Color Shifts in Waterless Offset - Trapping or Dot Gain in Halftones?

Trapping effects in waterless offset were studied with two inks, two halftone types of screening and two coated fine papers as variables to determine which contribute most to color shifts. A scientific test-form was used giving independency on the type/construction of the waterless offset printing press. Color gamut shifts were measured from 2229 patches by running the waterless printing process optimal to press contruction, with linearly adjusted ink keys, printing the test-form at similar fulltone print densities. Thus, the ink coverage was kept as equal as possible in all process inks and ink key positions, with an observed thermal process stability. Determining the relationship for halftone trapping through color gamut measurements by spectrophotometric means revealed that halftone screening and ink composition dominated over effects from coated fine paper properties. Differentiating dot gain effects and trapping effects for halftones is difficult while using only densitometry and spectrophotometry. Through spectral measurements we may still determine how different variables induce color shifts, however we can not clearly determine the actual reason. For waterless offset, using no dampening solution, color shifts in halftones seem to arise mainly from dot gain. This means that correct pre-press adjustment is crucial to control accurately, i.e. through compensations in each process inks separately, or by using color management profiling, to maintain highest possible print quality in the chosen production based printing configuration with locked consumables.