Offset printing is an indirect printing process. A typical offset press generally has three cylinders – the printing plate cylinder, the rubber blanket cylinder and the substrate or impression cylinder. There are several other components that make up an offset press. All these components work together to produce the printed substrate.
Components Of A Typical Offset Press
A typical offset press is made of a feeding tray to supply paper to the feeding system, a set of cylinders to create the printed image, a roller train to ink the image areas and dampen non-image areas of the image and a system for removing the paper from the printing system.
- Feeding System: The feeding system refers to the mechanism that feeds paper or any other substrate to be printed upon into the press for printing. In sheet fed presses the substrate is stacked together in a tray. Vacuum devices called sucker feet pick up each sheet of paper from the stack and feed it into the press one at a time. For web fed presses a rollstand is used. A large continuous roll of paper is mounted onto the rollstand that maintains the appropriate amount of tension while feeding the paper to the press.
- Printing System or Cylinders: Offset presses generally use three cylinders. The plate cylinder, the blanket cylinder and the impression cylinder. The plate cylinder is mounted with the led plate on which the image is drawn right side up. This image is transferred onto the blanket cylinder in reverse or wrong side up. The blanket cylinder then passes the image onto the impression cylinder – once again right side up.
- Inking System: The inking system in offset presses is made up of a fountain, which holds the ink and a set of rollers known as the roller train. In general, the more rollers in a roller train the better the press. A roller draws ink from the fountain into the roller train. The ink is milled to the required thickness and brought to the final rollers in the roller train known as the form rollers. These apply the ink to the plate.
- Dampening System: The dampening system also consists of a fountain and a set of rollers. These apply the fountain solution to the plate to keep the non-image areas from getting inked.
- Delivery System: This is the mechanism that collects the printed substrate from the press and assembles it in a neat manner. In sheet fed presses the printed sheets of substrate are carried from the press into a tray where they are neatly stacked. In web fed presses there are two options for delivery systems. A roll-to-sheet press cut the continuous substrate into single sheets after printing as per specifications. A roll-to-roll press carries the printed susbtrate from the press to a rewinding unit where the substrate is would onto a spool.
Types Of Offset Presses
There are two main kinds of offset presses:
- Sheet fed offset press: In a sheet fed press the substrate is fed to the press in single sheets and each sheet is printed individually. The printed substrate is then collected in a tray and stacked neatly. Sheet fed presses often give better quality than web fed presses. However, the latter can be more economical for larger runs.
- Web fed offset press: In a web fed press the substrate is fed to the press from a continuous roll. After printing the roll is either rewound onto a spool or cut into sheets depending on the requirements. Web fed presses print at a much higher speed than sheet fed presses and hence are more economical for larger runs.
Thus, the initial investment in an offset printing press can be fairly high but the actual cost of offset printing turns out very economical.