G7 has quickly become a recognized term in the printing industry and, at Techkon, we’ve embraced it from its inception. It’s interesting that many packaging printers have asked some variation of two closely related questions: “Does this apply to flexo?” and “What are the differences between performing G7 on flexo and offset presses?”
We’ve noticed in our work with printers around the world that the number of G7 implementations for flexo appears to be lagging compared to G7 for offset. Why is that? My initial hunch was that much of the G7 literature and marketing must be aimed at commercial printing, but a few Google searches has quickly eliminated that assumption.
In one of the first results to appear, Idealliance defines G7 as an “industry-leading set of specifications for achieving gray balance” and “the driving force for achieving visual similarity across all print processes.” (Emphasis mine.)
Wikipedia is even more emphatic, describing G7 as “…a printing procedure used for visually accurate color reproduction by putting emphasis on matching grayscale colorimetric measurements between processes. The method is used in many applications of printing such as offset lithography, flexography and gravure.”
Photo courtesy of Techkon
In fact, the website for this very magazine has numerous stories relating to G7 certification, such as one from January 2018 discussing Taylor Communications’ G7 Master Printer Certification of its integrated color management system for producing digital, durable labels at its plant in Radcliff, KY.
Why All the Questions About G7?
Despite G7’s applicability to flexo, the web is littered with questions such as, “Does anyone know for sure if a facility can be certified G7 for flexo printing?” and, “Is G7 Master Certification available for flexo?” (both actual questions posted on print forums). The resulting threads on those forums are even more interesting, with discussions ranging from flexo’s use of non-standard inks and different ink transparencies to substrates not always being paper. (Note—while the substrates used in flexo may not meet one of the published CRPCs, the G7 calibration process is substrate-relative. Therefore, the methodology is absolutely applicable to any substrate type and color.)
Search “G7 for flexo” and you’ll find that many G7 experts and vendors have published excellent content aimed at eradicating the “myth” of it not being applicable to the print process. This suggests we’re not the only ones hearing these questions.
But there’s another reason—one that’s potentially much simpler to understand—for some of the confusion. When discussing our products with printers, especially our inline quality control solution, most conversations follow a very predictable pattern. There has been an ongoing explosion in brand proliferation that is putting unprecedented demands on printers to deliver shorter runs with higher color accuracy and greater color consistency. To remain profitable, printers are under enormous pressure to shorten makeready, reduce waste and produce more jobs in less time. That’s exactly what’s driving them to look at a solution for tighter process control and real time, on-press color management. Therefore, could it be that printers just have little time to find, read and digest the hundreds of online articles regarding G7 to see if it’s a possible solution for them?
While I can only hypothesize why G7 for flexo raises so many queries, I can at least help to clarify the issue and point out a few of the G7 differences as related to flexo. However, first, let’s start with a quick introduction to the G7 methodology.
What Is G7?
G7 can be summed up as a simple and effective way to calibrate CMYK devices. Unlike other print specifications, which rely primarily on dot gain, G7 is a colorimetric methodology that has been mathematically formalized in CGATS TR015.
G7 has precise definitions with several key elements:
- G7 is a specification for grayscale appearance and applicable to any CMYK printing process
- G7 defines two elements: tonality and gray balance
- Tonality represents the weight of the CMY and K curves, and is referred to in G7 as Neutral Print Density. Tonality in G7 can be defined by density or L*a*b* values
- Gray balance represents how gray should appear to the human eye. In G7, it is defined in terms of CIE Lab a* and b* values
- In addition to being a specification, G7 also is a calibration method that can be used to calibrate any CMYK device
Why Use G7?
One of the most valuable things about G7 is its applicability to any CMYK printing system. When used across multiple systems, each can produce a result visually similar to the others, even if they are completely different print processes. This is often referred to as common visual appearance. For example, a plastic bag produced using G7 and a folding carton produced using G7 will have similar tonality and gray balance.
This means a brand or print buyer can have one proof and achieve similar results across many printing methods and locations. It is the common G7 tonality and gray balance that achieves this. This common visual appearance was very difficult to achieve prior to G7 and required many different proofs and sets of color corrections.
So now that we have introduced G7 and explained its role and value in the pressroom, let’s look at how G7 is different in flexo environments compared to offset.
Press Test Form, Printing Standards & Ink Standards
The G7 press calibration test forms from the Idealliance website can oftentimes be too large for narrow web flexo presses. In this case, flexo printers have the option to make their own test that will fit their press sizes. Custom test forms must include at least two P2P51 targets on the form and keep in mind that when space is an issue, there are mini and micro P2P forms that can be used. In addition, Idealliance will also accept the much smaller G7 Verifier target for submissions.
Data courtesy of Techkon
Calibrating a flexo press to G7 is often accomplished using a combination of ISO standards for flexo and litho, including the use of CGATS21/PAS 15339 characterization data. Table 1 illustrates the various printing standards used for calibrating a flexo press using G7 methodology.
Because the ISO color standard for inks referenced in ISO 12647-6 has been withdrawn, it is generally recommended that most flexo printers target ink color values of a published CRPC such as GRACoL 2013.
In addition to CIE Lab values, flexo inks are also targeted using hue angles (h°). The recommended tolerance for hue angle is within +/- 2 degrees.
Minimum Dot & Averaging
Minimum dot refers to the ability of a plate system to image or create a dot. While newer flexo plate making systems can produce a complete tonal range, many older systems may not be able to image down to 1 percent. In these cases, a hard drop-off will result where there are no dots present below a given threshold. To accommodate these situations, Idealliance allows printers to ignore gray balance and NPDC errors under 20 percent, which can be helpful when preparing flexo plates for G7 Master submission.
Submissions to Idealliance from flexo presses are allowed to use an average of three samples instead of requiring a single sample to meet the G7 requirements. This means that when calibrating your press, you do not need to rely on a single sample to pass; instead, you are allowed to read multiple samples and average them together to achieve a passing set of print samples.
Conclusion
G7 adoption leads directly to higher profitability and is absolutely applicable to all CMYK printing technologies. Faster makeready, less waste and easier color matching mean you can complete more jobs per shift with less cost per job. And, as we’ve discussed here, G7 makes it easier to match the same print condition on all presses—flexo and offset—and even in different locations, providing both printers and customers with more confidence in their final printed results.
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