WATERLESS OFFSET PRINTING
The main differences between offset litho and waterless offset are the printing plate and the inks. Instead of relying on a printing plate that is water receptive in the non-image areas and ink receptive in the image areas, waterless plates have an ink receptive image area and a non-ink receptive non-image area. The ink receptive image is very similar to the polymers used for offset litho plates, except that the polymer does not have to be photosensitive, or capable of ablation by thermal laser. This polymer layer is coated with a film of silicone polymer that is less than two microns thick. This silicone polymer completely rejects specially formulated waterless inks. The plate is developed by removal of the silicone polymer layer.
The plate is fitted to the plate cylinder of an offset press in exactly the same way as an offset litho plate. Both types of plates are made in a similar thickness.
Apart from the way in which the silicone non-mage layer rejects waterless ink, the other main difference is that the image layer is below the silicone.
This means that every halftone dot is in the form of a cell with walls of silicone. There is no dampening solution or emulsified ink present to interfere with the quality of the halftone image. The ink film thickness can be varied considerably without major changes in dot gain.
An essential part of waterless printing is press temperature control. This is particularly important in the areas of plate and inking rollers. Refrigerated dampening systems fitted to modern high speed presses have the effect of cooling the plate and blanket cylinders, and to some extent the ink roller train. Sufficient extra cooling is provided for the inking roller train by means of chilled water passing through hollow oscillating rollers. If the refrigerated dampening system is removed this extra roller cooling is seldom sufficient for waterless printing. It is necessary to boost flow and cooling power to a level that can maintain the desired temperature for the type of press and running speed. The best systems allow for separate temperature control of each printing unit. Any system should be capable of maintaining a suitable plate temperature to within 2 degrees centigrade.