Investigation into the runnability of coating colours at the blade at high production speeds

There is a sustained trend of ever rising coating speeds which is targeted to achieve a further productivity increase of modern coating plants with large machine widths. However, the objective of increased production speeds poses problems of many kinds irrespective of the original speed level to be enhanced. As far as blade coating is concerned, this objective frequently entails several different subtasks. The limiting factors of sheet run, flying splice or drive control can be largely overcome by mechanical means. On the other hand, the problems involved in application and metering techniques are far more complex in nature. High production speeds are often accompanied by more blade wear, streaking, skip coating and- most frequently- stalagmite formation or bleeding at the blade. Stalagmite formation in particular is influenced by a multitude of parameters such as coating base, coating colour composition, solids content water retention, colour rheology as well as the coating conditions including type of blade, blade geometry or blade angle. If the undesirable phenomena of stalagmite formation and bleeding at the blade are to be avoided it is highly essential that the critical solids contents are determined with different coating pigments, binders and cobinders as well as pigment blends. The investigations have revealed that the two phenomena are more or less pronounced depending on the metering elements employed-even without a base paper being used The profile of the flow when passing through the metering gap is primarily disturbed by fibres, pigment agglomerates or blade tip defects. All these factors tend to aggravate stalagmite formation or bleeding at the blade. Commercial coating colours of good and poor runnability were studied for their rheology in the high-shear range by means of a high pressure capillary viscosimeter that was modified in design to incorporate ''mini-blades''. These rheological studies have confirmed the feasibility of characterizing the runnability of coating colours in the laboratory by means of a high pressure capillary viscosimeter equipped with ''mini-blades''.