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.