PRINTING PRESSURE IN LETTERPRESS HALFTONE PLATES

Abstract

This chapter discusses the evaluation of printing pressure, as a function of applied load and printing area, in letterpress halftone plates. In letterpress printing, pressure arises through the fact that the clearance between the printing forme and the impression cylinder is less than the total thickness of packing topped by the paper sheet: this combination of packing material and impression sheet will, therefore, undergo a deformation, the amount of which will determine the resulting pressure. When a solid area alone is being printed, it is relatively easy to measure, by means of a suitable transducer, the total applied load and to calculate the linear pressure—in terms of force per unit length—knowing the width of the printing plate. The real difficulty arises when halftone plates are being printed and, above all, when halftones of various values are combined in the same printing forme. In this case, it would be desirable to be able to determine not only the mean pressure over the forme as a whole but also the distribution of the total load over the various zones of the forme.