Commercial Printing


After completing a layout, most in-house designers send large jobs out to a commercial printer. While laser printers and affordable color printers are suitable to smaller tasks, professional quality reproductions require hardware that most companies do not own.

It is impractical to print 1000 copies on a laser printer, unless your company has one of the larger, faster models. At 8–16 pages per minute, you could wait a long time for 1000 copies, and no one else could print in the meantime. The issue is more complex if you want to use color. The cost difference alone can make using a commercial press more practical. Desktop color output starts at more than 15 cents per page. This price per page does not include the cost of the printer. A commercial press can charge as little as 5 cents per color for a 1000 page run.

Beyond the Desktop

Prepare to be insulted when you contact commercial printers. While some smaller print shops are happy to print copies from whatever output you give them, larger and more experienced shops tend to demand a diskette from you, without any output. Commercial printers assume that in-house designers cannot produce acceptable output on affordable equipment.

Typesetters

When a commercial print shop requests a diskette to create it own output, it is not because the printer wants to earn a few extra bucks. Print shops use phototypesetters or direct-to-plate technologies. Relatively speaking, these devices are equivalent to laser printers with resolutions of 1270, 2540, or 3386 dots-per-inch. Some newer devices can even create output at 3600dpi. We assume your laser printer cannot match 3600dpi, but some are getting amazingly close results.

You might ask a print shop what their typesetter’s out is, just in case the shop outputs most documents at 1270. If 1270 is the shop’s resolution of choice, then you can easily match the output in-house. In fact, 1200dpi with “enhancement” technologies is more than sufficient for any document printed on standard papers.

Scanners

A Crossfield drum scanner can cost more than $200,000.

Commercial presses also tend to have scanners with capabilities far beyond the desktop flatbed scanners. These special scanners are known as drum scanners. Drum scanners can cost hundreds of thousands of dollars, but their resolutions are amazing. Drum scanners often exceed true optical resolutions of 4000dpi. These scanners have advanced color matching capabilities, guaranteeing that the scanned image is nearly perfect.

Offset Printing

Commercial print shops use several methods to transfer images to various media. Some of these methods have limited applications. For example, silkscreen printing is a method commonly used to transfer inks to clothing and three-dimensional surfaces. The primary printing method for documents is offset printing.

Offset printing is based on a principal familiar to most school children: oil and water do not mix. An offset press does not use an impression to transfer images. A flat sheet of either paper or thin metal, known as a plate, is created using a photographic technique. A clear negative image of a page is placed on the plate. Then, a burst of light is aimed at the plate. Where the plate is exposed to light, a chemical reaction occurs. The exposed areas of the plate now attract water, while the unexposed areas retain oil-based inks.

One plate per color is created for every one to 16 pages of a document. If more than one page is on a plate, the pages are cut after printing.

The plate is then wrapped around a large cylinder. Plates are positive images, meaning you can read them just as you can the original document pages. As the cylinder spins, a tray of water deposits a thin film onto the plate. A tray of ink is located after the tray of water, in relation to the spinning of the plate.

As the cylinder encounters the ink tray, ink is deposited only on the dry areas of the plate via an intermediate roller. A series of rollers thins the ink, preventing a build-up that would produce smears. As the plate continues to spin, the ink builds up only where there is no water.

A web press is fed paper via large rolls. A sheet press moves paper from stacks, much as a copier or laser printer does.

The image on the cylinder is transferred to another roller, covered by a thin rubber sleeve. The rubber-covered roller is now a negative image, meaning it is inverted. Finally, the rubber roller comes into contact with the document’s paper. As the negative image is transferred, it once again becomes a positive image – and therefore, readable.

Shading and Color

If you want to create good layouts, you have to understand how commercial printers work with shades and colors. Liquid inks are not actually mixed to form shades on a page. Using a magnifying glass, look closely at a newspaper photo. You will see dots of four primary substrate colors. It is your responsibility to create documents with this process in mind.

Screens

Laser printers only have black toner. Likewise, liquid ink printing presses only use solid colors. As mentioned earlier, dots of color mixed with small blank gaps create the optical illusion of shades.

