Printing
Letterpress
Offset Printing
Gravure
Other Printing Processes
Letterpress
Letterpress, also known as relief printing, is the oldest of the printing methods and probably the easiest to understand.
When we print a woodblock or use a rubberstamp, we are printing by letterpress. The area to be printed is raised; when the surface is inked, the surrounding area, being lower, receives no ink and therefore does not print. The ink is transferred from the printing plate directly to the paper by means of pressure.
Because letterpress plates involve photography and engraving, they are called photoengraved printing plates (more often referred to as engravings, cuts, or zincs), and the platemaker is called an engraver. Letterpress plates are usually made of zinc or magnesium, although copper, which is harder, is used for fine details and long runs.
To make a letterpress plate, a photographic negative is brought into contact with a light-sensitive metal plate. The plate and negative are exposed to a light that passes through the clear part of the negative and strikes the plate. The exposed areas are light-hardened. The plate is then dipped into an acid bath. The hardened image area of the plate is impervious to the acid, while the remaining areas are etched to the desired depth. This leaves the image area raised and ready to be inked.
The plate is then "blocked"; that is, mounted on a piece of wood or metal to make it type-high (.918"). After the plate is blocked, it is inked and a proof is pulled. A copy of the proof is sent to the client and the plate is sent to the printer.
Not all letterpress printing requires a printing plate: type that is handset or cast can simply be inked and printed. If an illustration or halftone is to be combined with the type, then an engraving is made for that piece of art only. Most letterpress jobs are an assortment of type and engravings all "locked up" together on the press. An advantage of this system is that elements can be changed without the expense of having to make an entirely new plate.
There are times when the printer may wish to convert from one printing method to another. This is particularly common in cases where a job that was originally printed by letterpress is to be printed by offset lithography. In this case, the printing plates are converted to film from which a new plate is made. Three conversion systems are Brightype, Cronapress, and Scotchprint. (See Glossary.)
In recent years a great number of jobs, especially newspapers, have been converted from letterpress to offset. This is not because of a lack of quality in letterpress printing but because of the time involved in plate-engraving. To counteract the flow of business away from letterpress to offset, the letterpress industry has developed the photopolymer direct relief (PPDR) printing plate. This is a photosensitive plastic plate which when mounted on metal can be used as a regular letterpress plate. The photopolymer plate drastically reduces the time required to make a letterpress plate, which is one of the major advantages of offset printing, while at the same time retaining the letterpress advantages of being able to use cheaper paper and ink.
CHARACTERISTICS OF LETTERPRESS
Capable of high-speed, high-quality printing of jobs of a variety of sizes in black and white and in color.
Consistent quality throughout run.
Printing plates are generally more expensive than offset, but considerably less expensive than gravure.
Capable of printing from handset or machine-set type as welt as from original or duplicate printing plates.
Duplicate plates are relatively expensive. The most commonly used duplicate plates are electroplates, plastic plates, rubber plates, and stereotypes.
Prints best on book papers. If halftones are to be printed, the paper must be calendered or coated. Also, accepts papers of any thickness from the very thinnest to cardboard, depending on the type of press.
When printing from original metal type or engravings, any part of the job can be changed without the expense of having to make entirely new plates. This does not apply when printing from one-piece duplicate plates or from curved plates (rotary).
Among the uses of letterpress are short-run printing from type and engravings: jobs requiring numbering (tickets, forms, etc.) and imprinting, imprint changes, on-press die-cutting, slotting. perforating. embossing. debossing, etc. Also used for fine long-run publication work where the cost of the press plates can be economically amortized over a number of printings.
Proofing is relatively inexpensive.
Lays down an ink film thicker than offset but not as thick as gravure.
Requires a lot of makeready time to compensate for the varying thicknesses of materials used: type, plates, engravings, etc.
Presses There are three basic types of letterpress printing presses: platen, flatbed cylinder, and rotary.
