Ghosting and vanishing dots in heatset printing


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Ghosting and vanishing dots in heatset printing

  1. 1. Ghosting and vanishing dots inheatset printingBackgroundIn recent years “Ghosting” and “Vanishing Dot” have become more and more prominent issues in WebOffset Heatset Printing. They come along with the increasing share of CTP plates and the general decreaseof alcohol levels. It is a fundamental defect of the Heatset printing process and became an unacceptablecommercial issue especially for high quality magazine jobs.The Ghosting defect is characterised by the appearance of faint replicas of high coverage shapes from oneside in half tone fields on the opposite side. In these areas the printed dots of one colour are printed smallerthan they are copied onto the plate. This effect is observed on presses that have an angle different from 90°between the connecting line of the centres of both blanket cylinders and the paper web (Figure 1). Ghostingappears always on that side of the blanket which the paper leaves last (closer to the dryer) if a high coverageimage is located on the opposite side (Figure 2). As the name implies, this printing defect seems to appear,disappear and reappear without any obvious reason or change in printing conditions. Ghosting can appear inall shades. All printing units can be affected independently including the first unit (usually black). Figure 1. Release angle defines ghosting appearance The digital universe of paper and printing knowledge 11/03/13 ●
  2. 2. Figure 2. Appearance of ghostingVanishing Dot is a constant decrease of the printed dot in the half tone areas as the printing processproceeds. All colours are reported to “vanish” although the most notorious are Magenta and Cyan. To thehuman eye, this is especially prominent in skin tones. Vanishing dots is most often seen on the opposite sideof the web compared to ghosting – however sometimes seen also in the other side. Figure 3. Appearance of vanishing dots The digital universe of paper and printing knowledge 11/03/13 ●
  3. 3. The only possibility to continue quality printing is to wash the blankets - this is causing press down time andwaste (see Figure 4 as an example how washing cycle influences dot-%). Register moving devices like Auto-Cycling systems are known to extend the washing intervals but they do not cure the problem. Figure 4. Example of CLCC data (washing is marked with black bar)Over the years Sappi has done by itself and on different cross-industrial working groups extensive researchon both subjects. In this article a summary of those results is presented.MechanismThe underlying reason for halftone dot decrease in both ghosting and vanishing dots has been quite clearlyproven to be a microscopic build-up between the dots. The build-up is approximately 10 microns thick andingredients typical for ink and paper can be identified. Figure 5. SEM pictures from blanket; build-up (left), clean blanket (right). The digital universe of paper and printing knowledge 11/03/13 ●
  4. 4. Figure 6. Cross-section of blanket with build-upThe hypotheses of the mechanism are shown on Figure 7 . As ink build-up between dots increase inthickness the halftone dot transfer from plate to blanket and from blanket to paper becomes more difficult. plate ink ink blanket accumulated material Figure 7. Schematic picture of hypotheses on dot decreaseHowever the reason why build-up occurs or why it occurs locally in case of ghosting has not been fullyexplained. Different hypotheses have been presented related to ink/water balance, plate’s water carryingcapacity, and blanket’s release properties. The digital universe of paper and printing knowledge 11/03/13 ●
  5. 5. VariablesIn the following different printing variables are discussed including paper, ink, fountain solution, blanket andplates.Paper has been very often accused as the main cause of these problems and it is true that frequency ofboth vanishing dots and ghosting seemed to increase in the late 90’s with the development of new types ofhigh quality coated heatset papers.However paper is also probably the most tested variable and so far no direct link to any of paper’s rawmaterials or characteristics has been found. Still with the following variables these problems seem to happenmore often. Usually seen with high quality LWS or MWC papers with grammage range from 60-90 gsm (especially ghosting – vanishing dots seen also with higher grammages) Ghosting or vanishing dots are not usually seen with very rough papers (matt grades with PPS >4)On the other hand both phenomena have been seen with following variables: Different coating formulations from pure calcium carbonate to pure clay Different ink setting speed or fountain solution absorption Different stiffness’sInk and fountain solution has been also studied extensively as those are the main variables besides paperink offset process. Most popular hypotheses on how the material build-ups between halftone dots is that inkparticles are emulsified into fountain solution and carried onto non-image area of the blanket where they startto accumulate. Inks with clearly lower tack level have seen to decrease the rate of vanishing dots andghosting. Components that increase lubrications and washing behaviour of the fountain solution has beenseen also to decrease the rate of the ghosting and vanishing dots.Typical for these variables as for others also is that it is very difficult to get consistent results from one trial toanother and especially from one press to another. That indicates that these are multivariable problems withinterdependencies and thus difficult to solve.Blanket is believed to be crucial especially for ghosting appearance. It is known that different blankets havedifferent release properties and also the motif of the layout influences locally on web release. This can beseen clearly on the pictures taken during printing (Figure 8). Figure 8. Photos taken during printing showing release pattern; pilot-scale heatset press (left), commercial heatset press (right) The digital universe of paper and printing knowledge 11/03/13 ●
  6. 6. The change from film-based plates to CTP happened approximately at the same period as ghosting andvanishing dots problems were seen more frequently. And as stated earlier one of the most popularhypotheses is connected to emulsification where printing plate also plays important role. CTP plates havedifferent water carrying capacity than conventional plates. However as with earlier variables no singleparameter with plates has been found to effectively decrease the problems. This also supports multi-variablehypotheses.SummaryVanishing dots and ghosting are probably most difficult printability problem is heatset offset that has beenseen in last 20 years. A lot of research has been done to solve this issue by different institutes as well ascross-industrial groups. It is clearly a phenomenon that is linked closely to the basic fundaments of heatsetoffset printing with multi-variable nature. Its fluctuating nature makes the studies of different variablesextremely difficult.However Sappi is still taking part on efforts to solve this vexing problem via research projects with cross-industry partners. The digital universe of paper and printing knowledge 11/03/13 ●