Ghosting in Heatset Web Offset Printing (Part II)

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1 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 *correspondence address: gerd.meder@bertelsmann.de 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 (Part II) The ghosting printing problem in heatset web offset printing is understood as the reduction of the tonal values of the printout on one side, caused by a motif or colour combination pattern on the reverse. This is clearly not just a case of the image on the reverse showing through or the penetration of oil or water from the ink on the reverse. Reference 1 presented a model for the occurrence of ghosting. Ghosting was explained as interactions during the separation of the paper web from the rubber cylinder (Fig. 1) as a result of different 56

2 Das Papier Das Papier Summary Ghosting in Heatset Web Offset Printing The ghosting printing problem will be examined on the basis of test samples and a test form (sample printouts and rubber blankets). It tack forces on the front and reverse and the consequent continuous reduction in dot size. This reduction in dot size leads to a reduction in the tonal values and is the reason for lightening. Ghosting, which only occurs after a certain number of rollovers and which largely depends on the materials used paper, ink, fountain solution, rubber blanket is controlled by the tack forces and thus by the colour combination on the two sides. will be shown that the study of the deposits on the rubber blankets and the comparison with the deposits typical of the offset technique bring about a greater understanding of the phenomenon. The influence of the colour combinations on the two sides (front and reverse of the printed page) on ghosting will be examined in greater detail here. First let us look at a number of examples of ghosting in practice. Fig. 2 displays a print sample in which only the colour black was printed on both sides. The contour and darker area around the number on the reverse side can be clearly seen. The darker area is the one that is not affected by the number and ghosting is caused by the black reverse side around the number. Fig. 3 gives an example in which the red letter P on the reverse can be clearly seen as a bright image in the orange area (CMYK tonal values of the print plate specified in the image). Clearly the magenta on the reverse side has influenced the magenta on the front. Fig. 4 shows a grey area (CMYK, 30 % each) whose reverse produces strong ghosting (in the area of the hair) with equally high tonal values in all colours. Which colour on the reverse causes the ghosting? Fig. 5 presents a cyan yellow area on the ghosting side which is influenced by the colour combination on the reverse. The tonal values for all colours entered in the images show that ghosting occurs in points 1 and 3, in other words where the colour on the reverse is black

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5 These real examples of ghosting provoke the question: which ink on the front (ghosting side) is actually influenced by which ink on the reverse? Obviously it cannot be assumed that the ghosting effect only occurs between two identical colours, i. e. within a single print unit (Fig. 1). We must assume that direct ghosting (within the same print unit) and indirect or consecutive ghosting (consecutive print units are affected) exist as shown in Fig. 6. Which combinations of colour on the front and back produce ghosting? The combinations for direct ghosting are black-black, cyan-cyan, magenta-magenta and yellow-yellow. There are many more combinations for indirect ghosting. Fig. 7 demonstrates a number of combinations. The figures are intended to explain how an ink applied in a print unit (e. g. magenta in print unit 3) passes through the following print units by means of the paper web. When it comes to indirect ghosting, the question is whether a subsequent colour, e. g. the cyan on the reverse, can influence the black on the ghosting side (tack force difference FS-C in Fig. 7) or is it rather the colour previously applied to the reverse side, black, that influences the subsequent ghosting colour, cyan (tack force difference FC-S in Fig. 7). Is an influence only relevant over one colour interval (subsequent print unit only) or can magenta also be influenced by black and what happens when the colours are overprinted? To answer these questions, we shall consider the test form already used in 1, Fig. 8, in which different colours were combined for the ghosting side and the reverse side. Fig. 9 exhibits the rubber blankets for the ghosting side belonging to the test print copy. The black rubber blanket presents the ghost effects in all colour combinations that contain black, while the cyan rubber blanket shows the ghosting effects in all colour combinations containing black and cyan. The magenta rubber blanket evidences ghosting effects for all colour combinations that contain black, cyan and magenta. Interestingly, all rubber blankets feature increased ghosting effects in the colour combinations that feature colours that precede their own colours. For example, in the case of the cyan rubber blanket, the CMYK and MYK colour combinations (K before C) feature greater ghosting than the colour combination CMY, even at much lower tonal values. In the case of the magenta rubber blanket (Fig. 10), the combinations K40-C and K40-M (M or C after K!) do not feature indirect ghosting, but C40-K and M40-K certainly do (K before M or C!). During indirect ghosting, the inks on the upper and lower rubber cylinders do not connect with the paper at the same time (as is the case with direct ghosting) but rather the already partially penetrated ink applied to the underside of the paper web comes in contact with the fresh ink of the upper rubber cylinder. At a printing speed of about 8 m/s, the previous ink (the emulsion) had about 0.1 seconds to transfer oil and water to the paper. Thus, this dot will become tacky at an earlier stage or will remain tacky for longer if a larger amount of ink is applied, leading to increased tackiness. However, it is precisely these differences in tack that are the decisive reason for ghosting. To summarize: A subsequent colour (e. g. magenta) cannot influence the previous colour on the other side (e. g. black), but a previous colour (e.g. black) can influence the subsequent colour (e. g. magenta), Fig. 10. The degree of this printing error mainly depends on the type of ink and, in particular, on the ability of the paper to absorb water and oil. However the tendency remains basically unchanged. Fig. 11 presents the influence of twosidedness of paper. After the motif has been printed on side 1, the paper web was turned and printed on side 2. This example evinces that even if it was not possible to eliminate ghosting, a marked difference could be achieved. Is the printing error the lightening on the ghosting side caused by the colour combination and thus by the tack forces always the same or can a page even be darkened? To examine this issue in quantitative terms, the lab values (Fig. 12) were measured in the ghosting areas. Figs. 13 and 14 present the L/L0-values, (L0 =L of 20 %) for different combinations of the upper and lower colour combination. Fig. 13 shows that ghosting only occurs with a ratio of tonal value tile / tonal value ghosting side = 2. In this example, direct ghosting only occurs in magenta. The upper images in Fig. 14 point out the influence of the preceding and subsequent colours. In the case of the paper type examined here, black on the tile side has the greatest influence on magenta, followed by magenta itself on magenta. The other combinations do not lead to lightening, i. e. ghosting, but rather cause darkening! Fig. 15 again clearly demonstrates that subsequent colours do not cause preceding colours to ghost, but also indicates that previous colours can have no effect if the subsequent colour is applied on both sides. The preceding colour has sealed the tile side like a film, thus creating different conditions for the subsequent tack. Once again, these curves are valid for the ink and paper used. If both are changed, the quantity of the resulting ghosting also changes, although the described working mechanisms remain valid. In order to further examine these working mechanisms, the input and output angle of the web was changed in the magenta print unit by installing deflector rollers, Fig. 16. In variant V1 the input and output of the web is unchanged, while in variant V2 the input of the paper web has been changed in such a way that the paper web runs symmetrically. In contrast, in variants V3 and V4 the output web was diverted either to run out symmetrically (V3) or to be compulsorily guided on the lower cylinder (V4). 5

