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G7™ Method for Flexographic Press Calibration

Article · January 2008

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3 authors, including:

Xiaoying Rong Malcolm G Keif

California Polytechnic State University, San Luis Obispo California Polytechnic State University, San Luis Obispo
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G7™ Method for Flexographic Press
Calibration
Michael Bayard, Xiaoying Rong*, Malcolm Keif*

Keywords

GRACoL, G7, calibration, flexography, color reproduction

Abstract

The purpose of this study is to apply the G7 method for calibration on a

four-color flexographic press. G7 controls and calibrates a press using
colorimetric values rather than tone value increase. Two press runs were
completed to calibrate a flexographic press. The first press run provided
data for creating a control curve called Neutral Print Density Curve
(NPDC) for the press run with a specific combination of inks and substrate.
The printed P2P target was used to build the RIP curve and cutback the
digital file. The calibrated plates then ran for the second time to check the
efficiency of calibration. By applying the NPDC curve, the second press
run produced better neutral colors visually.

Introduction

GRACoL (General Requirement for Applications in Commercial Offset

Lithography) has been widely accepted as printing guidelines in the
industry. This method was developed using the lithographic printing
process. It describes how to approach a consistent color reproduction by
printing on standard substrates with standard ink sets. In the previous
versions of the guidelines, printing quality was controlled by color density
and TVI (tone value increase). The current version of GRACoL results in a
closer visual match between the proof and the output on press. Gray
balance is determined by combining 50% cyan, 39% or 40% magenta and
yellow. Based on different substrates, this combination changes slightly.

New York University
*Graphic Communication Department, California Polytechnic State
University, San Luis Obispo, CA 93405
email:[email protected]
SID (solid ink density) and TVI (tone value increase), used for controlling
gray balance, vary from substrate to substrate because SID and TVI are
device dependent parameters. This control requires different SID and TVI
aim values for different substrates. In GRACoL7 (2007), NPDC (Neutral
Print Density Curve), highlight, mid-tone and shadow control points are
introduced. The gray balance is determined by reaching a*=0 and b*=-2 in
an L*a*b* color space. NPDC curve is used to replace TVI control. G7™ is
based on GRACoL 7 specifications. The “G” refers to calibrating gray
values. The “7” refers to the seven primary colors values defined in the
ISO 12647-2 printing standard: Cyan, Magenta, Yellow, Black, Red, Green
and Blue. The G7 method can be used to calibrate a press and proofer,
which eliminates much of the work normally done by ICC color
management.

The G7 (2007) combination of cyan, magenta and yellow for gray is

achieved by using substrates and inks defined in ISO 12647-2 (2004).
Flexography use different ink sets which is defined in ISO 12647-6 (2006).
Printing sequence and drying mechanism are different than offset
printing. By focusing on gray balance control, G7 is possible to calibrate
any printing process including flexography.

This study focused on the validation and efficiency of G7 method in

calibrating a flexo press. By following the guidelines of G7, the quality of
gray and color reproduction was evaluated.

Experimental

Test Target

A test target was built following G7 guidelines (Figure 1). This target
includes a P2P target, a gray stripe of 50% black, a gray balance stripe of
50% cyan, 40% magenta and 40% yellow, G7 color bar, and two test images.

Materials

DuPont Cyrel® DFM digital plates with shore A 65 degree and

EskoGrphics Cyrel Digital Imager (CDI) were used for platemaking. The
plates were made at 150 lpi using circular dot shape. Screen angles were
set as cyan at 8o, magenta at 38o, yellow at 83o and black at 68o. The plates
were produced at 68oF. Plates were mounted using 3M 1015 mounting
tape, a medium compressible tape.

MACtac Kromekote, a gloss-coated pressure-sensitive label stock, was

used to print. The colorimetric values of this label stock is L* 95.05, a* 1.11,
b* -1.58.
Figure 1: Designed test target for calibration.

Water-based flexo inks from Water Ink Technologies, Inc. were used. The
process color inks were custom-made to meet the colorimetric
requirements in ISO 2846-5 (2005). Extender from the same supplier was
obtained to extend the ink as necessary.

Apparatus

A Mark Andy 2200 7 inch flexo press was utilized for all press runs. The
press was set to manufacturer’s specification. Color sequence was yellow,
cyan, magenta and black. Press was run at 100 feet per minute. Ink
viscosity, ink pH, press speed and other press settings were kept consistent
throughout the first and second press run.

Results and Discussions

The first set of plates were made without any curve adjustment except a
bumb-curve – used to compensate for exposure in the highlights. CIELAB
values for the solids of process colors on paper were measured by an X-
Rite 500 series spectrodensitometer at illuminant D50 and 2o observer.
Solids of process colors were adjusted to follow ISO 12647-6:2006(E)
standards with delta Eab less that 5. The following table listed the average
delta Eab for solids of process colors (Table 1) during the first press run.

