OFFSET PRINTING

Offset printing, also called offset lithography, or litho-offset, in commercial printing, is a widely used printing technique in which the inked image on a printing plate is printed on a rubber cylinder and then transferred (i.e., offset) to paper or other material. The rubber cylinder gives great flexibility, permitting printing on wood, cloth, metal, leather, and rough paper.

The first characteristic of offset printing is that the printing and the nonprinting elements are parts of a single continuous surface. The difference is simply that the printing parts of the surface repel water when moistened but absorb the ink with which they are coated, whereas the nonprinting parts absorb the water and repel the ink.

The form used in offset printing is a metal plate the surface of which has been prepared to divide it into the parts with these opposite properties.

The second characteristic of offset printing is that the ink is not transferred directly from the printing form to the paper but is first transferred to a rubber blanket. It is the ink retained by the blanket that is transferred to the paper.

Offset presses utilize three principal elements: the plate cylinder; the blanket cylinder; and the impression cylinder, which applies the paper, in sheets or a roll, against the blanket cylinder.

Before a print job can go to an offset press the page designs created on a computer must be separated by the types of inks that will be used to make up each page.

For years, a photo-engraving process exposed color-separated films onto photosensitiveplates. Today, computer-to-plate (CTP) systems use digitally controlled laser engraving systems to etch the imaging data from digital files onto aluminum plates. CTP systems have reduced the time and expense involved in setting

The prepress process for offset printing often includes more elaborate proofing methods, such as imposition proofs that enable buyers to check that all of the pages of a published magazine will appear in the desired sequence.