1. Color and Commercial Printing
Image Resolution and Line Screen
One of the greatest challenges designers and publishers face is ensuring that the color in their printed artwork looks the way they intend. Accurate color reproduction requires a solid understanding of color basics and how color is displayed and printed on different devices, as well as good communication between the designer, the prepress provider, and the commercial printer.
This chapter introduces fundamental printing concepts and terminology and describes several processes that are commonly used to produce commercial printing.
Describing Color
Objects appear to be certain colors because of their ability to reflect, absorb, or transmit light; we perceive this light as color. Our eyes are sensitive enough to perceive a nearly infinite range of colors in the spectrum of visible light—including many colors that can’t be displayed on a computer screen or printed on a commercial printing press.
We describe color in terms of three characteristics: hue, saturation, and brightness. These qualities are traditionally represented graphically on a color wheel. Brightness has to do with the quantity of light reaching your eye—the brightness of a surface depends on how reflective it is. Hues depend on wavelength and are identified by color names; a hue corresponds to a direction on the color wheel. Saturation, sometimes called chroma, refers to a color’s vividness.
Two colors have the same hue and brightness but differ in saturation if one appears whiter or more neutral. Spectral colors—the colors of a single wavelength of light from a prism—have maximum saturation. The saturation of a pure spectral color can be reduced while keeping the brightness steady by diluting the color with white light. On a color wheel, which has spectral colors along the rim, white at the center, and uniform brightness, saturation corresponds to distance from the center of the wheel.
Each type of device used to create a color publication—be it a scanner, computer display, color desktop printer, or commercial printing press—reproduces a different range of color, or color gamut. Even similar devices, such as two computer displays made by the same manufacturer, can show the same color differently. You can view more vivid colors on your display than you can print on a desktop printer or a commercial printing press. Special inks can also create printed colors that can’t be represented on a computer display. In addition, scanners, digital cameras and computer displays use different models to describe color from those used by desktop printers and commercial presses. As colors move from the computer display to the printing press, they’re converted from one color environment to another, resulting in sometimes-dramatic changes.
A. Saturation B. Hue C. Brightness D. All hues
Color models
Designers can use different models to select and manipulate color, corresponding to the way color is generated in different media. On a television screen or computer display, a spot on the screen emits varying amounts of red, green, and blue (RGB) light that combine to define the spot’s color. When you manipulate color (using an image-editing program, for example), you have the option of working in the RGB model and specifying colors by their red, green, and blue components.
The color printing process uses four transparent inks: cyan, magenta, yellow (CMY), and black (K). The black is used to define detail in images, to deepen shadows, and to print type and graphics in black. When you define color using computer software, you have the option of working in either the RGB or the CMYK mode, and you may decide to convert images from one color space to the other.
Red, green, and blue are the additive primaries of light. If you combine 100 percent of red, green, and blue light, you see white. If none of the additive primaries are present, you perceive black. When discussing printing inks, cyan, magenta, and yellow pigments are the subtractive primaries. They filter components of red, green, and blue from white light, and you see what remains. For example, if a printed sample absorbs all the red light striking it and reflects the green and blue, its color is cyan.
If you combine 100 percent of cyan, magenta, and yellow ink on paper, the result is complete absorption, or black—in theory. Impurities in printing ink pigments cause the combination of cyan, magenta, and yellow to create a color that is not quite black; black ink is added to compensate for that.
Some computer applications, like Adobe Illustrator, require that the entire document be in one color space—RGB or CMYK. When you create a new document in Illustrator, you must select one of the two. After an Illustrator document is created, you can switch it to the other color system by changing the Document Color Mode under the File menu. Spot colors, like those defined with the Pantone color system, can be used in either document color environment.
Image-editing programs such as Adobe Photoshop require an image to be in one of several color spaces (typically RGB or CMYK) but provide several color selection options besides RGB and CMYK. These include the hue, saturation, brightness (HSB) palette, which lets you choose colors by the color wheel method; and the Lab color model, which uses the coordinates of colorimetry. When you’re working in Photoshop, you can select a color using any of its palettes even though the document exists in the RGB or CMYK color space.
Adobe InDesign documents readily accept color images in a variety of color spaces (typically RGB, monochrome, or CMYK) without requiring any conversion. InDesign shares the Color Settings of Adobe Photoshop; when you open a document, it adopts the default color settings it finds in Color Settings.
The three additive primaries of light are red, green, and blue. When combined in pairs, they make cyan, magenta and yellow. When all three are combined, you see white. Computer displays emit light in additive colors, displaying it as pixels on the screen.
When working with printer’s ink, the subtractive color primaries are used—cyan, magenta, and yellow. They are called subtractive because they act as filters, subtracting color from white light. When mixed in pairs, they make red, green, and blue. When combined, they make a muddy brown color, a result of slight impurities in the pigments used to make ink. To get a good black, printing processes add black ink to supplement the cyan, magenta, and yellow primary colors, thus CMYK process color.
Color gamuts
The range of colors a device can reproduce, capture or display is called its color gamut. The gamut of most output devices, including printers, is a fraction of the visible color spectrum. The color gamuts of different devices typically overlap but don’t coincide; these differences often result in the same color appearing different in different contexts. To help guarantee consistent color across output devices, computer applications use color management systems. These systems embed files with color profiles to ensure consistent color through all stages of production.
