When I embarked on the Flexo Quality Consortium’s (FQC) Narrow Web Near Neutral Calibration Project, I was not sure what to expect. G7 seemed to be a buzzword that people throw around like dice at a casino. The fact is, G7 significantly increases odds of “winning” when printing with flexography. G7, aka near neutral calibration (American National Standards Institute TR015), is simply a way to control the process and give printers a unified achievable target.
The overarching purpose of near neutral calibration is to simplify workflows and increase the visual appearance of print samples from flexo to other print types. Flexographers print on a variety of different substrates and utilize different ink vendors, not to mention countless other variables that exist from plant-to-plant and at times, job-to-job.
The objective of the narrow web near neutral project, as stated in the charter was to:
- Create a dataset from 10 documented flexographic pressruns printed following G7 specifications
- Compare the resulting narrow web dataset to existing datasets
- Determine if the narrow web dataset is comparable to one of the existing datasets, or if multiple datasets are needed
- If a new dataset is needed, determine if it is consistent and repeatable within itself
Some examples of existing datasets are General Requirements for Applications in Commercial Offset Lithography (GRACoL) and Specifications for Web Offset Publications (SWOP). They were created in a process similar to this FQC project, where many pressruns, performed under standard conditions, were analyzed to determine the average ability of that printing process with those conditions.
Gray Balance & Density Curves
G7, as defined by Idealliance, is both a definition of grayscale appearance, and a calibration method for adjusting any CMYK imaging device to simulate the G7 grayscale definition. G7 yields a visual match between different imaging systems using simple one-dimensional curves. It enables shared appearance between different printing devices or specifications when additional color management is not available.
G7 is the basis for GRACoL on #1 paper (TR006), SWOP on #3 paper (TR003), SWOP on #5 paper (TR005) and Flexographic Image Reproduction Specifications & Tolerances (FIRST’s) flexo on white polyester substrate (TR007). G7 utilizes one of the implementation methods of the new ISO 10128 standard for near neutral calibration. A key benefit of G7 is that it is device independent.
The G7 neutral print density curve (NPDC), gray balance definitions and calibration methodology are the same for any imaging technology, regardless of substrate, colorants, screening technologies, etc. The NPDC at the heart of the G7 grayscale definition was derived by analyzing the neutral tonality of typical ISO commercial offset printing using computer-to-plate technology.
Targets & Techniques
Before any of the runs were started, the UV or water-based ink from the different vendors needed a certificate of analysis proving it was International Standards Organization (ISO) 2846-5 compliant. The IT8.7/4 target was then printed on a variety of papers and films with and without laminates on narrow web flexographic presses. It was important that the IT8.7/4 targets run under normal conditions to assure a real life setting, proving that anyone can do it.
Metrics, Methodologies & Meaning
- The objective of the narrow web near neutral project was to create a dataset from 10 documented
flexographic pressruns printed following G7 specifications
- Print conditions were meticulously recorded usingFlexographic Image Reproduction Specifications & Tolerances (FIRST) 4.0
- The hypothesis was that narrow web presses under multiple conditions and substrates could print to
CGATS TR006 (GRACoL20006 V2) aims
- Narrow web flexographic printers that print using ISO2846-5 ink and adhere to ISO12647-2 printing aim points, can achieve color that is undistinguishable from offset samples
- Near neutral calibration provides a tool for common appearance without limiting gamut size
Print conditions were meticulously recorded using FIRST 4.0. The chart shows the Lab Color Space (LAB) chroma and hue values that needed to be achieved during the pressruns, specified in ISO 12647-2. The hue angel was the value that was most important to achieve during the pressrun.
None of the pressruns had any issues hitting the specified targets. After the press was set and the hue angel was within tolerance, the press ran for five minutes at normal speed and print samples were collected every minute for 10 minutes to obtain the 10 samples that would then be analyzed for each pressrun.
Spectral data was collected from the 10 printed IT8.7/4 targets from each pressrun with the same X-Rite i1iO Spectrophotometer in patch mode with UV filter (if a substrate had no Optical Brightening Agents [OBAs] the UV and non UV reading will be the same). The data was then analyzed. Comparisons were made to other established datasets to determine potential similarities and differences due to the different variables (ink type/substrate type/with and without varnish.) Comparison of the narrow web data was made with the existing flexo wide web dataset, the gamut of narrow web is significantly larger.
The 10 pressruns were staged to gather data, with the press aims being ISO 2846-5 inks and ISO 12647-2 printing. The hypothesis was that narrow web presses under multiple conditions and substrates could print to Committee for Graphics Arts Technical Standards (CGATS) TR006 (GRACoL20006 V2) aims.
When looking at the gamuts of all 10 datasets, compared to GRACoL, the volume of series #5 was much larger than other datasets and taken out for this comparison.
The remaining nine datasets were very close to CGATS TR006 (GRACoL20006 V2) aims. Making an average of the nine datasets and comparing to GRACoL20006 V2 aims. Figure 1 illustrates all data compared, with GRACol in the center. Figure 2 shows an average of the nine sets, G7 calibrated and compared to GRACoL2006 V2.
Measured average and max International Commission on Illumination (CIE) delta-E values from the 1,617 patches of the IT8 7.4 between pressruns—both linear with only ISO 12647-2 solid aims, and with G7 or ANSITR015 aims applied—were documented.
If an offset printer is achieving GRACoL print standards, based on the results of this project, narrow web flexographic printers that print using ISO2846-5 ink and adhere to ISO12647-2 printing aim points, can achieve color that is undistinguishable from offset samples.
FQC researchers concluded that narrow web flexo can use GRACoL 2006 as a common communication tool. Two other findings reported: printers can print larger gamuts providing value add; near neutral calibration provides a tool for common appearance without limiting gamut size.
About the Author: Shawn Oetjen is senior flexographic instructor within the Graphics Arts/Printing Department of Dunwoody College of Technology, Minneapolis, MN. He is an active leader in the Twin Cities Flexo Association, a 2010 recipient of FTA’s President’s Award and a frequent speaker at FTA’s Annual Forum and Fall Conferences. Most recently he co-chaired the Flexo Quality Consortium’s Narrow Web Near Neutral Calibration Project with Mike Buystedt of Flint Group.
Oetjen specifically thanked Buystedt for continually assuring that the project moved forward; then complimented Steve Smiley for the data analysis and John Raburn of Vertis for doing a lot of behind the scenes work on analyzing the data. Participating companies list out as follows: Flint Group Packaging and Narrow Web, Meyers Printing, Smyth Companies, Dunwoody College of Technology, Sun Chemical Corp. and Environmental Inks and Coatings division of Siegwerk.