How Stuff Works: Offset Printing Press

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Because I grew up around printing presses and in a printing company, sometimes I take for granted how cool the print process actually is. I forget what an interesting place printing companies can be to work in until I tour a client or prospective customer around our building to show them what we do.

One of the things I like to explain is how the print process actually works as most people have no idea how a printing press gets the job done. Specifically, how offset printing works.

The principle of offset printing is the basic tenet that oil and water don’t mix. My dad was famous for his school demonstrations on career day(s) where he would load up an ink knife and lick it in front of the class to show the ink wouldn’t transfer onto his tongue.

(As a side note, we had an employee who tried doing that one time when a group of students came to tour the building and he got a little too aggressive with the pressure … it didn’t end well.)

The print process starts with good design. A good graphic designer that really knows the print process can do a lot to help you save money and make sure your print project runs well on press and through the bindery.

Next, the print design is sent to pre-press where it is prepared for the press. Because design is done in single pages or layouts, pre-press is where the entire job is considered and laid out into press sheets that can be combined in bindery to produce your final piece (more on this in another post).

The final step in pre-press is to burn plates. The print process I grew up with required large cameras and plate burners to produce negatives, strip those negatives into press sheet layouts, and then burn the negatives onto a plate.

A fresh plate before processing

These days all of that is computerized and we utilize high-quality print-ready PDF’s to lay the job out, separate colors, and rip it into files the imagesetter can understand and translate to a plate.

A simplified explanation of a plate is simply that it’s an aluminum sheet with two special characteristics. The imagesetter exposes the image area of the plate to light which “burns” the image into the plate using a compound that repels water and is receptive to oil, while the rest of the plate is receptive to water.

Imagesetter at Searles Graphics

The plates are then put on the press where the plate first contacts a roller that coats the non-image area with water, followed by a roller that coats the remainder of the plate (the image area) with ink (remeber the whole ink and water don’t mix thing?).

Because the plate is also saturated with water, it wouldn’t do much good to put that in contact with a sheet of paper, so the plate is run against a blanket cylinder where the image is transferred – or “offset” – onto the blanket. This is what ultimately transfers the ink onto the paper and is how offset printing gets its name.

Hopefully I didn’t bore you too much and feel free to stop by or contact us if you’d like to take a tour or lick an ink knife sometime (we may not actually let you lick an ink knife)!

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