2008 Science and Technology Das Papier to the original molecules of the ink particles. Naturally, such analyses contain particles of the paper coating as well as the particles of ink. However it seems unlikely that these are the cause of the build-up. It is more probable that, from a particular adhesion level of the ink particles onwards, particles are torn away from the paper coating. Fig. 4: Development of ghosting effect on blankets and print copies; glance at blankets and print copy. Ghosting side, upper half of testform The question is why, when ghosting occurs, parts of the matrix surface change over from ink carrying to non-ink carrying areas as time progresses and why the dot gets smaller. The examination of the behaviour of a drop of ink (or rather a drop of emulsion because the drop of ink is emulsified with the moistening agent water and additive during printing) under technical conditions has proven to be extremely complex. Technical marginal conditions to be taken into account in an analysis such as this include nip, accelerated separation, surface energies Fig. 5 Das Papier 2008 T59

5 Das Papier Science and Technology 6/2008 and water/oil depletion at the contact surfaces, formation of the meniscus, etc. Whatever the explanation, the question remains why the ratio of dots on the front in relation to the reverse is of decisive importance for the occurrence of ghosting. All enlargements show that the transfer of ink in the dot on the ghosting side is smaller. A lower transfer of ink means that the tack in other words the force with which the drop of ink adheres to the rubber blanket and the paper decreases. The size of the dots on the reverse remains unchanged. Because when ink is transferred, the emulsion drop between the rubber blanket and the paper is split, the cohesion is of decisive importance. What is the impact of the change during printing? Upon what physical parameters does it depend? Fig. 7: Direct and indirect ghosting In 4 Keiter uses the laws of fluid dynamics to derive the separating force of a flowing material. The formula (Fig. 11) shows that the force for separating a drop between two plates is proportionate to power 4 of the drop radius and, as a corollary, proportionate to power 3 of the ink drop thickness. Viscosity is applicable in a linear pattern. It should be noted that the adhesion of an ink (tack) and its viscosity are not identical, so that this formula can only be a good approximation. In particular, the viscosity does not record the fact that, while viscosity remains unchanged, the adhesive effect at the limits of the drop can change. Nonetheless, this formula shows the enormous influence of the size of the dot area reducing this means greatly reducing the tack. Fig. 12 shows the influence of different dots and therefore tacks on the top and bottom sides of the paper web. If the rubber cylinder is arranged symmetrically and the dots are uniform in size, then the paper web is not deflected. If the upper dot is larger, it deflects the paper web to the upper cylinder, while if the lower dot is larger, it deflects the paper web to the lower cylinder. The upper and lower rubber cylinders in all heatset rotary print units are tilted Fig. 6: Glance at blanket magenta 5 Das Papier 2008 T60

6 6/2008 Science and Technology Das Papier towards each other to a certain extent. If the upper cylinder is placed behind the lower cylinder, looking along the paper path, this is referred to as a 1 o clock setting, the other alternative being an 11 o clock setting. Fig. 13 shows the influence of different dot sizes with the two cylinder settings. It is important that ghosting occurs whenever the paper web becomes separated from the cylinder as a consequence of the different dot sizes and different tack. Because the different dot diameters on the top and bottom sides already exist at the start of printing, the process that finally leads to ghosting must be as follows: Fig. 8: Dots on magenta blanket 200 x magnified. Ghosting field M20Y20; tile field MY, 20 % to 100 % 1. Different tacks at the top and bottom cause the paper web to be deflected. The web adheres to the rubber cylinder on the reverse side to the ghosting side. The tensioning of the paper web pulls the web away from the cylinder. 2. The deflection of the paper web causes the transfer of ink to the ghosting side (smaller dot) to be disrupted. 3. The disruption to the transfer of ink leads to a change whereby the dot on the ghosting side is reduced 4. The reduction of the ink transferred in the dot causes a further increase in the difference in tack and thus leads to an increase in the deflection of the paper web. Fig. 9: Dots on magenta blanket. Ghosting field M20Y20; tile field MY, 70 % Fig. 10: Dots on magenta blanket. Ghosting field M20Y20; tile field MY, 70 % 5. However, another effect must be added, namely as a result of rolling over the dots, a change to the adhesion of the remaining emulsion or to the deposits on the rubber blanket on the tile side. If this were not the case, ghosting, which only occurs after a certain time with M20-K40 for example, would be immediately apparent with larger K values, e.g. M20-K60. However, the evaluation of the prints shows that this is not the case. In all tile fields the start of ghosting becomes apparent with the same number of rollovers. The process of reducing the dot on the ghosting side, changing the tack forces and separating the web each reinforce one another. As rollover increases this leads to increasing ghosting. As a result Das Papier 2008 T61 6