By having a description of each device’s color gamut in the workflow, a program that performs color management can coordinate color reproduction at each stage of a print project—scanning, design, proofing, and print—and help the designer achieve acceptable and predictable color in the final copy.
Color management with ICC profiles
The ICC profile is a standardized software component developed by the International Color Consortium for describing the color behavior of any device. These profiles can be embedded in images and used to process colors when scanning, viewing, and printing. Profiles provide a description of the color behavior of each device in the workflow so that each application performing color management can reliably manage color precision at each stage of a print project.
Adobe added ICC color management as a foundation technology in Photoshop beginning with version 5.0. Today, nearly every image that is processed through Photoshop contains an identifying embedded ICC profile (Photoshop calls this the Color Working Space). These embedded ICC profiles provide information about colors as they were captured by the digital camera or scanner and make possible reliable color reproduction without guesswork.
When you’re working with programs like Adobe InDesign, color management lets you create page-layout documents that use images in their original color space; these images can then be printed by one or more printing processes without converting the original images. For example, you can place photos in RGB color, CMYK color, or grayscale in a page layout, and then print them to CMYK, Hexachrome, monochrome, or another destination color environment without needing to convert the original images. The processing of the color occurs at print time, guided by the embedded profiles and the color-management software running on the computer printing the document.
Using this method, a single document can be printed both on glossy paper on a sheet-fed press and to newsprint on a roll-fed press without modifying the original document images. The color characteristics of the output are optimized for the printing process, paper, and ink according to the output profile that defines how the color is converted for the specific printing process.
For more information on color management, see “Using a Color Management System” on page 60.
Prepress Terms
Blueline
A diazo (UV-exposed and self-processed) photo print made to proof pagination, image position, and type. Bluelines have been made mostly obsolete by the digital revolution.
Camera-ready
Said of text or artwork ready to be photographed by a process camera.
DPI
An abbreviation for dots per inch. Refers to the resolution at which a device, such as a monitor or printer, can display text and graphics.
Continuous-tone art and line art
Continuous-tone art is art, such as photographs, that consists of shades of gray and color gradations. It’s distinguished from line art, such as a line drawing, which has no tonal variation. If you look closely at continuous-tone art, you can see that shades of gray or color blend smoothly without breaking into dots or other patterns. When the art is printed, the corresponding regions are reproduced as arrays of different-sized dots printed in the colors used on the press.
Dot gain
Many variables—from ink to paper surface and press used—affect the size of halftone dots. A certain amount of dot gain, or increase in halftone dot size, occurs naturally when wet ink spreads as it’s absorbed by the paper. If too much dot gain occurs, images and colors print darker than specified.
Dot gain is one of the characteristics taken into account when color-management systems are applied.
(See page 110 for more information on dot gain.)
Halftone dots in a color proof
Halftone dots after printing
Halftone screens
Ink is an all-or-nothing medium in the sense that any spot on the paper is either inked full-strength or not at all. To simulate shades of gray or color on a commercial press, the image must be broken into arrays of dots of various sizes using halftone screening.
In the case of black-and-white photography, black dots are used to simulate shades of gray. Areas where the dots are small appear light gray, and areas where the dots are large appear dark gray or black. The human eye is tricked into seeing tonality by its ability to average the tiny printed dots into the background paper. You “see” gray when you’re really looking at small printed black dots on a field of white paper.
To achieve a satisfactory range of color, the printing press superimposes four arrays of dots—in cyan, magenta, yellow, and black ink. A region with larger dots appears darker than a region with smaller dots. The positioning, or register, of the four arrays on top of each other is critical to quality printing.
Any visible pattern of interference between the four arrays is distracting. To minimize the chance of interference, each array is oriented at a different angle on the press.
Image resolution (ppi)
Image resolution is the number of pixels displayed per unit of length in an image, usually measured in pixels per inch. An image with a high resolution contains more, and therefore smaller, pixels than an image of the same dimensions with a low resolution. For best results, use an image resolution that is greater than the printer’s resolution (a factor of 2x is appropriate).
Halftone screen with black ink
Halftone screens with process inks at different screen angles; correctly registered halftone dots form rosette patterns.
Knocking out and overprinting
When artwork involves two objects or colored regions that overlap each other, a designer can choose either to let the top object eliminate, or knock out, what is beneath it or to allow overprinting.
In most cases, you want an object to knock out the one below it, to avoid unintended color blends. However, you can use overprinting to create special effects or to hide errors in press register (see “Trapping” on page 24).
Adobe Illustrator and InDesign both feature an Overprint Preview menu selection that can help you see the effect of overprinting colors.
Line screen (lpi)
Line screen, also called screen ruling or screen frequency, is the number of halftone dots per linear inch used to print grayscale or color images. Line screen is measured in lines per inch (lpi)—or lines of cells per inch in a halftone screen. It gets its name from acid-etched lines on glass screens that were originally used in graphic arts cameras to divide an image into microscopic circles of confusion—which, by varying exposure, create halftone dots. The electronic evolution of the halftone uses a virtual screen to create its halftone dots.
Machine resolution
Output devices like film imagesetters and platesetters have extraordinarily high resolution. Their minimum imageable mark is called a device pixel (sometimes called a machine spot). A 300-dpi laser printer uses a 1/300" square device pixel; a 600-dpi printer uses a 1/600" square device pixel. Film imagesetters, which are capable of much higher resolutions, can make a mark as small as a 1/3600" square dot. Modern platesetters have resolutions as great as 1/5000". By comparison, most computer displays work with device pixels that are 1/72" square—quite coarse, compared to printing processes.
