Although the most advanced commercial printing companies are moving to digital printing, many printers still use the proven offset method that has been the standard for commercial printing for more than a century.
While the quality of digital printing has improved dramatically, offset printing using metal plates still provides the highest print quality.
Offset printing uses printing plates to transfer images onto paper or other substrates. These plates are usually made of thin metal sheets, but in some cases the 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 a higher degree of accuracy than plates made of other materials.
In the production stage, called prepress, the image is "burned" onto the plate using a photomechanical or photochemical process. A plate is made for each color of ink used in the print job.
The plate is attached to a plate cylinder on the printing press. Ink and water are applied to the cylinder. The image on the plate is transferred to the intermediate cylinder and then to the plate, with the ink adhering only to the imaging area of the plate. The ink is then transferred to the paper that passes through the press.
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Print jobs printed with black ink only require only one plate. Jobs printed with red and black inks require two plates. Typically, the more plates needed for a print job, the more expensive it is.
When it comes to color photos, things get even more complicated. Offset printing requires separating the color image into four ink colors - cyan, magenta, yellow and black. the CMYK file ends up as four plates running on four cylinders at the same time on the press. CMYK is different from the RGB (red, green, blue) color model you see on your computer screen. The digital files for each print job are checked and adjusted to minimize the number of plates needed to print a project and to convert color images or complex files to CYMK only.
In some cases, there may be more than four plates. Additional plates are required if the logo must appear in a specific Pantone color, or if metallic inks are used in addition to the full-color image.
Depending on the size and quantity of the final printed product, multiple copies of a document may be printed on one large sheet of paper and then trimmed to a certain size.
When a print job is printed on both sides of a sheet, the prepress department may plate imposition the image to print all fronts on one plate and all backs on the other; this imposition is called sheet imposition. This imposition is used when one side of the paper has a different texture than the other, or when the print job consists of versions of the same front and multiple backs.
The front and back can be imaged on the same plate in either a work-and-flip or work-and-tumble layout. In these methods, a single sheet is usually more expensive because it requires double the number of plates. Depending on the size of the project, the amount of ink and the size of the paper, the prepress department chooses the most efficient way to impose the project onto the plate.
Metal plates are expensive. The more plates needed, the higher the setup cost of the print run.
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In screen printing (a popular fabric printing process), the screen is the equivalent of a 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 only suitable for short runs, where there is no need to trap tight or contact colors. Plan your design so that if you want to save money, you can use paper plates efficiently. Not all commercial printers offer this budget option.
The digital printing process does not use printing plates. It requires a different type of press and is ideal for short runs, quick turnaround, affordable short runs and personalized variable data printing. Not all commercial printing companies have offset and digital presses.
