offset lithography

Offset printing is a commonly used printing technique where the inked image is transferred (or "offset") from a plate to a rubber blanket, then to the printing surface. When used in combination with the lithographic process, which is based on the repulsion of oil and water, the offset technique employs a flat (planographic) image carrier on which the image to be printed obtains ink from ink rollers, while the non-printing area attracts a water-based film (called "fountain solution"), keeping the non-printing areas ink-free.

Offset lithography is one of the most common ways of creating printed matter. A few of its common applications include: newspapers, magazines, brochures, stationery, and books.

Many modern offset presses use computer to plate systems as opposed to the older computer to film workflows, which further increases their quality.

Advantages of offset printing compared to other printing methods include:

• Consistent high image quality. Offset printing produces sharp and clean images and type more easily than letterpress printing because the rubber blanket conforms to the texture of the printing surface.
• Quick and easy production of printing plates.
• Longer printing plate life than on direct litho presses because there is no direct contact between the plate and the printing surface. Properly developed plates running in conjunction with optimized inks and fountain solution may exceed run lengths of a million impressions.
• Cost. Offset printing is the cheapest method to produce high quality printing in commercial printing quantities.

Disadvantages of offset printing compared to other printing methods include:

• Slightly inferior image quality compared to rotogravure or photogravure printing.
• Propensity for anodized aluminum printing plates to become sensitive (due to chemical oxidation) and print in non-image/background areas when developed plates are not cared for properly.
• Time and cost associated with producing plates and printing press setup. As a result, very small quantity printing jobs are now moving to digital offset machines.

How ink is transferred from the ink duct to the paper has several steps.

1. Ink duct roller delivers ink from the ink duct to the ink pyramid.
2. The drop roller transfers ink from duct roller to distribution roller. It is never in contact with both rollers at the same time.
3. The distribution rollers evenly distribute the ink. The first distribution roller picks up the ink from driving rollers, and the last distribution rollers transfer the ink to the form rollers.
4. The transfer rollers transfer ink between the ink-absorbing and ink-delivering driving rollers.
5. Driving rollers roll against the distribution rollers and either absorb or deliver ink, depending on their placement.
6. Ink form rollers transfer ink from the last distribution rollers on to the printing plate.
7. The printing plate transfers the ink to the offset cylinder(typically called blanket cylinder) usually covered with a rubber 'blanket'.
8. The paper is then pressed against the blanket cylinder by the impression cylinder, transferring the ink onto the paper to form the printed image.