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Commercial Printing Processes

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Intro
This tutorial is an introduction to the different types of printing processes used in the commercial printing industry. In some ways, the printing industry uses an entirely different set of technological skills than most of us are familiar with. The following information may help those of you that support the graphics art industry. There are two important terms that we need to introduce before we get started:

Image Carrier: The device that carries the inked image to the substrate. This device can vary depending on the printing process being used.

Substrate: This is the material that is being printed on such as paper, cardboard, vellum, etc.

Relief Process
Letterpress:
This is the oldest of the major printing processes and is not very commonly used anymore. This process uses plates which were first made of wood and later they were made of metal. The print image on the plate is raised and when inked, the plate is placed in contact with a platen that holds the paper and the image is transferred.

Flexography:
Flexography is most commonly seen in the packaging industry as it provides the flexibility to print on a wide variety of surfaces. The process is very similar to Letterpress except for the fact that the plate used is made of a flexible material such as rubber or plastic.

Intaglio Process
Gravure:
Gravure is used for a variety of purposes such as magazines, catalogs, packaging, tablecloths, wallpaper, etc. This process is sort of the opposite of the relief processes described above in that the print image is recessed through an engraving/etching process and uses a metal cylinder as the image carrier. When the substrate comes into contact with the cylinder, ink is transferred from the recessed cells on the cylinder to the substrate.

Screen Process
Serigrapic Printing(Silk Screen):
In this process a squeegee forces ink through a mesh or screen. The non-image areas are blocked so that the ink does not pass through. Screens can be made of a variety of materials including polyester, nylon or metal. This is the only printing process in which the ink passes through the image carrier. Common uses include T-shirts, mugs, mousepads, etc.

Planographic Process
Lithography:
This is the most commonly used commercial printing process and also the most complicated. It is used to create magazines, books, newspapers, reports, brochures and much more. The fundamental concept behind this process is that "ink and water don't mix". This type of printing is done on an offset press that uses an indirect printing process. This means that the image carrier and the substrate do not come into contact with each other. These presses can be either sheet fed or web fed in nature. Sheet fed presses send individual sheets through the press while web fed presses use a very large roll of paper.

The first step is to create a plate which is typically made of aluminum, polyester or paper depending on how long the run is and how durable the plate needs to be for archiving. There are several different prepress workflows in existence with the newest method being CTP(computer to plate) technology. Traditionally, plates are created through a time consuming prepress process that involves creating CMYK color separations and then developing large film negatives from which the plates are made. CTP improves the time and effort involved considerably by using a device(looks like a large printer) that accepts electronic files and then images the plates, removing the necessity for film development.

Once the plate is created, it is wrapped around the plate cylinder and water is applied to the non-image areas and ink is applied to the image areas. The plate cylinder then comes into contact with the blanket cylinder and the image is transferred to it. The blanket cylinder is has a rubber blanket wrapped around it that picks up the image from the plate cylinder. The substrate passes between the blanket cylinder and the impression cylinder where the image is passed to the substrate. Commercial presses are very fast and have very high quality output.

Digital Printing
Digital:
Digital printing is typically used for shorter run paper based jobs such as reports, newsletters, books, brochures, etc. Digital printing has some distinct advantages over the other printing processes. The other processes are incapable of producing variable data which is the ability to create sections of a job that vary from page to page. An example of this would be a phone bill in which your name, account number, address, etc are printed on the bill. Many digital printers can pull this information from a database and incoporate it into a job. This is possible because, unlike the other processes, a digital printers image carrier is imaged on a page by page basis. This can't be done on an offset press(for example) where permanent plates are fixed to a drum. Digital printing comparitively has very little prepress time and provides the ability to easily edit a job. Digital Printing's weaknesses are it's comparitive lack of speed, resolution and color reproduction, however, technological advances are improving these factors all of the time.

So how does it work? Digital printing uses a positive electric charge on the image areas of a drum while non-image areas possess a negative charge. The drum is passed by negatively charged toner particles that are attracted to the positively charged areas of the drum. The paper passes across the drum and the toner is transferred. From here, the paper passes through a fusing unit that uses heat and pressure to bond the toner to the paper. Digital printing cuts down on much of the prepress labor involved with using an offset press and is very popular for shorter run jobs where the speed and quality of a press is not needed.

Comparison

Process Image Carrier Image Transfer Substrate Versatility Run Length Cost

Letterpress Lead Direct Low Short High

Flexography Rubber Direct High Long Moderate

Gravure/Intaglio Copper/Chrome-Plated Direct Moderate Very Long Very High

Screen Printing Polyester, Nylon or Metal Direct(Through image carrier) High Short to Long Low

Lithography Polyester, Paper or Aluminum Indirect High Short to Long Low to Moderate

Digital Photo-receptor Direct Low Short Very Low

As previously mentioned, the Letterpress is no longer very common due to it's low substrate versatility, short run lengths and high costs. So of the remaining 5 processes, which is the best? Well, that question really can't be answered as each serves a purpose and handles certain types of jobs. As printing technology progresses, there is becoming a bit of competition between the Digital and Lithographic(offset press) markets. While larger presses are much faster than any production digital printer, they are also very expensive(in the millions of dollars for a large one). Both digital printers and presses are becoming faster and less expensive which will continue to perpetuate the odd competition between these dissimilar pieces of equipment.

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