The gray patches on the test target printed with un-calibrated plates
showed a visual hue shift or cast. The colorimetric values of the gray
balance stripe were a* 0.90 and b* 15.42.
Black Cyan Magenta Yellow
Average
3.23 5.46 4.65 6.33
delta Eab
Table 1: Delta Eab for solids of process colors.

The P2P target printed with un-calibrated plates was measured by a

SpectroScan. The data were used to create NPDC curves, which were
applied later in making the plates for second press run. IDEALink™
Curve software was used to create the NPDC curves (Figure 2 and Figure
3). NPDC curves were applied to calibrate the RIP. The second set of plates
were made with NPDC RIP curves applied. The platemaking process was
the same as the first set of plates.

Figure 2: Un-calibrated curves of gray balance and TVI.

Figure 3: Gray balance correction created by IDEALink™ Curve.

The calibrated plates were mounted and printed at the same conditions as
the un-calibrated plates. During the second press run, delta Eab values for
solids of process colors were around 6.

The test target printed with calibrated plates showed significant

improvements in gray balance, although the color shift can still be
detected visually. The colorimetric values of gray balance stripe from the
second press run were a* 0.81 and b* -6.77.

There were some press parameters that limited the repeatability. The
Mark Andy 2200 flexo press tested does not have automated impression
adjustment. There is no ink viscosity control system on press. Impression
pressure was adjusted manually. Even though there was no press setting
change after the first press run, there were some difficulties in achieving
the same colorimetric values for solids of process color. Due to the
limitation of environment control, ink viscosity changed slightly, resulting
in the need to use extender to dilute the inks.

G7 uses neutral density (ND) combined with highlight range (HR),

shadow contrast (SC) and highlight contrast (HC) to control tone
reproduction. Prior to G7, this used to be controlled with TVI (tone value
increase) alone. To simplify the quality control, ND, HR, HC and SC
integrate the effects of density control and TVI control. The colorimetric
values of solid process colors from the first and second press runs were
controlled within the tolerance. These colorimetric values are directly
related to solid ink density (SID). HR, HC and SC are directly related to
TVI. Without precise impression pressure control, achieving consistent TVI
is difficult. This also leads to inconsistent gray reproduction.

NPDC curves created by IDEALink™ Curve software were based on ISO

12647-2 ink set. The flexo ink set defined in ISO 12647-6 is slightly different
than the offset inks. Although the G7 method is based on colorimetric
values rather than solid ink density, the NPDC curve is adjusted by
densities of gray at different tone values. The difference in colorants may
create the color shifts in gray correction in flexo printing. During the first
press run, dots smaller than 10% showed a decrease in size on the press
sheet. IDEALink™ Curve may also have limits in creating curves with
data that appears to have dot sharpening instead of dot gain.

Conclusions

The G7 method introduces a new philosophy for press calibration and

control. This method controls the press with colorimetric values to achieve
a better visual match. Theoretically, it can be applied to any color
reproduction regardless of printing methods. With the results from the
flexo press calibration, the G7 method demonstrated its ability in
calibrating a printing press other than offset printing. The significant
improvement in gray balance proved that colorimetric control could
achieve a better visual match.

The G7 method was developed from offset printing using standard inks
and paper defined in ISO 2846-1 (1997). The difference in ink colorants
may result in different densities at different tone values when printed
with flexo inks. Using the curved created by offset data, the calibration
may not be as precise as for offset printing.

The G7 method requires a highly stable and repeatable process. Without

automated press adjustments and ink viscosity and pH control, the
consistency of flexo printing is challenging. Colorimetric values of solid
process colors may change when ink viscosity and pH change during
printing. Densities of halftone printing may change when impression
pressure is not consistent from one press run to another.

More tests need to be done to study the efficiency of the G7 method to print
processes with different ink colorants other than those for offset inks.

Acknowledgments

The authors would like to thank Mark Andy for supporting the Graphic
Communication education at Cal Poly. The authors also thank Water Ink
Technologies for supplying testing inks. Special thanks to Mark Mazur of
DuPont for his expertise and comments. We also like to thank Cal Poly
Professor Brian Lawler for helping collecting data.

References

GRACoL7 (2007)
IDEAlliance® GRACoL® Specifications 2007 (GRACoL 7)
G7 (2007)
IDEAlliance® G7™ Proof-to-Press Process
ISO 12647-2 (2004)
ISO 12647-2:2004(E)
Graphic technology – Process control for the production of half-
tone colour separations , proof and production prints, Part 2: Offset
lithographic processes
ISO 12467-6 (2006)
ISO 12467-6:2006(E)
Graphic technology – Process control for the production of half-
tone colour separations , proof and production prints, Part 2:
Flexographic printing
ISO 2846-5 (2005)
ISO 2846-5:2005(E)
Graphic technology – Colour and transparency of printing ink sets
for four-colour-printing, Part 5: Flexographic printing
ISO 2846-1 (1997)
ISO 2846-1:1997 (E)
Graphic technology – Colour and transparency of ink sets for four-
colour-printing, Part 1: Sheet-fed and heat-set web offet
lithographic printing

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