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Process Color



An examination of three variables that affect color reproduction.



Published July 20, 2005
Related Searches: Anilox rolls Color management Pressure sensitive
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Once upon a time, there was a man who ordered proofs for a certain freesia-scented fragrance. He supplied the disk with the completed artwork and patiently awaited the result. Although the scent is ordinarily associated with the color purple, the proof he received back was green. Angry at the first prepress house for printing the wrong color, he turned to another. The same thing happened.

The problem? “His monitor was not calibrated, and the monitor was tinted red. The yellow and blue he made up looked like it had red in there, so he never added magenta,” says James R. Kadlec, president of Advanced Prepress Graphics in Wood Dale, IL.

A new proof and a couple of hundred dollars later, the problem was fixed by a wave of the prepress house’s magic wand and a little magenta. But don’t be fooled, process color stories don’t always have a happy ending.

According to the Glossary of Terms for Pressure Sensitive Labels, published by the Tag and Label Manufacturers Institute, process printing is defined as: “Printing from a series of two or more halftone plates to produce intermediate colors and shades. In 4-color process, the colors are yellow, magenta, cyan and black.”

Four little colors, a series of plates, and lots of dots. Sounds simple enough, but there are many variables that affect process color. Among that long list are artwork separation, ink density and dot gain.

Artwork separation:
The man with the purple-turned-green freesia labels did not properly understand artwork separation: the act of taking a piece of art and separating it for CMYK printing. If you ask a lot of converters, neither do many of the designers employed by end-use customers today:

“It’s not all the time, but more the case than not, we’ll receive a separation. We’ll also receive a non-industry standard proof, in other words, something done off of an inkjet where someone has gone to their management to show, and the management falls in love with it. Now, it becomes the thing to match and it doesn’t represent what the file looks like whatsoever,” says André Michaud, graphics director for Dow Industries in Wilmington, MA.
“We’ve got customers who might send a file that’s got 20 spot colors in it. Well, that’s going to cost them an arm and a leg, so usually what we do is try and convert it to process. And doing that can drastically change a color,” says Terie Syme, operations manager for Prestige Label Company Inc. in Burgaw, NC.
“The $1,000 artist with the Mac in the basement takes a photograph and attempts to scan it on his $100 scanner. Resolution issues and color balance issues are there galore, along with minimum dot issues, because you’re dealing with somebody fresh out of college with a very limited amount of practical application knowledge of color separation,” says Kadlec.

The digital age is upon the narrow web industry, affording designers the luxury of scanning their own pictures and sending them through e-mail to prepress houses and converters. Previously, converters could “control their own destinies,” says Michaud, but now, they are left to clean up the sometimes-frequent messes of well-meaning but uneducated designers.

These messes include dealing with the CMYK color gamut; communicating color effectively; and compensating for certain press phenomena in the design.

For those new to 4-color process, there are a couple of general facts to remember. First, there are limitations to what a press can do. The CMYK color gamut is not as large as RGB. It is very possible to design something in Adobe PhotoShop that looks great on the monitor, but is impossible to reproduce on press using only 4-color process. For instance, bright oranges, greens and violets are difficult to reproduce without also introducing spot colors.

In addition to dealing with a limited color gamut, those involved with artwork separation must also be aware of the color management process. Scanners, monitors and inkjet printers, if not calibrated correctly, are not reliable predictors of color. As was the case in the freesia mix-up, what’s on the file may not be what the end user really wants. Instead of judging color solely by what it looks like on the monitor, there are other methods that communicate color more effectively.

“What the amateur artist needs to do besides look at their non-calibrated $100 monitor is to work by the numbers. They’ll be able to predict the final printed brown, blue or green by selecting from the process equivalent using a PMS process color equivalent chart. Unless you’re going to use a spot color,” says Kadlec.

Aside from color considerations, factors such as color trapping and dot gain need to be examined during the concept stage. There are steps one can take to predict what a press will do, therefore ensuring accurate color separations.

“The first thing that really needs to be done is a fingerprint of the press,” says Ron Rex, plant manager for West Essex Graphics in Fairfield, NJ, a prepress flexo supplier. “Then you are able to do your separations accordingly because you’re understanding dot gain and you’re understanding the parameters of the press.”

To fingerprint a press, a converter prints with a set of plates that have known values, called targets. Once completed, these targets allow a converter to see where the gains and losses are on press. A curve is created based on the information ascertained.

