Printing nip

Figure 7. Charge distribution and ink deformation in a gravure printing nip with...

To retain effectiveness at reasonable power supply voltages, the covering resistivity should not be too high, otherwise the voltage drop between the charging unit and the printing nip proper becomes too high to obtain effective results. 

Post-printing nip capillary sorption of ink and ink vehicles is discussed using Lucas-Washburn theory and the influence of the rate of capillary sorption on ink holdout, show through and set off are discussed. Finally, the long-term migration of oil vehicles over fibre surfaces by spreading with the attendant loss of paper opacity is described. 

A slight redistribution of the ink can occur after the paper leaves the printing nip due to aspiration as the network struc-... 

In this review, ink-paper interactions during and after the printing nip will be examined starting with the least complicated... 

Figure 4. Splitting of free ink film in printing nip. (F = fraction of free ink film split that stays with paper.)...

In 1947, Chapman- developed a instrument to measure the optical contact between a flat glass prism and the surface of paper. The idea of measuring the smoothness of paper under pressure simulating the compression of paper in a printing nip has been explored by several authors since- -2--. Perhaps the most striking result of this work is the low fraction of contact that is achieved. Under printing compression optical contact with smooth newsprint is about 20 percent and seldom above 50 percent for coated papers. 

Clearly, the deformation of the fluid ink in the printing nip must raise the total contact considerably. One way of calculating the effective contact between ink and paper is to measure the fraction of the paper surface covered with ink when printing with a solid printing form. Using a black ink it is possible to calculate the fractional coverage from the reflectances of the printed and unprinted paper ... 

Two important factors in the generation of tack forces in a printing nip are cavitation and ink rheology. 

Elasticity at high shear rates is also bound to influence tack force in the nip of an offset press. Besides shear rate the elasticity is Influenced by the molecular weight and molecular structure of the dissolved polymers, and interactions between the pigment particles, the vehicle, and the dissolved polymer molecules. Of particular interest is the effect of the molecular mobility of stiff-chain polymers such as polyindene on elasticity and tack force at the shear rates encountered in an offset printing nip. 

It is difficult for students to understand or acknowledge that their teachers are human (some are more human than others). Since I am human (despite what my students might report), I can and do make mistakes. If there are mistakes in this book, they are my sole responsibility, and I am sorry. If you find one, PLEASE let me know so that it can be corrected in future printings. Nip it in the bud. 

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