Offset printing is a commonly used printing technique in which the inked image is transferred (or “offset”) from a plate to a rubber blanket, then to the printing surface.
When used in combination with the lithographic process, which is based on the repulsion of oil and water, the offset technique employs a flat (planographic) image carrier
on which the image to be printed obtains ink from ink rollers, while the non-printing area attracts a water-based film (called “fountain solution”),
keeping the non-printing areas ink-free. The modern “web” process feeds a large reel of paper through a large press machine in several parts, typically for several metres, which then prints continuously as the paper is fed through.
Offset printing today
Offset lithography is one of the most common ways of creating printed materials. A few of its common applications include: newspapers, magazines, brochures, stationery, and books. Compared to other printing methods, offset printing is best suited for economically producing large volumes of high quality prints in a manner that requires little maintenance. Our offset presses use computer to plate systems as opposed to the older computer to film work flows, which further increases our print quality.
Advantages of offset printing compared to other printing methods include:
- Consistent high image quality. Offset printing produces sharp and clean images and type more easily than, for example, letterpress printing; this is because the rubber blanket conforms to the texture of the printing surface.
- Quick and easy production of printing plates.
- Longer printing plate life than on direct litho presses because there is no direct contact between the plate and the printing surface. Properly developed plates used with optimized inks and fountain solution may achieve run lengths of more than a million impressions.
- Cost. Offset printing is the cheapest method for producing high quality prints in commercial printing quantities.
- A further advantage of offset printing is the possibility of adjusting the amount of ink on the fountain roller with screw keys. Most commonly, a metal blade controls the amount of ink transferred from the ink trough to the fountain roller. By adjusting the screws, the gap between the blade and the fountain roller is altered, leading to the amount of ink applied to the roller to be increased or decreased in certain areas. Consequently the density of the colour in the respective area of the image is modified. On older machines the screws are adjusted manually, but on modern machines the screw keys are operated electronically by the printer controlling the machine, enabling a much more precise result.
Due to the time and cost associated with producing plates and printing press setup, very small quantity printing jobs may now use Mpress’s digital offset machines.
The offset printing process
The actual process of printing is quite involved. One of the most important functions in the process is pre-press production. This stage makes sure that all files are correctly processed in preparation for printing. This includes converting to the proper CMYK color model, finalizing the files, and creating plates for each color of the job to be run on the press.
Every printing technology has its own identifying marks, as does offset printing. In text reproduction, the type edges are sharp and have clear outlines. The paper surrounding the ink dots is usually unprinted. The halftone dots are always hexagonal though there are different screening methods.
Sheet-fed refers to individual sheets of paper or rolls being fed into a press via a suction bar that lifts and drops each sheet onto place. A lithographic (“litho” for short) press uses principles of lithography to apply ink to a printing plate, as explained previously. Sheet-fed litho is commonly used for printing of short-run magazines, brochures, letter headings, and general commercial (jobbing) printing. In sheet-fed offset, “the printing is carried out on single sheets of paper as they are fed to the press one at a time.” Sheet-fed presses use mechanical registration to relate each sheet to one another to ensure that they are reproduced with the same imagery in the same position on every sheet running through the press.
Web-fed versus sheet-fed
Sheet-fed presses offer several advantages. Because individual sheets are fed through, a large number of sheet sizes and format sizes can be run through the same press. In addition, waste sheets can be used for make-ready (which is the testing process to ensure a quality print run). This allows for lower cost preparation so that good paper is not wasted while setting up the press, for plates and inks.
MPress Offset printing uses inks that, compared to other printing methods, are highly viscous. Typical inks have a dynamic viscosity of 40–100 Pa·s
There are many types of paste inks available for utilization in offset lithographic printing and each have their own advantages and disadvantages. These include heat-set, cold-set, and energy-curable (or EC), such as ultraviolet- (or UV-) curable, and electron beam- (or EB-) curable. Heat-set inks are the most common variety and are “set” by applying heat and then rapid cooling to catalyze the curing process. They are used in magazines, catalogs, and inserts. Cold-set inks are set simply by absorption into non-coated stocks and are generally used for newspapers and books but are also found in insert printing and are the most economical option. Energy-curable inks are the highest-quality offset litho inks and are set by application of light energy. They require specialized equipment such as inter-station curing lamps, and are usually the most expensive type of offset litho ink.
Ink and water balance is an extremely important part of MPress offset printing. If ink and water are not properly balanced, the press operator may end up with many different problems affecting the quality of the finished product, such as emulsification (the water overpowering and mixing with the ink). This leads to scumming, catchup, trapping problems, ink density issues and in extreme cases the ink not properly drying on the paper; resulting in the job being unfit for delivery to the client. With the proper balance, the job will have the correct ink density and should need little further adjustment except for minor ones. An example would be when the press heats up during normal operation, thus evaporating water at a faster rate. In this case, our pressman will gradually increase the water as the press heats up to compensate for the increased evaporation of water. MPress pressmen generally try to use as little water as possible to avoid these problems.
Offset lithography became the most popular form of commercial printing from the 1950s (“offset printing”). Substantial investment in the larger presses required for offset lithography was needed, and had an effect on the shape of the printing industry, leading to fewer, larger, printers. The change made a greatly increased use of colour printing possible, as this had previously been much more expensive. Subsequent improvements in plates, inks, and paper have further refined the technology of its superior production speed and plate durability. Today, lithography is the primary printing technology used in the U.S. and most often as offset lithography.