Technical Information

To achieve predictable and repeatable color on a flexographic press, you need good color management and reliable process control. Otherwise, your colors will really fluctuate. While the initial work may require some effort, it's worthwhile. The results will lead to greater customer satisfaction, and you'll save time and money through faster setup times and fewer reprints.

eXact eliminates problems in flexographic process control

If you are proofing on an uncalibrated press, if the plates you make are unverified, and if the press run does not have proper color measurement, there is no way to tell if the color you are viewing is correct. Color management ensures that your equipment produces consistent, accurate color in each job and between jobs.

You cannot rely on your eyes to judge color accuracy. Human color perception is subjective and limited by external factors, while color measurement equipment is objective and without the same limitations. In the flexographic workflow, spectrophotometers can be used for quality control of proofing and press output, as well as for calibrating proofers and presses. Contact us if you need help choosing the instrument best suited to your needs.

The International Organization for Standardization provides ISO 12647-6 as a set of printing standards. ISO 12647-6 is specific to flexography. This standard not only defines solid ink density target numbers, but also looks at actual color values. The best L*a*b* targets are established by the printer for a given substrate.

Here are some resources that can help:

• The FTA (Flexographic Technical Association) has developed a specification manual called FIRST, which includes general guidelines for producing high-quality prints according to ISO 12647-6.

• EFIA (European Flexographic Industry Association) also provides guidelines and manuals on flexographic printing practices.

Process control begins with establishing target values and then adjusting the process to achieve those values. For production, you need more than just the hue angle of the ink and the L*a*b* of the substrate color. Production targets must include specific L*a*b* values for solid inks and overprints, as well as TVI values. These values can come from published target values or from custom values developed on your press. Most importantly, establish standard target values for production under each press condition you run.

The first step must be to optimize the press settings. This will determine the best combination of line screens, anilox rolls, ink strength, plates, mounting tape, and other press settings to suit your chosen substrate and application.

Next, determine the targets or target values that will be used when measuring control targets in production. The patches to be measured include the printing ink colors and any spot colors, gray balance patches, overprint/ink patches, and tone scales that will be used in production. There will also be many test elements, including vignettes, impressions, slanders, registration, and other visual assessments of print quality. The measurements of the patches will be recorded as production targets.

Measurements of samples throughout the run will be used to help calculate the expected variation in this print run. If the variation is greater than the specified production tolerance, the specifications or pressure conditions, or both, must be adjusted. These new targets and tolerances can also be used for future runs. When preparation or production measurements show that you are outside the tolerance range, you know there is a problem that needs to be addressed. The earlier it is found and addressed in the production process, the more cost-effective the production process will be.

A good proof represents the target achievable by the press. It can be used to obtain customer approval to run the job and to show the customer what they want to achieve. In some cases, they are not even made by the print provider. If you are running custom press conditions, you will need to provide your own measurements or ICC profiles.

Most inkjet proofing systems can simulate the press using ICC profiles or measurement files from characterized reference print conditions

Incorrect lighting conditions can alter the way you see color. In fact, insufficient light can alter the color of the proof in a different way than the printing paper. If the press operator views the proof and print samples under incorrect lighting and makes changes based on what they see rather than what is measured, the color will be wrong.
Always view your prints under lighting conditions that meet ISO standards. In addition to D50 (daylight), you may need to view production samples under other simulated store or home lighting to ensure that different parts of the packaging (such as labels and cartons) continue to match on the shelf or in the consumer's home.

Control targets are the most important tool for monitoring pressing behavior. It should include patches representing ink, overprint, tone value increase, and gray balance. You can measure these bars and then compare them to target points and tolerances called "standards".

Sometimes these standards are defined by ISO, while other times print conditions will require you to define custom standards by test.

Correct ink density does not necessarily mean good color. Take these images as an example...they have the same ink density, but look very different. Which one is correct? The problem may be related to tone value increase, overprint, or ink smudging. But these images show that we cannot rely solely on ink density to achieve accurate color.

G7 is an implementation of ISO standard 10128, focusing on near-neutral calibration. It is widely used in North America and gaining recognition worldwide. G7 can help you achieve consistent gray balance using many different printing processes and applications. The main idea behind G7 is that it uses plate curves to define a method of neutral gray balance from highlights to shadows. To achieve gray balance, each color will have a unique plate curve and its own TVI.

Once you reach this stage, you need to document everything and communicate it to everyone involved in the workflow. Standard Operating Procedures (SOPs) provide direction, help solve problems, and make training new employees easier. When SOPs are correctly developed and implemented, they also ensure that everyone in the organization operates in the same way, regardless of their shift or location.

ISO has established industry standards, and each region has developed specifications and best practices to achieve these standards. To produce repeatable, accurate colors on a flexographic press, you must operate the system to meet these specifications and standards, use proofs that simulate the same printing conditions, and view your prints under correct lighting.