Press characterization: Key to flexo process control

Consistency = predictability, as printers who’ve done their press fingerprinting homework discover.

Flexography has the potential to approach and competitor the print quality of competitor lithography and gravure if flexo converters are willing to pay the price. That price is awareness and control of printing variables. This type of control can be obtained with a press fingerprint—or as it is more commonly called today, press characterization. Characterization, when done correctly, has the potential to control variability in the current process.

A press characterization doesn’t necessarily improve your printing process; however, it makes the process more consistent and thus more predictable. The characterization can be costly both in terms of money and time. However, it is money and time well spent if your goal is to produce higher-quality graphics, efficiently and consistently.

The objective
The characterization targets are designed to show the elements that are common to most print jobs.

These elements include:

• solid density blocks
• trap targets
• tone gradations
• print registration
• vignettes
• UPC barcodes in both directions
• minimum fine line
• minimum type fonts
• positive and reverse type
• the parallelism of the anilox to the plate, and of the plate to the substrate
• other elements that are common to the product mix of specific print operations.

If the above elements are included in actual production jobs and data from the fingerprint are reproduced, the printing process has the potential to be more consistent.
The only element that should not be included as part of the characterization process is a pictorial image built with the process colors. First of all, the dots haven’t been reduced and second, there tends to be a conscious or unconscious tendency to try to “make the image look good.“

In a perfect world, a printer would be using only one set of inks, one type of printing plates, one set of anilox, and only one type of substrate. This, of course, is hardly ever the case. Because of the many ink systems, plates, etc., a group of knowledgeable individuals must be assembled to help identify print variables.

The team
When a company has decided to put the time, energy, and resources into a press characterization, it is crucial to carefully select the characterization team participants. These members need to have decision-making power in the development of objectives, and they should also have a good understanding of the flexographic process.
A team leader needs to be identified to coordinate activities and be the ultimate decision-maker.

Those selected should represent the following areas:

Graphics – Either employees or external members, depending on the company. Usually, an internal person should be involved who has a basic knowledge of the job mix and expectations of both current customers and potential customers.

Press – Ideally a press operator with expertise in the printing process.

Inks/Solvents – This member is usually a supplier, with expertise in ink chemistry and all the parameters of the ink system chosen, such as viscosity requirements, water inks, pH, solvent blends, and additives.

Anilox rolls – A representative from the chosen anilox supplier should be present in development, and for the press run, to lend his/her knowledge of engravings and cell volumes specific to the types of graphics to be reproduced.

Note: For those converters that are just getting into process printing or for those that have just dabbled, a banded anilox roll trial before the actual fingerprint run is desirable. This is a very valuable tool in the anilox selection process.

Plates – This position is critical because it not only relates to the types of plates to be used but the entire system to include plates, mounting tapes, sleeves, and or cylinders. This position would need to be provided from both internal and external sources, ideally including plate suppliers.

Substrates – An employee handling this role should know what substrates customers are requesting. Every substrate type will have different data, so access to the various substrate manufacturers is desirable as printability issues arise.

External members should be suppliers with whom the printer wishes to pursue long-term partnerships. The process of press characterization is time-consuming and costly, so to achieve the best result a close working partnership should be developed.

The press run

At this point control variables have been identified, the target plates have been mounted and installed, and the substrate and ink have been brought to the press. Each team member needs to be assigned a specific function; for instance ink measure and control, collecting and identifying the print samples coming off the press for data evaluation, and keeping a record of variable changes for each sample taken from the press.

The team should be present at the press-side to evaluate samples, rather than attempting to bring samples to a conference room for the evaluation process. This is confusing, and there is also a chance for poor data transfer. Press characterization is very much a hands-on process.
During the actual press set-up, it is tempting for team members to keep adjusting the settings to get the absolute best sample that can be printed. That would be a mistake. Remember, every adjustment that is made needs to be reproduced in the live job for characterization data to be valid.

Instead, let the press crew set up the job. There should be a very good reason for any other members of the team to reset the press operator’s settings. These press personnel is going to have to reproduce what they are doing every day in production. When a fingerprint or characterization is completed it is only going to be of value when all the same values and conditions are duplicated. For example, if plate material is changed a new fingerprint should be run.

The operator
To achieve reproducible settings, the operator should be trying to get to the minimal verifiable pressures to get ink to the plate from the anilox and then to the substrate. Setting verification is a critical element in obtaining usable data. If there isn’t a way to verify printing pressures then the press operator becomes a variable, and data may be contaminated.

Now that the settings have been achieved, various samples should be obtained from the following:

• areas of certain dot size
• dots that are of specified color and density targets in a certain specified range
• the ink system is used to show good trapping characteristics
• the printing inks.

Ink density and ink lay are important components relative to the variability of the process. It is during this fingerprint test that the printing inks can be optimized. An assumption can be made that anytime the above characteristics are reproduced the area looked at will appear the same as it does during characterization. Consistency = Predictability.

As ink viscosity varies, the number one variable to look at its density. If there is a point where the viscosity is dropped, within the parameters of the ink system, and density improves (must still be in target range), optimum viscosity has been reached.

Density improves because the ink lay is better on the solid (wetting out), and ink transfer is improved. If the ink density at this point is higher than the recommended target, an extender should be considered.

The data
The characterization process is going to be only as valid as the documentation of the variability control within the process.

The following must be documented:

• set-up of each printing station
• station number
• anilox roller for each deck
• ink system viscosity and/or pH
• color
• plate type and tape used
• in-between dryer changes
• substrate information
• tension settings.

If full documentation was recorded, the finished print samples can now be evaluated for density, trap, and dot gain. This is valuable information that can be used for developing separations for different print jobs. When a job comes out to press now and is set up using the parameters of the fingerprint test results, the chances for success are greatly increased because the process is now under control.

Although the chances for success are greatly improved when a fingerprint or characterization is done, there is still no guarantee that every process job will now work in the press. There will still be times when jobs will need to be redone to meet customer expectations.

The importance of partnerships with suppliers when working through the parameters of a fingerprint trial cannot be overemphasized. This relationship is just as important in everyday working operations, to keep the process under control.

Flexographic printing can and should be treated more as a science and less as an art form if it is to continue to meet and exceed customer requirements now and into the future. Performing successful press characterization, or fingerprinting, has the potential to keep the process under predictable and numerical control.