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Photoengraving is a process that uses a light-sensitive photoresist applied to the surface to be engraved to create a mask that shields some areas during a subsequent operation which etches, dissolves, or otherwise removes some or all of the material from the unshielded areas. Normally applied to metal, it can also be used on glass, plastic and other materials. A photoresist is selected which is resistant to the particular acid or other etching compound to be used. It may be a liquid applied by brushing, spraying, pouring or other means and then allowed to set, or it may come in sheet form and be applied by laminating. It is then exposed to light—usually strong ultraviolet (UV) light—through a photographic, mechanically printed, or manually created image or pattern on transparent film. Alternatively, a lens may be used to project an image directly onto it. Typically, the photoresist is hardened where it receives sufficient exposure to light, but some photoresists are initially hard and are then softened by exposure. A solvent is used to wash away the soft parts, laying bare the underlying material, which is then bathed in or sprayed with the acid or other etchant. The remaining photoresist is usually removed after the operation is complete. In the graphic arts, photoengraving is used to make printing plates for various printing processes, reproducing a wide variety of graphics such as lettering, line drawings and photographs. The same procedure is used to make printed circuit boards, foil-stamping dies and embossing dies. It is also used to make nameplates, commemorative plaques and other decorative engravings. It can be used to make flat springs, levers, gears and other practical components that would otherwise be fabricated from sheet metal by cutting, drilling, jigsawing or stamping. A very high degree of precision is possible. In these applications, it is properly called photochemical machining, but the terms photochemical milling, chemical milling and photoetching are sometimes used. A similar process called photolithography is used to make integrated circuits. ==Methods== One method of photoengraving produces a shallow depression in the metal. This is used for intaglio printing plates or for decorative purposes. It is also the same method used for printed circuit boards. The engraving is usually made in copper or brass. The process can be done in open trays but is much more effective if the etchant (often ferric chloride) is sprayed onto the metal. When ferric chloride is used as the etchant, no metal parts other than titanium can be used in the etching equipment. Decorative engraving is often filled by spray-painting then sanding to remove the paint from the raised parts of the engraving. Another method produces a deep engraving with sloped shoulders. In this method, the metal (usually zinc or magnesium) is held face down and a mixture of nitric acid and a soap-like oil is splashed onto it. As the acid etches the surface, the oil adheres to the edges of the exposed area. This progressively reduces the area being etched, resulting in a sloped edge; a single dot will end up as a cone-shaped mound protruding from the etched area. This method is used for printing plates (the shoulder supports the printing surface), foil stamping dies and embossing dies. Decorative engravings made by this method may go through a second process to produce a decorative background. The raised parts and their shoulders are painted with an etchant-resistant material and a pattern of etchant-resistant material is applied to the deep parts of the engraving. The resist for the background may be another photoengraving or may be randomly splashed on. The engraving is etched again for a short time to produce a raised pattern in the background. Decorative engravings of this type may also be spray-painted and sanded as in the previous method. In traditional print shop practice, a special very-large-format camera is used to image the source material either directly onto the photosensitive coating, or onto a sheet of photographic film which is then developed and contact-printed onto the coated plate. In large-scale commercial printing, computer-driven optoelectronic equivalents began to replace these methods in the 1970s. In the case of line cuts (graphics in solid blacks and whites without gradations of gray or color), the photoengraving is done on zinc, and the result is called a zinc etching. In the case of halftone cuts, the work is done on copper. The halftone effect is accomplished by photographing the subject through a wire or glass screen, which breaks the image up into a pattern of dots with sizes corresponding to the local brightness of the image; the larger dots create the darker areas, the smaller dots the highlights. The finer the screen, the finer the detail possible in the printed product. Halftones made with a screen having 65 lines to the inch are considered coarse. Those having 150 lines to the inch are considered fine. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「photoengraving」の詳細全文を読む スポンサード リンク
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