Understanding the scope of your printing project, the budget constraints, and the different processes available is essential for successful print planning. Several factors should be considered when choosing a print vendor and process. What degree of quality is appropriate to the message you are conveying? What is your company’s budget? What is the process that will get your project completed at the quality that you desire? For example, a flyer announcing the grand opening of a grocery store may not be printed the same as a sales sheet trying to sell mansions in Beverly Hills. Although the two projects are the same in dimensions, the scope of each project is different. These differences can have a dramatic influence on the vendor you choose, the type of printing process that you employ, and your overall budget.
Lithography
Most printing today is offset lithography. Lithography is a term describing the printing process where the image area and the non-image area co-exist on the same plane. This is in contrast to letterpress or gravure printing, where the image area is raised or depressed in relation to the non-image. The lithographic process requires a small amount of oil and water – the greasy, oil-based ink is attracted to the image areas and water is attracted to the non-image areas. The fundamental chemical reaction we remember as "oil and water don’t mix" keeps the two regions separate.
The term lithography was derived from the Greek language, and literally means "stone writing." Lithography was performed on stones made from a variety of calcium carbonate, called lithographic stones. And by 1817, presses were built around this process using lithographic stones.
Offset Lithography
Offset lithography was invented by accident – when paper was misfed into the press, the rubber blanket would roll over the stone plate, picking up the image onto the blanket. The next sheet through would have the image on both sides and surprisingly, the image from the blanket side was sharper and of higher quality than the image from the lithographic plate-offset lithography was born. By 1930, offset lithography became the dominant form of printing in the United States. Shortly thereafter, printing with lithographic stones became almost non-existent.
In modern times, lithography refers to the use of aluminum plates that have taken the place of lithographic stones. Offset lithography refers to lithographic printing in which the inked plate transfers the image (offsets) to a rubber blanket, which, in turn, transfers to the surface. There are different forms of offset lithography; they include sheet-fed offset lithography, Web offset printing, direct lithography and waterless printing.
Offset lithography is the most common printing method used today. It is marked by high- quality, long print runs and is relatively inexpensive when compared to other methods. Offset plates are easy to make (most generally from a photographic process from negative or positive film-see the "Pre-press tutorial"). However, plates for offset lithography are increasingly being imaged directly from computer files. Imaging plates directly by thermal or light-based lasers produce "digitally pure" plates that many say is an improvement on the photographic method of producing plates from film.
Sheet-fed offset lithography refers to what the name implies: Discrete sheets of paper fed one at a time into the press, utilizing the offset lithography process. Sheet-fed offset is generally used when the paper is of higher quality or heavier weight, usually above 50# (lbs.) and usually the best option if you are reproducing artwork.
Web-fed printing refers to rolls of paper, or Webs of paper feeding the press. Derived from the newspaper boom of the mid-1800’s, high-speed perfecting presses that were fed from rolls of paper were invented. At this time, the process was still largely letterpress and lithography was viewed as a process for artwork and artistic endeavors. Finishing devices such as folders increased the efficiency of these presses.
Technical issues slowed the development of offset lithography until the 1930’s when the photographic plate-making process that replaced lithographic stones finally became perfected. By 1960, the industry began taking advantage of heat-set inks, drying systems and inline folders of the earlier letterpress systems, and Web-fed offset lithography began to explode. Newspapers and publishers moved away from letterpress to Web-fed offset lithography.
Web-offset lithography is highly efficient and may work best when paper weight is under 50# (lbs.). Because Web presses "pull" a continuous piece of paper through the press, much lighter weight papers can be used. A job may be right for Web if the number of impressions is more than 25,000. Many Web presses have the ability to do folding for complicated direct mailers and brochures.
Different kinds of Web presses include:
- Full Web
For sixteen-page signatures (the assembly of pages for a plate on press) whose flat trim size is typically 23" x 35". Can be called a sixteen-page Web. This type of press uses rolls 35" to 40" wide. - Three quarter Web
For eight-page signatures whose flat trim size is typically 17" x 22". Three-quarter Webs use rolls 22" to 27" wide. - Half Web
For eight-page signatures whose flat trim size is typically 17" x 22". Half Webs use rolls 17" to 20" wide and are also called mid-sized Webs. - Mini Web
For brochures, newsletters, and other products whose flat trim size is typically 11" x 17". Mini Webs use rolls 11" to 14" wide. - Form Web
For business forms, direct mailers, catalog sheets, stationery, and other products whose flat size is typically 8.5" x 11". Form Webs use rolls 8.5" to 10" wide.
