Offset Printing is a widely used commercial printing technique that produces consistent and high quality printed materials. As a general rule, this technique is used for production runs of 1,000 or more pieces. The offset printing method is also commonly used for gang or combo runs.
Offset printing presses use a sequence of rollers (also called cylinders or drums) to deliver ink to the paper.
The Roller Sequence of an Offset Press
The first roller has a thin aluminum or plastic printing plate formed around it. This roller is called the Plate cylinder. The plates are custom made for each print job and contains the images that are to be transferred to the paper. The plates are treated so that certain areas attract ink while other areas attract a water solution which repels ink.
As the plate cylinder spins, it contacts rollers that distribute a water solution onto the plate and other rollers that distribute ink onto the plate. Because the plate has been specially treated, the ink adheres to the image areas while the water solution repels ink from the non-image areas.
The next roller in the sequence has a rubber blanket around it. This roller is called the Offset or Blanket cylinder and turns in the opposite direction of the plate cylinder. As the plate cylinder rolls against the offset cylinder, the water on the plate is squeezed away and the ink on the plate is transferred to the rubber blanket. The offset cylinder receives a mirror image of the ink design to be printed.
The final roller in the sequence is called the Impression cylinder. It turns in the opposite direction of the offset cylinder. It is a clean steel cylinder that presses the paper against the rubber blanket to transfer the ink to the paper. The image on the paper is now identical to the image on the plate.
Now you know why this printing method is called offset printing: the image is not applied directly from plate to paper, it is offset onto a middle roller before being applied to the paper. And because the surface of the middle roller is rubber, it reduces wear on the printing plate and prolongs its usable life.
Each Ink Color has its own Set of Rollers
It is important to note that each ink color applied to the paper has its own set of rollers as described above. Hence, an offset press used for four color process printing has four sets of rollers in succession, one for each of the four CMYK colors. Likewise, a two color offset press will have two roller sets in succession.
The Paper Feed
Offset printing presses are often described by the method used for feeding paper into the press, namely sheet-fed or web-fed.
A sheet-fed offset press uses single sheets of paper that are fed into the press one after the other. Sheet-fed presses are used for many popular printed items such as brochures, flyers, letterhead and booklets. Depending on the project, the sheets used may be the same size as the finished printed piece or they might be larger sheets that are trimmed down to the finished size after being printed.
A web-fed offset press uses a long, continuous roll of paper that is fed into the press from a giant spool. The paper feeds into the press through a series of tension rollers which keep the paper taut. After printing, the paper is trimmed to the desired size. Web presses run faster than sheet-fed presses and are used for high volume printing such as newspapers, magazines, books and catalogs.
Offset printing is a fascinating process that produces very high quality printing. Let us know if you have any questions about offset printing. Also, keep Medius in mind for any upcoming printing projects you may have. We make the printing process easy for you.