498
T. Scherzer
specific setting parameters of the printing press. Any change of one of these parameters may lead to a change of the conversion of the printed layers. Generally, printing
is a high-speed process, which rapidly produces large amounts of printed sheets.
Therefore, sufficient conversion must be achieved before stacking in order to avoid
blocking of the sheets. Moreover, adequate wipe resistance is required. It would
be therefore desirable to monitor the conversion continuously during the printing
process in order to ensure high quality and uniformity in this way.
However, monitoring the conversion in a printing machine is extremely difficult.
So far, no in-line method for control of the conversion exists, and also its off-line
determination is usually impossible in a printing plant, since no adequate analytical
equipment for these measurements such as conventional IR spectroscopy is available
at a printing press. Typically, indirect methods such as the determination of the
coefficient of sliding friction with simple test equipment are used. NIR spectroscopy
would open completely new possibilities for direct quantitative (or at least semiquantitative) monitoring of the conversion during the printing process.
For developing a calibration model, calibration samples with well-defined thickness have to be prepared. As outlined above for coatings (paragraph 21.2.3.3.), variations of the thickness seriously disturb measurement of the conversion and may
completely prevent quantitative analysis. Therefore, great care was bestowed on the
preparation of printed layers with constant thickness. For each color, a number of
printed paper stripes were prepared using a printability tester. The application weight
depended on the color of the ink and varied between 0.8 g/m
2 (cyan) and 1.2 g/m
2
(yellow). Moreover, samples with a clear lacquer were prepared with about 3 g/m
2 .
Printed layers were irradiated with different UV doses in order to obtain samples
with a broad conversion range. After recording the NIR spectra, discrete calibration
models were developed for each color and the printing varnish. FTIR/ATR spectroscopy was used as reference method. As an example, the PLS calibration of a
yellow UV printing ink is shown in Fig. 21.13 [35].
It is apparent that the scattering (and consequently RMSEP) is higher than in case
of coatings with thicknesses, that are at least one order of magnitude higher (paragraph 21.2.3.1.). Nevertheless, it can be clearly seen that there is a linear correlation
between reference conversion and predicted signal, which clearly proves that the
conversion can be predicted from NIR reflection spectra by adequate chemometric
tools. Generally, RMSEP for all inks is in the order of 3–4.5%. Due to the somewhat
higher thickness of the layers of the printing varnish, RMSEP of the corresponding
model was found to be only 2.3%. External validation tests confirmed the precision
of the predictions [35, 36].
Using the various PLS models, the conversion of UV-cured ink and varnish layers
was monitored directly in a sheet-fed offset press. It is obvious that only the position
in the delivery unit behind the UV lamps can be used for mounting the probe head
(see paragraph 21.2.5.1.). Except of the spatial limitations mentioned above, the
probe head is exposed to vibrations, dust (e.g. talcum powder used for oil-based
printing inks handled at the same press), heat, airflows, etc. at this position. Moreover,
the printed sheet, which is fixed by clamps at its front edge only, glides on an air
cushion through the delivery system, which may lead to sheet flutter. Variations of the
T. Scherzer
specific setting parameters of the printing press. Any change of one of these parameters may lead to a change of the conversion of the printed layers. Generally, printing
is a high-speed process, which rapidly produces large amounts of printed sheets.
Therefore, sufficient conversion must be achieved before stacking in order to avoid
blocking of the sheets. Moreover, adequate wipe resistance is required. It would
be therefore desirable to monitor the conversion continuously during the printing
process in order to ensure high quality and uniformity in this way.
However, monitoring the conversion in a printing machine is extremely difficult.
So far, no in-line method for control of the conversion exists, and also its off-line
determination is usually impossible in a printing plant, since no adequate analytical
equipment for these measurements such as conventional IR spectroscopy is available
at a printing press. Typically, indirect methods such as the determination of the
coefficient of sliding friction with simple test equipment are used. NIR spectroscopy
would open completely new possibilities for direct quantitative (or at least semiquantitative) monitoring of the conversion during the printing process.
For developing a calibration model, calibration samples with well-defined thickness have to be prepared. As outlined above for coatings (paragraph 21.2.3.3.), variations of the thickness seriously disturb measurement of the conversion and may
completely prevent quantitative analysis. Therefore, great care was bestowed on the
preparation of printed layers with constant thickness. For each color, a number of
printed paper stripes were prepared using a printability tester. The application weight
depended on the color of the ink and varied between 0.8 g/m
2 (cyan) and 1.2 g/m
2
(yellow). Moreover, samples with a clear lacquer were prepared with about 3 g/m
2 .
Printed layers were irradiated with different UV doses in order to obtain samples
with a broad conversion range. After recording the NIR spectra, discrete calibration
models were developed for each color and the printing varnish. FTIR/ATR spectroscopy was used as reference method. As an example, the PLS calibration of a
yellow UV printing ink is shown in Fig. 21.13 [35].
It is apparent that the scattering (and consequently RMSEP) is higher than in case
of coatings with thicknesses, that are at least one order of magnitude higher (paragraph 21.2.3.1.). Nevertheless, it can be clearly seen that there is a linear correlation
between reference conversion and predicted signal, which clearly proves that the
conversion can be predicted from NIR reflection spectra by adequate chemometric
tools. Generally, RMSEP for all inks is in the order of 3–4.5%. Due to the somewhat
higher thickness of the layers of the printing varnish, RMSEP of the corresponding
model was found to be only 2.3%. External validation tests confirmed the precision
of the predictions [35, 36].
Using the various PLS models, the conversion of UV-cured ink and varnish layers
was monitored directly in a sheet-fed offset press. It is obvious that only the position
in the delivery unit behind the UV lamps can be used for mounting the probe head
(see paragraph 21.2.5.1.). Except of the spatial limitations mentioned above, the
probe head is exposed to vibrations, dust (e.g. talcum powder used for oil-based
printing inks handled at the same press), heat, airflows, etc. at this position. Moreover,
the printed sheet, which is fixed by clamps at its front edge only, glides on an air
cushion through the delivery system, which may lead to sheet flutter. Variations of the