Rheology and the Letterpress Printing Process

Since most printing inks do not obey the Newtonian flow equation, their rheological properties cannot be characterised by a single coefficient of viscosity. However, in the majority of printing inks the shearing stress-rate of shear relation becomes linear at high rates of shear, allowing the now curve to be defined by two quantities, viz. U the “plastic viscosity” and f the “yield value”. The validity and the usefulness of this procedure are discussed. The effects of the U and f values of an ink on its printing performance are considered in outline.