1 - The Phenomena of Moire
Very often, when one speaks about
halftone printings in screenprinting, "coarse or fine haltones” one of the terms
which is going to return often in the
conversation will be that of the "moiré".
In some cases, one could almost believe
that it is really the only source of
problems for us!…
"Oh, if we didn’t have moiré, that would
go better! " We saw that it would be
better to exclaim :" Oh, if we had no
fabric for the screens! ". The moiré is
indeed only one of the problems in direct
relation with our expensive and
indispensable fabric.
A moiré effect, it is, let us remind it, a
priori "a geometrical image engendered
by the superposition under certain
angles, of various elements such as lines
containing a regular spacing between
them".
We feel, as printers, the effects of"moiré" obtained by the superposition
of lines of dots under certain angles :
these sorts of more or less geometrical
drawings which result from optical
vibrations. If one can sometimes find
them a certain graphic interest, they
are very harmful to the good output of
an halftone printing, that it is in single,
four -or six colours process!
If this problem is relatively less striking
with coarse dots (less than 55 lines /
inch – 22/cm), it does not there exist
less.
Indeed, and it is a supplementary
difficulty with which collides the screen
printer who wants to print halftone
reproductions, we have the doubtful
privilege to have two chances of moiré,
against only one for the offset and flexo
printers!
And it is even worse in the screen
printing on textiles because a third
source of possible moiré comes to be
added from the textile itself…
In addition to the moiré effect that can
have the films among them, we have
also the moiré that each of these films
gives under certain angles with regard
to the fabric of the screen! But this
second problem which arises at the
moment of the stencilling, we shall talk
again later.
The moiré is already annoying for screen
printing of what I call " coarse dots ",
that is to say between 35 and 85 lines /
inch (14 and 34 / cm), but it disturbs
infinitely more those who, as me, print
in fine and very fine line halftone.
In fact in fine line halftone - 100 to 175
lines / inch (40 to 70 / cm) and especially
very fine - 200 and 300 lines / inch, (80
and 120 / cm) this problem of moiré
stays maybe the most crucial of the
diverse problems we can have. |
2 - The Moire Number One :
The "INTER-FILMS" Moire
The first work, in all the graphic printing
processes using "regular" dots to
reproduce halftones, is to avoid the "moiré" between the films, or, indeed,
between the programming of the RIP in
CTS (either in the ink jet with inactinic
wax – such as the Lüscher, for example,
or with UV or soon with the violet laser –
such as the Kiwo ScreenSetter or the
Sign-Tronic).
I think you all know what it is the problem
about : to avoid moiré effect, it is
necessary that the line of dots of the
halftone films makes an angle of 30 °
with each of the other films...
We have then only 3 angles, for example
0° (or 90°), 30° and 60°... If we print
using the “trichromatic” process, no
problems. But in four colour process, it
is necessary to find a fourth angle.
To do it, one puts, in all the printing
process, the film of the yellow at 15°
only from 2 of the other films of the
colour separation : then, an effect of
moiré will occur - he is known as "cross
moiré effect " - one of the moiré that my
friends from Autotype name a "truemoiré".
As the "optical value" (in intensity or
density) of the yellow is very weak, this
moiré will not be too visible (at least in
offset). In analogical, the photoengravers had
games of halftone screens " preangulated"
or had angulated "turrets"
in their scanners. In digital this
programming is made by the software
of imaging, the RIP (Raster Image
Processors), or with fixed data or, top
of the top, with the possibility of varying
angles up to the 1/10-th of degree.
In Europe offset printers (films or CTP)
uses the standard DIN 16547: yellow at
0°; black at 45°, magenta at 75° (or
45°); cyan at 15°. Or, also, black at 75°
and magenta at 45 °.
In the USA, one often puts the yellow
at 90°, the magenta at 105°, the cyan
in 75° and the black in 45°.
The result is moreover the same
naturally!
An other solution often recommended
in screenprinting: is the use of the
angles of the flexography which are the
angles above, increased or decreased
of 7,5° : 22,5° instead of 15° for
example.
This cross moiré can be more annoying
in screenprinting (because of the
thickness of the ink deposit, in UV
especially), then some authors
recommend solutions which I shall
consider friendly as strange, and sometimes quite unrealistic! : different
mixture of halftones dot counts, yellow
in stochastic (FM, random, I shall talk
later about it).
3 - Solution for the Visible
Cross Moire : By the
Sequence of the Printed Colours
Because this cross moiré exists, I accept
it, but I manage to make it nearly
invisible :
My solution is then always to print the
yellow in last position.
In screenprinting we deposit, under the
best technical conditions, an ink layer
much more important than in offset,
especially with U.V. inks, because there is no "loss of thickness by evaporation" of solvents or water, in spite of all the
techniques of reduction of the printed
coat which I spoke about the UV inks
systems : the d printed dots in
screenprinting have a certain relief, even
small but real. When several coats of
dots are printed, we name it “dots build
up”.
We have just seen that the film of the
yellow, which is necessarily, as seen
above, at 15° only of 2 of the other
films in the four colour process, created
with these 2 other films a "cross moiré ".
This moiré, which is very visible when
one stacks the films of separation, sees
itself only little in offset (because of the
grey optical value of the primary yellow
close to O and of the thinness of the ink
coat - less than 2 µ).
It does not see itself in screenprinting
provided that the yellow is printed last.
If on the other hand, one prints the
yellow in the second position, either after
the cyan, or after the magenta,
according to the "classic" (but
erroneous) recommendations, in
screenprinting - especially U.V. - the
cross moiré will become visible if the
cyan, the magenta, or the black, are
overprinted onto the yellow.
The reason is that these colours have a
strong density or "a optical" intensity
(grey value), this moiré becomes
optically rather visible or very visible.
This can be even worse when one prints
the yellow in the first position - an other
big traditional classical idea for many “advices givers”! |
