The Role Printing Plates Play in the Printing Process

Lithography machine in printing shop
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Although state-of-the-art commercial printing companies are moving to digital printing, many printers still use the tried-and-true offset printing method that has been the standard in commercial printing for more than a century.

Offset Printing Process

Offset lithography — one of the most common ways to print ink on paper — uses printing plates to transfer an image to paper or other substrates. The plates are usually made of a thin sheet of metal, but in some instances, plates may be plastic, rubber, or paper. Metal plates are more expensive than paper or other plates, but they last longer, produce high-quality images on paper, and have greater accuracy than plates made of other materials. 

An image is put on the printing plates using a photomechanical or photochemical process during a stage of production known as prepress — one plate for each color ink to be printed. 

Printing plates are attached to the plate cylinders on the printing press. Ink and water are applied to rollers and then transferred to an intermediary cylinder (blanket) and then to the plate, where the ink clings only to the imaged areas of the plate. Then the ink transfers to the paper.

Prepress Plating Decisions

A print job that prints only in black ink requires only one plate. A print job that prints in red and black ink requires two plates. In general, the more plates that are needed to print a job, the higher the price.

Things become more complicated when color photos are involved. Offset printing requires the separation of colored images into four ink colors — cyan, magenta, yellow and black. The CMYK files eventually become four plates that run on the printing press at the same time on four cylinders. CMYK is different from the RGB (red, green, blue) color model you see on your computer screen. The digital files for every print job are examined and adjusted to minimize the number of plates needed to print the project and to convert color images or complicated files to only CYMK. 

In some cases, there may be more than four plates — if a logo must appear in a certain Pantone color, for example, or if a metallic ink is used in addition to full-color images.

Depending on the size of the finished printed product, several copies of the file may be printed on a large sheet of paper and then trimmed to size afterward. When a job prints on both sides of the sheet of paper, the prepress department may impose the image to print all fronts on one plate and all backs on another, an imposition known as sheetwise, or with both the front and back on a single plate in a work-and-turn or work-and-tumble layout. Of these, sheetwise is usually the most expensive because it takes double the number of plates. Depending on the size of the project, the number of inks and the size of the sheet of paper, the prepress department chooses the most efficient way to impose the project on the plates.

Other Plate Types

In screen printing, the screen is the equivalent of the printing plate. It can be created manually or photochemically and is usually a porous fabric or stainless steel mesh stretched over a frame.

Paper plates are usually suitable only for short print runs without close or touching colors that require trapping. Plan your design so that paper plates can be used effectively if you want to save money. Not all commercial printers offer this option.