What the process of offset lithography?

Offset lithography is the workhorse of printing. Almost every commercial printer does it. But the quality of the final product is often due to the guidance, expertise and equipment provided by the printer.

Offset lithography works on a very simple principle: ink and water don’t mix. Images (words and art) are put on plates (see the next section for more on this), which are dampened first by water, then ink. The ink adheres to the image area, the water to the non-image area. Then the image is transferred to a rubber blanket, and from the rubber blanket to paper. That’s why the process is called “offset” — the image does not go directly to the paper from the plates, as it does in gravure printing.

Now, let’s look at the steps in the printing process.

Step One: Pre-Press Production:

Before the job can be printed, the document must be converted to film and “plates.” In the case of How Stuff Works Express, film negatives are created from digital files. Images from the negatives are transferred to printing plates in much the same way as photographs are developed. A measured amount of light is allowed to pass through the film negatives to expose the printing plate. When the plates are exposed to light, a chemical reaction occurs that allows an ink-receptive coating to be activated. This results in the transfer of the image from the negative to the plate.
There are different materials for plates, including paper (which produces a lower-quality product). The best plate material is aluminum, which is more costly.

Each of the primary colors — black, cyan (blue), magenta (red), and yellow — has a separate plate. Even though you see many, many colors in the finished product, only these four colors are used (you’ll also hear this called the four-color printing process — it’s a little like the three-color process used in television).

We’ll talk more about this later as we explore color and registration control.

Step Two: The Press Run

The printing process used to print How Stuff Works Express is called web offset lithography. The paper is fed through the press as one continuous stream pulled from rolls of paper. Each roll can weigh as much as 2,000 pounds (1 ton). The paper is cut to size after printing. Offset lithography can also be done with pre-cut paper in sheetfed presses.

Web presses print at very high speeds and use very large sheets of paper. Press speeds can reach up to 50,000 impressions per hour. An impression is equal to one full press sheet (38 inches x 22 and three-fourths inches), which is 12 pages of How Stuff Works Express.

Even when a 1-ton roll of paper runs out, the presses do not stop rolling. Rolls can be spliced together as the web press is running by using festoons. Festoons are a series of rollers that extend up into a tower. A few moments prior to the splice occurring, the festoons will move up into the tower, pulling in large amounts of paper. At the moment the splice occurs, the rolls of paper stop rotating for a split second, at which point the paper is taped together automatically. As the newly spliced roll begins to pick up speed, the festoons begin to drop out of the tower at a rate predetermined by the speed at which the press is operating. The press operator never has to adjust the press controls during this operation.

The press has to maintain a constant balance between the force required to move the paper forward and the amount of backpressure (resistance) that allows the paper to remain tight and flat while traveling through the equipment.

The Inking Process
Ink and water do not mix — this is the underlying principle of offset lithography. The ink is distributed to the plates through a series of rollers. On the press, the plates are dampened, first by water rollers, then ink rollers. The rollers distribute the ink from the ink fountain onto the plates.

The image area of the plate picks up ink from the ink rollers. The water rollers keep the ink off of the non-image areas of the plate. Each plate then transfers its image to a rubber blanket that in turn transfers the image to the paper. The plate itself does not actually touch the paper — thus the term “offset” lithography. All of this occurs at an extremely high speed.

The paper is left slightly wet by all of the ink and water being applied. Obviously, there is a risk of the ink smudging. The smudging is avoided by having the paper pass through an oven. The oven is gas fired, and the temperature inside runs at 350 to 400 degrees Fahrenheit (176 to 206 degrees Celsius).

Immediately after leaving the oven, the paper is run through a short series of large metal rollers that have refrigerated water flowing through them. These chill rollers cool the paper down instantly and set the ink into the paper. If this were not done, the ink would rub off on your fingers.

Color and Registration Control

Color and registration control is a process that is aided by the use of computers. Registration is the alignment of the printing plates as they apply their respective color portion of the image that is being printed. If the plates do not line up perfectly, the image will appear out of focus and the color will be wrong. A computer takes a video image of registration marks that have been placed on the press sheet. Each plate has its own individual mark. The computer reads each of these marks and makes adjustments to the position of each plate in order to achieve perfect alignment. All of this occurs many times per second while the press is running at full speed.

Color control is a process that involves the way in which the ink blends together, and is tied closely to the plate registration. The amount of ink that is released into the units depends on how much ink is needed to achieve a desired look. The ink is adjusted via the control panel that is part of the overall control console. Prior to being placed on the press, the plates are scanned and the data is then transferred to a micro cassette. This serves as the “master” that directs the release of ink to pre-set values.

Step Three: Bindery
The bindery is where the printed product is completed. The huge rolls of now-printed paper are cut and put together so that the pages fall in the correct order. Pages are also bound together, by staples or glue, in this step of the process.

In the case of How Stuff Works Express, a machine called a stitcher takes the folded printed paper (called press signatures) and collates them together. Then stitches (staples) are inserted into the signatures, binding them together.

The final components in the stitcher machine are the knives, which trim the paper to the final delivered size. The product is then ready to be shipped to the end destination. printing company!

  1. I have read this great article today! The dampening system (Unit) is an essential part of every commercial printing press, I found this article that explains in details about Dampening system types.

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