Characterizing the influence of white ink coating weight and print layer film type on chromaticity in gravure printed flexible packaging

Abstract

 When printing transparent substrates, a white ink underlayer is frequently required to reproduce saturated colors. Nevertheless, the cost of adding this underlayer makes it a potentially attractive cost reduction opportunity. The primary objective of this study was to investigate the influence of white underlayer coating weight (g/m²) and print layer film type on the chromaticity of reverse printed structures representative of those found in gravure printed flexible packaging. To analyze the influence of white ink coating weight (Ctg Wt) and print layer film type on chromaticity, six design of experiments (DOEs) were conducted. Each DOE explored the relationship between three levels of white ink coating weight and three print layer film types for one ink (Magenta, Cyan, Yellow, Orange, Violet, or Black). The DOE results showed that both coating weight and film type have a statistically significant effect on chromaticity (P < 0.005). For Magenta, Cyan, Yellow and Orange inks, white underlayer coating weight had the dominant effect on chroma (the values of ΔC*_ab typically between 4 and 7) while the effect of print layer film type was relatively minor (the value of ΔC*_ab is typically < 1). For Violet ink, white ink Ctg Wt was still the dominant effect (the value of ΔC*_ab was 7), but print layer film type had a more pronounced effect (the value of ΔC*_ab was 3). The relationship between coating weight and chroma over the range from 0.49 g/m² to 1.95 g/m² was linear for all inks (R2 > 0.99). Finally, when printing Magenta, Cyan, or Orange ink a coating weight reduction of 1 g/m² (from 1.95 g/m² to 0.98 g/m²) resulted in a color shift of the ΔE₀₀ ~2.5, and a 1.5 g/m² reduction (from 1.95 g/m² to 0.49 g/m²) resulted in a color shift of ΔE₀₀ ~3.5. Yellow, the weakest ink, was more affected (the ΔE₀₀ ~4 for 1 g/m², and ΔE₀₀ ~5 for 1.5 g/m²), while Violet, a hue where the human visual system has less sensitivity to chroma differences, was less affected (the ΔE₀₀ ~1.5 for 1 g/m², and ΔE00 ~2 for 1.5 g/m²). For achromatic Black ink, lightness (L*) was chosen as the response variable. Although white ink Ctg Wt and print layer film type had statistically significant effects on L*, the differences attributable to these effects were at or below the threshold of visual detection.