Platen. A press in which two flat surfaces come together to make an impression: one surface holds the printing plate; the other, called the platen, provides the pressure necessary to make the impression on the paper. There are two kinds of platen presses: the platen flat-bed (which was the press used by Gutenberg) and the platen clamshell (also called a jobbing platen). The clamshell is still in use today, mainly in commercial shops for short-run jobs of stationery, business cards, etc.
Flat-bed Cylinder. As the name implies, this press consists of a flat bed, which supports the printing plate or type form, and a cylinder which replaces the platen in providing the necessary pressure. These presses are usually larger and faster than platen presses. Excellent for booklets, catalogs, etc.
Rotary. Unlike the platen and the flat-bed cylinder presses, in which the type or engravings to be printed are flat, the rotary press has a curved printing plate that either fits on a cylinder or wraps completely around it. This permits printing at high speeds and is ideally suited to long runs of high-quality work, A rotary press can print from individual sheets of paper (sheet-fed) or from a continuous roll (web-fed). Other rotary presses, called perfecting presses, can print both sides of the paper at the same time.
Offset Printing
Offset Lithography
Of the three major printing processes offset lithography is the most recent. It is a highly refined form of lithography invented in 1799 by a German named Aloys Senefelder. Lithography, which means "stonewriting," is based on the principle that water and grease do not mix. The image to be printed is drawn with a special grease crayon on a slab of highly polished limestone. The stone is then sponged with a solution of water, gum arabic, and acid. This solution is rejected by the greasy image area and absorbed by the nonimage area. When the stone is inked, the opposite happens: the ink, which is greasy, is accepted by the image area and rejected by the non-image area. To print the image, a sheet of paper is placed over the stone, pressure is applied, and the image is transferred to the paper. Properly prepared, a lithograph stone can produce hundreds of high-quality prints.
The commercial form of lithography is offset lithography, more commonly known simply as "offset. " (Offset refers to the method of transferring the image from the plate to the paper, which we will discuss later.) In offset lithography the flat stone is replaced by a thin, flexible metal printing plate designed to wrap around a printing cylinder.
The first step in making an offset plate is to "strip" both the line and halftone film negatives into their proper positions according to the mechanical. This makes up what is called a "flat." Before the actual offset plates are made, a photographic paper proof is made from the flat that shows the client the exact position of the elements. These proofs are called by various names, reflecting their general color: Van Dykes, browns, silvers, blues, or salts. After the proof has been approved, the plate is made and the job printed.
Offset plates are made photographically. The plate, which may be aluminum, stainless steel, or a specially processed paper, is coated with a lightsensitive chemical similar to that used on photographic paper. The flat is brought into contact with the plate and exposed to a high-intensity light. The plate is then either processed by hand or put through an automatic processor where it is developed and made press-ready. (That is, the plate is chemically treated so that the image area will reject the water solution and accept ink and the non-image area will accept the water and reject ink.)
As the platemaking process for offset is much simpler than for either gravure or letterpress, the printer will very often make his own plates rather than depend on the services of an outside platemaker or engraver.
CHARACTERISTICS OF OFFSET
Capable of printing jobs of a variety sizes in black and white and in color at a relatively low cost,
Requires more attention than letterpress or gravure to maintain image consistency throughout run.
Printing plates are relatively inexpensive and require only a short time to make as compared with letterpress and gravure plates.
Duplicate plates are inexpensive.
Printing plates can be either negative-working or positive-working (deep etch); that is, made from either film positives or film negatives.
Although the quality is highest on smooth or coated papers, offset can print effectively on rough-surface papers as well.
Corrections require making a new plate, but plates are inexpensive.
Proofing can be done either on the production press or special proofing presses, Proofing on the production press is expensive. Pre-press proofs made from the film are usually furnished in the form of blueprints, Cromalin proofs (if process color), or by overlay systems such as the 3M system of color proofs.
