Ghosting in Heatset Web Offset Printing (Part II) - Zellcheming

6/2008 Science and Technology 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.
Das Papier 2008–T57
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
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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.