In a previous post, we talked about the newer technology of digital printing (and digital presses) that is currently transforming the printing industry. Today, we’re going to talk about traditional offset printing — what it is, how it works, and the two types of offset printing presses.
A Brief History
The first offset printing press appeared in 1875 in England. Invented by Robert Barclay, the offset printing press was a further refinement of a printing technology invented by Alois Senefelder in 1796 known as lithography. The fundamental idea behind the development/creation of the lithographic printing process is the fact that water and oil do not mix. Unlike all previous printing technologies where a printed impression was affected by pressing paper onto an inked, raised surface (woodblock printing) or by forcing paper into an inked, depressed surface (intaglio printing), lithography used oil-based inks applied to a flat surface — initially a stone (lithos is Greek for “stone”) — where non-printing areas would be etched with a combination mild acid and arabic gum that would hold water. Hence, when the oil-based ink was applied to the stone it would adhere to only the non-etched areas of the stone. Paper pressed onto the flat stone would then absorb the ink.
What is an Offset Press?
Offset presses are built utilizing a series of rollers (metal cylinders), arranged in a specific sequence, that transport paper through the press and imprint the paper with the desired design/image/text. The first roller in the sequence, known as the plate cylinder, accepts a thin metal or plastic “plate” onto which the material to be printed has been, not unlike the lithographic stone, etched (traditionally via photographic methods now via laser technology) or treated such that non-printing areas of the plate attract a water solution that repels ink. Additional rollers contact the plate cylinder as it spins distributing water and ink to the plate.
Next in the sequence of rollers is the offset or blanket cylinder on which a rubber “blanket” is affixed that picks up the ink from the plate cylinder as the water solution drains away. The rubber blanket now holds a mirror of the design on the printing plate.
Finally, the last roller in the sequence, the impression cylinder, presses the paper against the rubber blanket and picks up the ink. Because the image from the printing plate is not printed directly to the paper, but is transferred to the middle cylinder (offset) before the third cylinder creates the impression on the paper, the term “offset” is used to identifye this type of printing press.
What about Color Printing?
In a multi-color offset press, each ink color has its own set of rollers. While 2-color and 4-color (CMYK) presses are most common, 8-color presses are prevalent and there are even 12-color presses available in today’s print industry.
Two Kinds of Paper Feed
Offset presses are commonly identified by the technology used to feed paper through the press. If paper is sent through the press one sheet at a time, the press is referred to as a sheet-fed press. On the other hand, if paper is fed continuously into the press from a roll of paper, the press is referred to as a web press. A common acronym used to designate a web press is SWOP which simply means Standard Web Offset Press.
Generally, sheet-fed presses are more prevalent and are found in small to medium-sized print shops. Web offset presses are typically found in larger commercial print operations because they run much faster than sheet-fed presses and can produce greater quantities of items like newspapers, catalogs, magazines, books, etc., in shorter periods of time.
If you have any questions about offset printing, give us a call at 330-597-8560. We’re happy to help you get the most out of your printing project.
