Method of Improving Print Performance in Flexographic Printing Plates

Abstract

A method of making a relief image printing element from a photosensitive printing blank is provided. A photosensitive printing blank with a laser ablatable layer disposed on at least one photocurable layer is ablated with a laser to create an in situ mask. The printing blank is then exposed to at least one source of actinic radiation through the in situ mask to selectively cross link and cure portions of the photocurable layer. Diffusion of air into the at least one photocurable layer is limited during the exposing step and preferably at least one of the type, power and incident angle of illumination of the at least one source of actinic radiation is altered during the exposure step. The resulting relief image comprises a plurality of dots and a dot shape of the plurality of dots that provide optimal print performance on various substrates, including corrugated board.

1 . A flexographic relief image printing element comprising at least one photopolymer layer on a backing layer, wherein a plurality of dots in relief are created in the at least one photopolymer layer by selectively exposing the at least one photopolymer layer to actinic radiation to selectively crosslink portions of the at least one photopolymer layer and separating and removing the uncrosslinked portions of the at least one photopolymer layer, and wherein said plurality of dots comprise at least one characteristic selected from the group consisting of:
 a) a planarity of a top surface of the dot is such that the radius of curvature of the top surface of the dot, r t , is greater than the total thickness of the at least one photopolymer layer; and   b) an edge sharpness of the dots is such that the ratio of the radius of curvature at the intersection of the shoulder and the top surface of the dot, r e , to the width of the top of the dot, p, is less than 5%; and   wherein the plurality of printing dots exhibit a compound shoulder angle such that an angle nearest the top surface of the dot, θ 1 , is greater than 70° and an angle nearest a dot floor attachment, θ 2 , is less than 45°.   
     
     
         2 . (canceled) 
     
     
         3 . (canceled) 
     
     
         4 . (canceled) 
     
     
         5 . The flexographic relief image printing element according to  claim 1 , wherein the ratio of r e :p is less than 2%. 
     
     
         6 . The flexographic relief image printing element according to  claim 1  wherein a dot relief of the printing element is greater than about 9% of the overall plate relief. 
     
     
         7 . The flexographic relief image printing element according to  claim 6 , wherein the dot relief of the printing element is greater than about 12% of the overall plate relief. 
     
     
         8 . A plurality of relief dots created in a relief image printing element, wherein the relief image printing element comprises, in order, a laser ablatable mask layer, at least one photopolymer layer, and a backing layer, wherein a plurality of dots in relief are created in the at least one photopolymer layer during a digital platemaking process, the digital platemaking process including the steps of (i) laser ablating a laser ablatable mask layer to create an in situ negative; (ii) selectively exposing the at least one photopolymer layer to actinic radiation through the in situ negative to selectively crosslink portions of the at least one photopolymer layer and (iii) separating and removing uncrosslinked portions of the at least one photopolymer layer, and
 wherein said plurality of relief dots comprise at least one geometric characteristic selected from the group consisting of:   (a) a planarity of a top surface of the relief dots, measured as the radius of curvature of the top surface of the dot, r t , is greater than the total thickness of the photopolymer layer;   (b) a depth of relief between the relief dots, measured as a percentage of the overall plate relief, is greater than about 9%; and   (c) an edge sharpness of the dots is such that the ratio of the radius of curvature at the intersection of the shoulder and the top surface of the dot, r e , to the width of the top of the dot, p, is less than 5%; and   wherein the plurality of printing dots exhibit a compound shoulder angle such that an angle nearest the top surface of the dot, θ 1 , is greater than 70° and an angle nearest a dot floor attachment, θ 2 , is less than 45°.   
     
     
         9 . The plurality of relief dots according to  claim 8 , wherein said planarity of the top surface of the relief dots is such that the radius of curvature of the top surface of the dot, r t , is greater than the total thickness of the photopolymer layer. 
     
     
         10 . (canceled) 
     
     
         11 . (canceled) 
     
     
         12 . (canceled) 
     
     
         13 . The plurality of relief dots according to  claim 8 , wherein the depth of relief between the relief dots is greater than about 12% of the overall plate relief. 
     
     
         14 . The plurality of relief dots according to  claim 8  wherein the edge sharpness of the relief dots is such that the ratio of r e :p is less than about 2 percent. 
     
     
         15 . (canceled) 
     
     
         16 . (canceled) 
     
     
         17 . The flexographic relief image printing element according to  claim 1 , wherein an average surface roughness of the relief image printing element is less than about 100 nm. 
     
     
         18 . The flexographic relief image printing element according to  claim 1 , wherein an average surface roughness of the relief image printing element is between about 700 and about 800 nm. 
     
     
         19 . A plurality of relief dots created in a relief image printing element, wherein the relief image printing element (i) laser ablating the laser ablatable mask layer to create an in situ negative; (ii) laminating an oxygen barrier membrane to a top of the relief image printing element; (iii) selectively exposing the at least one photopolymer layer to actinic radiation through the in situ negative and the oxygen barrier membrane to selectively crosslink portions of the at least one photopolymer layer; and (iv) developing the relief image printing element to separate and remove uncrosslinked portions of the at least one photopolymer layer, and
 wherein said plurality of relief dots comprise at least one geometric characteristic selected from the group consisting of:   (a) a planarity of a top surface of the relief dots, measured as the radius of curvature of the top surface of the dot, r t , is greater than the total thickness of the photopolymer layer;   (b) an edge sharpness of the dots is such that the ratio of the radius of curvature at the intersection of the shoulder and the top surface of the dot, r e , to the width of the top of the dot, p, is less than 5%; and   (c) a depth of relief between the relief dots, measured as a percentage of the overall plate relief, is greater than about 9%; and   wherein the plurality of printing dots exhibit a compound shoulder angle such that an angle nearest the top surface of the dot, θ 1 , is greater than 70° and an angle nearest a dot floor attachment, θ 2 , is less than 45°.