Vincent DiTrolio, founder of DiTrolio Flexographic Institute in Broadview, IL, says fingerprinting a press “is a good starting point,” however, “the problem with fingerprinting the press is you are taking into account mechanical attributes also.”

Mechanical attributes vary from run to run. In order for a fingerprint to be 100 percent accurate all the time, a converter would have to test all the different variables — substrates, anilox rolls, repeat cylinders, etc. For this reason, some converters forgo the fingerprinting process and ask trade shops what averages they are using. Then they just apply the averages.

A sample fingerprint evaluation chart from Trinity Graphic USA.

Other people choose to go a step further, starting a library of information based on previous jobs run. “You need to put a little bit more information in the waste areas of the plates, and do a little bit more of a constant fingerprint,” says DiTrolio. “You are going to apply the tonal ranges on the side and see exactly what your gains are on each job. You are going to collect more data.”

Ink Density
There isn’t one ink density standard that will work all the time. Although the Flexographic Technical Association has suggested standards for narrow web flexo applications, called Flexographic Image Reproduction Specifications and Tolerances (FIRST), “ink density is a subjective conversation because of hues,” says Kadlec. The best ink for the job depends on the application, substrates and anilox rolls, as well as other factors such as scuff or chemical resistance.

As far as ink density is concerned, the anilox roll is a vital key to what type of ink strength a converter needs. “To achieve printed densities targeted in the prepress process, you must choose the right combination of ink and anilox roll, making sure the inks are appropriate for the size of anilox cell that you’re putting them on,” says John Kalkowski, marketing manager for Sun Chemical Ink in Northlake, IL.

“There’s a trend toward using finer anilox cells, which can result in better printing of fine detail. The anilox controls the amount of ink transferred, and the smaller cells of a fine anilox carry less ink in each individual cell. That usually requires a higher ink pigment strength to achieve densities, but allows you to print with a finer ink film,” he says.

Choosing the correct ink density is an important component to producing quality work. If the ink’s pigment is too weak or too strong, it can throw off the colors entirely. Likewise, if a company is compensating for ink that is too weak by using a lower line anilox roll and laying down a thicker film of ink, it can cause a host of other problems.

“If you use a lower line anilox roll when you transfer the ink, you are going to transfer more ink than needed most of the time,” notes Kadlec. “Where does that ink go? It goes between the dots in the printing plate, therefore you get bridging or you get doughnuts. The ink will start building up around the edges of the dot, and you’re going to be printing more dot gain than you would desire. That’s where ink density comes in, using a finer anilox roll.”

While choosing the correct ink density for the job is important, of equal importance is maintaining consistent ink density on press and from run to run.

“If ink density is not monitored, it may be that once you’ve produced a piece, you can never get back there,” says Michaud. “When you consider process printing, you are printing four different colors and if your density is not maintained through the run, not only do you get color shift, you get color balance issues.”

Evaporation can wreak havoc on maintaining consistent ink density. “Some folks in the narrow web industry don’t have ink pumps, they have their ink well, and they are going to have to deal with evaporation a little bit more than folks who have an ink pump,” says Rex.

When evaporation occurs, the inks get more viscous and the pH level drops. If the pH level is too low, “your print is going to start to get dirty, your color is going to start getting dark, your plate itself will start to clog up. If you look at the dots, you’ll see your dots start to bridge,” says DiTrolio.

Evaporation is not the only cause of inconsistent ink performance. Sometimes ink density problems can be traced back to the ink manufacturer. “You have to rely on your ink supplier really, to make sure they are producing a consistent ink strength and that the hue of the pigment is correct. Those are big factors,” says Kalkowski.

Fortunately, ink density is easy enough to monitor and control. “Most of the time, what we experience is the folks that are having a problem with ink density, it is an issue press side. That is, they either do not have a densitometer to check their ink density, they are not following procedures press side, they are on substrates that might not be treated properly — those types of issues,” says Rex.

With the proper tools, maintaining consistent ink density is “something that is easily done. It’s a science, not an art anymore,” says DiTrolio. Density, viscosity and pH level can be measured using densitometers, zahn cups and pH meters.

Through careful monitoring of the inks on press, ink density can be controlled throughout the run. Achieving consistency from run to run involves one more step: keeping careful records.

