Duplex printing

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Duplex printing is a feature of computer printers and multifunction printers (MFPs) that allows the automatic printing of a sheet of paper on both sides. Print devices without this capability can only print on a single side of paper, sometimes called single-sided printing or simplex printing.

Consumer and low-to-medium volume office printers use a duplexing unit that reverses a piece of paper after the first side has been printed. Often, on MFPs, this is combined with a reversing automatic document feeder (RADF) to allow for duplex scanning also. Higher volume printers may in essence have two print engines in a single device, and are able to print both sides of the paper in a single pass.

Duplex print devices, depending on options, software, and printer settings, can print single-sided page to single-sided page (1:1) or double-sided page to double-sided page (2:2). Many can also combine single-sided pages into a double-sided page format (1:2). Double-sided booklet formats (2:2 with a center fold) are also available, depending on optional outputs from the printer.

Duplexed documents can be printed as if to be bound on either the short edge or the long edge. This functionality is mostly available on printers that come with a duplexer. It can be verified in the printers properties whether or not a specific printers supports the settings.[1] Long edge binding in portrait mode allows for pages to be turned side-to-side like a book, and the pages be right side up. Short-edge binding allows the pages to be oriented correctly if they are flipped vertically, as in a notepad. This second form of printing/binding is sometimes known as "tumble." If the printing is done in landscape mode, these concepts are transposed since the print direction is different.

Single-sided printers can still print duplex jobs; however, the user has to manually turn the print job over and re-initialize the printing of the document. In that process, it is important to determine which edge and side the printer prints on. If the pages are re-initialized on the wrong side and print face, the printing will occur on the wrong direction. A simple print test is required to define the print face by writing a letter on a blank sheet and running it through the printer.[2]

In commercial printing (books, magazines, newspapers, etc.), the term applied to imparting an image to both sides of the substrate at the same time is 'perfecting' and is commonly achieved—especially in lithography—by passing the substrate through a perfecting drum, thus turning the sheet over after the first side is printed. The turned sheet then continues its way through the press, being gripped at the opposite edge whilst the second side is printed. This in effect tumbles the job; therefore, accurate sheet sizing is a must to ensure accurate backing up of the job.

Some printers, such as multiple magicolors, are capable of recognizing a command telling them to print double-sided, but need a duplex unit attachment to enable them to independently print duplex; otherwise, the printer stalls and doesn't print until the command has been retracted and changed.

Some of the original Duplex Units were designed by David Precht in collaboration with Gary Starkweather while working at Xerox.[3]

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