Wet peeling process

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Wet print processes are used for reproduction and, like dry toner processes ( xerography ), work on the principle of electrophotography .

With wet extraction, a drum (indirect method) or a special paper ( direct method ) is electrically charged and locally discharged when exposed to light. The toner adheres as dry toner systems to the unexposed areas of the drum or of special paper on which the charge is remained.

In contrast to dry processes (xerography), however, the toner is in the form of a suspension in an insulating liquid.

In the direct process, paper coated with zinc oxide is used, which is electrically charged and then exposed directly to an optical image of the original. It is then wetted with the liquid developer, the toner particles of which adhere to the charged areas of the paper.

The indirect method is described below.

Differences to the dry toner process

The first major difference is the transfer of the toner onto the drum. In today's dry toner systems, this is done using special brushes and a powdery (dry) developer (mostly iron powder ). However, the toner cannot be arbitrarily fine in order to remain free-flowing. In the wet peeling process, on the other hand, the toner is present as a suspension in a transfer liquid; it can therefore be much finer and also consist of droplets.

The exposed drum moves through the bath of toner and transfer liquid, the toner particles sticking to the unexposed areas - but not the transfer liquid itself. This uses up the toner, but hardly any of the transfer liquid. For this reason, in addition to the transfer liquid supply, such systems have a supply of toner concentrate which contains the toner particles.

The second difference is the transfer from the drum to the paper . In contrast to the dry peeling process, there is no transfer unit in the wet peeling process. The drum comes into direct contact with the paper, transfers the fine layer of toner, which then dries on the paper and does not have to be fixed any further.

advantages

Due to the extremely fine toner particles and the direct application to the paper, the quality of the printed product is far superior to that of a dry toner system, especially in the area of ​​tinting and gradients . There is no plastic paint application and the problem of the toner flaking is also unknown.

disadvantage

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The transfer liquid , the toner and the concentrate make a highly toxic chemical cocktail. Furthermore, the devices are extremely care-intensive and maintenance-intensive. The transfer liquid tends to dry out, which can only be prevented to a limited extent. If the device is used infrequently, the toner will clump and it will be difficult to clean the drum. The biggest technical hurdle was cleaning the drum after each run. Even with high-quality devices, the streaks characteristic of a photocopy of the time showed up very quickly. Even the indirect method cannot print on all papers.

For the direct process, a special paper coated with zinc oxide is required, which could only be printed on one side.

meaning

The dry toner systems that emerged in the mid-1980s, after initial skepticism on the part of users, almost completely replaced the wet peel process within a very short time. The disadvantages are too severe and drying systems have now also achieved a very good quality.