An offset printing plate is a thin sheet (usually made of aluminum, sometimes polyester) used in the offset printing technique.

What is it and how does it work?

Material and thickness: The plates are typically made of aluminum and have a thickness ranging from 0.15 to 0.4 mm.

Planographic printing: Offset is a form of planographic (flat-surface) printing. There is no physical height difference on the plate itself between the areas that are printed and those that are not.

Water and grease: The process relies on the principle that water and grease (ink) repel each other:

1. The plate is first dampened with water. The non-image areas (the white spaces) absorb the water.

2. Next, greasy ink is applied. The water repels the ink, so the ink only adheres to the dry, exposed image areas.

Indirect printing process: The ink on the plate is first transferred to a rubber cylinder (a rubber blanket) and is then printed onto the paper from that cylinder.

One plate per color: A separate printing plate is required for each printing color (often CMYK—cyan, magenta, yellow, and black—or a specific PMS color).