The Printing Process

Offset printing is something almost every commercial printer is involved in. However the quality of offset printing often depends on the experience of the printer and the equipment used. There are three main steps in the printing process.

• Pre-press Production: Once the artwork has been approved it needs to be converted to film and plates for printing. The film negatives are created from the digital files. The images from the negatives are then transferred onto the printing plates much like developing photographs. Different materials from paper to aluminum are used to produce plates. Each of the four colors – C, M, Y and K has a separate plate.
• The Press Run: Offset printing works on the principle that oil and water do not mix. The press run or offset printing process or offset press is made of the feeding system, the printing system or three cylinders, the inking system and the delivery system. There are different types of offset presses as well that are used for the press run. The press run starts with a blank substrate and ends with a printed substrate.
• Bindery: The bindery is the stage when the printed substrate is given the final treatment before it is declared ready for use. If the substrate is in the form of a large roll, it is cut into required size sheets. It is glued, bound, stapled and collated before being shipped to the final destination.

Thus, the offset printing process involves a number of stages and the hard work of many professionals. It is no wonder that offset printing is a large industry in itself and the applications of offset printing are increasing day by day.

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:

1. 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.
   
2. 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.