“How do they get consistent? They keep the gray balance bars, and the color bars, samples of those, in the job jacket. They write down their formula, they write down the brand of ink, anilox rolls — all the variables — the type of plate materials, the ink rotation,” says Kadlec.

Kadlec says that on each job, there should be color bars and gray balance bars on the edges of the plate. This provides the converter with more accurate information than a fingerprint of the press, because it’s specific to the mechanical functions used for that particular job.

Six and seven colors
Four color process isn’t the only game in town. Now, printers can choose to employ 6- or 7-color processes as well. Relatively new to the scene, these processes offer a larger color gamut than CMYK printing and more brilliant hues.

Hexachrome, a process using modified CMYK plus orange and green, debuted commercially in 1995. “Hexachrome’s color gamut is about 50 to 60 percent larger than the highest quality 4-color process. The Hexachrome color space is almost 90 percent of the color space enveloped by Pantone colors,” says Richard Herbert, president of Pantone Inc. in Carlstadt, NJ, and inventer of the Hexachrome process.

A comparison of color gamuts.
courtesy of Pantone, Inc.
The Opaltone Matching System, commercially released for the flexo industry in 2000, widens the color gamut by using CMYK plus RGB. Matthew Bernasconi, technical director for Opaltone in Charlotte, NC, estimates that using CMY + RGB allows for 3.06 million hues, excluding grayness. This compares with the 30,000 hues (excluding grayness) that can be printed using just CMY inks.

While these two processes have been in place for several years, 4-color process is still the most popular printing method. Despite the advantages offered by 6- and 7-color process, there are some barriers to overcome before widespread implementation is realized.

In Hexachrome, “the main barrier is the workflow. The printing industry has been very accustomed to wo
rking in a CMYK workflow. Once the data has been crunched to CMYK space, Hexachrome offers no value. The key is to keep the artwork from design through prepress in RGB color space and PANTONE spot colors. Once that occurs, Pantone offers our low-cost Hexware tools that make Hexachrome extremely easy,” says Pantone’s Herbert.

For Opaltone, the main barrier is printing press technology. “Press registration is critical to the Opaltone process. If your press cannot hold 0.005" then you will struggle to be successful,” says Bernasconi. “Education is another issue. The industry simply needs to be aware there is ‘another way’ to reproduce color in print cost-effectively. Any breakthrough in technology is a paradigm shift in thinking. It just takes time to get the message out there.”

Dot gain
Dot gain. The very phrase can illicit heart palpitations in narrow web printers. But here’s the good news: “Dot gain can be measured with densitometers or spectrophotometers, so it’s a measurable thing. And it is predictable. Since it’s measurable and predicable, that means you can find ways to control it,” says Kalkowski.

Dot gain occurs when the dot on the film and the plate is smaller than what’s actually printed on the substrate. It is impossible to eliminate altogether, but it can certainly be planned for and controlled.

Excessive dot gain has two main causes. First is “where somebody may be trying to hold too much of a dot on the plate and through overexposure they are actually making the dot bigger than what it should be on the negative,” says Robert Smithson, CEO/Director of Technology for Trinity Graphic USA in Sarasota, FL. Second is mechanical problems, such as “the printer putting too much pressure on the plate during impression or bad selection of anilox rolls, or the material that’s printed on can be too absorbent and create too much dot gain.”

Besides exposure and mechanical dot gain, printers must also be aware of optical gain. It’s “when the eye looks at something there can be a little halo that’s generated by the light,” says Kalkowski. Optical dot gain is most often a problem when the substrates readily absorb the inks.

Minimizing dot gain should not be the only concern of narrow web printers. Maintaining a consistent, predicable dot gain is just as important.

“No matter what your dot gain is, if you know that it is consistent, you can always adjust your separations and your plates to take care of the dot gain, and that’s what you need to be primarily concerned about,” says Kalkowski.

While the main causes are overexposure and impression, there are many, many other factors that contribute to dot gain. If converters are struggling to manage dot gain, “they should be checking for all the variables in the process, mainly the inks. They should also ensure that their anilox rolls are cleaned 100 percent after every time they use them, and also they should have on hand densitometers that will be able to measure screen areas when printed on the substrate they are printing,” says Smithson.

If that doesn’t work, there are other resources available. “I would urge them to talk with both their ink suppliers and their press manufacturers. Both of them should have the capabilities to help the converter identify what the cause may be,” says Kalkowski.



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