Process for preparing printing plates from photopolymerizable materials by the use of a masking technique

Claim 1

A PROCESS FOR PREPARING A PHOTOPOLYMERIZED PRINTING RELIEF HAVING IMPROVED UNIFORMITY OF POLYMERIZATION IN SHADOW AND HIGHTLIGHT AREAS, SAID RELIEF PREPARED FROM A PHOTOPOLYMERIZABLE ELEMENT COMPRISING A SUPPORT BEARING A LAYER OF A PHOTOPOLYMERIZABLE COMPOSITION COMPRISING (1) A PREFORMED COMPATIBLE POLYMER BINDING AGENT, (2) A NON-GASEOUS, ADDITION-POLYMERIZABLE ETHYLENICALLY UNSATURATED COMPOUND CONTAINING AT LEAST ONE TERMINAL ETHYLENIC GROUP CAPABLE OF FORMING A HIGH POLYMER BY PHOTO-INITIATED ADDITION POLYMERIZATION IN THE PRESENCE OF AN ADDITION POLYMERIZATION INITIATOR THEREFOR ACTIVATABLE BY ACTINIC RADIATION AND (3) FROM 0.0001 TO 10 PERCENT BY WEIGHT, OF THE LAYER OF SUCH AN INITIATOR WHICH IS INACTIVE THERMALLY BELOW 85*C., WHICH ELEMENT REQUIRES A FIRST AMOUNT OF EXPOSURE TO ACTINIC RADIATION TO OBTAIN A SUITABLE DEPTH OF POLYMERIZATION IN HIGHLIGHT AREAS AND A SECOND, SUBSTANTIALLY LESSER AMOUNT OF EXPOSURE TO OBTAIN A SUITABLE DEPTH OF POLYMERIZATION IN SHADOW AREAS; WHICH PROCESS COMPRISES DIRECTING ACTINIC RADIATION IN AN AMOUNT AT LEAST EQUAL TO SAID FIRST AMOUNT OF EXPOSURE AT SAID ELEMENT THROUGH A HALFTONE NEGATIVE HAVING HIGHLIGHT AND SHADOW AREAS, SAID NEGATIVE HAVING AN REGISTER THEREWITH A MASKING LAYER HAVING COMPLEMENTARY AREAS TO SAID NEGATIVE HIGHLIGHT AND SHADOW AREAS, THE DENSITIES OF SAID COMPLEMENTRY AREAS BEING MODULATED SUCH THAT THE MASKING AREA COMPLEMENTARY TO SAID NEGATIVE HIGHLIGHT AREA IS ESSENTIALLY CLEAR AND TRANSPARENT TO ALLOW SAID FIRST AMOUNT OF EXPOSURE AND THE MASKING AREA COMPLEMENTARY TO SAID NEGATIVE SHADOW AREA HAVING INCREASED DENSITY TO ALLOW SAID SECOND AMOUNT OF EXPOSURE.