Method of pre-exposing relief image printing plate
View Patent ↗The present invention is directed to a method of selectively pre-exposing a photosensitive printing element prior to imagewise exposure in order to remove oxygen from the photosensitive layer prior to imagewise exposure. The invention is usable in a computer-to-plate process to produce flexographic relief image printing elements.
1. A method of producing a relief image printing element comprising:
providing a photosensitive printing element, said photosensitive printing element comprising a photosensitive layer disposed on a base substrate, wherein a desired image is creatable in a surface of said photosensitive layer by selectively curing said surface to create cured and uncured areas in said surface of said photosensitive layer;
selectively pre-exposing portions of said surface of said photosensitive layer with actinic radiation at a selected first intensity of a radiation source to quench dissolved oxygen in the selectively pre-exposed portions of the photosensitive layer prior to imagewise exposure wherein the selected first intensity is not an intensity that cures the photosensitive layer;
selectively imagewise exposing said surface of said photosensitive layer with actinic radiation at a selected second intensity of said radiation source to create cured and uncured areas; and
selectively removing uncured portions of the photosensitive layer to reveal the relief image;
whereby the selective pre-exposure of the photosensitive layer provides increased definition of the relief image surface.
2. The method according to claim 1 wherein the radiation source is a laser.
3. The method according to claim 2 , wherein the laser is an ultraviolet laser.
4. The method according to claim 1 wherein said portions of said surface are selected from the group consisting of highlight dots, isolated dots, fine lines, and one or more of the foregoing.
5. The method according to claim 1 , wherein said pre-exposing step is performed through a film or negative that provides for varying levels of opacity in accordance with a desired final image.
6. The method according to claim 5 , wherein the radiation source is selected from the group consisting of arc lamps, fluorescent lamps, plasma lamps, and light emitting diodes.
7. The method according to claim 1 , wherein the pre-exposing step is performed using a radiation source selected from the group consisting of lasers, LED arrays, and other focused actinic radiation beam sources, wherein intensity and position of the radiation source are controlled, whereby a preferential pre-exposure of the portions of the surface of the photosensitive layer is achieved.
8. The method according to claim 7 , wherein the laser is used to project an inverted image of a desired pattern on the surface of the photosensitive layer using the first intensity of the laser, wherein the inverted image has an intensity inversion as compared to the second intensity used to imagewise expose the photosensitive layer, whereby areas that receive the lowest intensity during the pre-exposing step receive the most intensity during the imagewise exposing step.
9. The method according to claim 8 , wherein immediately after the pre-exposing step, the photosensitive layer is exposed to the laser in an image of the desired pattern on the surface of the photosensitive layer at the second intensity of the laser.
10. The method according to claim 9 , wherein the second intensity is between about 20 mJ/cm 2 and about 2000 mJ/cm 2 .
11. The method according to claim 9 , wherein the radiation is applied at the second intensity for about 30 nanoseconds to about 3 microseconds.
12. The method according to claim 8 , wherein the first exposure dose, received by the fine feature areas, is between about 5 mJ/cm 2 and about 250 mJ/cm 2 .
13. The method according to claim 8 , wherein the radiation is applied at the first intensity for about 1 millisecond to about 5 seconds.