The Role Printing Plates Play in the Printing Process

Before digital printing, offset printing ruled

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.

Although the quality of digital printing has vastly improved, offset printing with metal plates still offers the highest possible printing quality.

Offset Printing Process

Offset printing 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 are made of plastic, rubber, or paper. Metal plates are more expensive than paper plates, but they last longer, produce high-quality images on paper, and have greater accuracy than plates made of other materials. 

An image is "burned" on a printing plate using a photomechanical or photochemical process during a stage of production known as prepress. One plate is made for each color of ink in the print job. 

Printing plates are attached to the plate cylinders on the printing press. Ink and water are applied to rollers. The image on the plate transfers to an intermediary cylinder and then to the plate, where the ink clings only to the imaged areas of the plate. Then the ink transfers to the paper that runs through the press.

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 specific Pantone color or if a metallic ink is used in addition to full-color images, additional plates are needed.

Plate Imposition and Cost

Depending on the size and quantity 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 print job prints on both sides of the sheet of paper, the prepress department may impose the image to print all the fronts on one plate and all the backs on another, an imposition known as sheetwise. This imposition is used when paper has a different texture on one side than the other, or the print job consists of the same front with multiple versions of the back.

The fronts and backs can be imaged on the same plate in a work-and-turn or work-and-tumble layout. Of these approaches, sheetwise is usually more 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.

Metal plates are expensive. The more plates that are needed, the higher is the setup cost for a print run.

Other Plate Types

In screen printing, a process popular for printing on fabrics, 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 budget option.

The Rise of Digital Printing

The digital printing process doesn't use printing plates. It requires a different type of printing press and is ideal for short runs, fast turnarounds, affordable short runs, and personalized variable data printing. Not all commercial printing companies have both offset and digital printing presses.