On-press Color Control

Closed-loop color systems, combining color measurement hardware and process control software to continually monitor and adjust ink densities on press, are easy to justify even in lean times. Used in conjunction with CIP4 ink key presetting, these systems can yield substantial reductions in makeready time and paper waste.

Sheetfed presses are relative newcomers to closed-loop color, with most relying on near-line measurement of press sheets by a scanning spectrophotometer at the console. If you're still relying on a handheld densitometer to measure, you might wonder about the challenges and benefits of implementing a closed-loop solution. Heidelberg color specialist Chuck Koehler, whose experience dates back to his years at GATF, recommends a combination of tools: Heidelberg's Image Control and Axis Control near-line spectrophotometers or the InPress built-in spectro, which moves color measurement inside the last press unit. Heidelberg's new “Near Neutral” calibration utility (previewed at IPA last month) and the updated Prinect Color Toolbox are also part of the offerings. After installing such tools, Koehler works through his implementation plan:

1. While running an ink density optimization test form, he optimizes ink key preset data using the Color Assistant module (ISO 12647 ink values are resident in Image Control to make this easier).
2. Once the test form is “flat lined” (all keys perfectly set), Koehler prints “raw” color calibration test forms at standard LAB values.
3. Using the Near Neutral calibration utility, the raw press sheet is compared to a standard profile (such as GRACoL) or to a user “house standard.” Near Neutral calculates platemaking correction curves needed to achieve a visual match. (Heidelberg's MetaDimension RIP automatically imports these curves through its Calibration Manager.)
4. Remake the plate using the new curves, then run it at the desired color values to verify results.
5. On all subsequent production, include one of Heidelberg's “mini-spot” targets so that Prinect Color Toolbox's quality monitoring software can detect if/when the calibration should be updated.

When using Image Control, Koehler recommends building a database of print job elements such as reference values for screen builds. “This system can match crossover images on separate signatures, even on different stocks,” he says. “It lets you run to gray balance very effectively.”

For web offset, color control pioneer Graphic Microsystems (GMI), now owned by AVT, has introduced Spectral Press Expert (SPX), a time, trend and artificial intelligence application. It provides real-time feedback on press conditions, but also collects data throughout the run, compiling trends and issuing warnings. SPX compares running press data to known standards (including SWOP, GRACoL and Fogra), or users own data sets. Once a print style has been defined, SPX offers direction on controlling press performance; it can even recommend new plate curves if current print results are too far away from your chosen standard. GMI says SPX is scoring high marks during in-field beta testing.

In addition to monitoring color and gray balance, the detailed information captured by SPX can help identify a host of mechanical problems, such as slurring or bad couplings. After the press run, SPX generates a concise report that certifies compliance with a particular print standard to share with clients or store as a record of your process control.

GMI's measurement tools are often teamed with MAN Roland's Pecom press consoles. As the press management component of MAN's printnet networking and integration platform, Pecom connects third-party scanners to both web and sheetfeds. MAN Roland's printservices consulting team provides customized project management assistance for implementing closed loop on webs.

KBA pairs Densitronic S color control with its Logotronic management system. Densitronic S monitors print density and color quality; Logotronic tracks and retains run data for reporting.

There are many more options, such as Perretta Graphics' CLC 2500, or Web Printing Controls Closed Loop Color and CLC Plus systems augmented with Process Manager to drive color, register and web alignment control; QuadTech's Color Control System with a new CCD-based scanning spectrophotometer able to measure spectral data for 10 color bar patches in a single image capture; LithoTel Software Design's ColorLinx computer and scanning spectrophotometer package, or systems for offset and flexo control from Eltromat (shown facing page). These tools provide an accurate and efficient way to implement gray balance-based quality assurance methods including GRACoL's G7 or System Brunner.