Phantom – ghost – the image of something that does not exist in reality.
The word “ghost” in the printing is defined as the appearance of a parasitic image, inexplicable from the standpoint of a particular printing individual, which occurs on the printed impression during printing.
The appearance of printing “ghosts” is periodically observed, both in web and sheet printing. The shape and look of the “ghosts” are quite varied. To combat this “off-world phenomenon”, the chief technologist department of Pushkinskaya Ploschad, together with a number of leading European laboratories and research institutes has been collecting and will collect the information on this issue and analyze the reasons for the ghost images. Thus far, four types of ghosts have been described and investigated.
The ghost images were classified; each class of ghosts has been given a scientific definition and the mechanism of their formation has been described.
GHOSTING
DOUBLING
CHEMICAL GHOSTING
PHANTOMING
Only three of them were under scientific research ghosting, doubling и chemical ghosting
Chemical ghosting (CG) was the first to be investigated. In foreign publications, this type of ghosts is often called “chemical ghosting”, which can be translated as a chemical ghost.
CG means that a mirror image, graphic objects printed on the front side with metallic or fluorescent pantone ink appear on the reverse side, as well as the ghost images of dense colorful areas with high total ink values. The “ghost” image is formed due to local reticulation (cracking) of the ink layer surface, resulting in a visual change in the ink surface gloss.
CG appears on the back side of covers, front side of which is varnished with WD and/or UV varnish. If the printed sheet is pulled out of the press immediately after printing, the effect is almost imperceptible. The effect occurs in the pile through sometimes quite long time (from 30 minutes to 3 hours).
The problem arises due to the interaction of the desiccant components of the sheet ink with some components of the paper. In the process of this chemical interaction, the free radical compounds are generated, and they catalyze the oxidative polymerization. Under this influence, the speed of ink fixing multiplies, which increases the internal tension of the ink layer and results in the formation of cracks on the ink layer surface. The deformation is so great that in some cases there is the formation of a well-marked terrain, similar to hot stamping.
There are other theories explaining the formation of CG in which a major role in accelerating of ink fixing is assigned to certain components of ink or paper, but the general conclusion of all researchers is that it occurs due to the interaction of the paper components and ink fixing products through close contact of the front and back sides of the sealed paper in the pile. (Learn more about the mechanism of CG formation in the article by Uwe Berthold “Ghosts are no longer a mystery” PrintCom Russia No.19/20-2008).
One of the features of this type of ghost is that it is practically impossible to measure and record it with instruments. Neither the glossmeter, nor the densitometric or spectrometric instruments detect it by quantity, making it difficult for research. For several years, this phenomenon has not been observed. It can be corrected with more WD-varnishing of the ghosted impression surface, which smoothes the gloss, and the ghosts disappear.
Tonal ghosts – are the images on the ink screen tints, formed by the local slight change of the tone on the relatively flat halftone fills, resulting from the local changes in the tonal grow of the screen dots.
The main distinguishing feature of the tonal ghosts compared to mechanical ghosts is that they are not a reflection of graphical elements on the reverse side of the paper web, and they have a unique shape depending only on the configuration and mutual arrangement of the pull-off forces between the paper web and the rubber.
The tonal ghosts often appear on the pages that are closer to the edge of the paper web.
In most cases the tonal ghosts are geometrically unstable, i.e. they change their shape during the printing process. According to the theory, the ghost is formed by double-tapping of the paper web to the offset rubber on the cylinder displaced during the printing.
This occurs at a certain localization of the forces of ink layers separation along the offset blanket. As a result, the appearance of these ghosts depends on the rheological properties of the ink layers (ink thickness, adhesiveness and viscosity) and on the configuration of the contact areas on the rubber. Accordingly, ghosting can be reduced by increasing the tension of the paper web and/or reducing the adhesiveness of the ink. For example, by adding the supply of the fountain solution.
Tonal ghosts are seen also when printing on the sheet-fed presses, where the cases of double-tapping of the offset blanket by the paper are also recorded. However, the reason for double-tapping is different from the web-fed presses. In the sheet-fed printing, it occurs on the thin paper due to entanglement of the tail section of the paper sheet during its approach to the contact area.
Mechanical ghosting – is a negative mirror image of graphical objects on the reverse side of the paper web, appearing on plates, reproduced by screen dots with percentage in the range of 15-80%.
UPPER SECTION
LOWER SECTION
Mechanical ghosting
The below data are provided by Sappi, a leading global manufacturer of paper.
In 2009, the chief technologist department of Pushkinskaya Ploschad has contacted Sappi to conduct an analysis of the deposits formed on the surface of the offset rubber during printing. The samples of scrapes and pieces of spent rubber-fabric blanket with the typical deposits formed at printing on coated paper were transferred.
As a result, a report in the form of two presentations with very vague conclusions was drawn up. All samples in the correct scotch tests and rubber scrapes showed the presence of ink particles and coated layer (calcium and clay).
The main result of the research conducted by Sappi was the scientific evidence that whitish overlay on the offset rubber was mainly composed of calcium deposits and clay (kaolin), i.e. the particles of the coated layer. This conclusion was confirmed by the pictures of blushing on the offset rubber taken at 320-fold magnification.
The picture clearly shows that the calcium deposits are formed on the spaces around the screen dots and the thickness of deposits is significantly greater than the thickness of the ink layer. Naturally, this leads to an increase in pressure between the offset cylinders of the upper and lower sections and, consequently, to an increase in dot gain on the reverse side in the areas of calcium deposits.
Analysis of elements
Rubber surface topography
Dynamics of the ghost formation that is evident due to a gradual increase in the intensity of the ghost image after cleaning the offset blanket.
Presence of the white low-soluble stubborn deposits on the rubber
The formation of the mechanical ghost and the degree of its manifestation are not affected by the blanket tension or the rheological properties of the ink. For example, an increase in supply of fountain solution only increases the degree and promptness of appearance of calcium ghosting.
These ghosts are formed only on coated paper
THE MAIN AND THE ONLY METHOD OF DEALING WITH THE CALCIUM GHOST IS ADDING THE ANTI-PEELING ADDITIVE IN THE FOUNTAIN SOLUTION
FORMATION OF THE IMAGES OF REAL OBJECTS – LETTERS, SILHOUETTES, ETC., ON THE BACKGROUND SCREEN TINTS,
their formation cannot be justified by logic or scientific explanation.
In my practice, there were only two cases of manifestations of real ghost images. Both cases have been on the M600 Harris printing press. On April 30, 2006, during printing a luxury magazine on paper Galerie Fine, a shadow of a woman on the wall appeared in the corner of the spa hotel advertisement. The printing workers stopped the press and checked the files and forms. No shadow of a woman was found.
Just to be on the safe side, they changed the whole set of the forms. Nothing changed. They tried to vary the paper tension and changed the rubber on all sections. The shadow did not disappear, moreover, in spite of all actions performed it remained in the same place, the shape did not change even slightly. The recheck of all digital information on this page, films, flats, and forms did not reveal the presence of the shadow on the wall. After a day of “shadow fighting”, it was decided to rearrange the order to another, exactly the same printing press, and if it does not disappear, print as it is. The shadow disappeared!
The second incident took place about a year later, on the same web press. The shadow of the letter “B” appeared on the background tint of the advertisement of a prestigious luxury foreign car. As in the previous case, no reason for its occurrence has been detected. But this time, the workers already knew how to solve the problem, and they immediately placed the forms on the other press standing next. The ghost did not appear. Such ghosts did not appear anymore.
According to experts, ghosting is still considered a mysterious phenomenon in the heatset web offset printing.
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