Details:
Summary:
A series of 11 coated papers, ranging from single coated LWC to multicoated woodfree grades, was tested extensively on a heatset web offset press under chosen conditions shown to create problems of heavy build-up on the blanket occurring at the non-image area of a later unit on stochastic mid tone areas where no image existed on the reverse side - termed reverse piling. The nature of the deposit, and the fact that it corresponded to the one side image area only, led to hypothesise an interaction between the printed image on the coated paper and the fountain solution gaining access at each subsequent unit from both the reverse side and the black image side where no further colour was applied. Raising the temperature on the press by reducing fountain solution cooling made the phenomenon significantly worse when using an alcohol-free fountain solution. It is shown by using methods of permeability, porosimetry, ink tack development and coating structure network modelling, that the problem is linked to the combination of two main properties: low absorption potential by the basepaper matched by a lack of permeability of the coating when and only when a predominance of nanopores (< 0.1 ?m) is present - a feature needed in modern press rooms for fast setting and the required speed of turn around of the printed product. The phenomenon of reverse piling in this case of fountain solution interaction is described using the novel term, "aquapiling".