Misregister
Paper sometimes stretches and shifts as it absorbs moisture and is pulled through a press. Printing plates can also be mounted out of alignment. These factors can cause multicolor jobs to print out of register, resulting in slight gaps or hue shifts between adjacent colors. Trapping and overprinting can conceal some of these flaws. Misregister can also cause images to appear blurred or out of focus. If a press has printed out of register enough to cause images to appear unsightly, the press run should be made again with colors in register.
Moiré patterns
When process-color separations are printed, the arrays of dots for each color are oriented at different angles to minimize interference patterns. The screens are positioned so that the dots form a symmetrical pattern called a rosette, which the human eye merges into continuous-tone color.
The relationship between the screen angles is critical. Occasionally, a pattern in a photo (woven furniture and herringbone fabrics are common culprits) interferes with one or more screen angles, causing a noticeable pattern of interference lines called a moiré pattern. These patterns are also caused by attempting to print photos that have been scanned from already-printed material. Moiré patterns from printed sheets can be removed by some scanner software and also by techniques in Adobe Photoshop. It isn’t advisable to scan printed material, because the result will almost certainly produce a moiré pattern.
When moiré patterns show up in normal printing processes, it can be an indication of a problem in the prepress or platesetting software.
PDF (Portable Document Format)
PDF is a document format developed by Adobe for handling documents in a device-and platform-independent manner. It allows files to be viewed, transmitted, printed, and archived in a single format. The PDF format works on all major operating systems, including Mac OS, Windows, and Unix. Adobe Acrobat software provides for the conversion of documents into PDF and allows documents to be created from any application on any computer platform. When converted into PDF, documents can retain a full range of color, graphics, and high-quality typography. Reduced-resolution PDF files make it possible to transfer them efficiently over the Internet for copy checking and on-screen proofing.
PostScript
The PostScript language is a page description language developed by Adobe as a way to describe to a printer the image on a page. The introduction of PostScript printers created the electronic publishing revolution. PostScript has become the standard way for a computer to communicate with a printer, imagesetter, or platesetter.
Process colors
In the four-color printing process, color is reproduced using transparent pigments of cyan, magenta, and yellow (CMY). These are called process colors. In theory, process colors create shades of gray when combined in equal combination and black when combined at full strength. Because of impurities in the inks, however, equal amounts of the three don’t produce neutral gray, and full-strength inks combine to create a muddy brown. To achieve contrast and detail in shadows, and to assist in maintaining neutral grays, black ink (also transparent, and identified by the letter K) is added to the three process colors.
Using black ink to replace neutral combinations of C, M, and Y is also economical for printing and helps to maintain the neutrality of midtones.
RIP (Raster Image Processor)
The RIP interprets the PostScript code sent from a computer application and then translates that code into instructions for the marking engine that marks the pixels on the paper, film, or plate. A RIP is built into all PostScript desktop printers and is a separate component for imagesetters and platesetters. Some RIPs are software based.
Separations
To print color artwork and images on a commercial press, each page is separated into component images called color separations. Traditionally, separations were created photographically through colored filters, with the results exposed onto large sheets of film. Today, separations are created digitally. There are usually four separations per page, one for each of the CMYK process colors and one for each spot color being used.
Within each separation, photographs are screened into an array of halftone dots (or similar patterns). Type, line-art illustrations, and similar graphics are either printed as solids of colors or screened into halftone patterns according to the assigned values in the originating document.
Designers generally don’t produce separations; instead, they provide complete digital files to the printer for production. In modern printing, the separation of colors in a printed document is done as part of platesetting.
Spot colors and tints
Spot color refers to color printed using inks other than process colors. Each spot color is produced using a single ink and printing plate. You can choose from among hundreds of different spot-color inks.
Spot color may be used to reproduce colors not within the CMYK gamut. A spot color may also be used to bump, or boost, the density of a process color. Spot color is often used to save money when only one or two colors are needed—a job can then be printed on a less-expensive two-color printing press. (See page 54 for guidelines on choosing spot colors.)
A spot color printed at 100 percent density is a solid color and has no dot pattern. A tint is a lightened spot or process color created by printing that ink with halftone dots. This process is typically referred to as screening.
Trapping
The quality of a printer’s work depends on getting the different inks to print in register—that is, exactly aligned with each other. If one or more inks print out of register, white gaps may appear between adjacent objects where the paper shows through, and there may be fringes of unexpected color. To minimize the effects of misregister, commercial printers use a technique called color trapping: adjacent colors are intentionally set to overprint along common boundaries. Trapping can be done manually in an illustration or an image-editing program, but today much of it is done by sophisticated processes in prepress production software.
Undercolor removal (UCR) and gray-component replacement (GCR)
At any point on the page where the three CMY inks are used at combined percentages to produce gray, the combination can be replaced by black. To avoid having too much ink on the page (which can cause drying problems), printers use techniques called undercolor removal (UCR) and gray-component replacement (GCR). With UCR, cyan, magenta, and yellow colors are reduced slightly to put the right amount of ink on the page and thereby allow the ink to dry. With GCR, black ink replaces much of the cyan, magenta, and yellow in neutral gray areas according to a complex formula.
