In-line monitoring of the conversion in UV-cured printed layers by NIR spectroscopy in an offset printing press

• The conversion in printed ink and varnish layers was determined by NIR spectroscopy.
• The thickness of the layers was 1.2 g m−2 for the ink and 2.8 g m−2 for the varnish.
• Quantitative analysis of the spectral data was based on chemometric methods.
• In-line monitoring of the conversion was carried out at an offset printing press.
• The predicting errors were 4.5% for the ink and 4.0% for the varnish.

In this study, it was demonstrated that the conversion in printed layers of UV-cured printing inks and varnishes can be determined in-line in a printing press by NIR reflection spectroscopy supported by chemometric methods. Layers were printed with a thickness of ∼1.2 g m−2 for a printing ink and ∼2.8 g m−2 for a printing varnish. Quantitative analysis of the spectral data was carried out with calibration models based on the partial least squares (PLS) algorithm. In-line monitoring tests were performed at a large-scale sheet-fed offset printing press in order to evaluate the predicting performance of the calibration models under process conditions. Different conversions in the layers were induced by variation of the irradiation dose. The predicting errors were 4.5% for the ink and 4.0% for the varnish. The printing speed was found to have no significant influence on the precision of the measurements.

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