Relief printing processes work on the same principle as a rubber stamp. Ink is applied to the raised portions of the printing surface, and is then transferred by pressure to paper or some other substrate. Two forms of relief printing—letterpress and flexography—are currently in use, distinguished by the physical characteristics of their printing surfaces and inks. Letterpress printing is accomplished using a hard metal or plastic printing surface and a highly viscous ink. Flexography employs a soft rubber or plastic printing surface and a fluid ink.
Letterpress, the oldest form of printing, originated with the invention of movable metal type in the middle of the 15th century and was for five centuries the only viable mass printing process. In the mid-20th century, letterpress printing, despite its superiority in the clarity of impression and in the density of ink, lost its predominance to lithography, a much faster process.
Originally, letterpress printing surfaces were prepared by assembling thousands of pieces of metal type on which individual letters or letter combinations were cast in relief to create pages of text called type forms. Ink was applied to the raised areas of the form and then transferred under pressure to paper or vellum. Woodcuts and engravings could be combined with type to produce composite pages containing both text and graphics.
The first letterpress printing plate was created by making a plaster mold of a type form and then casting a metal duplicate of the original, called a stereotype. Stereotyping became an extremely important technology during the Industrial Revolution because it yielded a one-piece printing surface that could be used in place of the original type form on a variety of automated printing presses. Curved stereotypes cast from papier-mâché molds were used on rotary letterpresses for printing daily newspapers until the early 1970s, when hot-metal machine typesetting was largely replaced by computer typesetting.
Another important duplicate plate, called an electrotype, was made by electroplating a thin layer of copper onto a wax impression of the original type form and then filling the resulting copper shell with type metal. Electrotypes retained more detail from the original relief surface than stereotypes and were therefore preferred to stereotypes for higher-quality letterpress printing.
In the late 1950s a radical new way of making relief printing surfaces was introduced; it employed a soluble plastic that hardened upon exposure to ultraviolet radiation. Since then a large number of photopolymer plate materials have been created. A thick coating of photopolymer on a metal or plastic support can be exposed to ultraviolet light through a piece of film that allows the light to pass through only those areas that will transfer ink. The photopolymer hardens, or polymerizes, in these areas, and the remaining unexposed coating is washed away with water or some other solvent. The result is a relief printing surface than can be mounted directly on a printing press.
In a variation of this process, a liquid photopolymer that solidifies when exposed to ultraviolet radiation is spread on a paper or plastic support. After exposure the unexposed liquid is blown away with air. These plates can be made rapidly and are therefore most suitable for newspaper printing, where deadlines are critical.
High-speed rotary web presses and photopolymer plates have allowed letterpress to remain competitive in some areas, such as in newspaper printing, despite the fact that lithography is now the uncontested leader among printing processes.
| Flexographic Printing |
The soft plates and highly fluid inks used in flexography make the process ideal for printing on nonporous materials such as foil laminates and polyethylene. Originally, all flexographic plates were made of molded rubber, which is still the preferred material when multiple copies of the same image are needed on a single printing cylinder. Rubber plate molds are impressions of original relief surfaces, such as type forms or engravings, and are normally used to make several duplicate rubber plates. The preparation of a printing cylinder using molded rubber plates is a time-consuming process because many rubber plates are mounted on a single cylinder and each plate must be carefully positioned in relation to the others.
In the 1970s photopolymer plate materials were introduced, and the time required to manufacture and mount a set of plates was reduced significantly. This has allowed the process to enter new markets, most notably newspaper printing. In addition, water-based inks can be used in flexography, eliminating the need for toxic solvents.
Flexographic printing presses are simple in design because the fluid ink is easily distributed to the printing surface without an elaborate inking system. Printing is usually done on rolls or webs of substrate rather than on cut sheets, and the printed rolls are then converted into finished products in a separate manufacturing process.
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