6/2008 Science and Technology Das Papier Das Papier 2008–T www.ipwonl<strong>in</strong>e.de
These real examples of ghost<strong>in</strong>g provoke the question: which <strong>in</strong>k on the front (ghost<strong>in</strong>g side) is actually <strong>in</strong>fluenced by which <strong>in</strong>k on the reverse? Obviously it cannot be assumed that the ghost<strong>in</strong>g effect only occurs between two identical colours, i. e. with<strong>in</strong> a s<strong>in</strong>gle pr<strong>in</strong>t unit (Fig. 1). We must assume that direct ghost<strong>in</strong>g (with<strong>in</strong> the same pr<strong>in</strong>t unit) and <strong>in</strong>direct or consecutive ghost<strong>in</strong>g (consecutive pr<strong>in</strong>t units are affected) exist as shown <strong>in</strong> Fig. 6. Which comb<strong>in</strong>ations of colour on the front and back produce ghost<strong>in</strong>g? The comb<strong>in</strong>ations for direct ghost<strong>in</strong>g are blackblack, cyancyan, magentamagenta and yellowyellow. There are many more comb<strong>in</strong>ations for <strong>in</strong>direct ghost<strong>in</strong>g. Fig. 7 demonstrates a number of comb<strong>in</strong>ations. The figures are <strong>in</strong>tended to expla<strong>in</strong> how an <strong>in</strong>k applied <strong>in</strong> a pr<strong>in</strong>t unit (e. g. magenta <strong>in</strong> pr<strong>in</strong>t unit 3) passes through the follow<strong>in</strong>g pr<strong>in</strong>t units by means of the paper web. When it comes to <strong>in</strong>direct ghost<strong>in</strong>g, the question is whether a subsequent colour, e. g. the cyan on the reverse, can <strong>in</strong>fluence the black on the ghost<strong>in</strong>g side (tack force difference FSC <strong>in</strong> Fig. 7) or is it rather the colour previously applied to the reverse side, black, that <strong>in</strong>fluences the subsequent ghost<strong>in</strong>g colour, cyan (tack force difference FCS <strong>in</strong> Fig. 7). Is an <strong>in</strong>fluence only relevant over one colour <strong>in</strong>terval (subsequent pr<strong>in</strong>t unit only) or can magenta also be <strong>in</strong>fluenced by black and what happens when the colours are overpr<strong>in</strong>ted? To answer these questions, we shall consider the test form already used <strong>in</strong> 1 , Fig. 8, <strong>in</strong> which different colours were comb<strong>in</strong>ed for the ghost<strong>in</strong>g side and the reverse side. Fig. 9 exhibits the rubber blankets for the ghost<strong>in</strong>g side belong<strong>in</strong>g to the test pr<strong>in</strong>t copy. The black rubber blanket presents the ghost effects <strong>in</strong> all colour comb<strong>in</strong>ations that conta<strong>in</strong> black, while the cyan rubber blanket shows the ghost<strong>in</strong>g effects <strong>in</strong> all colour comb<strong>in</strong>ations conta<strong>in</strong><strong>in</strong>g black and cyan. The magenta rubber blanket Das Papier Science and Technology evidences ghost<strong>in</strong>g effects for all colour comb<strong>in</strong>ations that conta<strong>in</strong> black, cyan and magenta. Interest<strong>in</strong>gly, all rubber blankets feature <strong>in</strong>creased ghost<strong>in</strong>g effects <strong>in</strong> the colour comb<strong>in</strong>ations that feature colours that precede their own colours. For example, <strong>in</strong> the case of the cyan rubber blanket, the CMYK and MYK colour comb<strong>in</strong>ations (K before C) feature greater ghost<strong>in</strong>g than the colour comb<strong>in</strong>ation CMY, even at much lower tonal values. In the case of the magenta rubber blanket (Fig. 10), the comb<strong>in</strong>ations K40C and K40M (M or C after K!) do not feature <strong>in</strong>direct ghost<strong>in</strong>g, but C40K and M40K certa<strong>in</strong>ly do (K before M or C!). Dur<strong>in</strong>g <strong>in</strong>direct ghost<strong>in</strong>g, the <strong>in</strong>ks on the upper and lower rubber cyl<strong>in</strong>ders do not connect with the paper at the same time (as is the case with direct ghost<strong>in</strong>g) but rather the already partially penetrated <strong>in</strong>k