Book Printing.

printing press procedures

How Offset Printing Works

Offset printing is a technique during which an inked image is transferred, or “offset” from a plate to a rubber blanket, then to a printing surface. Offset printing is often combined with lithographic printing, which uses the repulsion of oil and water to produce a flat image carrier. This is often referred to as offset lithography.

Offset lithography is the most common type of color printing for high-volume jobs. Now we’ll discuss in more detail how offset printing works and the advantages to using it.

Photo offset printing
This is the most common type of offset printing. Photo offset printing involves using light-sensitive chemicals and photographic procedures to offset images and text from original materials to printing plates. Original materials can be an actual print photo or a digital file. Nowadays, almost everything is printed from a digital file. It’s easier, faster and more accurate to use digital.

Each of the primary colors used in printing – cyan, magenta, yellow and black – have a separate plate.

Web offset lithography
The “web” part of web offset lithography is a paper feeding process whereby the paper is pulled from rolls into the printing press as one continuous stream, or web of paper. Each roll can weigh up to 1 ton (2,000 pounds). The paper isn’t cut until printing is done. Offset lithography can also be used with individual sheets of paper in sheet-fed presses, but web offset lithography is much quicker.

Web presses can print up to 50,000 impressions per hour. An impression is one full press sheet (38” x 22-3/4”).

How the ink gets to the paper
Remember that oil and water do not mix. The oil is the ink in this case. The ink is dispersed to the plates by a series of rollers. Then, on the press, the plates are dampened by water rollers, and then ink rollers. The rollers disperse the ink onto the plates.

The plate’s image area picks up the ink from the ink rollers and the water rollers keep the ink from transferring to the non-image areas of the plate. Each plate transfers its image to a rubber blanket that in turn transfers the image to the paper. This whole process happens with lightning speed.

With all the water and ink sloshing around, you would think there’d be a lot of smudges to clean up. Well, that’s been taken care of – by putting the paper through a gas-fired oven that reaches 350-400 degrees F, and immediately after the oven, the paper goes through large rollers that have refrigerated water in them. These chill rollers cool the paper instantly, which sets the ink on the paper.

Advantages of offset printing include:

Consistently high quality images. Offset printing makes images and lines sharper more easily than other printing methods because the rubber blanket is flexible and therefore can conform to the texture of the printing service.

Production plates can be made quickly and easily.

Longer printing plate life. The plates last longer because there is no direct contact between the plate and printing surface.

High speed and high volume printing.

Costs less the more you print. Most of the price of printing is wrapped up in the preparation of the production plates and anything else that happens before the first page is printed. Once everything is prepared, the more you print will only cost you the price of paper and ink, which is minimal compared to setup costs.

About the Author

Kaye Z. Marks is an avid writer and follower of developments in color printing industry and how these improvements can benefit small to medium-scale business.

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