What are the primary reasons that printers have excessive color variation?

Answered by Dan Remaley, Senior Technical Consultant Process Controls, PIA/GATF

In your experience, what are the primary reasons that printers have excessive color variation? What's the tip-off that a particular printer has a serious problem?

There are several fundamental reasons for color variation; the majority can be eliminated with good process control. The printing process should be measured at each step. Scanning, proofing, platemaking, and presswork need to be controlled within industry standards and tolerances.

Scans should be in gray balance and color corrected. The proofing system should match the print condition through color management and/or tone reproduction curves. Each proof should have a measurable, as well as visual, target for conformance to the standard. These targets should contain L*a*b* values, tone scales, and gray balance information.

Every plate should have a target to measure the dot values and exposure. The target should be placed in the plate bend area and will not print. Film-based products need a UGRA scale with microlines—a resolution of 6–8 microlines is considered the correct exposure. This generally equals a 50% film value becoming 54% on the plate. Digitally imaged plates also need a plate control target, although with CTP we are more concerned with the highlight dots 1-2-3-4-5% and the basic curve of the 25-50-75% values.

The printing press has the most variability of all these items. Press sheets should have a color bar that can be used to measure solid ink density, midtone (50%) dot gain (TVI), print contrast (75%), and midtone gray balance (50C/40M/40Y). The pH and conductivity should be measured and checked often, as well as roller stripes, roller durometer, and pressure settings.

The tip-off to a serious problem is what, and how often, do you measure? Do you have separate and distinct plate curves for Y-M-C-K? Color control and management starts in the pressroom and then works backward to the proofing device. I look for checklist sheets, for proofs, plates and press sheets, showing the measurement and variability of each step in the process. You can’t control what you don’t measure.

This question was answered by Dan Remaley, Senior Technical Consultant Process Controls, PIA/GATF. For questions, comments or on-site help, contact Dan at dremaley@piagatf.org or (412)259-1814.
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