Offset printing uses metal plates engraved with the image to be printed. Ink is added to the plates and transferred to a rubber belt, then to paper. The paper is cut to size to complete the process. Digital engraving techniques are commonly used for plate creation. Proper plate placement is crucial to avoid errors.
The offset printing process is one of the oldest printing methods and one of the most widely used in the commercial sector. Unlike inkjet printing and laser printing, which begin by taking digital signals and printing them, the offset printing process begins with the operator creating metal plates into which the image to be printed is etched. The plates are then placed on the printer cylinder and properly aligned. A rubber band is used as a medium for the ink, which is then applied to the paper. After the paper has been cut, the offset printing process is complete.
Plates are used to carry ink in offset printing. The plates themselves are engraved with the printed image and ink is added to the plates during the printing process. Plates were traditionally made by hand, but this is difficult and can greatly impede the offset printing process, so most plates in the early 21st century are made digitally using engraving techniques that easily mimic the image being printed.
After the plates are complete, they are placed into the cylinders of the offset printer. There are different cylinders for different parts of the offset printing process and the plates must be placed in the correct order. Otherwise, color errors and printing errors may occur; for example, attempts to duplex may result in two different documents from two separate print jobs being printed on opposite sides of the same sheet of paper. Plate placement varies based on the exact offset printing method used and the number of cylinders.
At this point the offset printer is turned on and a rubber band passes to take the ink from the plates. To push out the ink, water and ink are combined so that the ink is cleanly imprinted on the rubber belt. The image in the rubber belt is reversed for now. After printing, the rubber belt is usually cleaned and reused in other models.
The ink from the rubber tape is transferred to the paper; the rubber belt is reversed before reaching the paper so that the image transfers in the correct orientation. Most offset printers have large rolls of paper, so the offset printing process isn’t complete yet. A cutting mechanism must cut the offset paper roll correctly to make sheets of suitable size. If your offset printer uses sheet instead of a roll, which is rare, but not uncommon, then a cutter is usually not needed or trimming is a low priority function.
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