engleski

Nonprinting Areas on the Offset printing plates What we know-what we should know

Majority of the offset printing plates are today built of aluminium sheets. To ensure surface characteristics needed in the printing process, aluminium surface is mechanically, chemically and electrochemically processed. All those processes are made to make a specific micro roughness of aluminium-oxide anodic layer. Micro roughness and aluminium-oxide layer have on all printing plates similar characteristic, but vary depending on the supplier and printing process in which printing plate will be exploited. In the final stage of the printing plate production, aluminium-oxide surface is covered with a photoactive layer which enables image transfer on the plate in conventional or CtP plate making process. This layer must change its solubility in defined developing solution by electromagnetic irradiation. After irradiation plates are processed in defined developing solution which dissolves parts of the photoactive layer to expose aluminium-oxide surface - nonprinting areas. Aluminium oxide is as well as aluminium amphoteric, which means that is soluble in both acid and alkaline solutions. On the other hand, majority of the photoactive layers present on the market today are soluble in highly alkaline solutions. This fact makes developing process highly important. After dissolving photoactive layer, developing solution is in contact with aluminium-oxide layer and dissolves peaks of the micro roughened layer. This fact causes degradation of wetting properties and makes control of the wetting characteristics obligatory, knowing that wetting properties of the printing plate's nonprinting areas are one of most important variables in achieving high quality printing. Definition of wetting properties of a solid surface is done by observing contact angle results, surface free energy determination, its polar and dispersive part.

Offset printing plate; developing process; contact angle; surface free energy; pH value; electrical conductivity

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