Offers the designer great creative freedom: a great variety of substrates and finishes can be printed; vignette halftones are easy to produce; soft and subtle tones are easily reproduced.
Presses. There is a wide variety of offset presses available, ranging in size from small office duplicators to huge web-fed perfecting presses capable of printing an entire book in a single run.
All offset presses are of the rotary type and have three cylinders: a plate cylinder, around which the printing plate is wrapped; a blanket cylinder, onto which the image is offset; and an impression cylinder, which presses the paper against the blanket cylinder.
In operation, the printing plate first comes into contact with dampening rollers. These wet the plate with a solution of water, gum arabic, and acid. This water solution is accepted by the non-image area and rejected by the image area. Next, the plate is inked. The ink, repelled by the water solution in the non-image area, is accepted only by the image area The inked image is then transferred, or "offset," onto the blanket cylinder, which in turn transfers the image to the paper.
The reason the image is offset onto the rubber blanket rather than printed directly on the paper is because the offset plate is very delicate and the abrasiveness of the paper's surface would cause damage. In addition to extending the life of the plate, the rubber blanket, because it is compressible and conforms to the slightest degree of texture on the paper's surface, makes it possible to print on rough papers.
Gravure
Gravure is the commercial form of intaglio printing. Intaglio is the second oldest printing process, dating back to 15th century Germany. The original process is very simple: the image to be printed is cut or etched into the surface of the plate; the plate is then inked and wiped clean, leaving ink only in the incised areas. When printed under pressure, the paper draws the ink out of the etched areas, transferring the image onto the paper.
Gravure plates are curved to fit around a cylinder. Like letterpress plates gravure plates are photoengraved; however, unlike letterpress plates, the area to be printed is lowered, not raised. To print, the entire surface of the plate is inked and then wiped clean with a flexible steel scraper called a "doctor blade," leaving ink only in the etched areas.
The unique aspect of gravure printing is that all copy, continuous-tone and line, must be screened. This includes type. The screening process is much more complicated than that used for either letterpress or offset lithography. However, the purpose is the same: to break the image up into thousands of microscopic dots which when etched in acid become tiny cells, varying in depth and diameter. The tonal gradations of the printed image are determined by the depth of the cells: the deep cells hold more ink and therefore print darker tones; the shallow cells hold less ink and print lighter tones.
Presses. Gravure printing is done on a rotary press that is either sheet-fed or web-fed (rotogravure). Sheet-fed gravure is used most often for short runs and to reproduce works of art. The rotogravure press is designed to run at tremendous speeds and is ideally suited to long-run jobs such as mass-circulation catalogs, newspaper inserts, cigarette packages, postage stamps, etc.
CHARACTERISTICS OF GRAVURE
High-quality, high-speed printing of black and white and of color.
Consistent quality throughout run.
Richest blacks and widest tonal range of all the printing processes.
Most economical for long runs at high speed (web-fed). However, with careful preparation and attention to detail, short-run jobs can be printed at a cost competitive with that of quality letterpress or offset.
Plates or cylinders are more expensive than either letterpress or offset plates, but they last longer.
Duplicate plates or cylinders are expensive.
Capable of printing on a wide range of surfaces. Although the highest quality is attained on smooth or coated stock, gravure can also produce high-quality work on inexpensive uncoated papers. An example of this is the Sunday supplements printed by rotogravure (web-fed gravure).
Corrections are expensive because a new printing plate must be made.
Proofing is much more expensive than for either letterpress or offset.
Gravure printing is recognizable by the fact that the entire image area is screened, including the type. For this reason, gravure printing is better suited to reproducing continuous-tone images than to reproducing type (especially type with fine serifs or strokes and in sizes of less than 8 point).
Able to most closely simulate the continuous-tone effect.
Other Printing Processes
Not all our printing demands are met by the three major printing process. Today, our printing needs are highly diversified; printing is no longer restricted to ink on paper, but can be found on a wide range of materials such as glass, plastics, and metals, to name just a few. To satisfy this demand, printing methods have been diversified and specialized. Here are a few of the more popular printing processes with which the designer should be familiar.
