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Runability Resistance

Summary:

Two separate fields of interest were evaluated in this project namely scratch tendency and early wearout during print runs of directly imaged plates.

A direct relationship of the characteristic scratches (5 types) and the manual plate transport mechanisms has been found through empirical and laboratory experiments. A method for determining the scratch bias of a plate in a relative or qualitative way is finalized at present.

It was found that the Sutherland scratch method can be used to create characteristic scratches. It is part of a developed method for determining if a plate is prone to scratches, termed marking index. It was further found that surface roughness correlates (estimated by means of gloss measurements) well with scratch tendency. Furthermore practical hints have been collected to provide corrective actions for the two governing factors namely plate model and plate handling.

Laboratory apparatus, which allow to stress plates both in a mechanical and chemical way, were put into operation. The general suitability of these methods for testing plate wearout was evaluated on selected plate samples. In most of the cases modifications had to be carried out in order to stress the plates in the desired way. The integration of typical materials and process parameters into these test methods was part of the investigation. It showed that none of the tested apparatus and methods is capable of applying mechanical and chemical wear mechanisms similar to the offset printing process.

Hence a test method has been developed that allows an indication of plate wear out by using minimal amounts of paper in a normal printing press. Testing this new method showed that substrate particles penetrate into the printing unit and strongly contribute to a reduction of plate permanence.

Beside this press tests laboratory tests have been evaluated to check printing formes with respect to their chemical resistances against dampening solution and washes. Components of dampening solutions have been identified that have a direct bearing onto the respective imaging layers.

Here a new test method has been developed that incorporates these findings. In that light it was found that runability problems with burnt plates are not directly related to the used damping solutions nor washes. Here the main reason could be found in mechanical problems during printing.



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