Printing with GCR separations allows neutral colors to be printed with better color consistency. This is particularly beneficial on high-speed web-fed printing presses and for screen printing, where maintaining gray balance can be challenging. Sheet-fed printing is more commonly done with UCR separations, because the presses run more slowly and neutral balance is easier to control.
Computer Graphics
Vector graphics
Vector graphics are created in illustration programs like Adobe Illustrator. The objects in vector graphics are made up of mathematically defined curve and line segments. You can edit such a graphic by moving and resizing the entire graphic or selected components. Curves in vector graphics are determined by the points you select for the lines to pass through; you change the shape of a curve by dragging its control points.
Because the objects that form them are defined mathematically, vector graphics take up comparatively little space and aren’t tied to a particular resolution. When displayed or printed, the graphics are calculated to fit whatever screen or printer is used. Vector graphics are therefore considered resolution-independent and scaleable. Unlike raster images (described next), they can be output to different-sized screens or printing technologies of varying resolution at any size without any quality loss.
Photos placed in vector illustrations don’t share their independence of resolution. Too much enlargement of an illustration containing a placed photo will reveal visible pixels.
Raster images
Digital photographs are raster images, also called bitmap images. They exist in a rectangular grid of small squares called pixels. Each pixel contains data that describes its gray level or color value. Raster images take up more space on disk than vector graphics do, often tens or hundreds of megabytes.
Because a raster image is made up of a fixed number of pixels, the dimensions at which the image is output depend on its resolution. A square raster that is 100 pixels wide, for example, will have a resolution of 100 pixels per inch if printed in an inch-wide square. If printed at twice the size, however, it will have half the resolution in both dimensions and thus one-quarter the overall resolution.
Unlike vector graphics, which are edited by altering mathematical lines and shapes, raster images are edited by manipulating groups of pixels. Because a raster image inhabits a grid, problems can be associated with enlarging it or moving it to a grid with more available pixels. In a simple enlargement, the individual pixels are enlarged and may become visible as jagged lines. In transferring an image to a grid with more pixels, the colors or gray levels for the new pixels must be inferred by a process of interpolation that may cause blurring.
Vector graphics describe shapes mathematically.
Bitmap images describe shapes with pixels.
Image Resolution and Line Screen
Spatial resolution, typically some number of dots per inch, corresponds roughly to the ability to render graphic detail. On a computer screen, it’s the number of pixels per linear inch (ppi); on an output device, it’s the number of dots printed in a linear inch (dpi); on a scanner, it’s the number of pixels sampled per linear inch of the scanned image. The resolution of a raster image depends on the size it’s printed as well as on the pixel dimensions of the file. Pixel dimensions determine file size; and, as a practical matter, file size is related to storage and processing efficiencies.
If you’re scanning images from film or prints, you have control over the resolution, and you can choose whatever value you think will produce the best results. If the resolution is unnecessarily high, files will take a long time to display and print; if the resolution is too low, the quality of output will suffer. The choice of resolution should be based on the intended printing process—which usually defines the halftone screen frequency—and the subject matter of the image.
As a general rule, the scan resolution should be double the lines per inch (lpi) you intend to use for halftone screening, although this can vary. With naturally textured images, such as images of water and foliage and many portraits, you may get good results with an image resolution that is one-and-a-half times the screen frequency.
You can also use lower resolutions with nontraditional forms of screening, such as stochastic (frequency-modulated) screening. Use high resolution where detail is critical and where lines must be sharp, as with images of electronic products, jewelry, and machinery. Because resolution and reproduction size balance each other, accepting less than the traditional 2:1 resolution ratio means that an image can be reproduced at a slightly larger size.
Image resolution: 72 ppi
Image resolution: 300 ppi
Image resolution: 300 ppi
Output resolution: 300 dpi
Image resolution: 300 ppi
Output resolution: 2400 dpi
65 lpi: Coarse screen used for screen printing (simulated)
133 lpi: Fine screen often used for magazines (simulated)
85 lpi: Average screen often used for newspapers (simulated)
150 lpi: Fine screen often used for commercial printing (simulated)
Printing Technologies
Frequency-modulated and hybrid screening
Traditional halftone screening arranges image dots of varying size into a regular grid pattern. Frequency-modulated (FM) screening (also called stochastic screening) arranges dots in irregular clumps rather than in an orderly formation. Screens made this way don’t have repeating beats and are generally free from moiré patterns. To use FM screening, you need special software or a platesetter that supports the process.
Traditional halftone screening uses the size of the dot to modulate between gray levels: larger dots for darker shades, smaller dots for lighter shades. Frequency-modulated screening controls the level of gray or color by varying the composition of clumps and how close the clumps are to each other. It distributes dots semi-randomly but still controls the number of dots in each space: more dots produce a darker effect, fewer dots produce a lighter effect.
In addition to its freedom from moiré patterns, FM screening has another advantage over traditional halftones. Because it uses mostly small dots to make its impression, images usually display more detail in the highlights. On the other end of the tonal scale, colors that should be solid may appear grainy. Stochastic screening also has measurably greater dot gain than an equivalent conventional halftone process, which requires strict control in the platesetting process.
Recent advances in stochastic screening have delivered hybrid screening processes that put FM patterns in the highlights while using conventional halftone dots in the shadows.