Screen Printing. Screen printing, also known as silkscreen printing, is a stencil process. The stencils are either cut by hand or photographically prepared. They are bonded for strength to a screen (silk, nylon, Dacron, or fine metal mesh) and stretched over a wooden frame to create a smooth flat surface. A squeegee is then used to force the ink through the open (image) area of the screen onto the surface to be printed.
Because the inking in screen printing is heavier than in other printing processes, it produces excellent results on a variety of smooth and rough surfaces: metal, glass, ceramic, wood, plastic, fabric, cardboard, and of course, paper. This makes screen printing ideal for a great many commercial projects, such as street signs, posters, and wallpapers.
A major advantage of screen printing is that because of the opaque quality of the ink, white or any light color can be printed on a dark surface in one application. And there are a great variety of inks available, including metallic inks and fluorescent colors. There is even a special ink that is used for screen printing electronic circuits.
Although the speed of screen printing does not compare favorably with that of the three major printing processes, it has attained an important place in the printing industry because of its versatility and unique advantages.
Flexography. Flexography is a form of relief printing in which a rubber printing plate and fast-drying ink are used. The printing process is similar to that of rotary letterpress: the rubber plate is mounted on a printing cylinder and generally prints from rolled stock (webfed).
Flexography is a versatile printing process, printing from one to six colors and using webs up to 100" wide. It can be used to print on paper, cellophane, foil, or plastic and is used for printing such diverse items as bubblegum wrappers, wallpaper, and shower curtains.
Flexography is particularly popular with the packaging industry.
Because the printing plate used in flexography is rubber it can be stretched; this unique advantage permits the printer to vary the width and length of the printed piece somewhat. Another feature is that changes can be inserted directly into the rubber plate by simply cutting out the old copy and replacing it with a patch. This operation is much easier than making a new plate. And the printing cylinders are interchangeable, making it possible to change jobs quickly and easily.
Flexography does have its limitations, however. The ink has a tendency to spread, making it difficult to print a clean, sharp halftone. For this reason it is not unusual to use gravure cylinders on the press to print the halftones and flexo for the line work only. Also for this reason, small type sizes (6 point and smaller) should be avoided, and if type is to be reversed, it should be bold enough so that it will not fill in.
Letterset. In letterset, also known as indirect letterpress, the image is first offset onto a rubber blanket and then transferred from the blanket to the paper.
The purpose of letterset is to combine the quality of letterpress with the convenience of offset, One of its advantages is that it allows a printer with an offset press to do letterpress work. Letterset printing is ideal for printing on metal or plastic (cans, labels, cartons) and offers consistency of printing quality and color.
Collotype. Also called photogelatin. The only process in which continuous-tone copy can be reproduced without the use of a screen. The basic concept is similar to that of offset lithography, in which water and ink do not mix. In collotype, moisture and ink do not mix. The printing plates are aluminum and are coated with a gelatin solution. The relative softness or hardness of this coating dictates the continuous-tone values: the soft areas accept more water and less ink; the hard areas accept less water and more ink. However, unlike offset lithography, in which the image is first offset, in collotype the image is transferred directly from plate to paper. Collotype is an inexpensive, short-run process used extensively for one-sheet, two-sheet, and three-sheet posters; point-of-purchase displays; and printed transparencies used in back-lighted displays.
Thermography. Although thermography is a finishing process rather than a printing process, the designer should be familiar with it. Thermography is an inexpensive way to achieve an engraved look and is frequently used to print business cards, invitations, decorative papers, and greeting cards. The engraved effect is produced by the use of heat and special printing materials. The plate is printed with a slow-drying ink which is dusted with a powder. When heated, the powder and the ink fuse to give the type a raised, or engraved, look. Printing can be done in a number of colors, including gold, silver, and copper. The finish may be either dull or glossy.