The lithography name comes from lithos, stone and graphia. It’s best described as a planographic process, which is printing from a smooth surface, a plate, to a substrate, which is generally onto paper. Nowadays lithography is called Offset or Offset Lithography.
HOW DOES LITHO PRINTING WORK?
The lithography plates are rough and covered with a photosensitive/light sensitive emulsion.
There are two ways that the plates are made: exposure from a light source with film on top of the plate, or using a machine that exposes the plate by using computer lasers. Both ways create a negative of the image, releases the emulsion and transfers a positive image to the emulsion. Then the emulsion is chemically treated to remove the unexposed areas, similar to the way a photograph is developed.
Once the printing plate is made the image is rendered grease receptive and hydrophobic (water repelling). The non-printing areas are rendered hydrophilic (water attracting), and ink repelling. The plate is mounted onto the plate cylinder and when it comes into contact with the rollers that are wet, the water adheres to the negative (rough) portions of the image and the rollers with ink adhere to the positive (smooth) portions of the image.
Next, a cylinder covered with rubber, which is called a blanket, is rolled over the plate. The blanket squeezes out the water and picks up the ink. Now when the cylinder rolls over the paper, the ink transfers the image. This process is called “offset lithography” because the image is offset to the drum before being applied to the paper.
Advantages of Litho printing:
- Consistently high quality images
- Quick and easy production of printing plates
- Longer printing plate life
- Cost effective
- Can do short, medium and long press runs
- Both sheet-fed and web presses are used: sheet-fed press is used for printing advertising, books, catalogues, cards, posters, packaging, direct mail inserts, vouchers, and art reproductions. Web press is used for newspapers, pre-printed newspaper inserts, advertising literature, catalogues, books, and magazines.