A web press is a type of printing press which accepts paper in rolls, rather than in individual sheets. Most web pressed are rotary presses, meaning that the plates used for printing are mounted on a cylinder. The rolls of paper or “webs” unspool past this cylinder, which can come in a variety of widths, and then coil back up at the other end of the press. Web presses are used for large-scale print runs in which tens or hundreds of thousands of copies are needed. Most books and newspapers are printed on web presses.
Three different printing techniques can be used with a web press. In offset lithography, one of the most common, an image is transferred to a roller from a plate. This method is rapid and highly efficient, although it can be prone to smearing. Rotogravure involves the use of an engraved cylinder or a cylinder with an engraved plate mounted to it, while flexography uses a flexible raised plate to transfer ink to the paper.
Huge web presses capable of handling really big jobs usually are heat-set, meaning that once the paper rolls past the cylinder with the plate, it is exposed to set the ink so that it will not smear. Smaller cold-set presses require more careful paper handling, because the ink is not exposed to ink, so it will be prone to smearing until it sets. Both are capable of continuous feeding, using splices when a roll of paper reaches the end.
In some cases, a web press can print on both sides of the paper simultaneously, as might be needed with a book or newspaper. In other instances, the paper is coiled as it comes out of the press and fed back through to print the other side. In either case, once the paper has been printed, it can be cut, folded, stapled, glued, and sewn into the desired shape or size. Automated machines for all of these tasks are available.
Running a web press requires a high degree of skill. The pressperson must be able to keep the tension of the paper consistent so that the press does not stop running, and he or she must keep on the supplies of ink and continuously check the output of the press to confirm that it is printing properly without any misregistrations. A small mistake can be costly when a web press involved, as it costs a lot of money to halt or redo a print run, making attentiveness at every stage critical.