Knowledge about Offset Printing Technique
What is offset printing?
Offset printing is a technique in which inked images are pressed onto rubber sheets (offset plates) first and then transferred from the rubber sheet onto paper. When used with lithographic printing, this technique avoids the problem of water adhering to the form along with the ink.
Advantages of offset printing technique
High image quality – sharper and cleaner than direct printing from the printing plate onto paper because the rubber blanket presses evenly onto the printing surface.
Ability to print on various surfaces, including non-flat surfaces (such as wood, fabric, metal, leather, and rough paper).
Easier plate-making process.
Longer lifespan of printing plates – because they do not come into direct contact with the printing surface.
Offset printing has become the most popular printing technique in commercial printing. However, for personal hobbies and interests, some people still prefer to create high-quality prints using direct printing. Some people still like the indented and raised lines left on the paper from direct printing. Some books of this kind are even printed using typesetting plates made from lead type, which is quite old printing technology.
Offset printing principle
Offset printing is a flat printing method, where image information is displayed on a printing plate with photosensitive properties to create printing elements that attract ink and non-printing features that attract water. In addition, the image on the printing plate must be in the correct orientation, meaning it should be in the same direction as the paper it will be printed on.
Structure of offset printing machine
An offset printing machine for sheets consists of major components such as a paper supply unit, one or more printing units, transfer devices to move the paper through the engine, a paper output unit, and additional support components such as a machine control panel.
Usually, a printing unit in a sheetfed offset printing press has three main cylinders along with a dampening system and an inking system:
Plate cylinder: a metal cylinder that carries the printing plate with ink-receptive image areas and water-receptive non-image areas.
Blanket cylinder: a cylinder covered with an offset rubber blanket comprising a fabric layer and a synthetic rubber layer to transfer the image from the printing plate onto the substrate.
Impression cylinder: a cylinder that always contacts the blanket cylinder to transfer the inked image onto the paper or other substrates.
Dampening system: a dampening roller that soak the printing plate with a dampening solution containing various additives such as acid, gum arabic, isopropyl alcohol, or other fountain solutions.
Inking system: ink rollers that apply ink to the printing plate. Other essential components in a single-color or multi-color offset printing press include:
- Feeder: a device that sucks paper and other substrates from the paper feeder and delivers them to the first printing unit.
- Transport system: usually a set of gripper cylinders that transport paper through the press.
- Delivery system: a device that receives printed sheets and piles them evenly on the delivery table.
The moistening system in sheetfed offset printing supplies water-based dampening solution or fountain solution onto the surface of the printing plate before it is inked. The dampening solution keeps the non-image area on the scale wet, so it does not accept ink. It is applied to the entire printing plate.
However, the non-image areas are designed to accept water and repel ink on the plate, and this is achieved by the presence of a thin layer of gum Arabic during the plate-making process. This layer is hydrophilic or water-loving, while the image areas are hydrophobic or water-repelling. Water itself can be used to dampen the plate. Some sheetfed offset printing presses can use water alone to print short runs. However, this ink-repelling layer gradually wears off as the container is reused on the media. The chemicals in the fountain solution supplement this layer’s ink-repelling ability.
Ink transfer system
The ink supply system in sheetfed offset printing performs the following four essential functions:
Transfer ink from the ink fountain roller to the ink plate. Distribute the thick ink layer into a thin, even layer on the transfer rollers. Apply ink to the image areas on the scale. Remove ink residue from the ink rollers from previous printing.
Structure of the ink supply unit:
Ink fountain: contains ink needed for printing.
Ink duct roller: an alternating roller in contact with the ink fountain roller and the first roller of the ink supply system, usually the metering roller in the ink supply system.
Form roller and plate cylinder: are synchronously moving rollers connected by gears and chains that not only rotate but also move horizontally back and forth with the tube axis from left to right of the ink fountain, the two side walls of the ink fountain, the doctor blade and ink fountain roller, and are responsible for applying a thin layer of ink on the rollers and removing the previous ink layers on the ink duct roller.
Intermediate rollers: are moving rollers based on contact with other connected rollers in the transmission system; medium rollers located between the impression cylinder and the ink duct roller are responsible for providing ink for the printing process and are often called metering rollers – when they come into contact two other rollers or are called ink transfer rollers – when they contact only one roller, for example, like a form roller. Plate cylinders: include 3-4 plate cylinders of different diameters, which come into contact with and apply ink to the printing plate.
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