Coating Thickness and Its Importance in Lithographic Platemaking and Performance.

Coating thickness is discussed as a major factor in lithographic platemaking and plate performance. It is pointed out that the popular theory, i.e., light sensitivity is a direct function of relative humidity, neglects to consider the effects of coating thickness. It is suggested that much of the apparent change in sensitivity may be caused by differences in thickness, due to different drying rates, rather than by changes in moisture content of the dried layer. The relationship between coating thickness and tone control, image development, and plate durability on the press is discussed. The prime factors which influence coating thickness are given as being: solution density and viscosity, whirler speed, and drying rate. It is emphasized that absolute humidity within the whirler has a major effect upon drying rate, whereas, relative humidity has an indeterminate effect.