If you’ve never heard of offset printing before, you may not be familiar with everything it involves. Offset printing is how we got our start.
Offset Printing Definition
Offset printing, sometimes referred to as lithography or litho-offset, is a widely used printing technique in commercial printing where an inked image on a printing plate is printed on a rubber cylinder and then transferred to paper or other printing materials. The rubber cylinder itself gives you a ton of flexibility, allowing you to print on metal, wood, cloth, leather or rough paper.
With offset printing, the print is not raised above the surface of the printing plate (as in letterpress) nor is it sunk below it (like intaglio or gravure printing). Instead, it’s flush with the surface of the plate; thus offset is classified as a planographic method of printing.
Offset printing, which is a development of lithography, is based on the principle that water and grease do not mix. Due to this, a greasy ink can be deposited on grease-treated printing areas of the plate, while nonprinting areas, which hold water, reject the ink. The offset plate is commonly made from zinc or aluminum, or sometimes zinc and aluminum together, with the surface treated to render it porous and then coated with a photosensitive material. Exposure to an image hardens the coating on printing areas; the coating on nonprinting areas is washed away, leaving wetted metal that will reject ink.
Modern offset printing is done on a press composed basically of three rotating cylinders:
- Plate Cylinder – This is where the metal plate is fastened.
- Blanket Cylinder – This is an impression cylinder which pressed paper into contact with the blanket cylinder.
- Plate Cylinder – Contacts moistening rollers which deposit moisture in granulations of the metal.
A series of inking rollers then pass over the plate, and the ink is rejected by the water-holding areas and accepted by the greasy image. The inked image is transferred to the rubber blanket and is then offset to the paper traveling around the impression cylinder.