Development of a method for pre-damping in laboratory offset
printing units

Abstract: At the Iggesund mill there are several methods and equipments in laboratory
scale that simulate the offset printing process in order to evaluate mottle
tendency or ink tack for instance. However, it is not possible to study the
effect of pre-damping prior to printing on these properties, which mostly is
the case in industrial printing.
The purpose of this work was therefore to develop a method for pre-damp
application used prior to laboratory offset printing in order to learn more
about the ink/fountain solution/substrate interaction. This method was used
to study the effect of pre-damp on ink tack measurements and on water
interference mottle tendency in laboratory scale.

The initial trials were focused on ink tack measurements without pre-damp in
order to achieve knowledge about this type of measurement. The effect of
different parameters used within the ink tack measurements was also
investigated.
The ink tack measurements were rather sensitive to the method of ink
distribution procedure, the positioning of the tack disc and the fastening
of the paperboard sample. Additionally, the printing speed, the printing
pressure and the hold time affected the measurements. However, measurements
in the cross direction and the machine direction of the paperboard were
compared, and did not show a significant difference. Different levels of
tackiness of the ink were also tested. For instance, an ink that previously
had caused delamination failure in industrial offset printing showed a
relatively high ink tack.

Four different paperboard qualities were also compared. The small
differences in ink tack between the samples were explained by the
differences in coating characteristic resulting in different ink absorption.
The same paperboard qualities were also used in an inter-comparison between
different laboratories measuring ink tack (Iggesund, STFI and Imerys). The
same machine settings and the same type of ink was used in the
investigation. Although the four qualities always were ranged in the same
order, both the level and the shape of the ink tack curve differed in
between the three laboratories. One explanation given was that different
tack discs were used in the investigation.

The method developed for pre-damp application was based on condensation of a
water film on a cooled down metal disc. The effect of the temperature on the
metal disc surface (inside a fridge), the dwell time inside a desiccator and
the time for condensation in a conditioned room on the amount of water
developed on the disc surface was investigated. The amount of water
developed and transferred could be related to the resulting print density or
transferred ink amount. A lower print density or transferred ink amount
implied a larger amount of water applied to the sample.
The experiments showed that this procedure is rather uncertain and that the
tests are difficult to reproduce. However, a lower temperature of the damp
disc inside the fridge in general resulted in a larger amount of water
developed onto the disc surface.

The pre-damp application was used prior to printing and ink tack
measurements. The maximum and the shape of the ink tack curve only changed
slightly when using pre-damp in comparison to the measurements without pre-
damp. In general, the ink tack curve in the later phase was lowered when
using pre-damp, indicating a faster ink setting and drying. This may be
attributed to that the viscosity of the ink decreases when blended with
water. Another explanation given was that penetration of water into the
substrate leads to swelling and consequently a rougher surface and a lower
ink tack. A lower amount of ink transferred in some cases may be another
explanation.

The pre-damp was also applied prior to printing in Prüfbau in mottle
evaluations, creating water interference mottle. Two paperboard samples with
different levels of performance in industrial printing were compared. The
sample with less good performance also appeared worse in this investigation
with respect to mottle, both with and without pre-damp before printing. This
sample had lower coat weight according to burn out pictures and also a lower
contact angle in comparison to the better sample.

A correlation was also achieved between the water interference mottle
tendency obtained in laboratory printing and in a full scale printing trial,
respectively. Only three qualities were included in this investigation.
Further experiments must be performed to really see the correlation between
these two methods.