Analysis of the shortness of offset ink as a function of tack on paper by comparing elastic and Hencky strain extension

Abstract

We conducted a study to differentiate between the effects of the extensional length of an offset ink and its film split tack behavior related to the blanket paper ink film separation on a press. Two separation techniques were used to study these phenomena separately: constant force and thus varying acceleration, giving sensitivity biased toward ink length; and constant extensional strain rate (Hencky strain rate), giving sensitivity biased toward ink on paper tack and viscosity. In general, high intrinsic ink tack level seems to correspond to high ink on paper tack value. Both methods identify the tack cycle as ink oil is removed by capillary action into the substrate. The extensional strain rate matches more closely the film splitting effect of the print cylinders, identifying effects of extensional viscosity. In contrast, the constant force technique is sensitive to length of an ink as a result of tack build. A range of inks of different shortness (inverse length) was analyzed using the two methods on coated paper. Development of tack differed for the range of inks according to shortness, and thus the way they responded to the two measurement techniques. Differentiation was obtained between the behavior during tack rise, the balance between adhesion and maximum internal tack, and the tack fall over time. The development of shortness as a function of ink on paper tack provides new insights into the basis for piling, paper strength demands, and runnability under different press dynamics.