High-fidelity color printing
Much research has gone into expanding the range of colors produced by process color printing. One of the solutions is to print additional inks, such as red, green, and blue; or green and orange. The addition of these inks produces a much larger color gamut but poses challenges to the traditional color separation process (and requires a printing press with more than four units). Stochastic screening plays an important role in the success of high-fidelity color printing because it allows additional inks to be printed without risk of duplicate screen angles and moiré patterns.
Pantone Hexachrome is a commercial high-fidelity color system supported by Photoshop, InDesign, and QuarkXPress. This system enables printing with six process inks (CMYK plus orange and green) to produce a greater gamut of colors on press. Six-color printing presses are common today, making Hexachrome and other high-fidelity printing more practical.
Conventional halftone screening
FM screening
Imagesetters
An imagesetter is a device for marking photographic film with the data necessary for printing. These machines use lasers to expose rolls of graphic arts film to produce either positive or negative images that are later exposed to an aluminum printing plate for printing. Driven by a Raster Image Processor (RIP), imagesetters produce very high resolution text, graphics, and images to film.
A generation of newer platesetters has replaced most imagesetters in the printing industry, but film imagesetters are still used in the screen printing and flexographic printing fields where film sheets are a necessity of image processing.
Direct-digital printing presses
In direct-digital printing, presses are networked to workstations that create PostScript files from digital documents, process document components, and send the files to the press. The presses don’t require film or, in some cases, printing plates. Some direct-digital presses transfer digital information onto electrophotographic cylinders instead of plates and use toner to print four-color pages. Other presses expose the digitized pages directly to special plates mounted on the press.
Direct-digital printing produces fast turnaround times, low production costs, and the ability to personalize publications. Digital printing is often used for variable-data printing, on-demand printing, or short-run color printing, where a small number of copies are printed. It isn’t suited for high-volume print jobs.
Variable-data printing
Digital presses create the opportunity for a new kind of printed marketing using presses that can change the copy and images for every impression they print. Called Variable-Data Printing (VDP), these documents can have varying text, personalized graphics, photos that change according the demographic information of the recipient, and more.
VDP is created with a combination of page-layout applications, database records, libraries of variable images, and special software that defines which components go together for each recipient.
VDP products have been shown to improve the response to printed marketing efforts. They’re compelling because they bring individualized printing out of the category of junk mail and into the category of one-to-one marketing. When created with properly filtered data, these individualized marketing pieces are often read with more interest by their recipients, and response rates are measurably better.
Computer-to-plate systems
Computer-to-plate (CTP) devices work like film imagesetters, but they expose an image directly onto an aluminum or polyester surface rather than a piece of film. This eliminates a costly and time-consuming step on the way to press. CTP produces higher print quality by avoiding generations of film processing and exposure, and it features higher resolution than its film counterparts.
CTP devices accept PostScript files and can produce images with either conventional screening or stochastic pattern imaging. Platesetters also reduce the printing industry’s dependence on photo chemistry and produce output with no environmentally dangerous waste byproducts.
The plate is held to the internal drum by vacuum
Plates are hand-fed into this machine. Some platesetters have auto-feed
Offset Lithography
You can choose from several different processes to print a publication: flexography, intaglio (gravure), screen printing, and offset lithography are the most common. The method you choose depends on your budget, your choice of a commercial printer, and the printed results you want. Because offset lithography is the most common printing process, we use it here to explain the basics of commercial printing.
Offset lithography involves printing from a flat printing surface. The printing plate holds ink because the image area is treated to make it chemically receptive to oil-based ink but not to water—not because the image area is raised (as in flexography) or etched (as in gravure).
A multicolor offset press has a separate printing unit for each ink being printed. For example, if you’re using process colors and one spot color in a print job and your commercial printer’s press can handle five inks, a printing unit will be set up for each ink. The paper will then pass through each unit in succession. If the press handles fewer inks, your printer will print two or three inks first, stop the press and change the inks, and then run the paper through again to print the remaining inks.
Platemaking Using a laser or photographic process, a printer exposes the document onto a flat plate with a smooth coating and then processes the plate to remove the nonimages areas. Nonimage areas are porous aluminum, which is attractive to water.
Dampening The plate is mounted on a cylinder. When the press starts, the plate comes into contact with dampener rollers first. Dampening solution (water plus additives) flows constantly from a fountain through a series of rollers to the plate cylinder. The last roller dampens the entire printing plate.
Inking Next, the ink roller applies oil-based ink to the plate. Thick ink flows from another fountain through a series of rollers, which distribute the ink thinly and evenly. When the last ink roller contacts the dampened printing plate, it smoothly distributes the ink across the water-resistant image area. The adjustment of ink and water must be balanced before printing can be done.
Printing The unique roller in an offset press is the blanket cylinder, which carries away a reversed image from the plate and transfers this image to the paper. The blanket has some resiliency and gives slightly when pressed against paper, so the image can transfer evenly to smooth or textured papers. The blanket is also formulated to accept ink but reject the dampener solution, leaving most of the water behind.
In the printing step, the paper—in individual sheets—passes between the blanket cylinder and the impression cylinder. The blanket cylinder carries the ink from the plate, presses against the paper, and transfers the ink onto the paper. On web-fed offset presses, two blanket cylinders belonging to two press units print both sides of the roll simultaneously, so there is no impression cylinder.
