Method of Improving Print Performance in Flexographic Printing Plates
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 photo curable layer. Diffusion of oxygen 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 dots and a dot shape that provide optimal print performance on various substrates, including corrugated board.
1 . A flexographic relief image printing element comprising a plurality of dots in relief, and wherein at least a portion of 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 photopolymer layer;
b) a shoulder angle of the dot is such that either (i) the overall shoulder angle of the dot is greater than 50° or (ii) θ 1 is greater than 70° and θ 2 is less than 45°; and
c) an edge sharpness of the dots is such that the ratio of r e :p is less than 5%.
2 . The flexographic relief image printing element according to claim 1 , wherein shoulder angle of at least a portion of the dots is such that the overall shoulder angle is greater than about 50°.
3 . The flexographic relief image printing element according to claim 2 , wherein the shoulder angle of at least a portion of the dots is such that overall shoulder angle is greater than about 70°.
4 . The flexographic relief image printing element according to claim 1 , wherein the shoulder angle of at least a portion of the dots is such that θ 1 is greater than 70° and θ 2 is less than 45°.
5 . The flexographic relief image printing element according to claim 1 , wherein the ratio of r e :p is less than 2% for at least a portion of the dots.
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 . Relief dots created in a relief image printing element and forming a relief pattern, wherein said relief dots are created during a digital platemaking process, and wherein said 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 shoulder angle of the relief dots is such that (i) the overall shoulder angle is greater than 50°, or (ii) θ 1 is greater than 70° and θ 2 is less than 45°;
(c) a depth of relief between the relief dots, measured as a percentage of the overall plate relief, is greater than about 9%; and
(d) an edge sharpness of the relief dots, is such that the ratio of r e :p is less than 5%.
9 . The 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 i , is greater than the total thickness of the photopolymer layer.
10 . The relief dots according to claim 8 , wherein said shoulder angle of the relief dots is such that the overall shoulder angle is greater than 50°.
11 . The relief dots according to claim 10 , wherein the shoulder angle of the relief dots is such that the overall shoulder angle is greater than about 70°.
12 . The relief dots according to claim 8 , wherein the shoulder angle of the relief dots is such that θ 1 is greater than 70° and θ 2 is less than 45°.
13 . The 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 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 . The relief dots according to claim 8 , wherein said relief dots comprise the following geometric characteristics:
a) a planar top surface, such that the radius of curvature of the top surface to the dot, r t , is greater than the total thickness of the photopolymer layer;
b) an overall shoulder angle of the relief dots is greater than 50°;
c) a depth of relief between dots, measured as a percentage of the overall plate relief, is greater than about 9%; and
d) a ratio of r e :p is less than about 5%.
16 . The relief dots according to claim 8 , wherein said relief dots comprise the following geometric characteristics:
a) a planar top surface, such that the radius of curvature of the top surface to the dot, r t , is greater than the total thickness of the photopolymer layer;
b) a shoulder angle of the relief dots is such that θ 1 is greater than 70° and θ 2 is less than 45°;
c) a depth of relief between dots, measured as a percentage of the overall plate relief is greater than about 9%; and
d) a ratio of r e :p is less than about 5%.
17 . A method of using a photocurable printing element comprising a laser ablatable mask layer disposed on at least one photocurable layer to manufacture a flexographic printing plate comprising a plurality of printing dots, the method comprising the steps of:
(a) selectively laser ablating the laser ablatable mask layer to create an in situ mask and uncovering portions of the photocurable layer;
(b) selectively applying a diffusion barrier over at least portions of the uncovered photocurable layer;
(c) exposing the laser ablated printing blank to at least one source of actinic radiation through the in situ mask and the diffusion barrier to selectively cross link and cure portions of the photocurable layer,
wherein the diffusion of oxygen into the at least portions of the photocurable layer is limited by the diffusion barrier; and
wherein at least a portion of said plurality of dots comprise at least one characteristic selected from the group consisting of:
i) 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 photopolymer layer;
ii) a shoulder angle of the dot is such that either (i) the overall shoulder angle of the dot is greater than 50° or (ii) θ 1 is greater than 70° and θ 2 is less than 45°; and
iii) an edge sharpness of the dots is such that the ratio of r e :p is less than 5%.
18 . The method according to claim 17 wherein the oxygen diffusion coefficient of the diffusion barrier is less than 6.9×10 −9 m 2 /sec.
19 . The method according to claim 17 wherein the oxygen diffusion coefficient of the diffusion barrier is less than 6.9×10 −10 m 2 /sec.
20 . The method according to claim 17 wherein the oxygen diffusion coefficient of the diffusion barrier is less than 6.9×10 −11 m 2 /sec.
21 . The method according to claim 17 wherein the diffusion barrier is applied as a liquid and is selectively applied to portions of the printing element and then dried to a solid or semi-solid before exposure of the printing element to actinic radiation.
22 . The method according to claim 21 wherein the liquid is applied by ink jet.
23 . The method according to claim 17 , wherein shoulder angle of at least a portion of the dots is such that the overall shoulder angle is greater than about 50°.
24 . The method according to claim 17 , wherein the shoulder angle of at least a portion of the dots is such that overall shoulder angle is greater than about 70°.
25 . The method according to claim 17 , wherein the shoulder angle of at least a portion of the dots is such that θ 1 is greater than 70° and θ 2 is less than 45°.
26 . The method according to claim 17 , wherein the ratio of r e :p is less than 2% for at least a portion of the dots.