How Does a Rotary Printing Press Work?
The Machine
The rotary press was invented in the mid-1800s by Robert March Hoe. While rotary presses have evolved somewhat to accommodate automation through electricity, the core machine remains more or less unchanged. The typical rotary press functions by passing a substrate (the material to be printed onto) between two rotating drums. One of the drums will be partially submerged in a large inkwell. This drum will have the image to be applied to the substrate curved over its surface, either on a specialized plate or the drum itself will be engraved with the image. As the drum turns toward the substrate, its surface is scraped by a doctor blade. The function of doctor blade is to ensure that excess ink does not reach the substrate. At the point of contact between the substrate and the inked image, the second drum or impression cylinder presses the paper against the inked drum, creating the image transfer. Rotary printing presses are used to do three main types of printing: rotogravure, flexography and offset printing.

Rotogravure
Rotogravure printing is the most direct form of rotary printing. This process makes use of copper cylinders that are partially immersed in ink wells. The cylinder itself is engraved through etching, forming a multitude of small impressions in the cylinder. These impressions are filled with ink as the cylinder rotates in the inkwell and then the ink is transferred onto the substrate as it passes between the etched cylinder and the impression cylinder. This process can be used to create color images by using several etched cylinders immersed in different ink colors, though the ink must be dried between color applications.

Flexography
Flexography, also referred to as flexo, typically uses flexographic plates to accomplish the image transfer onto the substrate. The plates are created using a number of methods, but the end result is the same: a plastic or rubber plate, with the desired image in a mirrored, raised relief. The flexography process employs a three-roller system rather than a two-roller system. One roller is partially immersed in the inkwell. This transfers ink to the raised surfaces of the plate, which is wrapped around its own cylinder. The plate is scraped by the doctor blade. The substrate is then passed between the plate cylinder and the impression cylinder, effecting the ink transfer from the plate to the substrate.

Offset Printing
Offset printing is a more complex process that shares as much with lithography as with rotary printing. In this process, a lithographic plate is used on the plate cylinder. As the plate cylinder rotates, water is applied to the plate and then ink is applied. The lithographic plate is treated in such a way that water will be attracted to the areas of the plate where no image should be printed. This water will repel the ink in those areas. The image portion of the plate is treated to repel water and only the ink will stick to it. The ink image is transferred onto an offset cylinder, which is typically made of rubber. The rubber picks up the ink. The substrate is then pressed between the offset cylinder and an impression cylinder where the ink is transferred to the substrate.