General Overview of Printing Processes
The five major printing processes, lithography, letterpress, flexography, gravure, and screen printing, 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, which is paper in our case, either directly or indirectly. In direct printing the image is transferred directly from the image carrier to the substrate.
Examples of direct printing 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 one of four types: relief, planographic, intaglio, or screen. 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. 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. 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. 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.
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. Press refers to actual printing operations. 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.
Prepress consists of those operations required to convert the original idea, such as a photo or sketch, for a printed image into a printing plate or other image carrier. Prepress steps include composition and typesetting, graphic arts photography, image assembly, color separation, and image carrier preparation. With the exception of image carrier preparation, the prepress process is similar for the five major printing processes. Plateless process do most of the prepress steps using a computer.
During composition, text, photographs and artwork are assembled to produce a "rough layout" of the desired printed image. The rough layout is a detailed guide used in the preparation of the paste-up or camera ready copy from which an image carrier can be produced.
Traditionally, rough layouts and pasteups were composed by hand using: drafting boards; light tables; various paste-up tools such as technical pens, rulers, and cutting tools; and adhesives. The text used in the paste-up was typeset and printed mechanically. However, composition has changed dramatically with the advent of computers. Both type and artwork can be generated and edited using computers. Computer systems can be equipped with both optical character recognition and photographic image scanners and digitizers so that pretyped material and photographic images can easily be incorporated into the document being composed. With the systems now available, the computer can directly drive the typesetting and image carrier preparation processes once the page or entire document is laid out and ready for printing.
Typesetting operations assemble the type characters into pages. There are a number of methods of typesetting including manual assembly of pieces of metal type (letterpress), mechanical assembly of lines of type, and phototypesetting. Until the 1950s, the majority of typesetting was performed using the Linotype machine which produces a "slug" or line of type from molten metal. Similar machines produced single characters of type. Today phototypesetting devices have almost completely replaced manual and mechanical methods of typesetting.
Phototypesetting devices, first demonstrated in the late nineteenth century, were introduced commercially in the early 1950s. They rapidly overtook the Linotype and similar machines in importance. In phototypesetting, individual type characters or symbols are exposed onto photographic film or paper. In early mechanical phototypesetting units, entire fonts of characters were stored as negatives on film. In the later generations of computer-driven phototypesetters, the image is generated electronically, and, in the latest generation of units, a laser is used to project the image onto the photographic film or paper. Phototypesetting produces high contrast, high resolution images ideal for printing purposes. Other computer driven output devices, which include strike-on, line, ink-jet, and laser printers are used extensively in-plant printing applications.
Copy Assembly and Process Photography
Copy assembly consists of bringing all original work (text, pictures, and illustrations) together and preparing photographic images. The photographic images are in the form of either positive or negative films and are used for photomechanical image carrier preparation. Copy must be set up correctly to ensure the finished image carrier will produce a high quality print. Assembled copy that is ready for the photographic process is called a flat. When copy of various sizes and shapes is assembled for transfer to film the process is called image assembly or stripping. The printing industry depends heavily on the use of highly specialized photographic equipment, methods, and materials to produce high quality printed material. Process photography refers to the photographic techniques used in graphic arts. Prior to the invention of electronic page making systems, virtually all printing processes employed photomechanical methods of making image carriers.
Two important types of photography used in the preparation of image carriers are line and halftone photography. Neither of these processes can be used to print a true continuous-tone photograph (i.e., a photograph with intermediate or graduated tones) though halftone can achieve the illusion of continuous tones. Letterpress, lithography, screen printing and some gravure methods involve both these types of photography.
Line photography is used to produce high contrast images on film. Image areas on the film are solid black; little or no illusion of intermediate tones can be achieved with this method.
As noted above, by using halftone photography the illusion of intermediate tones can be achieved for letterpress, lithography, lateral dot gravure, and screen printing. In halftone photography, continuous-tone images are broken down into high-contrast dots of equal density but varying sizes and shapes. (Depending upon the type and quality of printing being done, the density of dots varies from 24 to 120 per centimeter). If, for example, very small dots are used in one area of an image, that area appears to be lighter than those areas of the image where larger dots are used. This occurs because more of the lighter color substrate remains visible in the areas where the very small dots are used.
Some form of image carrier is used in each of the five printing processes that now dominate the industry. The image carrier, often a plate, is used to transfer ink in the form of the image to the substrate. The image carrier must pick up ink only in the areas where ink is to be applied to the final image on the substrate. It must also reject ink in the areas of the image where it is not wanted. Relief plates used in letterpress and flexographic printing have raised areas that pick ink up from the inking source. Non-printing areas are recessed below the level of the inking rollers and therefore are not coated with ink.
The reverse of a relief plate, the printing areas of a gravure image carrier are recessed below the level of the non-printing areas. The depressions, referred to as cells, pick up small amounts of ink as they pass through an ink fountain. The ink is then passed to the substrate from the cells. The surface of the plate is constantly scraped clean with a doctor blade so that no ink is retained except in the cells. Most gravure presses use a cylindrical image carrier, although some sheet-fed gravure presses and intaglio plate printing presses use a flat plate.
