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| In the third part of the series on moire effects, Professor Stephen Abbott of Autotype international, looks at the subject of moire look -alikes |
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Steven Abott-
Director Research & Technical Autotype International Ltd.,UK
Steven Abott recieved his Oxford PhD in Chemistry for work carried out at Havard. After a Post-Doctoral position in Strasburg he went to work at ICI on new product developments in electronics, imaging & data storage business. As research & technical director of autotype he has been responsible for ensuring a constant stream of new product, & also for providing the Science behind the coating and printing techniques used. A lot of this science has come from a fruitful partnership with professor Phil Gaskels team at University of Leeds where Steven is visiting Professor. He is a frequent speaker at International confrences where he is passionet About the need to apply good science to coating & printing
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In the previous two articles we discussed
how moiré effects are created in screen
printing, what causes them to occur and
some proven recipes for success.
Unfortunately, anything that looks like an
unwanted regular pattern in part of a print
becomes known as a moiré effect; many of
them, however, are not. These pseudo moirés
can still cause considerable problems in the
screen print process and it is therefore
important to be able to recognise and cure
them.
Skipping
Skipping can be identified by unpleasant dotty
areas, which can usually be seen in darker
regions of a print. In particular, the effect is
often found when squeegee pressure is backed
off to avoid flooding in darker areas.
Skipping is caused as a result of the dots
being too high when printing one dot on top of
another and/or insufficient squeegee pressure.
Under a loupe or microscope, skipping can be
identified by halftone dots that are normally
broken up into several smaller dots; alternatively, the dots may be much smaller than
expected and can often be located in the valleys
that form between previously printed dots.
Stracking Transfer
The second pseudo moiré effect is known as
stacking transfer and is extremely difficult to
spot. It resembles a normal moiré but comes
and goes from print to print, making it hard to
pin down and resolve unless you know
specifically what you are looking for.
To identify stacking transfer, you need to
look at the regular patterns formed by the real
image dots; if you can then see extra dots
between the real ones this is stacking transfer.
It may seem odd to look for dots that should
not be present, but once you have adjusted
your approach it becomes relatively easy to identify this particular pseudo moiré effect.
Stacking transfer is essentially caused by
excess ink going underneath the stencil when
printing on top of previous dots; the stencil
will then transfer this excess ink onto the next
print. If there is perfect registration between
prints, this will not cause a problem; however,
minor changes in registration will result in extra
unwanted dots that appear as a moiré-like
pattern.
Switching to a low EOM, low Rz stencil will
overcome the problems of stacking transfer
without creating skipping. Once again,
although the use of an emulsion will have the
desired affect it is an unnecessarily difficult
and expensive step; a better option is to use a
specialised film that is specially designed to
prevent skipping and stacking transfer, and to
consider the use of a solvent or water based
UV ink. |
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Postscript Banding
Postscript banding is only found in postscript
vignettes and occurs when you try and achieve
a high lpi screen from a low dpi imagesetter.
Unfortunately, the basic postscript can only
deliver a limited number of different grey levels,
so instead of a smooth vignette being
produced, distinct bands are formed.
This problem is extremely straightforward to
resolve by using a higher dpi imagesetter that
is designed to produce more grey levels;
alternatively, use a higher performance RIP that
can produce grey levels more effectively from a
given lpi/dpi combination or, as a final option,
reduce your lpi, although this will reduce image
quality.
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Mesh Patteming in solids
A further phenomenon is mesh patterning.
Although this is not a moiré or pseudo moiré
effect it nevertheless creates a similar type of
unwanted pattern. It can, however, be cured
very simply by using thinner thread diameters
and lower viscosity inks; note that if lower
viscosity inks are used, a lower Rz stencil will
be required. As a low EOM is always required
for high quality printing, a specialised film such
as Capillex CP should, once again, be used.
Pseudo moire effects are an interesting and surprisingly common phenomenon. They can, however, be easily identified and with a little care and planning be eliminated, helping to produce better quality prints.
Unstable Rosettes
The final pseudo moiré effect is that of
unstable rosettes. These generally occur in
four colour printing and are
caused when the registration
between colours is poor,
causing what should be strong
classical rosettes patterns to
become unfocussed or
unstable, resulting in a poor
quality print. The effect can
clearly be seen by inspection
using a loupe.
The difference between an
unstable and a classical
rosette is actually extremely
small in terms of moiré
mathematics; in other words
the effect can occur as the
direct result of small
fluctuations in process or setup conditions.
To eliminate the problem, it is important to
ensure that you have the best possible
registration and to use a RIP with an option for
stable rosettes, so that the potential for
incorrect registration is eliminated. Other
solutions include those previously outlined,
such as using thinner inks and producing high
quality dots using a low EOM, low Rz film.
One further point that is worth noting is
that the term topographic moiré is often used to
describe moiré effects. Interestingly, there is no
such phenomenon as a topographic moiré! The
closest to this supposed effect is the stacking
pseudo moiré discussed above.
As moiré effects occur as the result of
interference between regular patterns the effect
will be there regardless of whether those
patterns are topographic or perfectly flat.
Although surfaces with rough topographies
may show more visible moiré than their
smoother counterparts, assuming there is an
underlying moiré pattern in the first place, they
can be more correctly described as
topographically amplified moiré patterns.
Pseudo moiré effects are an interesting and surprisingly common phenomenon. They can, however, be easily identified and with a little care and planning be eliminated, helping to produce better quality prints while reducing downtime, wastage and costs. |
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