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发表于 2005-10-13 15:04:04
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Printing Method
Four major printing processes
Letterpress
Offset press / Lithography
Gravure
Screening press
Gravure / Intaglio Printing
In gravure printing, ink is transferred from a lowered surface to paper. The image area of a gravure plate is cut below or into the surface of the plate.
The entire surface of the plate is inked and then wiped clean. This leaves ink in the lowered areas of the plate. Paper is then pressed against the plate and ink transfers to it.
Paper is flexible. It can bend and stretch to get into the lowered areas of a gravure plate.
Gravure Plate Making
The first step in rotogravure is to expose a sheet of carbon tissue through a special screen and through line and continuous tone film positives. Carbon tissue is a coating of a light-sensitive emulsion on a paper backing sheet. Both type matter and illustrations are screened.
Light striking the emulsion of the carbon tissue causes it to harden. The degree of hardening depends upon the intensity of the light reaching the emulsion. The emulsion directly below the highlight areas becomes harder than the emulsion below middletone areas. Relatively little light passes through the shadow areas of the film and, therefore, the carbon tissue emulsion in these areas is relatively soft.
After exposure, the carbon tissue is adhered to copper-plated cylinder. After the tissue is adhered, the paper backing is removed and the unhardened gelatin emulsion areas are washed away with hot water.
The cylinder is then placed in an etching bath. The acid must penetrate the gelatin before it can etch the cylinder. Because the gelatin is softest in the shadow areas, the acid penetrates to the surface of the cylinder in these areas rather quickly. Gelatin in the highlight areas is rather hard, however, and the acid penetrates the highlight areas much more slowly.
The shadow areas are etched the deepest because they began to be etched first. Highlight areas are shallowest because they were etched last. Middletones are not as deep as the shadows, but they are deeper than the highlights.
The depth of the wells formed during etching determines the amount of ink each areas of the plate can hold. It is the ink variation that creates different tones in the printed image.
Letterpress
When ink is applied to raised surface and paper is pressed against the inked surface the ink is transferred to the paper. Relief and letterpress are terms that describe printing methods that use this principle. It transfers ink form a raised surface to paper.
Rubber stamp prints as this way.
On the plate, the letters should be backwards, then when transfer into papers, the letters would be “right reading”
Screen-process Printing
This printing method is to print by forcing ink through openings or holes in a stencil. After paper is placed under the printing screen, ink with a paint-like consistency is applied to the top of the screen. Finally, the ink is spread and forced through stencil openings onto the paper below the screen. This is done by pulling a rubber squeegee over the screen.
Screen Printing Plate making
Direct Photographic Stencils are prepared directly on the screen fabric. The screen is coated with a light-sensitive emulsion, then it is exposed to a source of light trough a film positive. Light passing through the clear areas of the film positive hardens the emulsion below. Because no light passes through the opaque areas of the positive, the emulsion behind these areas remains soft. After the stencil is exposed, the unhardened areas of the emulsion are washed away with water and form the openings in the stencil. Coat the screen under safelight conditions. Use a stiff piece to spread a thin, even coating of emulsion across the underside of the screen. After the emulsion dries, the screen is ready to be exposed. The sensitized screen is exposed through a film positive to a strong source of the light. During exposure the film positive and screen must remain in the tight contact. Then remove the unhardened emulsion representing the area to be printed by washing the screen under water until the image areas are clear. It persists about one minute.
Indirect Photographic Stencils are prepared by contact printing through films positive onto light-sensitive stencil materials. After the stencil is processed, it is adhered to the screen with water. A positive transparency is required to produce a photographic stencil. The image areas must be completely opaque. Non-image areas may be transparent or translucent. Positives may be prepared by contract printing a negative on process film. The stencil material is a light-sensitive emulsion on a plastic backing. A platemaker can be used to expose the stencil. A platemaker includes a vacuum frame for holding the film positive and stencil material in tight contact. Place the photographic stencil material, emulsion side down, on the rubber blanket of the vacuum frame. Place the film positive, emulsion side down, on the top of the stencil material. Light passing through the clear areas of the film positive will harden the stencil’s emulsion. Because no light passes through the opaque areas of the positive, the emulsion behind these areas will remain soft. The stencil is processed in a special developer. Remove the exposed stencil and place it, emulsion side up, into the tray of developer. Remove the stencil from the developer after about one minute. Then wash until the images appear. Adhere the photographic stencil to the screen immediately after washing.
Offset Press / Lithography
This kind of printing is based on the principle that grease and water do not mix.
The process works this way. First, a greasy image is placed on a flat plate. Usually the image may be placed on the plate photographically. Next, water is applied to the plate. The water will cover the non-image are of the plate. Water will be repelled from the image area because water and grease do not mix. The entire plate is then coated with ink. Ink is a greasy substance and adheres to the greasy image. Ink is repelled from the wet areas of the plate because water and grease do not mix. Paper is then pressed against the surface of the plate and the inked image is transferred to the paper. The printing image is formed chemically by making some areas of the plate grease receptive and water repellent, while others remain water receptive and grease repellent.
To minimize wear the image on the plate is first transferred to a rubber blanket. The paper does not make contact with the printing plate. |
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