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

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.Validerat; 20101217 (root