The converting of a photograph or other artwork into a dot pattern is known as applying a halftone screen. When a halftone screen is applied to a solid ink, the resulting shade is a tint. Photographs can be screened using a computer and scanner or using more traditional methods.

The simplest method to screen a photo is to place a mesh with tiny holes between the photographic negative and the photo paper used to produce a print. A more advanced method uses a device known as a photomechnical transfer machine, or PMT.

Paper Std. LPI LPI Range
Newsprint 85 65 to 100
Magazine Stock 130 120 to 150
Table Book 230 150 to 300

Printers discuss screens in terms of lines per inch. Screen settings do not correspond to dots per inch. These two measures work together to define the final quality of a document. Many designers wrongly assume that the greater the value of each measure, the better the output. The results of incorrect settings can be disastrous.

Different papers and inks have different tolerances. Smoother papers tend to bleed less than rough newsprint. Oil-based newspaper ink, as you might already know, smears easily. To complicate matters more, various typesetting devices have their own limits.

Laser Printer Limits

Assuming that you are preparing output on a laser printer, there are very serious limits to what can be accomplished. On any printer with a resolution less than 1200dpi, you cannot expect to reproduce black and white photographs – or artwork – with more than 72 shades of gray. This limit has to do with what happens when your output is used to create a printing plate.

Newer laser printers adjust the sizes of toner dots, allowing for greater detail and clarity. However, you cannot use this approach to create good master output for commercial printing. Halftone screens must be precise.

Printer or
Typesetter (DPI)
Max. Screen
Range (LPI)
Optical
Gray Tones
300 53 to 60 26 to 33
600 to 800 71 to 85 51 to 72
1000 to 1270 65 to 128 89 to 256
2400 or 2540 90 to 150 96 to 256
3386 to 3600 150 to 300 128 to 256

To avoid the phenomena of banding, make sure the screen ruling divides evenly into the output resolution.

A screen line is at least five typesetter dots wide. Fewer than five dots per line and the images produced are pixelated, meaning they look jagged. Consider trying to create a true dot in a coarse grid versus a fine grid.

DPI

Scanning Tips

Properly formatting a document begins when you first scan an image or create any bitmap art work. When you first scan a color image, set the input resolution to one and a half or twice the expected output line ruling. Do not confuse line ruling for the dots-per-inch of the output device.

Scan black and white art, known as line art, at the highest optical resolution of your scanner.

screen ruling x 2 = scan resolution

If you plan to distort or resize an image, then you need to modify the calculation for ideal scan resolution. Use the side you plan to enlarge the most or reduce the least – this will be the axis requiring the greatest input resolution – and multiply the scan resolution by this value.

screen ruling x 2 x (final measure / original measure) =
scan resolution

You do not need to scan most color images at resolutions greater than 600dpi.

If you plan to print a document at 1200dpi, with a screen ruling of 128 lines, with no scaling of images, then you need to scan images at approximately 200 to 300dpi. Over-scanning is a common mistake of new in-house designers. Over-scanned images look dull or blurry, even when they are not. If you scan every image at 1200dpi or better, you might fill your hard drive – but your documents will not automatically look better.

Scanning at unnecessarily high resolutions can also have an unexpected side effect. Computer programs often ask if you want to print at image resolution or printer resolution. If you scanned an image at 1200dpi and own a 300dpi printer, a 8.5 by 11-inch scanned image prints on four pages at the printer’s resolution.

Color Types

Color is the predominant reason to send a document to a commercial printer for reproduction. Many first-time designers mistakenly believe they can print a color page and take the output to a commercial printing press. The printing process is much more complex than many assume.

There are three different methods to include color in a document. The first is to merely substitute a color ink in place of black. Of course, there are very few occasions when printing all of a document in blue or some other color is appropriate. The two other methods rely on groups of halftone screens to print color. These methods are spot color and process color printing.

Spot Color

Spot colors are solid inks used to add colors and tints to a layout. When adding spot colors or tints to a layout, you must create one plate for each color used on a page, including black. Spot colors are never mixed on a page – you do not overlap halftone dots to create a mixed color.

Business cards and letterheads are generally printed using spot colors. Spot colors, because they are not the result of optical illusions, are more vibrant. A gray spot color always looks better than creating a screen gray.