1. The plate is dampened by the dampener rollers. The image area repels water.
2. Ink rollers apply ink to the “oleophilic” (image) areas on the plate.
3. Ink is transferred from the plate cylinder to the blanket cylinder. The water stays behind.
4. The blanket cylinder transfers ink to the press sheet.
Other Printing Processes
Flexography
Flexography uses a raised-image plate made of flexible photopolymer or rubber that prints directly onto a printing substrate. The flexible plate makes it possible to print on irregular surfaces such as pressure-sensitive label paper, plastic films, and corrugated cardboard. Because of the soft plates, flexography generates greater dot gain. Ink coverage on flexographic presses is defined by a special ink roller called the anilox roller.
Flexographic printing is required for some food packaging—it’s the only printing process that can print on materials that come in contact with food. It’s also common for printing on pressure-sensitive labels, shrink-plastics used on beverage bottles, and pharmaceutical products.
Intaglio printing (gravure)
Intaglio printing uses an engraved plate that carries the image. Flat intaglio printing usually goes by the name engraved printing and is used for fine business and personal stationery and announcements. Rotary intaglio printing is called rotogravure; it’s used for very-long-run commercial printing jobs like magazines and catalogs.
For rotogravure, the image is engraved on a copper cylinder; the surface of the plate represents the nonprinting areas. The plate cylinder rotates in a bath of ink, and the ink is retained in the engraved cells of the plate. As it turns, the cylinder comes in contact with a doctor blade, which removes the ink from the nonimage areas (like a squeegee). The cylinder then transfers the ink to the paper against an impression cylinder.
The cost of preparing rotogravure cylinders for printing and then doing the printing requires that this process be used for projects that run in the millions of impressions.
The quality of rotogravure printing is excellent, delivering some of the nicest results in the world. Publications like National Geographic magazine are printed using this process.
Screen printing
Screen printing is the relatively simple method of forcing ink through a screen stencil and onto a printable surface. Because screen printing lays down the ink up to 30 times thicker than lithography, the color is more dense and durable than it is in other printing processes. Screen printing uses photographically prepared stencils to transfer designs onto almost any surface, including posters, clothing, compact discs, and bottles. The screens are exposed from film positives made on an imagesetter.
Screen printing on fabrics may require using larger traps to compensate for misregister. A relatively low screen ruling of 35–55 lines per inch is appropriate for printing on textiles. One of the attractions of screen printing is the great variety of inks possible, including glittered and fluorescent inks.
Specialty printing and finishing
After printing, a number of processes can enhance the appearance of a product, including the application of varnishes, aqueous coatings, foil stamping, embossing, die-cutting, and thermography. Varnishes, which are clear or tinted inks, are usually applied on the same press that does the printing, but these coatings can also be added after printing. Varnishes and coatings can cover entire sheets or can be used to print over parts of a printed project to enhance its appearance. Many designers use varnish to augment the appearance of photos in a publication.
Relief printing processes add metallic or colored foil, patterns, raised panels, or raised lettering to the printed sheet. Most of these processes require the manufacture of a metal printing die (and sometimes a counter-die), but the cost of these tools isn’t extravagant. Other than the expense of the dies, finishing operations like these add just a few pennies to the cost of each printed piece.
Thermography is a finishing process that uses plastic resin to add an embossed effect to printing. After the ink is printed on the paper, it’s dusted with powdered resin, which sticks to the ink. The surface passes under a vacuum cleaner to remove the unused powder and then under a flash-heater, which causes the powdered resin to boil. The image rises in relief to produce an interesting effect. This process is often used to print business cards, stationery, invitations, and greeting cards.
Printing Terms
Aqueous coating Some printing presses have an extra unit for adding an aqueous (water-based) varnish to an entire sheet as it is printed. These coatings add a lovely gloss to a printed piece and make them more durable and less prone to fingerprints and smudges.
Anilox The flexographic printing process requires a special roller to control the density of the ink transferred from the ink metering roller to the printing plate. Anilox rollers are cylinders with microscopic cells etched into the surface. Each cell has a fixed volume, which defines the amount of ink the cell can hold. A doctor blade removes ink from the surface of the anilox as it turns.
Blanket In offset printing, a rubberized surface that transfers the inked image from plate to paper.
Blanket cylinder In offset presses, the metal cylinder around which the blanket is wrapped.
Cold-set printing Ink applied to paper on a printing press, then allowed to air-dry by evaporation of the vehicle that made the ink liquid. As opposed to heat-set printing, where the ink is dried by heating elements in the printing press.
Cover stock Heavy-weight paper used for the covers of magazines and publications.
Dampening In lithography, fountain solution (mostly water) is applied to the plate as it turns. Image areas are smooth (called oleophilic), which makes them attractive to ink. Non-image areas are porous (called hydrophilic), which makes them attractive to water. When in balance, the ink sticks to the smooth areas and the water to the porous areas.
Densitometer A device used to measure the density of ink (the ink film thickness). Density is the inverse of reflectivity.
Die A stamping tool used in embossing and cutting. Often requires a counter-die.
Dimensional stability A paper’s resistance to stretching or distortion, especially when damp.
Doctor blade A siff squeegee-like blade that removes excess ink on flexographic and gravure printing presses.
