2015年5月21日星期四

The Different Processes of Printing Mechines

The five major printing processes are distinguished by the method of image transfer and by the general type of image carrier employed. Depending upon the process, the printed image is transferred to the substrate either directly or indirectly. In direct printing the image is transferred directly from the image carrier to the substrate, examples of direct printing mechines are gravure, flexography, screen printing and letterpress printing processes.In indirect, or offset, printing, the image is first transferred from the image carrier to the blanket cylinder and then to the substrate. Lithography, currently the dominant printing technology, is an indirect (offset) process.

Image carriers (or plates) can generally be classified as four types: relief, planographic, intaglio, or screen.

1. In relief printing, the image or printing area is raised above the nonimage areas. Of the five major printing processes, those relying on relief printing are letterpress and flexography.

2. In planographic printing, the image and nonimage areas are on the same plane. The image and nonimage areas are defined by differing physiochemical properties. Lithography is a planographic process.

3. In the intaglio process, the nonprinting area is at a common surface level with the substrate while the printing area, consisting of minute etched or engraved wells of differing depth and/or size, is recessed. Gravure is an intaglio process.

4. In the screen process (also known as porous printing), the image is transferred to the substrate by pushing ink through a porous mesh which carries the pictorial or typographic image.

Each printing process can be divided into three major steps: prepress, press, and postpress.

1. Prepress operations encompass that series of steps during which the idea for a printed image is converted into an image carrier such as a plate, cylinder, or screen. Prepress operations include composition and typesetting, graphic arts photography, image assembly, and image carrier preparation.

2. Press refers to actual printing operations.

3. Postpress primarily involves the assembly of printed materials and consists of binding and finishing operations.

Within each process, a variety of chemicals are used, depending on the types of operation involved. Prepress operations typically involve photoprocessing chemicals and solutions. Inks and cleaning solvents are the major types of chemicals used during press operations. Depending on the finishing work required, postpress operations can use large amounts of adhesives.

This is especially true where the production of books and directories is involved. Of all the chemicals used in a typical printing plant, inks and organic cleaning solvents are the categories used in the largest quantities. Many of the chemicals used in the printing industry are potential hazards to human health and the environment.
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