The International Standards Organization has defined ISO 12647 as a set of Graphic Arts standards for printing. Included are eight parts:
Part 1: Print parameters and measurement methods
Part 2: Offset lithographic processes
Part 3: Coldset offset lithography on newsprint
Part 4: Gravure printing
Part 5: Screen printing
Part 6: Flexographic printing
Part 7: Proofing processes working directly from digital data
Part 8: Validation print processes working directly from digital data
Different parts of the world interpret these standards into their own specifications. For example, in North America IDEAlliance’s GRACoL and in Europe FOGRA39, are the specifications for offset printing that conform to ISO 12647-2 for a number one grade, coated paper.
ISO 12647-6, the flexo print standard, does not define solid ink density target numbers, but rather hue angle target values for solid ink color and recommended substrate color values. In other words, this measurement looks at the actual color value, not density.
Today we’ll take a closer look how ISO 12647-6 is utilized in flexographic printing applications.
Some of the graphic arts print standards provide specific color targets for production on specific types of substrates. For example, in offset printing, the standards are very specific to different grades of paper and the exact L*a*b* values for solids and overprints as well as target tone value increase or TVI (also known as dot gain). In flexo, the ISO standards only give the recommended hue angle for those colors, and it’s up to the printer to establish the best L*a*b* targets for a given substrate.
To comply with ISO 12647-6, you need more than the hue angle of the inks and the L*a*b* of substrate color. Production targets must include specific L*a*b* values for solid inks and overprints, as well as TVI values. These can be from published target values, or from custom values developed from your press.
The first (and perhaps the easiest) option is to optimize and fingerprint your press to match a published “characterized reference print condition.” There are seven new reference print conditions, each consisting of various size press gamuts. Defined as CGATS-21-2-2013, these references are part of a proposed ISO standard. The goal is to optimize and fingerprint your press in an attempt to come close to one of the seven print conditions. Next, adjust the solid densities to match the L*a*b* values to the closest characterized reference print condition, then calibrate the plates for the proper TVI.
Some of these conditions are based on other standards, such as web offset on coated paper. In the past, it was not uncommon for flexo printers to attempt to match the color of established specifications for offset printing like SWOP, GRACol, or FOGRA39. Now with the additional characterized reference print conditions, you may be able to find a characterized reference print condition that is easier to match with your particular substrate and press conditions.
In some cases, the printing substrate is just too different from any of the characterized reference print conditions, and the printer must establish a custom set of targets. Other times the customer may demand more color accuracy than can be accomplished through the use of reference print conditions. In these cases, press optimization and fingerprinting are limited by any published reference print conditions, and are pushed to the practical and sustainable production limits of the press. If process color production will be done on this substrate, an additional step – press characterization – must performed. This produces a prepress ICC profile that includes proper color separation and proofing that simulates the press condition.
The most important thing to remember is to establish standard target values for production with each press condition that you run.
The Flexographic Technical Association (FTA) is based in North America and has a branch in Europe. They have developed a specification handbook called FIRST. It’s a great tool for anyone doing flexographic printing because it includes general guidelines for producing quality prints based on the ISO 12647-6 standard for flexographic printing.
The European Flexographic Industry Association (EFIA) is another resource of guidelines and handbooks on flexographic printing practices.
X-Rite Color Services offers a two-day Print Process Control workshop to help flexo and offset printers understand and implement measurement-based process control.