Offset Printing - What is offset printing?
Any method of printing in which the image being printed is transferred from the image carrier to an intermediate carrier before being transferred to the substrate. The term can be applied to a number of analogue printing processes but is most commonly used to refer to offset lithography.
Offset printing was discovered accidentally; sheet fed lithographic presses were fed by hand, which meant that sometimes a sheet would be missed and the plate cylinder would transfer the image onto the impression cylinder instead of the missing sheet. When the next sheet was sent through, the image on the impression cylinder would be transferred onto the back of that next sheet and it was noted that the print quality of the image transferred from the impression cylinder was actually sharper and clearer than the image transferred from the plate cylinder itself.
Offset printing, therefore, introduces a third cylinder to the usual combination of an image cylinder (such as a plate cylinder or gravure cylinder) and an impression cylinder. This third cylinder is known as the blanket cylinder (or offset cylinder) and sits between the plate cylinder and the impression cylinder; the image is transferred from the plate cylinder to the blanket cylinder and then onto the substrate (which passes between the blanket cylinder and the impression cylinder).This means that offset printing is also a form of rotary printing, because the substrate passes between two cylinders (the blanket and impression cylinders).
Unlike non-offset forms of analogue printing, offset printing requires the image to be created on the plate cylinder in its original form; non-offset analogue printing processes need to start with a negative of the image, so that when it is transferred directly onto the substrate it will end up the right way round, whereas the image being printed by an offset printing process will be reversed as it transfers onto the blanket cylinder before being reversed back to its original form when it is transferred onto the substrate.
Offset printing is based on the concept of "set off"; usually considered a printing defect, set off occurs when wet ink transfers from the substrate to another surface that comes into contact with the substrate (e.g. when ink on the top of one sheet transfers to the back of a sheet that is placed on top of the first sheet). In offset printing, set off is the desired effect and the blanket cylinder is covered in a rubber-based blanket that facilitates the transfer of ink (and therefore the image) from the plate cylinder to the substrate.
Early blanket cylinders were covered with pure natural rubber, before the development of synthetic rubbers in the 1930s allowed the use of rubber compounds that were far more efficient at transferring ink. Conventional blankets are made of a rubber compound that is carried on a fabric backing. The compound contains a number of additives that are added to enhance key properties of the blanket, such as its ink receptivity, dimensional stability, durability, strength, and its ability to release from the substrate after the image has been transferred. The backing is made of two to four plys of fabric and up to eighty individual coats of rubber compound can be added to create the blanket. The finished blanket is vulcanised to further improve its dimensional stability and strength; this is a curing process that uses sulphur as a catalyst and converts the blanket into a more durable material by creating crosslinks between the chains of polymers in the rubber. Some offset printers use compressible blankets; these are similar to conventional blankets, but include additional layers between the plys of fabric that form the backing. These layers are made of soft, resilient materials (such as foam rubber, cork, or loose fabric fibres), which increase the overall thickness of the blanket but compress during the printing process without deforming or damaging the rubber surface or the fabric backing. This means that the blanket is more resilient and long lasting, and improves the ability of the blanket to conform to the surface of the substrate so it can produce a better print result even on uneven surfaces.
Offset printing can be done using a wide variety of printing inks to suit all kinds of substrates, but care needs to be taken with certain key properties, such as the viscosity and drying methods of the inks used. The viscosity of an ink influences its properties of tack and flow, and offset printing requires inks to be capable of remaining in the correct position, while also being transferred between the plate cylinder and the blanket cylinder, and between the blanket cylinder and the substrate. It is important that the inks do not dry too quickly (which would leave them in place on the plate or blanket cylinders), but they also need to dry quickly enough so that smudging doesn’t occur, particularly when printing in colour or at high speed and/or high volume. Offset inks tend to be tailored to the substrate being printed and whether the press is web or sheet fed. While both web and sheet fed offset presses use viscous printing inks, sheet fed presses usually require inks that are quicker to dry, as sheets are more likely to come into contact with other surfaces more quickly (usually when sheets are stacked at the end of the press). Therefore sheet fed offset printing tends to be done with quick drying inks that don’t require external influences to speed up the drying process. Web fed presses can use coldset, heatset, or energy curable inks. Coldset inks dry through absorption without external influences, heatset inks dry through evaporation that is assisted by drying lamps or heaters, and energy curable inks dry through polymerisation which is triggered by an external influence such as UV light, infrared light, or electron beams. Coldset inks are usually restricted to uncoated materials, which are usually better at absorbing printing inks than coated materials.
There are a number of advantages to offset printing. The rubber blanket means that high quality prints can be created that have both sharp, crisp lines for detail and a range of soft and rich tones of colour. The flexibility of the rubber allows an image to be transferred onto a variety of materials and substrates, as the rubber will both conform to and compensate for any unevenness in the surface of a substrate.
Offset printing is capable of high speed, high volume commercial printing, with low production costs and long lasting press functionality (for example, the printing plates last longer because they only come into contact with the soft rubber-based blanket cylinder, rather than a variety of substrates, which may or may not cause abrasive damage to the plate). Sheet fed offset presses produce small volumes of maps, posters, magazines, brochures, letterheads, and marketing materials, while web fed offset presses can be used for high volume printing of newspapers, magazines, brochures, stationery, direct mail, catalogues, books, and a whole variety of packaging materials, including metals, foils, and plastics. Both sheet and web fed offset presses are also capable of perfecting, or printing on both sides of a material; perfecting offset presses can print both sides simultaneously, and use two blanket cylinders (rather than one blanket cylinder and one impression cylinder) with each cylinder acting as the impression cylinder for the other blanket cylinder.
Offset printing can refer to any method of printing that uses the intermediary blanket cylinder, so the term could be applied to a variety of printing processes including offset flexography and offset gravure. However, the most common form of offset printing is offset lithography, which itself is the most common form of printing used for modern commercial printing presses.Go Back to Glossary