Ink series in web offset

The buyer of web offset inks, and also the printer at the offset rotation, is often faced with the question as to which of the many ink series offered on the market is best suited to his requirements. The various factors which have to be taken into account so as to make the right choice, and the reasons for the existence of varying ink formulations, are explained below.

The demands placed on the processing properties of an ink are ultimately oriented towards the customer's wishes, i.e. towards the expectations placed on the printed product. We can say with certainty that the conveying of information, be it the portrayal of a landscape in a travel brochure or of a food, e.g. meat or fruit in a newspaper supplement for a supermarket, or the design of a packaging, be it for a tin of food or a pack of potato crisps, has to be as authentic as possible. This ensures that the observer is able to recognize the image being portrayed, even if in part only sub-consciously, and that the observer feels that he is being addressed directly.

The first factor to impact on the result of a print is without doubt the choice of substrate. The surface structure, i.e. the glossiness or roughness of the paper surface, as well as the inherent color of the paper, which may range from a bright white to a more or less yellowish color, have a considerable impact on the quality of image reproduction. Smooth, glossy papers allow much more brilliant, high-resolution image reproduction than slightly yellowish, uneven substrates, where the paper fiber structure is still visible after printing.

Another - invisible - paper quality also has a not inconsiderable impact on the printing result. The capillary structure of the paper determines the extent to which the ink is absorbed, and this has a decisive impact on the color strength of the printed ink on the paper.

The surface solidity of the substrate has major consequences for the ink to be used. Any damage to the paper surface, e.g. the picking of paper fibers or coating particles, has a negative impact on the visual appearance of the printed matter, as well as on the continuous production run, and in particular with larger print runs.

An unsmooth full-surface print caused by the build-up of paper dust from paper fibers on the printing plate can only be overcome by repeated washing. While the rubber blanket is cleaned, the washing procedure also affects the printing plate, from which the contaminants are carried away via the paper web when the press cylinder is reset.

When evaluating the picking susceptibility of a paper, it is not only the tendency of the dry paper web to form dust which is of significance, but rather the property of the paper surface after contact with the water of the damping system. In the case of papers which tend to pick, individual paper fibers are very often straighten up to the surface after contact with the water of the damping system and come away from the surface of the paper. As a result they can easily be torn out due to adhesion with the rubber blanket and are then passed on to the printing plate by re-separation. Here they accumulate, particularly if there is also a lesser degree of material transport of ink towards the web due to low ink consumption.

In these cases the ink needs to be adjusted with respect to its tack in such a way that, on the one hand, it is not too sticky, so that this does not lead to increased picking of paper fibers, and so that, on the other hand, it has sufficient adhesion/cohesion so as to guarantee the continuous transport of paper dust via the printed paper web. In this respect one also has to consider that at this stage of the printing process the ink is in an emulsified state with fountain solution. There are findings that the water of the damping system is not only present in the ink in an emulsified state, as so-called "interior water", but also as so-called "surface water" where the ink and air come into contact.