All offset presses have these basic components in common: Feeding units bring paper into the press, register units make sure that the paper remains in place for each impression, ink units move ink evenly through a series of rollers to the plate cylinder, printing units make the impression on the paper, and finally, delivery units remove the printed paper.
Gravure
Gravure is a highly cost-effective printing method, ideally suited to high-volume printing with relatively inexpensive paper. The plate cylinders of a gravure press have millions of small cells that fill with ink during the process, and create images based on millions of tiny dots. Using lasers and diamond styluses, these cells are carved into the plate from scanned positives or digital information. By varying the depth of these cells, great control over the range of ink tones can be achieved. And gravure presses can be huge, ideally suited to print runs that have millions of impressions.
A number of examples can be seen in different publications where portions are printed using offset methods, and portions are printed using the gravure method. For example, if you look at a current issue of National Geographic Magazine, you probably can’t tell that the inside editorial-including both copy and photographs-is printed on a gravure press, while the cover stock and most of the outer-signature advertising is printed using offset presses. Why should this be the case?
Paper used in the gravure process needs to be relatively soft and have an extremely smooth surface. Where the rubber blanket used in offset printing can conform to paper irregularities, gravure presses have unyielding plates that come into direct contact with the paper. Any irregularities in the paper surface, when used on a gravure press, will miss contact with the cells carrying the ink.
There are, however, perceptible quality differences between the products that come from offset and gravure presses. Gravure, using tiny dots to compose images and words, lacks some of the range of that can be achieved through offset lithography, where all shading, depth and gradations can be used. The tiny dots that create gravure printing can create excellent images, but without the depth and final image quality of offset lithography. Some magazines combine the two methods to ensure the best blend of quality and efficiency.
Flexography
Flexographic printing has been developed for products like plastic wrappers for food, labels or heavy paper grocery bags. Under traditional offset presses, these substrates often don’t run well (if at all) because the one-dimensional blanket cannot conform to irregular surfaces.
With flexographic printing, the plates are made from soft rubber sheets with a raised image surface. The rubber blanket is then wrapped around a cylinder, and the process runs from there much like an offset press-a continuously shaded image can be transferred to the surface. But unlike the gravure process, smooth paper is not a requirement and unlike offset, the raised image on the rubber blanket is the only thing responsible for transferring the image, not the repellant properties of oil and water.
There are trade-offs in any production process, and flexographic printing exemplifies this-the process can’t provide the solid ink coverage or finely detailed images of offset. The flexibility of the rubber plates, the factor that allows them to print on awkward surfaces, makes them shift during the process, lowering the quality of dot gain and registration that can be achieved with offset.
Screen-printing
For all of the other printing processes described in this tutorial, there is expensive equipment like plates, imagesetters, and automated presses involved. But screen-printing can be accomplished with a few simple tools: screening fabric, a squeegee and ink.
First, a stencil of the image is created and the ink is applied – usually a lot of ink – and then the substrate is applied. The screen and stencil are soft, so highly irregular surfaces can be used, like clothing, three-ring binders and large signs.
Because the ink is applied heavily – up to 30 times more heavily than offset presses and five times more heavily than gravure – the colors tend to be vibrant and long-lasting. The heavy ink makes the process ideal for outdoor display and on frequently used items. Billboards and t-shirts are almost always printed in this process.
All that heavy ink takes a long time to dry, and registration can be a problem, as there’s often no mechanical process for aligning the substrate to the stencil. Thus, screen-printing may never be a viable option for high-quality print jobs. But for a few vibrant-colored, durable items, screen-printing is the best option.
Copyright 2000 PrintPlace.com, Inc. All rights reserved.
This article was last modified on August 9, 2000
This article was first published on August 9, 2000
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