496
T. Scherzer
since offset printing is a complex process, numerous factors may influence the
thickness of the layers. Consequently, it is indispensable to control the thickness
continuously during the printing process.
The thickness of colored printed layers can be monitored in-line by use of
reflectance densitometry, which relates the intensity of the reflected light to the color
density. However, this method is limited to standard colors (cyan, magenta, yellow
and black) only. It does not allow measurements of the thickness of layers with other
color shades. Moreover, it is not suited for transparent top coats, which are widely
used for the protection of printed layers, for upgrading their optical appearance,
for achieving special design effects or for finishing the printed layers with special
functional properties. It is obvious that the compliance of the required thickness (or
coating weight) is crucial for those properties. Up to now, the surface weight of clear
coats can be determined only off-line by gravimetry, which is a labor-intensive and
time-consuming procedure. The typical thickness of printed layers is in the order
around 1 g/m
2 ; it might be somewhat higher in case of clear coats. Due to the rather
low extinction coefficients, NIR spectroscopy is certainly not an obvious approach
to characterize such layers. Nevertheless, it was shown that similar methods like
those described above for coatings with thicknesses in the range of about 5–100 μm
(paragraph 21.2.2.) can be also used for process control in printing technology.
For monitoring of the coating weight, the probe head of the NIR spectrometer was
mounted either above the impression cylinder of the coating unit of a sheet-fed offset
printing press or in its delivery system behind the UV lamps. The first position allows
easy access and enough space for mounting, but enables analysis of the wet layers
only. In contrast, in the delivery unit the probe head has to be installed between the
moving elements of the sheet transport system, which strongly limits the available
space and requires special probe heads with reduced size and increased focal length.
However, this position allows monitoring the thickness of cured layers.
Reference data of the coating weight for calibration models were determined by
gravimetry. Test samples with different thickness of the layers were prepared with
a printability tester. Gravimetry was also used for crosschecking the data predicted
in-line. Blank sheets with known weights were numbered before laying them into
the feeding system of the press. After printing, the weights of the printed layers were
determined. The numbering of the sheets did not only enable an exact measurement of
the coating weights, but allowed also an unambiguous assignment of coating weight
and spectra to specific sheets.
During in-line monitoring, samples with different surface weights of ink or
varnish were obtained by systematic variation of the operating parameters of the
printing press. Investigations were carried out at printing speeds between 6000 and
12,000 sheets/h corresponding to about 90–180 m/min. NIR spectra were recorded
at a rate of 30 spectra/s. At 12,000 sheets/h, this corresponds to about 3 spectra per
sheet. Synchronization between NIR measurement and printing was achieved by use
of a sensor that detected the front edge of each sheet. The coating weight of printed
layers was checked by gravimetry after the end of the trial. As an example, Fig. 21.12
shows a result of in-line monitoring of the coating weight of printed layers of a clear
varnish [31]. The labels Opening and Speed refer to specific parameters of the offset
T. Scherzer
since offset printing is a complex process, numerous factors may influence the
thickness of the layers. Consequently, it is indispensable to control the thickness
continuously during the printing process.
The thickness of colored printed layers can be monitored in-line by use of
reflectance densitometry, which relates the intensity of the reflected light to the color
density. However, this method is limited to standard colors (cyan, magenta, yellow
and black) only. It does not allow measurements of the thickness of layers with other
color shades. Moreover, it is not suited for transparent top coats, which are widely
used for the protection of printed layers, for upgrading their optical appearance,
for achieving special design effects or for finishing the printed layers with special
functional properties. It is obvious that the compliance of the required thickness (or
coating weight) is crucial for those properties. Up to now, the surface weight of clear
coats can be determined only off-line by gravimetry, which is a labor-intensive and
time-consuming procedure. The typical thickness of printed layers is in the order
around 1 g/m
2 ; it might be somewhat higher in case of clear coats. Due to the rather
low extinction coefficients, NIR spectroscopy is certainly not an obvious approach
to characterize such layers. Nevertheless, it was shown that similar methods like
those described above for coatings with thicknesses in the range of about 5–100 μm
(paragraph 21.2.2.) can be also used for process control in printing technology.
For monitoring of the coating weight, the probe head of the NIR spectrometer was
mounted either above the impression cylinder of the coating unit of a sheet-fed offset
printing press or in its delivery system behind the UV lamps. The first position allows
easy access and enough space for mounting, but enables analysis of the wet layers
only. In contrast, in the delivery unit the probe head has to be installed between the
moving elements of the sheet transport system, which strongly limits the available
space and requires special probe heads with reduced size and increased focal length.
However, this position allows monitoring the thickness of cured layers.
Reference data of the coating weight for calibration models were determined by
gravimetry. Test samples with different thickness of the layers were prepared with
a printability tester. Gravimetry was also used for crosschecking the data predicted
in-line. Blank sheets with known weights were numbered before laying them into
the feeding system of the press. After printing, the weights of the printed layers were
determined. The numbering of the sheets did not only enable an exact measurement of
the coating weights, but allowed also an unambiguous assignment of coating weight
and spectra to specific sheets.
During in-line monitoring, samples with different surface weights of ink or
varnish were obtained by systematic variation of the operating parameters of the
printing press. Investigations were carried out at printing speeds between 6000 and
12,000 sheets/h corresponding to about 90–180 m/min. NIR spectra were recorded
at a rate of 30 spectra/s. At 12,000 sheets/h, this corresponds to about 3 spectra per
sheet. Synchronization between NIR measurement and printing was achieved by use
of a sensor that detected the front edge of each sheet. The coating weight of printed
layers was checked by gravimetry after the end of the trial. As an example, Fig. 21.12
shows a result of in-line monitoring of the coating weight of printed layers of a clear
varnish [31]. The labels Opening and Speed refer to specific parameters of the offset