BEST PRACTICES FOR PRESS FINGERPRINTING
Just like the tips of our fingerprints, the way each press lays down ink is unique. Each press prints at different speeds, pressures, dot percentages, etc. But printers must print customers’ products accurately and consistently. This is where press fingerprinting is useful.
This is why press fingerprinting is so important. Today’s printing industry is all about the customer. The newest and fastest presses, the best proofing processes, the most efficient workflows— none matter if the customer is not happy. If printers do not test the limits of their printing process, to know exactly how their press prints, they won’t be able to deliver the results
Carey Color has spent a lot of time working with printers to help them adopt elastomer sleeves as their preferred in-the round material. It’s come to rely on press fingerprinting and developed a set of best practices for the process. The goal is to help the printer control pressroom variables, achieve
tighter tolerances, improve print repeatability and match the proof consistently. How do we do that?
It starts with building a sleeve and continues by establishing a database of tightly matched parameters for each sleeve and fingerprint. We run an individual fingerprint for each set of variables (press, ink, substrate, etc.), so that when those variables change, we know what the printed piece is going to look like
When building a fingerprint file, it’s safe to only include the graphics, tables and linescreens that any press will print effectively. But you’ll typically find with a good profile that not all portions will succeed. Why? It’s important when building a file to include linescreens higher than typically printed, reverse copy smaller than usual and highlight dots you typically cannot
hold with the current process.
When simply preparing for something like moving from one manufacturer’s polymer plate to another manufacturer’s polymer plate, fingerprinting to determine minimum dot, dot gain, line screen and registration is the standard practice. But when embracing new technologies like elastomer and expanded gamut, the goal is to build a fingerprint that reaches beyond the boundaries of what you’ve printed successfully in
the past. With new technology, it’s good to know the threshold and sweet spot.
It’s important not to take the safe route—If a print shop has never printed higher than 120 linescreen, it should put 133 and 150 on the fingerprint, so that at some point, the print will fail. By doing this, a printer can understand exactly where its limitations lie. The printer may find that printing a higher density or a finer linescreen is a real possibility when it hasn’t been before.
PRODUCT ION CONDITIONS
Don’t roll out the proverbial red carpet when fingerprinting. The goal is to print in the same environment and process a printer normally would. This will create an apples-to-apples comparison between the fingerprinting job and a typical job. When preparing for a fingerprinting job, take these steps into consideration:
- Be mindful of what average printing conditions are and
attempt to maintain them for every print job
- Fingerprint with the same ink, density, pressure, speed
and operators normally used
- Mount the sleeves, bring the press up to speed, set impression,
check density and pull sheets for evaluation
A typical 1-color fingerprint should take less than 20 minutes from start to finish. The goal is not to create a product that cannot be reproduced on a day-to-day basis; rather, the goal is to develop a process that offers the best quality and repeatability on re-runs and in matching proofs.
Key components should be documented and recorded for future review. One option is to build a database for a printing process so future runs will be engraved and proofed with curves built from data collected in prior runs. By continuing to take readings from live production, a printer can later tweak the proofing and engraving curves.
After it is printed, it is important to precisely read the fingerprint and know what to look for. Here’s a breakdown of Carey Color’s process—remember, we are not always on the pressrun and not always capable of making density and impression changes while the sleeves are running:
- Visually check for imperfections like streaking or banding,
ink bridging and impression
- Evaluate visual consistency of ink laydown and density
- With a densitometer, measure dot gain in the following
- 1 percent to 5 percent
- 25 percent
- 50 percent
- 75 percent
- Enter the readings into software that plots dot gain characterization
curves. It’s important to take readings from
four places across the sheet and then allow the software
to determine an average
At times, run several different sleeves made with different elastomer compounds. The purpose of this is to find the best printing combination. By making changes in compounds and anilox a printer is able to pick the material and durometer best suited.
The goal in this first process is to build a dot gain characteristic that will match General Requirements for Applications in Commercial Offset Lithography (GRACoL) Standards (ranging from 18 percent to 25 percent dot gain). This process will be better defined after creating a 4-color fingerprint.
BUILDING IN 4-COLOR
Once a curve has been built, the dot percentages should translate across all channels. We then print a 4-color fingerprint to create International Color Consortium (ICC) Profiles for proofing and monitor calibration. In this process, the goal is to get the printer to G7 standards using CMY-to-black conversion patches. A typical target is a Color IT8 7.4.
We then measure the 4-color fingerprint to verify dot gain, wet traps and overprint for the purpose of building a profile for that specific press, substrate, ink and so on. ICC profiles are then built to define the achievable color gamut of the press with a 4-color process. The next step is to correlate the relationship between the input data and the printed result and
develop a proof that can match the press consistently.
What is most important to realize—and this is where most ICC Profiling falls short—is that just because a proofing target is within G7 (or any other proofing standard) specification, the proof may not be up to standards visually. Even if all colors are within a Delta E of +/-2, seeing the visual deficiencies and adjusting the proof to fix those deficiencies requires specific levels of expertise. This is why including common graphics
and images is important in a 4-color fingerprint file: To allow calibration of proofs for a visual match in addition to a specification match.
There are many challenges in this step. For instance, building a profile from a paper proof to a paper print is much easier than understanding how proofing on clear material overlaid on the printer’s substrate (like foil) will translate to the printed piece. This is where experience comes into play.
Building a proof, when proofing onto the printed substrate isn’t always an option, is tricky. The knowledge of an experienced specialist is essential to create a match.
EXPANDED GAMUT PRESS FINGERPRINTING
As more and more printers start to print using a standardized 7-color process, fingerprinting best practices remain as important as ever. Continuity and consistent horizontal density across the press sheet and from corner to corner is key for consistent press fingerprinting with expanded gamut. This is important because in a 7-color process, inconsistent densities
magnify just as much, or more than, traditional CMYK.
The evaluation process is similar in expanded gamut, but due to the complexity, specialized software is required to build the achievable color space and develop proofing processes to match the press. It is important when switching to expanded gamut to choose auxiliary colors that best match the color gamut of what is typically printed: Choose from
green, orange, red, violet and blue to achieve the widest color gamut.
Also, develop gray component replacement standards on prepress files to minimize ink density on the printed piece. Files that contain large Pantone solids and vector art are gray component replacement converted more aggressively, while CMYK images are converted less aggressively. This will get fingerprints up to color more quickly and consistently and result in better matching the proof to the printed piece.
With laser-engraved elastomer sleeves, variables like dot height, dot shoulder, relief depth and shoulder angle can be controlled. Therefore, when fingerprinting for elastomer, it is normal to include target areas with slight variations of dot height and shoulder angle. Because elastomer generally prints sharper, include a target with a slight bump curve in the
mid tone, which is usually a credible starting point.
Push targets with the highest possible linescreen ruling and include text and reverse text down to one point. The result is usually a very clean print with high ink densities and very predictable dot gain, which allows printers to print highlights more consistently by reducing impression/pressure.
THE NEXT ITERATION
About 90 percent of the work for press fingerprinting is done once the multichannel fingerprint is created. Proofs should be matched on press and with correct plate curves and proofing profiles, this should be very achievable. But there is always a need to evaluate live production.
New projects may reveal deficiencies in the proofing process because of unforeseen color combinations or visual inconsistencies that were not detectable with stock images. This is especially important with the first few post-fingerprint jobs.
Periodically providing printed material to a separator is key to continual improvement. It is vital to develop a proofing and printing process that, while consistent, can be adjusted slightly as new information about how the press lays down ink is discovered. Then, the goal is to run those jobs the same way, every time they are on press.