Before we understand the offset machine, let us first understand what is offset printing. In offset printing, the image or text to be printed needs to be transferred from a metal plate to a blanket or cylinder, and then transferred to the printing medium. The print medium (usually paper) is not in direct contact with the metal plate. "Offset printing" is a technical term for the process of transferring an image from a metal plate to a blanket and then onto a substrate or surface that is the final print medium, be it a book, magazine, newspaper or any other print medium.
The offset printing press was an accidental invention of New Jersey paper mill owner Ira Rubell in the early 20th century. He used a lithographic press to print bank deposit slips, and his press operator would occasionally miss a sheet, causing the press to print an image on the rubber surface of an impression roller. Thus, the next page fed into the printing press would be printed on two sides: one side with the image from the lithographic plate, and the other side with ink from the impression roller blanket. Rubell noticed that the images transferred from the blanket were sharper than those transferred from the slate. He teamed up with another printer to build a printing press that always first transferred the image to a blanket and then "offset" it onto a sheet of paper.
Gaining a deeper understanding of the capabilities of an offset press, it is important to note how image transfer occurs flawlessly and without spilling ink on non-printing areas of the substrate. It is known as an offset lithography process where the part of the substrate to be printed takes ink from an ink roller, while the rest of the substrate takes a soluble film from a water roller, keeping ink away from non-printing areas. The main components of an offset printing press are the ink roller, water roller, plate roller or cylinder, offset printing roller or cylinder, and blanket or impression roller.
There are two types of offset presses - sheetfed offset presses and web offset presses. In a sheetfed machine, the sheets are fed into the machine with the help of an automatic suction bar that uses mechanical registration to maintain the consistency of the print. These machines are great for short-term jobs like magazines, brochures, catalogs, and more. The web in an offset printing press is the huge web that is mounted on the machine to feed the paper continuously into the machine. This machine is ideal for high-volume printing jobs such as newspapers.