7 Das Papier Science and Technology 6/2008 of the tensile force of the paper web, the paper web again comes in contact with the cylinder an effect also known as release doubling because, due to the deflection of the paper web, a misalignment occurs, so that a second image of the matrix dots occurs at the second contact. Because the paper web is drawn back slightly as a result of the deflection, this mackling mark precedes the dot. The paper web diverts the stresses (tack forces) very locally by means of a membrane tension status like a shell (Fig. 14). Only in exceptional cases, for example with very thick papers, is there any sign of a plate effect. Thus, it is not enough during ghosting only to consider the difference of the tack forces, but rather the balancing conditions on the paper element are to be formulated (Fig. 15). Why does the tack force increase as the number of rollovers increases, what parameters influence this and why is the increase on the reverse of the ghosting side of decisive importance? Fig. 11: Calculation of tack for a cylindrical ink drop 4 The maximum nip pressure is in area t1- t2 (line pressure approx. 10N/cm). The Hertz formulae for the contact between two elastic bodies show that the distribution of pressure is symmetrical to point t2. However, calculations in relation to the pressure pattern for a continuous layer of ink between two rollers show that the pressure in the ink layer reaches its highest point before contact point t2, after which negative pressure can actually occur. The longer the paper web is In Fig. 16 the proportions of the paper web as it passes through the nip have been exaggerated to focus on the key points. The diagram shows that the underside of the paper web is flush with the lower cylinder from contact point t1 to the central point connection of cylinder t2u. The time between t2u and t1 is about 10-3 seconds (paper web speed 10 m/sec). Much later, i.e. after contact point t2o, the upper side of the paper web comes in contact with the upper rubber cylinder. If the tack on the lower rubber cylinder is higher than that on the upper rubber cylinder, the web on the lower cylinder will remain in contact until the paper web tension forces separation. The time for which the underside of the paper web is in contact with the lower cylinder is much greater than the time for which the upper side of the paper web is in contact with the upper cylinder. Because of the pliable rubber blankets and the overlap of the rubber blankets of about 11/100 mm each, there is no line contact, but rather the rubber blankets are flattened. Thus, even when there is a major difference between the upper and lower tack on the upper cylinder, there is a sufficient transfer of ink in the area of the nip. June 24 June 26, 2008 ZELLCHEMING, Emilstrasse 21, Darmstadt, Germany Phone: , Fax: zellcheming@zellcheming.de Expo: Z E L L C H E M I N G, B R I D G I N G T H E G A P B E T W E E N R E S E A R C H A N D I N D U S T R Y eastofmain OFFENBACH 7 Das Papier 2008 T62

8 6/2008 Science and Technology Das Papier Fig. 12: Deflection of paper with symmetrical setting of cylinders due to tack Fig. 13: Deflection of paper with unsymmetrical setting of cylinders due to tack Das Papier 2008 T63 8

9 Das Papier Science and Technology 6/2008 Fig. 14: Load balance on paper. Plate or shell (membrane tension status) in contact with the rubber cylinder, the greater the chance that water and oil from the ink will penetrate the paper. Both are drawn into the paper by means of the capillary forces. Estimates of the penetration times for water and oil in the capillaries show that times < 10-3 seconds are reached. These times are of relevance for direct ghosting, and even more so for indirect ghosting. Thus, in terms of ink transfer, the underside of the paper web (cylinders in 1 o clock setting) is clearly preferred. the capillary action and porosity of the paper, in other words the dynamic penetration of water and ink in the complex paper structure, change the emulsion drop decisively. The relative areas of the paper and emulsion and, in the final analysis, that of the rubber blanket surface to which the deposits adhere must be in a particular ratio to one another for ghosting to occur (Fig. 17). The interplay of the inconsistent materials and variations involved in the printing process lead to many different interactions. For this reason it should be understood that it was possible to mini- As a result of the higher pressure and the greater depletion of the ink drop in terms of water and oil, the tack of the lower ink point will increase more rapidly. If the penetration of water or oil in the paper is impaired, the potential for ghosting will be increased. However, during analysis, the laterality of the paper and the ratio of water/oil penetration should be examined in detail. The tack of the ink (emulsion) and the topography and surface energies of the rubber blanket and paper surface influence the tack. More importantly, Fig. 15: Load balance on paper element 9 Das Papier 2008 T64

10 6/2008 Science and Technology Das Papier Fig. 16 mize ghosting in concrete cases through the choice of a different paper, rubber blanket or ink, although a general solution could not be found. The future will show whether detailed improvements in the materials and/or coordination of materials will enable ghosting to be prevented, or at least minimized, during printing. References Fig Piette, P.; Lafaye, J.F.: Mechanical ghosting on web offset presses. Taga Proceedings 1989, Loibl, D.; Pöller, M.; Betzler, F.: Negativaufbauen im Rollenoffsetverfahren. Fogra Forschungsbericht Nr München, März Reinius, H.; Rousu, S.; Marttila, J.; Nygard, S.; Rautkoski, H.; Pietila, H.: Mechanical ghosting in web offset MAIN variables. Ink Maker 2004, No. 6, Keiter, S.: Haftung und Aufnahme von Druckfarben auf gestrichenen Papieroberflächen. Diplomarbeit 1998, Universität Dortmund Das Papier 2008 T