Processes and apparatus for producing photopolymerizable, cylindrical, continuous, seamless flexographic printing elements

Processes comprising providing a layer composite comprising a layer of photopolymerizable material and a removable substrate film disposed on one side of the layer of photopolymerizable material, wherein the photopolymerizable material comprises an elastomeric binder, an ethylenically unsaturated monomer and a photoinitiator; and (a) miter-cutting two edges of the layer composite to be joined, to form two miter-cut ends; (b) providing a hollow cylinder on a rotatably mounted support cylinder, wherein the hollow cylinder has an outer surface and is in a temporarily locked position relative to the support cylinder; (c) applying an adhesion-promoting layer to the outer surface of the hollow cylinder; (d) applying the layer composite to the hollow cylinder bearing the adhesive-promoting layer, wherein the layer composite is disposed on the hollow cylinder such that the removable substrate film faces away from the hollow cylinder and the two miter-cut ends lie substantially one on top of the other, but not overlapping; (e) removing the substrate film from the layer of photopolymerizable material to provide an exposed surface of the layer of photopolymerizable material; (f) contacting the exposed surface of the layer of photopolymerizable material with a rotating calender roll with heating to join the two miter-cut edges and form a processed hollow cylinder, wherein heating comprises a locally-acting heat source moved in an axial direction relative to the hollow cylinder along the entire width of the layer of photopolymerizable material such that the exposed surface is heated; and (g) removing the processed hollow cylinder from the support cylinder.