How can an industry have process control when one of the key components—anilox volume measurement calibration—is missing?
How
do you know what your anilox roller volumes are? Who measured them? How
were they measured? How do you know these bcm (billion cubic micron)
numbers are correct? You don't!
What is the missing piece? Calibration.
The
Center for Printability at Georgia Southern University has developed a
test method to certify the volume of a roller. A certified roller then
can be used as a calibration test gauge. Whatever method of volume
measurement you want to use, the certified roller would be used to
ensure your test device or method is accurate.
Yes,
roll scopes are calibrated. But they are calibrated using a National
Inst. of Standards and Technology (NIST) step gauge. Measuring a ceramic
cell is very different from measuring a height change on a flat
surface.
Why Calibrate? The Center for Printability conducted a series of tests to look at how measurements change based on the following:
Our
purpose was to describe the problem of volume measures, identify a
measurement method suitable for laboratory use, and develop a method for
certifying anilox volume.
A
banded mechanical roll was created in brass for the test. Since this
anilox was not intended to be a production roller, brass was used
because it does not corrode and therefore does not need chroming. The
bands are 400-line and 200-line. The volumes for each band will be
described in the project tests.
The
Flexographic Technical Association FIRST specifications describe two
measurement systems: a scope system and a liquid volume system. Added to
the study was a popular liquid strip volume test.
The four anilox volume measurement methods used in this test were:
The
following three comparisons attempt to answer the question of how well
one can rely on the bcm volume stamped on a cylinder. The tests record
the consistency of volume measurements based on the person, time, and
system.
For statistical purposes, each test condition was measured five times. The numbers reported are the average of these measures.
Operator-to-Operator Comparison Question: Are different operators able to produce similar measurement outcomes?
The
two test methods that are highly dependent on operator skill (liquid
volume and liquid strip) show the largest variation between users (see Figure 1.)
The roll scope results are much more consistent among operators but
still vary as much as 0.27 bcm at a 3.6 volume and 0.17 bcm at a 1.6
volume. That may not seem like much, but are you buying a 3.6 volume or a
3.87 volume roller? Is that difference important to you when you buy?
Conclusion: No matter what system is used, changing operators will effect the volume results.
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Day-to-Day Comparison Question: Does starting and stopping from day to day change the results?
The expert showed he/she was still learning the process for the liquid volume and liquid strip systems on the first day (see Figure 2).
The following three days show more operator consistency for each. The
results for the scope systems were mixed as well. Scope1 was more
consistent on the 200-line band, while Scope2 was more consistent on the
400-line band.
Conclusion: In relation to most of the measurement systems, the results changed from day to day measuring the same cylinder.
Systems Comparison What
is a similar result? Anilox cylinder manufacturers are reporting volume
numbers to the 0.1 place of a bcm, and some manufacturers are reporting
0.01 bcm numbers, e.g., 4.39 bcm. Keeping this in mind, you decide what
is similar.
Figure 3 was made by averaging all test data (35 measures) for each system.
From
low to high on the 200-line, there is a 25% difference in measured
volume and on the 400-line, a 51% difference. Even looking at the second
highest measurements, the difference is 10%.
Conclusion: The systems do not provide similar results based on a 0.1 bcm. Reporting volumes down to the 0.01 bcm seems unnecessary.
The Bigger Question: Which one of these numbers is correct? How can a volume number reflect the way ink fills and disperses across a cylinder?
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Certifying a Cylinder for Calibration The
Center for Printability believes the liquid volume test method is the
best for certification. The liquid method provides the most real
simulation of applying and wiping ink to an anilox. The center studied
the various methods and chose the tools, inks, and transfer papers that
provide the most accurate and consistent results. Repeated measures from
multiple locations provide highly reliable results.
The
most desired configuration of a cylinder for certification would be one
that is banded with each band no less than 3 in. wide. Ideally, the
cylinder would be from the same manufacturer that makes the production
cylinders.
Once
a reliable standard is available, all other measuring devices can be
adjusted to read nearly the same. Until this happens, any comparison of
anilox volume is suspect.
Dr.
Donald Armel is a professor of Printing Management at Georgia Southern
University and director of the Center for Printability. He can be
reached at donarmel@gasou.edu.
The views and opinions expressed in Technical Reports are those of the author(s), not those of the editors of PFFC. Please address comments to author(s). |