Offset lithography is the workhorse of the printing industry. Almost every commercial printer does some form of offset printing. But a high-quality final product is often due to the diligent planning, execution, and equipment provided by the commercial printer.
Offset lithography is based on a very simple principle: ink and water don't mix. Text and pictures are imaged onto printing plates, which are dampened first by water, then ink. Ink adheres to the image area, the water to the non-image area. Next, the image is transferred to a rubber blanket, and from the rubber blanket onto the paper. We call this process "offset" because the image does not go directly to the paper from the plate.
Step One: Making Printing Plates
Before any project can go on press, digital files must be imaged onto printing plates. Information from digital files is transferred to printing plates in much the same way as digital photographs are imaged. A measured amount of light, usually laser light, is used to expose the printing plate in a plate-making system. When exposed to light, a chemical reaction occurs that activates an ink-receptive coating on the printing plate. This results in the transfer of the digital image onto the plate.
Prior to the plate-making process, digital files are arranged on a computer in such a way as to maximize the use of the sheet of paper being used. This process is called imposition. Imposition also helps ensure that when the press sheet is cut and folded, that it is done in such a way to maximize efficiency in the manufacturing process.
There are different materials for printing plates, including paper (which produces a lower-quality product) and polyester. The best plate material is aluminum, which is more costly, but provides the quality and durability necessary to produce high-quality offset printing.
Each of the primary process colors -- black, cyan (blue), magenta (red), and yellow -- has a separate printing plate. Even though you may see thousands of colors in the finished product, only these four colors are used. You'll also hear this called the four-color or CMYK printing process – which is similar to the three-color or RGB color model used in television or video.
Because the human eye is an amazing averaging instrument, instead of millions of tiny dots, we see the colors and shapes that result from the overlap of the four process colors!
Step Two: The Press Run
Conquest Graphics uses the sheet-fed method of offset printing. Pre-cut paper is fed through the press as a stack or pile of press sheets. Sheet-fed offset presses print at very high speeds and use very large sheets of paper. Press speeds can reach up to 20,000 impressions per hour. An impression is equal to printing one side of a full press sheet (40 inches x 28 inches),
As a sheet of paper is fed into the printing press, it will move through a series of rollers and blankets where the ink is transferred onto the paper. A four-color press will have 4 or more “units” which are separate sets of rollers and blankets for each color to be oriented. As the press sheet moves from one unit to the next, another color will be transferred to the sheet of paper.
The Inking Process
Ink and water do not mix -- this is the underlying principle of offset lithography. The ink is transferred to the printing plates through a series of rollers. On the press, the printing plates are dampened, first by the water rollers, then by the ink rollers. These rollers move the ink from the ink fountain onto the printing plates.
The image area of the printing plate accepts ink from the ink rollers. The water rollers help to keep ink off of the non-image areas of the printing plate. Each printing plate then transfers the image to a rubber blanket that in turn transfers the image to the paper. The plate itself never actually touches the paper. This is what is meant by offset printing. All of these processes occur at very high speed.
The press sheet is left slightly damp by all of the ink and water being applied to it. Clearly, at this point in the process, there is a risk of the ink smudging. Smudging is minimized by applying a water-based coating to the press sheets. This coating is called an aqueous coating, and the coating may be either a glossy coating, or a dull coating. Once the coating is applied, the press sheets move through an infra-red dryer. Heat from infra-red radiation is used to quickly dry the ink and coating, so that smudging does not occur.
Color and Registration Control
Color and registration control are processes that are controlled by computers. Registration is the alignment of the printing plates as they transfer their respective color images to the press sheet. If the plates do not line up perfectly, the image will appear out of focus and the color will be blurry. As the press runs, a computer scans a set of registration marks that have been placed on the press sheet. Each printing plate has its own individual set of registration marks. The computer reads each of these marks and automatically makes on-the-fly adjustments to the position of each printing plate in order to achieve optimal alignment. All of this occurs many times per second while the press is running at speed.
Color control is a process that controls the placement of ink on the press sheet, and is related to plate registration. The amount of ink that is released into the color units depends on how much ink is required to achieve a certain ink density on the press sheet. In modern sheet-fed presses, color control is managed by a computer that is part of the control system of the press. During the press run, press sheets are automatically scanned, and the resulting measurements are used to adjust the amount of ink that is transferred from the ink fountain to the press sheet. This process helps ensure consistent and accurate color reproduction throughout the press run.
Step Three: Bindery
The bindery is where the printing manufacturing process is completed. Press sheets have been run through the press, allowed to dry, and transported to the bindery for finishing. Press sheets are then cut and folded (if necessary) into the appropriate configuration for the desired product. For brochure printing, sheets are cut down and folded on high-speed folding machines called “folders”. For catalogs and magazines, the press sheets are folded in nested order, so that they can be stitched together (stapled) to form the finished product.
In the stitching process, a machine called a “stitcher” takes the folded press sheets (called “signatures”) and collates them together. Then stitches (staples) are inserted into the fold of the signatures, binding them together.
The final components in the “stitcher” are the knives, which trim the paper to the final delivered size. The product is then ready to be shipped to the end destination.
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