Ghosting and vanishing dots in heatset printing

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11/03/13 ● www.sappi.com/houston

Ghosting and vanishing dots in heatset printing

Background

In recent years “Ghosting” and “Vanishing Dot” have become more and more prominent issues in Web Offset Heatset Printing. They come along with the increasing share of CTP plates and the general decrease of alcohol levels. It is a fundamental defect of the Heatset printing process and became an unacceptable commercial issue especially for high quality magazine jobs.

The Ghosting defect is characterised by the appearance of faint replicas of high coverage shapes from one side in half tone fields on the opposite side. In these areas the printed dots of one colour are printed smaller than 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). Ghosting appears always on that side of the blanket which the paper leaves last (closer to the dryer) if a high coverage image 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 in all shades. All printing units can be affected independently including the first unit (usually black).

Figure 1. Release angle defines ghosting appearance



Figure 2. Appearance of ghosting

Vanishing Dot is a constant decrease of the printed dot in the half tone areas as the printing process proceeds. All colours are reported to “vanish” although the most notorious are Magenta and Cyan. To the human eye, this is especially prominent in skin tones. Vanishing dots is most often seen on the opposite side of the web compared to ghosting – however sometimes seen also in the other side.

Figure 3. Appearance of vanishing dots



The only possibility to continue quality printing is to wash the blankets - this is causing press down time and waste (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 research

on both subjects. In this article a summary of those results is presented.

Mechanism

The underlying reason for halftone dot decrease in both ghosting and vanishing dots has been quite clearly proven to be a microscopic build-up between the dots. The build-up is approximately 10 microns thick and ingredients typical for ink and paper can be identified.

Figure 5. SEM pictures from blanket; build-up (left), clean blanket (right).



Figure 6. Cross-section of blanket with build-up

The hypotheses of the mechanism are shown on Figure 7 . As ink build-up between dots increase in thickness the halftone dot transfer from plate to blanket and from blanket to paper becomes more difficult.

plate

blanket

ink ink

accumulated

material

plate

blanket

ink ink

accumulated

material

blanket

ink ink

accumulated

material

Figure 7. Schematic picture of hypotheses on dot decrease

However the reason why build-up occurs or why it occurs locally in case of ghosting has not been fully explained. Different hypotheses have been presented related to ink/water balance, plate’s water carrying capacity, and blanket’s release properties.



Variables

In the following different printing variables are discussed including paper, ink, fountain solution, blanket and plates.

Paper has been very often accused as the main cause of these problems and it is true that frequency of both vanishing dots and ghosting seemed to increase in the late 90’s with the development of new types of high quality coated heatset papers.

However paper is also probably the most tested variable and so far no direct link to any of paper’s raw materials or characteristics has been found. Still with the following variables these problems seem to happen more 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’s

Ink and fountain solution has been also studied extensively as those are the main variables besides paper ink offset process. Most popular hypotheses on how the material build-ups between halftone dots is that ink particles are emulsified into fountain solution and carried onto non-image area of the blanket where they start to accumulate. Inks with clearly lower tack level have seen to decrease the rate of vanishing dots and ghosting. Components that increase lubrications and washing behaviour of the fountain solution has been seen 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 to another and especially from one press to another. That indicates that these are multivariable problems with interdependencies and thus difficult to solve.

Blanket is believed to be crucial especially for ghosting appearance. It is known that different blankets have different release properties and also the motif of the layout influences locally on web release. This can be seen 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 change from film-based plates to CTP happened approximately at the same period as ghosting and vanishing dots problems were seen more frequently. And as stated earlier one of the most popular hypotheses is connected to emulsification where printing plate also plays important role. CTP plates have different water carrying capacity than conventional plates. However as with earlier variables no single parameter with plates has been found to effectively decrease the problems. This also supports multi-variable hypotheses.

Summary

Vanishing dots and ghosting are probably most difficult printability problem is heatset offset that has been seen in last 20 years. A lot of research has been done to solve this issue by different institutes as well as cross-industrial groups. It is clearly a phenomenon that is linked closely to the basic fundaments of heatset offset printing with multi-variable nature. Its fluctuating nature makes the studies of different variables extremely difficult.

However Sappi is still taking part on efforts to solve this vexing problem via research projects with cross-industry partners.

Ghosting and vanishing dots in heatset printing

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