applied to the underside of the paper web comes <strong>in</strong> contact with the fresh <strong>in</strong>k of the upper rubber cyl<strong>in</strong>der. At a pr<strong>in</strong>t<strong>in</strong>g speed of about 8 m/s, the previous <strong>in</strong>k (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 rema<strong>in</strong> tacky for longer if a larger amount of <strong>in</strong>k is applied, lead<strong>in</strong>g to <strong>in</strong>creased tack<strong>in</strong>ess. However, it is precisely these differences <strong>in</strong> tack that are the decisive reason for ghost<strong>in</strong>g. To summarize: A subsequent colour (e. g. magenta) cannot <strong>in</strong>fluence the previous colour on the other side (e. g. black), but a previous colour (e.g. black) can <strong>in</strong>fluence the subsequent colour (e. g. magenta), Fig. 10. The degree of this pr<strong>in</strong>t<strong>in</strong>g error ma<strong>in</strong>ly depends on the type of <strong>in</strong>k and, <strong>in</strong> particular, on the ability of the paper to absorb water and oil. However the tendency rema<strong>in</strong>s basically unchanged. Fig. 11 presents the <strong>in</strong>fluence of twosidedness of paper. After the motif has been pr<strong>in</strong>ted on side 1, the paper web was turned and pr<strong>in</strong>ted on side 2. This example ev<strong>in</strong>ces that even if it was not possible to elim<strong>in</strong>ate ghost<strong>in</strong>g, a marked difference could be achieved. 6/2008 Is the pr<strong>in</strong>t<strong>in</strong>g error – the lighten<strong>in</strong>g on the ghost<strong>in</strong>g side – caused by the colour comb<strong>in</strong>ation and thus by the tack forces always the same or can a page even be darkened? To exam<strong>in</strong>e this issue <strong>in</strong> quantitative terms, the lab values (Fig. 12) were measured <strong>in</strong> the ghost<strong>in</strong>g areas. Figs. 13 and 14 present the L/L0values, (L0 =L of 20 %) for different comb<strong>in</strong>ations of the upper and lower colour comb<strong>in</strong>ation. Fig. 13 shows that ghost<strong>in</strong>g only occurs with a ratio of tonal value tile / tonal value ghost<strong>in</strong>g side = 2. In this example, direct ghost<strong>in</strong>g only occurs <strong>in</strong> magenta. The upper images <strong>in</strong> Fig. 14 po<strong>in</strong>t out the <strong>in</strong>fluence of the preced<strong>in</strong>g and subsequent colours. In the case of the paper type exam<strong>in</strong>ed here, black on the tile side has the greatest <strong>in</strong>fluence on magenta, followed by magenta itself on magenta. The other comb<strong>in</strong>ations do not lead to lighten<strong>in</strong>g, i. e. ghost<strong>in</strong>g, but rather cause darken<strong>in</strong>g! Fig. 15 aga<strong>in</strong> clearly demonstrates that subsequent colours do not cause preced<strong>in</strong>g colours to ghost, but also <strong>in</strong>dicates that previous colours can have no effect if the subsequent colour is applied on both sides. The preced<strong>in</strong>g colour has sealed the tile side like a film, thus creat<strong>in</strong>g different conditions for the subsequent tack. Once aga<strong>in</strong>, these curves are valid for the <strong>in</strong>k and paper used. If both are changed, the quantity of the result<strong>in</strong>g ghost<strong>in</strong>g also changes, although the described work<strong>in</strong>g mechanisms rema<strong>in</strong> valid. In order to further exam<strong>in</strong>e these work<strong>in</strong>g mechanisms, the <strong>in</strong>put and output angle of the web was changed <strong>in</strong> the magenta pr<strong>in</strong>t unit by <strong>in</strong>stall<strong>in</strong>g deflector rollers, Fig. 16. In variant V1 the <strong>in</strong>put and output of the web is unchanged, while <strong>in</strong> variant V2 the <strong>in</strong>put of the paper web has been changed <strong>in</strong> such a way that the paper web runs symmetrically. In contrast, <strong>in</strong> variants V3 and V4 the output web was diverted either to run out symmetrically (V3) or to be compulsorily guided on the lower cyl<strong>in</strong>der (V4). 5 Das Papier 2008–T