Method of making relief printing plates with concave printing areas

#This invention relates to relief printing plates.

#In various printing methods, relief printing plates are used. The desired printed image is defined by a plurality of areas raised relative to a surrounding level. Ink is applied to the surface of the raised areas and thereafter transferred from them on to the desired receptor, e.g. a sheet of paper or plastics film, by bringing the receptor into contact with the printing plate. In order to secure adequate transfer of printing ink to the surface to be printed it is necessary to bring the printing plate and sheet together under a certain pressure but this pressure tends to squeeze the ink lying on the raised areas to the edges of those raised areas. There is accordingly a tendency for the individual parts of the printed image each to be composed of an outer portion of high optical density corresponding to relatively high ink deposition, an annular region inwardly of that border of relatively low optical density and corresponding to the geometrical outline of the raised area on the printing plate and a central area of optical density intermediate that of the annular region and the dense border region. In some applications, particularly where high printing definition is not required and where even the annular region is relatively dense, this uneven ink distribution is of no consequence. However, in many high quality printing applications, particularly the printing of texture patterns e.g. for cartographic and other uses, this unevenness is highly disadvantageous, especially when the printed image is to be used as a master for the product e.g. of a planographic printing plate.

#According to the present invention there is provided a relief printing plate having a plurality of raised printing areas adapted to receive a film of ink, the upper surface of each area consisting of a relatively higher peripheral region and a relatively lower central region.

#A Nyloprint sensitised plate (ex B.A.S.F.) was taken and covered with a photographic negative of a texture pattern. The pattern was a regular array of circular dots on a square grid. Dot diameter was 0.66 mm and dot spacing 0.92 mm. A photographic positive with a dot image of identical array and spacing but with dots of size only 0.47 mm was then aligned over the negative, and the sandwich so formed contact exposed to diffuse light for 5.5 mins in a vacuum exposure frame.

#The plate was then washed in solvent for 10 mins to leave a relief printing plate in which each raised area was a circular area of upper edge diameter 0.66 mm with a depression in its upper face. This was used in a proofing press to print a dot pattern on to glossy stabilised cellulose diacetate film. The printed dots had a smooth density profile rising sharply at the edges of the image to a central gently arched curve.

#A lith film ("DuPont COD 7") negative of a 40%, 10.75 lines/inch dot pattern was punched, utilising a "Protocol" register punch in register with a Lith film (DuPont COD 7) positive of the same pattern, emulsion side down.