An Analysis of the Variables in Dry Offset Printing.

The practicability of dry offset for newspaper supplements is being investigated. There is no limitation on the types of copy which can be reproduced, but quality of reproduction is dependent on quality of copy. Preparation and handling of halftones is the most difficult phase. A 13-step gray scale is used, and dot sizes are checked. Densitometric control is employed. Neither offset negatives nor photoengraving negatives are suitable for dry offset, because loss of range occurs. For the process, highlight dots must be kept large enough for the engraver to hold, while shadow tones must be kept open enough for the pressman to hold. 120-line screens have been found most suitable for newsprint work. Plates of zinc, magnesium, and copper have been tried, the latter two being found best. Plates have ranged in thickness from 0.023 to 0.040 in. Sufficient metal is needed in the non-image areas to provide strength. Plates thinner than 0.023 in. are subject to buckling. Tolerance over the printing area is 0.001 in. Etching of curved plates is being investigated. If perfected, this method would simplify lock-up. An experimental machine for etching curved plates, using an air knife to control etching, is being built. Roller surfaces are critical and swelling from oil penetration or temperature changes must be controlled.