Predicting Spot Color Overprints

A continuing trend in package printing is the increasing complexity of jobs and the use of multicolor printing, while at the same time meeting brand owner demands to reduce time to market. Many packaging jobs require different combinations of inks, substrates, screening, and other variables. For example, a chocolate wrapper might have two browns; fruit juice might have oranges, greens, and reds.

The inability to accurately proof 'spot color overprint' or 'multi-channel profiling' or 'extended-gamut printing' poses a significant challenge—along with reliably communicating this color between all parties of the supply chain, from the brand manager to the printer. An effective and accurate way to predict the behavior of spot colors that print on top of each other is difficult to achieve. For example, if a 40 percent spot color green is overprinted by an 80 percent spot color red (green on top of red), what is the resulting color on a flexo press?

To make it even more complex, each printing process overprints differently. Flexo does not overprint like gravure—the ink film thickness and trapping properties are very different. Then, consider the unique information needed to accurately predict the overprint behavior of two custom spot colors: on a given flexo press, with a given anilox, at a given linescreen, with food grade inks. Using typical L*A*B* based measurement, the industry has become adept at making solid colors appear similar across different printing processes, but this leaves room for improvement. For a combination of four inks you need a CMYK IT8.7/4 test chart that actually prints 1617 patches—costly and time-consuming. The number of patches required to print grows exponentially with each additional ink.

Predicting ink behavior

An alternative method can be used to predict ink behavior on a specific substrate for each press setup—without the difficulties of proprietary chart-based press fingerprinting. This is accomplished by combining spectral modeling algorithms with spectral ink measurements that analyze the properties of each ink color, as well as the substrate's colorimetric properties. This information is applied to a specific printing process (flexo, offset, gravure). A profile can be created with a just the solid patches of the spot colors on the substrate, although better results can be achieved if multiple steps (50 percent, 30 percent, 10 percent, etc.) of the spot were measured. Traditional press fingerprinting charts can also be applied, along with existing press characterizations to refine accuracy.

This approach also allows for the substitution of the substrate within the profile (when ink behavior conditions are similar). In this case, the color values of one substrate can be replaced by the values of another substrate. This avoids the time-consuming and costly process of collecting redundant data because of a small change to the job parameters.

In many cases, the relevant measurement data can be gathered from control patches included in actual production runs and then saved and categorized according to print variables. This lets operators build an archive of measurements and combine existing measurements with new profiles, whenever a new combination of inks must be profiled. Accuracy is increased by adding more overprint readings. Then process-specific information is added (ink rotation, trapping, etc.), and the final press condition is simulated on a proof.

For a printer, this can have a substantial effect on performance. It allows for more packaging standardization and shorter production cycles.

Mastering multicolor proofs at CSW

CSW, Inc. provides integrated packaging services for consumer product companies (CPCs), including brand support, brand visualization, creative services, image engineering, flexographic printing plates, steel rule cutting dies, and workflow coordination. The company has a long history of partnering with brand owners, converters, design agencies, and retailers to produce practical solutions that maintain the brand performance and consistency consumers expect.

CSW's award-winning prepress team has been a recognized leader in color management since 1994. They keep a close eye on emerging print technology, especially looking for reliable color-management practices that follow internationally recognized standards.

"Color managing the standard, traditional 4-color process has been mastered by most prepress providers. Packaging, however, is based on spot colors," says Marek Skrzynski, CSW director of graphics, R&D. "When you consider the process of overprinting spots, which creates an unknown mix color, color reproduction in packaging is still pretty challenging. Many times we work with custom-mixed inks, beyond what Pantone or other ink suppliers offer. The ability to predict the behavior of overprints and the interaction of these inks is critical to managing color."

CSW was recently looking for a better way to represent spot color interactions with an inkjet proofing device, to predict multi-spot color overprints prior to seeing the result on press.

"After talking with GMG, we thought we might have found a way to move beyond conventional spot color separation and proofing," says Skrzynski. "When they visited us for the first time, we presented them with one of our most challenging separations to proof. Within minutes, their system, OpenColor, provided a proof with the same color fidelity as one that we had spent hours tweaking. Their live demo was done faster—and more accurately—than anything we had done in the past.

"While nothing can replace the data gained from reading a full press profile, OpenColor can use partial targets to confirm previous profiles and build upon them to create new profiles on the fly," continues Skrzynski. "Very often, we need to replace just one spot color for another—hard to find the press time to run new targets. This offers a reliable workaround to recalculate colors."

Many profile creators need data from lots of targets, but it can be difficult to read them all within one package development cycle. If CSW is working with a job that needs to be completed quickly, it can now read just some of the data and still deliver accurate color.

"It would be fair to say that OpenColor has cut our color management and proofing time in half," says Skrzynski. "Printers are not incurring the expense of makeready and press time for running 800 feet/minute for test charts. They are also seeing cost savings in the reduction of the consumables needed to print—and reprint—multiple overprint iterations on their proofing devices. Our own proofer is no longer tied up with such iterations, which kept other jobs waiting in the queue."

What's next?

Once the ability to predict overprint behavior is available, the next step will be for producers of packaging to eliminate inks not needed (usually C, M, Y, or K), achieving acceptable gamut through the use of mostly line colors on press. This will let prepress companies and converters eliminate colors within a job (without changing its appearance), reducing the press stations needed, while not compromising the line colors required. The creation of this type of artwork is very expensive and is often created with trial-and-error methods. As better ways of producing these separations evolve, the overall reduction of inks on press will further reduce costs across the entire supply chain. pP