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10 Das Papier Both in direct magenta-magenta ghosting and in indirect magenta-black ghosting the curves in Fig. 16 show that, because of the compulsory guidance of the output web, the separation of the web from the upper cylinder is no longer controlled by changing tack forces the ink transfer and thus the tonal values of the ghosting side remain constant over the entire tonal value range. With direct ghosting, the paper web is separated and the ink transfer is disrupted in V1 and V2 with a tonal value ratio of (80/40=)2. With indirect ghosting the error occurs even with a lower tonal value ratio because of the penetration of the black ink in the paper and the consequent increase in tack. Let s take a closer look at the dots of the three colour combinations marked in Fig. 8 for the four variants. Fig. 17, colour combination M20-M50, shows the already familiar pattern whereby dot M20 within the tile area (M50) is smaller than the dot outside. In Figs. 18 and 19 these dots were compared with those of variants V2, V3, V4. In the case of variant 2, the dots inside and outside the tile are largest and are almost the same size. Because the paper web feed is symmetrical, the penetration pattern on both sides is the same and the tack ratio does not change. In the case of variants 3 and 4, the dots are also of the same size, but much smaller than is the case with V1, V2. The transferred dots have extremely sharp edges thanks to the compulsory guidance of the paper web. In relation to the deposits in the ghosting area, the sharp separation means that the penetration of oil and water into the paper is minimal, the deposits on the rubber blanket increase and narrow the dot. In V3 and V4, a slower build-up is expected because of the long contact time and thus dots form that are less sharpedged. It is clear that V1 has the dots with the sharpest edges and V2 the dots with the darker core. Figs display the behaviour pattern of the dots for the colour combination M40-K50. With this indirect ghosting, the dots in V1 and V2 inside the tile area are much smaller than those outside the tile. The black ink (Fig. 22) has already penetrated and can build up a marked difference in tack from magenta in the magenta print unit. The compulsory guidance of the web at the outlet in V3, but in particular in V4, prevents ghosting here. Finally, let us consider Figs , the colour combination K20-K70. The paper input and output were only varied in the magenta print unit and the black print unit is completely unchanged. All variants feature almost no difference in the dot values; variant 2 features a less precise printout, while variant 4 has the printout with the sharpest edges. If, following these explanations, we look back at Figs. 2-6, it becomes clear that ghosting samples 1 and 2 are clearly direct ghosting, namely black-black (Fig. 2) or magenta-magenta (Fig. 3). In ghosting example 3, the measurement of the lab value indicates that the grey tone has moved into the red and indirect ghosting exists through black and cyan. Finally, ghosting example 4 also shows indirect ghosting black influences the cyan on the ghosting side. References 1 Meder, G.; Busse, W.; Coesfeld, M.; Frank, E.; Gombart, A.; van Hesteren, J.; Lawrenz, D.; Naydowski, C.; Resch, P.; Schölkopf, J.: Ghosting im Rollenoffsetdruck. Fogra-Tagung München, November 2007, Druckfarbe und Papier 10

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