engleski

Roughness of nonprinting surfaces of the offset printing plate depending on processing solution concentration

The plate making process is composed of two main processes: exposure of the photoactive layer and its chemical processing. The conventional diazo positive as well as most of the CtP lithographic plates has to be developed in a high alkaline solution (pH ≈ 12). During this processing, exposed areas of the photoactive layer are being removed from the nonimage surfaces of the printing plate. It is possible that processing solution dissolves a part of the aluminium-oxide layer too. The aluminium –oxide layer has porous structure with high specific surface which enables it to adsorb optimal amount of the fountain solution sufficient for achieving high quality prints. The aim of this paper was to determine relation between surface roughness parameters measured on aluminium-oxide surfaces and concentration of processing solution. The changes of the roughness of the aluminium-oxide are caused by his as well as aluminium's amphoteric properties (soluble in acid and alkali solutions) which makes it soluble in processing solution during developing process. For the purpose of this research a conventional diazo positive lithographic printing plate was developed in a processing solution with various concentration of sodium base. Obtained results showed that alkali concentration in the processing solution has significant influence on roughness parameters of the nonimage surfaces of the lithographic printing plates. One must take these results into account before plate making process starts. It is important to determine all explored process parameters which are significant for achieving printing plates of high quality.

chemical developing; printing plate; surface roughness;

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