Lithographic Offset printing is the most widely used form of printing in the world.
For a balance of cost effectiveness and high quality, no other printing method can compete in commercial printing. Although, it is coming under increasing pressure from advancements in Digital printing and Flexographic printing.
The Litho Offset printing process.
Pre Press Plate making.
Litho Offset printing is based on the theory that oil and water don't mix. The image to be printed is first burned onto a printing plate that is normally made from Aluminum with a special chemical coating. The plate is then put through a developer which separates the non image area of the plate from the image area that has had the image burned into it.
The result of this process leaves two very different areas on the printing plate. The image area of the plate is oliophilic and attracts ink, while the non image area is hydrophilic and attracts water.
A lithographic offset printing press has three cylinders.
The Plate Cylinder.
The top cylinder is the plate cylinder and as the name suggests, this is were the printing plate goes. The plate is held in place by a clamp at the bottom (grip edge) and at the top ( back edge).
The Blanket Cylinder.
Below the plate cylinder is the blanket cylinder. This is were a rubber printing blanket is placed and how offset printing gets its name. The image is transferred (offset) from the printing plate to the blanket. When the image is transferred to the blanket it becomes wrong reading or a mirror image of the printing plate which is right reading or appears as it would on the finished sheet.
The Impression Cylinder.
The image is transferred from the blanket to the paper, by the paper being squeezed between the blanket cylinder and the impression cylinder, which is the bottom of the three cylinders on the offset printing press. It is vitally important that the right pressure is set between these two cylinders as not enough pressure will lead the the ink not being transferred to the paper. While too much pressure can lead to dot gain or a slight distortion on the image on the sheet.
Inking and Damping System
Ink and water are transferred to the printing plate by using a system of interconnected rollers.
Ink is placed in an ink duct which has a metering roller that transfers ink to the rest of the inking rollers and then to the plate. The ink will be attracted to the image area of the plate but if there is not enough water on the non image area then the non image area can ink up as well.
Similarly to the inking system water is carried from a water trough by a system of rollers and transferred to the plate. The non image area of the printing plate will attract water and repel ink as long as there is enough water to do so. Alcohol is added to the water in an offset printing press to increase to surface area of the water. This allows the printer to carry less water and improve the quality of the print. Fountain solution is also added to the water to help control the PH balance of the water/ Alcoa mix.
Types of Offset Presses.
Sheet fed offset printing presses are the most common form you will find in the industry. Paper is fed into the machine one sheet at a time at high speeds and come out at the front or the delivery after traveling through the printing units.
Web printing is were paper is fed into the offset printing press by a large reel (roll) of paper. Travels through the printing units and cut into sheets at the delivery by a sheeter.
Lithographic Offset Printing is the worlds most dominant printing process. Everything from business cards to newspapers are printed by Lithographic Offset printing. The sheer volume of investment in Offset printing means it will continue to dominate printing for many years to come.