Electrophotographic (“EP”) printing involves transferring toner, or dry ink, to a substrate, such as paper, by means of an electric field and then fusing the toner to the substrate using a combination of heat and pressure. After fusing, the substrate is cooled, and excess charge is removed from the substrate. Conventionally, a release fluid is used during the fusing process to provide release of the substrate from the fusing roller. After fusing, cooling, and removing excess charge, the substrate exits the EP printing device, thereby completing the printing process. The substrate having an image fused thereon by an EP printing process is referred to as a “printed document” and may contain text, one or more images, or both. The low and medium density EP images are typically comprised of a halftone pattern of “dots” of individual dry ink particles. Image density increases as the amount of substrate covered by the dot pattern increases.
Commonly, the printed document subsequently is subjected to a finishing procedure. Examples of finishing procedures-include glossing, coating using ultraviolet (“UV”) radiation, and lamination. In the case of glossing, the printed document is subjected to a procedure that heats and casts the fused toner on the printed document to give it a glossy appearance. In the case of coating using UV radiation, the printed document is coated with a UV curable fluid and exposed to such UV radiation. In the case of lamination, a coating, such as plastic, is applied to the printed document and is heated under pressure to form a protective coating over the printed document.
For proper glossing, dry ink laydown must be continuous or offset will occur at the edges of the images. To that end, an inverse mask that applies more clear dry ink where there is less (or no) image is used. This results in continuous and thick layer of dry ink over the entire surface of print to be glossed.
The high gloss surface is generated by contact between a very smooth belt and a fused image in such a manner that sufficient heat is transferred to the image to cause it to completely conform to the smooth belt.