Dot gain The spread of dots during several stages of printing or platemaking, as measured by the increase in size of a midtone dot. When a 50% dot expands into a 60% dot, the gain is 10%. Average dot gain on offset printing is more than 20%. All printing processes experience dot gain, even ink-jet and toner-based printing.
Dry lithography and waterless offset A type of lithographic printing that uses a plate with a coating that repels ink without needing to be dampened. Ink is confined to the image area without the usual dampening. These plates are usually surfaced with silicone in the non-image areas.
Drying oven An oven used to dry paper after printing. In heat-set web-offset lithography, the paper passes from the press through a drying oven on its way to the folding and finishing units on the press.
Embossing Producing a raised image in paper by means of a die striking from the back of the paper into a counter-die at the front.
Engraving (Intaglio) A printing process in the family of Intaglio printing, where the image is engraved into a copper or steel plate. On the press, the plate is cleaned by a squeegee (doctor) blade, and the ink is then transferred by pressure and capillary action. Engraving is used for fine business stationery and currency printing.
Flexography Relief printing using a flexible printing plate—usually a photopolymer. The image is raised, as with an ordinary letterpress. Flexography is most commonly used to print on food product labels, pressure-sensitive labels, cardboard and other packaging.
Form A form is one side of a signature. A group of pages positioned so that when folded and finished, they come out in the right order and position.
Fountain The supply of ink for a lithographic press. Sometimes also the supply of dampening solution.
Gang run Two or more printing jobs run simultaneously on the same press sheet. Large sheets of paper are shared by several jobs and, after printing, are cut into separate jobs.
Gravure printing (Intaglio) Sometimes called engraved printing, gravure is printing by the Intaglio process where the image is engraved into a copper plate, ink is applied, then the residue is removed from the surface by a stiff blade called a doctor blade. This is the process used to print fine business stationery and currency. A variant is rotogravure, used to print very-long-run publications like the National Geographic.
Gripper edge The gripper edge of a sheet of paper is the leading edge where the sheet is grasped mechanically and drawn into the press.
Gripper margin Unprinted space allowed along the gripper edge of the printing medium.
Halo effect Occurs when ink builds up at the edge of an area, making the interior look lighter. Common in electrophotographic (toner) printing and some letterpress.
Hickey A donut-shaped imperfection in presswork caused by paper particles that get stuck to the blanket cylinder of an offset press.
Imposition The arrangement of pages for printing on a press sheet in such a way that they appear in correct order when the sheet is folded.
Impression cylinder A cylinder that presses paper into contact with an inked surface.
Ink coverage A percentage indicating the inked area of the paper. Also called dot area, it is the ratio of area covered by ink divided into the area measured.
Inkometer A tool for measuring the tackiness or stickiness of ink.
Inner form The part of an imposition that consists of inside pages. On a printed sheet, the inner form pages are those that will be on the inside when the sheet is folded into a signature.
Intaglio A family of printing processes where the image is engraved into the printing plate. Used primarily by two printing processes: gravure printing—used for fine stationery and currency printing, and rotogravure, used to print very long runs of commercial publications. The benefits include tremendous ink film thickness and the run-length of the plates, which will last for millions of impressions.
Lay-down sequence The order in which colors are printed.
Letterpress Relief printing directly onto the paper. The oldest form of printing. Raised areas of the plate hold the ink and transfer it directly. This is in contrast to intaglio or gravure printing, where sunken recesses hold the ink, and also in contrast to offset printing, where the plate has no relief.
Letterset Offset-letterpress printing. The image is defined by raised areas, as in the direct case. The raised areas get inked but do not come in contact with the paper. Instead, the image is transferred to a blanket, which in turn transfers it to paper. Also called dry offset. Resembles offset lithography in using a blanket as intermediary.
Moiré pattern Any of several interference patterns that show up in printed products a result of the nearly exact frequency of halftone dot patterns with other patterns in the artwork. Moiré is known by physicists as a beat pattern where two similar patterns occasionally match but not consistently, causing a visible wave-like pattern in an image.
Offset Printing that uses an intermediary surface called a blanket to transfer the image from the inked surface to the paper.
Perfector A type of printing press that prints both sides of the paper as it passes through the press. Web presses are usually capable of perfecting the job as it is printed.
Photopolymer plate Most flexographic printing is done with a relatively thin plate made of a polymer material and coated with a photosensitive surface, which is exposed from a film negative and processed to create a relief image. Rubber plates are used in some flexographic operations.
Piling A printing problem in which ink pigment accumulates on a plate or blanket.
Platen press A type of flat letterpress that uses a hinged clamping action to bring the paper and plate together.
Press proof A proof pulled from the press prior to an actual print run.
Press run or print run The number of copies in one printing.
Progressive proofs, or progs Proofs made on a press. Each color is shown separately, and various combinations are shown overprinted. Progs are used as a guide in adjusting the final color quality.
Register The correct alignment of colors during printing.
Register mark A mark to assist in making the correct alignment of overlay copy and color during printing in multiple colors.
Relief plate A printing plate with a raised, image-bearing surface. Letterpress and flexography use a relief plate.
Rotary printing Any method using a cylinder as the primary printing surface.
Rotogravure Intaglio printing on a rotary, web-fed press.
Rubber plate A flexible relief plate used in flexography.
