G7 and Flexographic Printing
G7 and Flexographic Printing
- Subject: G7 and Flexographic Printing
- From: Bill Whitfield <email@hidden>
- Date: Wed, 22 Aug 2007 20:12:19 -0700
Greetings,
Please note I am cross posting this to both the Colorsync and the
Printing Color lists - something Terry Wyse said he would appreciate
knowing. Also please excuse the length of this post. I took a few
extra words to explain some of the unique flexo press characteristics.
I am a flexo printer that is trying to use G7 methodology on film
based, not digital based, flexographic plates over multiple plants
and presses. Line screen is 150 in most plants and 175 in one plant.
In the past we used one plate curve, unique to line screen, for all
colors knowing it was not proper and now I am using IDEALink and the
G7 methodology to create separate curves.
One of our major problems is with gearmarks on the printed samples.
These gearmarks cause valleys and mountains in the curves reported
via MeasureTool. Offset people - gearmarks - think a "blush" in color
within the patch. If measured in the blush the TVI would be anywhere
from 5 to 10% higher than the non blush area of the patch. With great
input for Juergen (thanks heaps) I have created a target that
averages out these blushes by placing two P2P targets side by side
with one staggered a little and rotated 180 degrees from each other.
The stagger and rotation prevent the gearmarks from showing up in the
same row of patches.
I have the press run 2 separate impressions in one direction and then
flip the entire plate 180 degrees and run 2 more separate impressions
in the other direction to make 4 distinct runs. I then pull 25
samples out of 100 from each run and measure 10 samples (2 P2P
targets per sample) from each run. After measuring and averaging 40
samples (80 P2P targets) the curve becomes very smooth. Sidebar -
unfortunately my iO does not like to measure patches with gearmarks
in strip mode so rather than taking 1 minute to measure a P2P target,
it takes 10 minutes. Just enough extra time to do VERY LITTLE else.
Another challenge is adjusting color. Ink is in a pan and delivered
to the plate from; pan to rubber roller; from rubber roller to an
anilox roller; from anilox roller to the plate. Anilox roller - think
of a cylinder with small cups (aka cells) in it. The cells are like
film line screen (ie a 1000 line anilox roller has 1000 cells per
inch). Volume is the same for each cell but can be made different for
different line anilox rollers. Cell volume is measured in bcm's
(billions of cubic microns). The excess ink is "wiped" from the
anilox using a doctor blade. Thus an anilox roller is spec'd as 400,
2.38 bcm - 400 cells per inch, with each cell volume being 2.38 bcm's.
The plate cylinder "floats" between the anilox and stock with
adjustments available for plate cylinder contact with anilox and
plate cylinder contact with stock. A balance of contact to anilox and
stock must be maintained.
To adjust color you either reset the impression of the plate cylinder
and it relationship to the anilox and the stock, or you add
components to the ink. Impression is a delicate adjustment, by touch,
because too much impression will certainly change ink color, but also
affects TVI. Generally an ink is purchased strong and an extender is
used to achieve proper density. One could of course change the anilox
to a lower line count, larger volume, but the goal is to use standard
anilox line and volume and not be "juggling" anilox rollers on
production jobs to achieve color. Additionally there is a minimum
anilox cell count to printing line screen that needs to be honored.
While I have many questions about my results so far I will limit this
session to questions regarding instrumentation.
I use an iO table to measure the patches in the office and a 528 to
measure the press run on site. I have some discrepancies in numbers
between the two devices and would appreciate some feedback.
I use PatchTool to check dE values between presses when in the
office. I have placed reference values in my 528 for each of C, M, Y,
K, R, B. These values are from the adjusted G7 for flexo, ISO 2846-5
(taken from table 3 of a draft version of ISO 12647-6).
Below each color is; first line, LAB from iO with dE to 2846-5
standard from PatchTool; second line, LAB and dE to 2846-5 from 528
(note to myself - these measurements were taken from P2P on teeth
side on the 1080)
c L=55.00, a=-35.81, b=-49.73, dE=0.96
L=54.35, a=-34.20, b=-50.90, dE=1.02
m L=50.24, a=73.70, b=-6.50, dE=1.90
L=50.70, a=74.18, b=-4.25, dE=1.53
y L=90.07, a=-5.29, b=95.63, dE=2.97
L=90.08, a=-6.05, b=94.99, dE=2.80
k L=16.41, a=0.55, b=-6.50, dE=7.36
L=15.94, a=0.48, b=-3.15, dE=7.08
Well, after typing these numbers I realize that they are not that far
off, so I will use these numbers to ask another question or two:
1 - Other than the black, knowing the tolerance for G7 is 5 dE, would
you all feel comfortable running with these dE's to the standard? We
tend to run our blacks dark so we can run line work and process on
the same plate. No one wants gray type when they expect black type. I
have a gut feeling that the color of the black is very important
because gray balance from CMY is trying to match the black. What, if
any problems will I create by allowing our black to be this far off
in dE to the standard?
2 - What does it mean when a color "hooks" in the Analyze portion of
IDEALink? When I ran my linearized run there were no hooks, but when
I ran my 1st G7 curve there is a hook in the blue. The yellow,
magenta, cyan and green curve lookws the same on both runs, but as
mentioned the blue hooks and the red on the curve run, curves the
other way from the way the red ran on the linear run.
Sorry to have written so much, but spreading G7 over so may locations
and presses with tooling that does not match is very frustrating. My
first G7 run over three presses did not produce the results I
expected. If anyone is interested I will share the results in a
future post.
Thanks to all that respond (which means you took the time to read all
of this).
-Bill-
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