Planographic plates, used in offset lithography, have both the image and non-image areas on the same plane. The image and non-image areas of the plate are each defined by differing physicochemical properties. The image areas are treated to be hydrophobic (water-repellant ) and oleophilic (oil receptive). Ink will adhere to these areas. The non-image areas, on the other hand, are treated to be hydrophilic (water loving), and will not accept ink.
The image carrier in screen printing consists of a porous screen. A stencil or mask of an impermeable material is overlaid on the screen to create the non-image area. The image is printed by forcing ink through the stencil openings and onto the substrate. The stencil openings determine the form and dimensions of the imprint produced.
The primary method of image carrier preparation is the photomechanical process where a printing image is produced from a photographic image. Typically, with this process, a light sensitive coating is applied to a plate or other type of image carrier. The plate is then exposed to a negative or positive of a photographic image. The exposed plate then undergoes further processing steps.
There are other methods of image carrier preparation: manual, mechanical, electrochemical, electronic, and electrostatic. Some of these processes, such as the manual and the mechanical processes, are of little or no commercial importance. Other processes, such as the electromechanical preparation of gravure cylinders, are discussed within the gravure process description.
Photomechanical Image Carrier Preparation
Photomechanical image carrier preparation begins with a plate, cylinder or screen that has been treated with a light-sensitive coating. (The types of light-sensitive coatings used are discussed in the following section.) The coated plate is exposed to light that has first passed through a transparent image carrier such as a film positive or negative. The exposed plate is then processed to produce a plate with defined printing and non-printing areas. Typically, the exposed areas on the plate are resistant to the developing solutions used to process the plate, though in some cases the opposite is true. In either case, during processing the soluble areas of the coating are washed away while the insoluble areas remain on the plate. At this point image carriers produced from film negatives are essentially finished. The insoluble areas of coating remaining on the plate become the ink carrier during printing. Letterpress plates and lithographic surface plates are produced this way.
With image carriers made from film positives, the insoluble coating serves as a protective barrier during a further processing step called etching. The coating on this type of image carrier is often referred to as a "resist" because it resists the acid used to etch the plate surface. Image carriers produced by this method are used in lithography, gravure, and screen printing.
Light-sensitive Coatings
The three most important light-sensitive coatings used on image carriers are photopolymers, diazos, and bichromated colloids. Silver-halide and electrostatic coatings are used infrequently for special purpose plates used in duplicating equipment.
Most image carriers (printing plates) are made using any of a number of different types of photopolymeric coatings. These coatings are characterized by the type of reaction they undergo upon exposure to UV light: photopolymerization, photocrosslinking, photo arrangement, and photo degradation. A well known example of a photopolymer coating is Kodak Photo Resist (KPR), a photo cross-linking polymer, which is used in image carrier preparation for all major printing processes as well as in the preparation of printed circuit boards.
Depending on the type of image carrier being produced, the hardened photopolymer coating may remain on the image carrier as either the image or non-image area following processing. Photopolymer coatings are characterized by wearability, temperature and humidity stability, and long storage life. Some also exhibit good solvent resistance. For example, if baked prior to use, lithographic plates produced using photopolymer coatings can be used for press runs in excess of one million impressions.
Diazo Coatings, introduced in the printing industry around 1950, are used primarily for coating both presensitized and wipe-on lithographic surface plates. For presensitized plates, the diazo coating is applied by a machine called a whirler which spreads the coating on the rotating plate.
With wipe-on plates the coating is applied by the platemaker with a sponge or a roller applicator instead of by the usual whirler method. Diazo coatings are very thin and susceptible to abrasion and wear during the printing run and generally are used for short press runs of 75,000 impressions or less. However, pre-lacquered plates, plates supplied by the manufacturer with a lacquer impregnated in the plate coating, offer superior abrasion resistance and can be used for press runs in excess of 100,000 impressions. Most diazo plates have negative-process coatings, though positive process coatings are also used. Diazo coatings are used to presensitized deep-etch and bi-metal plates. Additionally, diazo is used to sensitize some colloid coatings.
The diazo resin most often used for plates is the condensation product of
4-diazodiphenylamine salt with formaldehyde. Diazo oxides such as
pyridol[1,2-a]benzimidazol-8-yl-3(4H)-diazo-4(3H)-oxo-1-
naphthalenesulfonate are also used (Kirk-Othmer).
Diazos are not usually affected by temperature and relative humidity and have a relatively long storage life. They can be processed by automatic plate processing machines which speed up production and result in much higher quality plates than manual methods. Automatic processing equipment can perform plate coating and exposure all in one continuous process. These machines are used extensively in newspaper printing.
Bichromated colloid coatings were widely used until the early 1950s; limited use continues today. They consist of a light sensitive bichromate and a collodion. The bichromate of choice is ammonium bichromate, with potassium bichromate used in special processes such as collotype. A collodion is an organic material that is capable of forming a strong continuous coating when applied to the image carrier. Colloid ions used for photoengraving are shellac, glue, albumin, and polyvinyl alcohol. Albumin, casein, alpha protein, polyvinyl alcohol, and gum arabic are used for lithography. Gelatin is used mostly for gravure, screen printing, and collotype. The colloid is formed when the finely divided bichromate and the collodion are mixed. Applied to the image carrier and exposed to light, the colloid forms an continuous, insoluble coating.
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