Screen printing A process in the porous family of printing processes. Used to print on garments, industrial products, and signs, screen printing is capable of laying down a tremendous amount of ink. The process involves making a stencil with fabric and a photo-sensitive coating, and using very thick plastic ink and a squeegee to force the ink through the stencil onto the substrate.
Sheet-fed press A printing press into which individual sheets of paper are fed, as contrasted with a web-fed press printing on a continuous roll of material—usually paper.
Show-through When the impression on one side of a sheet is visible on the other side, through the paper.
Signature A group of pages printed on the same sheet, front and back. After printing, the sheet is folded so that the pages fall in correct order.
Slurring A printing problem in which halftone dots appear elongated or smeared.
Tack Adhesive quality, or stickiness, of ink.
Trim marks Marks indicating where to trim.
Varnish A clear or tinted ink that adds a sheen or gloss or even a matte-surface quality to the press sheet. Varnish usually requires an extra printing plate with images for the desired areas to be coated. See also aqueous coating.
Web-fed press A printing press into which a continuous roll of paper is fed.
Work-and-tumble A type of sheet-fed printing in which front and back images share a single plate. The sheet is printed on one side, flipped end-to-end (along the gripper-edge axis), and printed on the other side. After flipping, the paper has a new gripper-edge.
Work-and-turn Another type of sheet-fed printing in which front and back images share a single plate. The sheet is printed on one side and then turned side-to-side so that the back side can be printed with the same plate. This does not change the paper’s gripper-edge.
Zinc engravings Line or halftone art as a relief image on zinc plates for letterpress printing. Other metals have also been used in the making of relief plates: aluminum, magnesium, copper, and brass.
Imposition and Binding
Most printing presses print large sheets of paper. Imposition is the process of arranging the pages of a publication so that when the sheets are printed and folded for binding, the pages are in the proper sequence.
Each press sheet is called a signature. Each side of the signature is called a form. The signature is bound with others as necessary and trimmed to create a finished publication.
The arrangement of pages for a form is usually done with imposition software in the printing plant. A folding dummy is constructed to model the final piece, and then information about the job’s folding requirements are put into the imposition software. The imposition software rearranges the pages in a document into the right order and orientation for printing and then sends the forms to the platesetter for imaging.
Binding is the process of gathering folded signatures using one of several methods. In saddle-stitch binding, signatures are gathered to form a common spine and then stitched with staples made from wire on the machine. Perfect binding involves gathering groups of signatures, grinding the bound edge to create a strong binding surface, and gluing the signatures at the spine with a hot-melt glue into a one-piece paper cover.
When signatures are folded, the inner pages of each move outward slightly. The more pages a signature contains, the farther the pages closest to the center of the signature move with respect to the other pages. This phenomenon is known as creep. The imposition software can compensate for creep by moving the pages of a signature inward in minute increments so that the trimmed signature doesn’t have an obvious image movement from the ends to the middle.
Imposed 16-page signature and folded signature
Inserted signatures
Saddle-stitch binding
Gathered signatures
Perfect binding
Creep occurs when the inner pages of a signature project outwards.
Creep affects the printed area during binding and trimming
Binding and Finishing Terms
Accordion fold Parallel folds, alternating in direction. Also called concertina fold.
Binding Any of many processes used to assemble pages or signatures.
Collate To gather pages in a particular order for binding.
Die A stamping tool used in embossing and cutting.
Die-cutting Cutting a paper product with a die. This usually refers to steel-rule die-cutting, where shaped cuts are made to printed products on a letterpress machine. Common uses include cutting slits for a business card to be inserted on a booklet, or cutting the flaps on pocket-folders. Flexographic printing uses rotary die-cutting, which is done on the printing press. Rotary dies are capable of intricate detail and extraordinary precision.
Embossing Producing a raised image in paper by means of a die striking from the back of the paper into a counter-die at the front. Embossing can be simple, called blind-embossing, where the paper is squeezed in the die-counter-die pair, causing an image or shape to be raised. Embossing can also be more complex, involving colored or metallic foils, or textures and heat, each of which imparts an image with both relief and color.
Endpapers The heavy paper at the front and back of a book, to which coverboard is glued. They hold the body of a book into the covers.
Form A form is one side of a signature. A group of pages positioned so that when folded and finished, they come out in the right order and position.
Glue binding A method of binding that depends on glue, also called perfect binding.
Jogging Vibrating a stack of sheets before binding or trimming. Used to bring the edges in-line.
Perfect binding An unsewn, flat-spined binding made with glue. Also called a glue binding.
Saddle-stitching A type of binding that uses wire stapling at the center of a magazine or pamphlet. Folded spreads are placed over a peaked frame called a saddle and stapled through the middle. The “stitches” are staples made from a roll of wire on the binding machine.
Side-stitching A method of binding that involves stapling through the spine of a publication from front to back. This prevents the book from lying flat when it is opened.
Signature A group of pages printed on the same sheet, front and back. After printing, the sheet is folded so that the pages fall in correct order.
Spine The backbone of a book, particularly of the binding.
Thermography An embossed effect obtained by applying resinous powder to a wet image and fusing it with heat.
Everything I’ve learned about printing: Keep an open mind; stay proactive; pick your battles; what you don’t know learn; it’s okay if the printed piece looks better than the proof; attitude is everything; and finally, as with this quote, no matter how much time you have to produce a piece, it always comes down to the wire.
—Gina Long, Lithomania, Inc., South San Francisco, CA
