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Different Types of Printing

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UNDERSTANDING PRINTING

Before you embark upon your career in printing, it would be wise to acquire an understanding of the various ways in which words and pictures are put on paper today. This understanding of the basics of printing will help you no matter which aspect of the industry you decide to enter. A list of printing terms and their definitions appears in the glossary (Appendix E) in the back of this book.

First, let's have a look at the various methods of printing:
  • Lithography



  • Letterpress

  • Flexography

  • Gravure

  • Screen printing
• Lithography

The lithographic method of printing is by far the most common, accounting for about one-half of all U.S. printing. It is also known as plan-o-graphic printing, offset lithography, and the photo offset process. This method of printing has become popular only in this century. Its simplicity has fostered the growth of thousands of instant printing centers in the United States. It is based on a simple natural phenomenon: Water and oil repel each other.

Here's how the process works: A thin sheet of metal is coated with an oil-based substance called an emulsion. The metal sheet, or plate, is placed in contact with a film, called a negative, on which an image has been photographically developed. The portions of the image that are to be printed are so dense that light cannot pass through them. Portions of the image that are not to be printed are left clear on the negative. A bright light is turned on briefly. Light strikes the emulsion coating on the plate wherever the negative has been left clear. The light causes a chemical reaction in the emulsion. Where the images exist on the negative, no light can pass through, and the emulsion beneath it remains undisturbed.

The plate is then removed from contact with the negative and washed in a chemical solution. The solution attacks the portion of the plate where light was permitted to pass and the emulsion is washed away, leaving the metal bare. Where light did not strike the emulsion, the solution has no effect, and the emulsion remains and hardens.

The plate is then strapped to a cylinder on the printing press. As it revolves, it is first coated with a thin application of water. The water will readily wet the bare metal. It will be repelled by the emulsion image left on the plate. As the plate continues turning on the cylinder, it comes into contact with an inked roller. You should understand that printing ink is an oil-based product. Since water and oil repel each other, no ink is deposited on the wet plate in the blank areas. However, the ink readily adheres to the emulsion-coated areas.

The plate cylinder now comes into contact with a rubber blanket cylinder and the image is transferred. As the blanket cylinder revolves, it next comes into contact with the printing paper, and the image is again transferred, this time to the paper. Because the image is transferred twice, it is "right reading" on the plate and on the paper (meaning reversed as to left and right). It is "wrong reading" on the blanket cylinder. This transferring of the image from plate to blanket to paper is why the process is called offset printing.

• Letterpress

Letterpress printing, also called relief printing, is the oldest of the printing processes. Whereas in offset lithography a thin metal plate is used, letterpress printing employs a much thicker, rigid metal plate or metal type. The plate has two surfaces-a raised surface, which is the original thickness of the plate, and an undercut surface. The undercutting of the plate is accomplished by etching the plate with acid. Areas that are to be printed are "stopped" by a gummy substance that resists acid. Those areas not to be printed are left bare. The plate is then immersed in a powerful acid bath.

The bare metal is eaten away by the acid, to a depth where the ink roller cannot reach the valleys. The plate is then washed clean and the acid resist removed. The plate is then strapped to the bed of the press or to a curved cylinder. As the press operates, the plate is brought into contact alternately with an ink roller and the paper to be printed. The plate picks up ink on its raised surface and deposits it on the paper. In letterpress, the image is wrong reading on the plate and right reading on the paper.

Until the later 1800s, letterpress was the most widely used process in the production of newspapers, magazines, books, and other volume products. In the United States and other industrial nations, offset printing has supplanted letterpress in most, if not all, these applications. However, letterpress is still common throughout Latin America and the Developing World.

• Flexography

Flexography is a printing process related to letterpress, in as much as it employs a raised printing surface, which is inked and applied to the paper. When you use a rubber stamp, you are using the basic elements of the flexographic process: you apply wet ink to raised flexible type and then press the type against a sheet of paper. The difference is that flexography employs a flexible plate, usually made of rubber or photopolymer, and specially prepared inks are used. Applications include packaging materials such as cardboard and plastic films.

• Gravure

Gravure printing is similar to letterpress printing in that a relatively thick and rigid metal plate is used. Ink is applied to the plate or type with a roller, and the plate is then pressed against a sheet of paper. The main difference is that instead of being raised, the image is dug out of the surface of the plate. (The process is also called intaglio printing, which means incised carving.) Ink is then applied to the entire surface of the gravure plate and wiped away on a roll of special paper. After the wiping, ink remains trapped in the grooves of the plate. The plate is then pressed against the paper with enormous pressure. The ink is absorbed by the paper and the image reproduced on its surface. In gravure printing, the image is also wrong reading on the plate.

Gravure technology has changed rapidly in the past few years owing to computer software and electronic imaging techniques. Conferences and journals dedicated to the subject are laden with advanced technical breakthroughs and procedures that challenge students and seasoned masters alike.

Applications for gravure printing are large quantities of books, magazines, catalogs, and Sunday supplements, in which the pictures require better-than-average fidelity in color, such as art reproductions. A related process, called engraving, employs the same techniques as gravure but on a smaller scale. Applications include money, letterheads, and all sorts of documents and certificates.

• Screen Printing

The fifth process, screen printing, is also called porous printing or silk screening. It employs a cloth screen stencil on which an image has been drawn by hand, or photographically exposed. Those portions of the image that are to be printed are left clear on the screen. Those portions not to be printed are stopped by an impermeable resist (a substance that cannot be penetrated). The screen is placed in contact with the paper, ink is allowed to flow freely over the surface of the screen, and then a squeegee is wiped across the screen, forcing the ink through the silk screen mesh onto the paper. Applications for the process include outdoor posters, point-of-purchase advertising materials, and other products normally printed in limited quantities.

• Other Printing Processes

Two new printing processes that are now in general use are ink jet printing and electrostatic printing. In the first process, tiny droplets of ink are propelled onto the surface of the paper. There is no physical contact between plate and paper, as there is in all other printing processes. Electrostatic printing deposits colored particles of resin that are held in place by a magnetic force and then fused to the paper to form an image.
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