Process Color

Process color uses four colors – cyan, magenta, yellow, and black – to create the illusion of more than 3000 colors. By printing dots of these four colors, just slightly skewed from each other, graphic elements such as photographs can be printed.

Process color should only be used for documents requiring photographs or reproductions of multi-color artwork. Four plates are created for each page of a document, making process color more time-consuming and expensive.

If you study any magazine or newspaper with color photographs, you can see the rosette pattern used to simulate colors. If the halftone screens are misaligned, an annoying moiré pattern results, which tends to be darker than the proper alignment. Also, moiré patterns produce noticeable rings of color instead of smooth gradients.

Aligning the plates to produce a rosette pattern in known as registering the colors. At the bottom of some newspaper pages a row of the primary colors is printed. This row of blocks or dots is known as a registration marks. When color plates do not align, the output is said to be out of registration, or misregistered.

Matching

Rule number one of designing color layouts on a personal computer: never trust the screen image to match the final product. To make sure that your desings print just as you envision them, ask your printer for a color chart. You will need to refer to this chart during the design process, since your screen will not match the final printed pages.

Most print shops rely on the Pantone color matching systems. Introduced more than 30 years ago, the Pantone systems use names and numbers to specify exact colors. Using Pantone color codes guarantees that two commercial printers, working with the same plates, will print exact duplicates.

Sample color chips are similar to those you find in the paint section of hardware stores – strips of paper with three or more sample swatches of color.

There are over 700 Pantone spot color codes and more than 3000 Pantone process color codes. Pantone, Inc., sells several tools for color matching. These tools include color charts, mixing charts, and sample chips of color. Pantone’s experts develop exact ratios of colors that produce other spot colors. Also, using a Pantone mixing chart, a spot color can be approximated using process printing.

Adjustments

Printing shaded or colors often requires fine-tuning. Several tricks used by commercial print shops can improve the quality of documents. You have to understand these procedures to provide the necessary output.

There are two primary reasons adjustments have to be made. First, paper can only absorb so much ink before the paper dissolves. Second, printing presses are not perfect – the plates do not always align perfectly.

Undercolor Removal and Gray Component Replacement

Undercolor removal, UCR, and gray component replacement, GCR, use black ink to enhance color images. GCR, the more popular of the two methods, adds black ink to create darker colors. At any point where cyan, magenta, and yellow dots appear together on a page, black is used as a substitute – a gray replacement. This avoids the “muddy” appearance of shadows created using only the three primary colors. Mixing the three primaries should produce black, but because of the imperfect nature of ink, instead produces either a midnight green or deep brown.

A benefit of replacing three colors with one is the reduction of ink on the page. Paper is not durable, so replacing three inks with one has a direct benefit. However, replacing ink with too much gray can cause areas to lose their textures. Shadows are not perfectly gray; a shadow is a darker mix of the base color.

Knockouts and Overprinting

Most computerized layout software takes care of making sure spot colors do not overlap. Overlapping two spot colors is known as overprinting. Only overprint black. Other colors mix, creating new colors; black stays black.

At a lot of newspapers, overprinting of black is frequently used to make layouts easier. Other colors, printed beneath the black, are hidden. Most software does not overprint black unless you select the option. Overprinting the ink avoids white gaps between the black images and any underlying colors.

A knockout is a blank area created on a color plate to prevent unwanted mixing of colors. If you are printing yellow letters on a blue background, an outline of the letters would be left blank on the blue plate. Then, when the yellow is printed, it is a true yellow, not green.

Trapping

A knockout should be slightly – very, very slightly – smaller than the area of color that will be printed. Inks are generally applied from lighter to darker. This means that in the above example, the yellow letters would be applied to the paper first, then blue ink would be applied to the page. The blue plate would feature a knockout. Because the knockout is slightly smaller than the letters, the blue ink overlaps the yellow. This area of overlap is known as a trap.

Advanced computer software, especially illustration packages, automatically print separations with small traps. A good press operator can produce seamless images with traps smaller than one-tenth of a point. If the printing press is less reliable, use a trap no greater than one-fourth of a point.



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Writer: C. S. Wyatt
Updated: 31-Dec-2013
Editor: S. D. Schnelbach