LETTERPRESS

Letterpress is printed by the relief method. It is the only process which can use type directly. Printing is done from cast metal type or plates on which the image or printing areas are raised above the non-printing areas. Ink rollers touch only the top surface of the raised areas. The surrounding (non-printing) areas are lower and do not receive ink. The inked image is transferred directly to the paper.

FLEXOGRAPHY

Flexography is a form of rotary web letterpress using flexible rubber plates and fast-drying solvent or waterbased inks. The rubber plates are mounted to the printing cylinder with double-faced adhesive. Plates are sometimes backed with thin brass or other metal sheets and attached to the cylinder with fastening straps for close register. Most anything that can go through a web press can be printed by flexography. Printing by the flexography process ranges from decorated toilet tissue to bags, corrugated board and materials such as foil, hard-calendared papers, cellophane, polyethylene and other plastic films. It is well suited for printing large areas of solid color. Inks can be overlaid to obtain high gloss and special effects. With some types of papers having excessively high absorptive quality, flexography is uneconomical because of the fluidity of the ink. However, inks can be formulated to prevent too rapid absorption.

THERMOGRAPHY (RAISED PRINTING)

Thermography is a process, which creates special effects in printing such as stationery, invitations, greeting cards, and paper decoration. A raised surface of printing resembling genuine engraving is formed without using costly engraving dies. Special non-drying inks are used in conventional printing, either by letterpress or offset, and the wet inks are dusted with a powdered compound. After the excess powder on the non-printing areas is removed by suction, the sheet passes under a heater, which fuses the ink and powdered compound. The printing swells or rises in relief to produce a pleasing engraved effect.

GRAVURE

Whereas letterpress uses a raised (relief) surface, gravure uses a sunken or depressed surface for the image. Gravure is an example of intaglio printing. The image areas consist of cells or wells etched into a copper cylinder or wraparound plate, and the cylinder or plate surface represents the non-printing areas. The plate cylinder rotates in a bath of ink. The excess is wiped off the surface by a flexible steel doctor blade. The ink remaining in the thousands of recessed cells forms the image by direct transfer to the paper as it passes between the plate cylinder and the impression cylinder. Gravure printing is considered to be excellent for reproducing pictures, but high plate-making expense usually limits its use to long runs. A distinctive feature for recognizing gravure is that the entire image must be screenedÑtype and line drawingsÑ as well as halftones. The gravure screen usually contains 150 lines per inch, about the same screen ruling as used in other processes. It is virtually invisible to the naked eye.

OFFSET LITHOGRAPHY

This is the fastest growing of the four major printing processes. Lithography uses the planographic method. The image and non-printing areas are essentially on the same plane of the surface of a thin metal plate, and the definition between them is maintained chemically. Printing is from a plane or flat surface, one that is neither raised nor depressed. Two basic differences between offset lithography and other processes are: (1) it is based on the principle that grease and water do not mix, and (2) ink is offset first from plate to rubber blanket, and then from blanket to paper.

SCREEN PRINTING

Formerly known as silkscreen, this method employs a porous stencil. A fine silk, nylon, dacron, or stainless steel screen is mounted on a frame. A stencil is produced on the screen, either manually or photo mechanically, in which the non-printing areas are protected by the stencil. Printing is done on a simple press by feeding paper under the screen, applying ink with a paint-like consistency to the screen, and spreading and forcing it through the fine mesh openings with a rubber squeegee.

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