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[BKARTS] more photopolymer



"Offset lithography plates are by definition flat (using the oil/water
> resist described earlier). Sheets printed in this way will always be
> flat. Photopolymer plates for offset lithography would not be an
> exception."  --Don Drake

That's right. Flexography or letterset are the correct terms for this type of printing, but "dry offset" is a misnomer that is sometimes used. There is a "waterless offset" process which uses a silicone coated plate. During plate development the silicone is removed from the image area of the plate. The ink adheres to the image area but not to the siliconized non-image areas. No water is needed. The image areas sit slightly below the non-image areas which creates a "well" to hold the ink. Another form of waterless offset printing is gravure offset which actually etches the image onto a plate which is inked, then surface ink is removed by a "doctor" blade. National Geographic magazine is printed this way. 

The photopolymer that is used in offset printing is a light-sensitive emulsion that coats the plate. Offset plates are made with a variety of materials including aluminum (the majority of plates used), plastic, and paper. Once an image has been transferred to the plate via a light source, the polymer hardens. Photopolymer that is not exposed to the light is removed. The photopolymer layer is very thin--these plates are quite different from the photopolymer plates that are used in letterpress and flexography printing. --h

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