How Litho Works

Posted on 9/16/2013 by Kevin Salter

Have you ever wondered how Litho printing works? Well it’s based on the principal that ink and water don’t mix.

The images you provide are transferred onto a printing plate by a laser. The plates have a roughened texture, and are coated with a thermal emulsion.

The plates are then chemically treated to remove the exposed part of the emulsion from the plate

When the printing plate is finished, the image area is rendered grease receptive and hydrophobic (so attracts ink and repels water). The areas on the plate that are non-printed are rendered hydrophilic (they attract water but repel ink)

The plates are affixed on a cylinder on the press and they rotate.

Through each rotation the plate is passed over dampening rollers, which flood the plate with water, which adheres to the rough surface, or negative parts of the image.

Then the plate is presented to rollers coated with ink, which adheres to the smooth or positive portions of the plate.

If at this point the image was transferred directly to paper, the paper would be dampened, so a cylinder with a rubber surface called a blanket is rolled over the plate. It squeezes the water away, but picks up the ink. In doing so, the image is transferred from the plate to the blanket, hence the term “Offset” Litho.

The image is then transferred from the blanket to the paper.

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