Flexographic printing mask with laser thermal imaging film

A method of forming a relief image, comprising:

• providing a relief-forming assembly comprising a mask and a relief-forming precursor in complete optical contact with the mask, wherein the mask includes:
• a transparent polymeric carrier sheet that is devoid of an ultraviolet radiation absorbing compound;
• a barrier layer on the transparent polymeric carrier sheet, wherein the barrier layer comprises non-crosslinked nitrocellulose and is devoid of an ultraviolet radiation absorbing compound;
• and
• an imaging layer on the barrier layer, wherein the imaging layer includes a non-silver halide thermally-ablatable material comprising non-crosslinked nitrocellulose having carbon black, and an infrared dye;
• imaged regions in the imaging layer, wherein the imaged regions have optical apertures that are substantially devoid of the carbon black and non-crosslinked nitrocellulose thereof;
• and
• non-imaged regions in the imaging layer, wherein the non-imaged regions have the non-crosslinked nitrocellulose, carbon black and infrared dye;
• exposing a relief-forming layer of the relief-forming precursor to curing ultraviolet radiation through the mask to form an imaged relief-forming layer with ultraviolet-exposed regions forming polymerized regions and non-exposed regions forming non-polymerized regions in the imaged relief-forming layer, wherein the exposing is with the mask having an optical density greater than 3.0 in the non-imaged region, and less than 0.20 in the imaged region;
• removing the mask from the imaged relief-forming layer;
• and
• developing the imaged relief-forming layer by removing the non-polymerized regions in the imaged relief-forming layer, thereby forming a relief image element having a relief image.