The most common kind of offset printing is derived from the photo offset process, which involves using light-sensitive chemicals and photographic techniques to transfer images and type from original materials to printing plates. In current use, original materials may be an actual photographic print and typeset text. However, it is more common—with the prevalence of computers and digital images—that the source material exists only as data in a digital publishing system.
Offset printing process consists of several parts:
the inking system (ink fountain and ink rollers);
the dampening system (water fountain and water rollers) ;
the plate cylinder;
the offset cylinder (or blanket cylinder);
the impression cylinder.
In this process, ink is transferred from the ink fountain to the paper in several steps:
The inking and dampening systems deliver ink and water onto the offset plate covering the plate cylinder.
The plate cylinder transfers the ink onto the blanket covering the offset cylinder.
The paper is then pressed against the offset cylinder by the impression cylinder, transferring the ink onto the paper to form the printed image.
The goal of any inking system is to place a uniform layer of ink across every dimension of the printing plate. The lithographic process is unique in that it requires the ink from rollers to pass in contact with the nonimage areas of the plate without transferring ink to them.
Inking systems are made up of several elements:
the ink fountain;
the ink fountain roller (or ink feed roller);
the ink ductor roller;
the ink distribution rollers;
the ink form rollers.
The ink fountain stores a quantity of ink in a reservoir and feeds small quantities of ink to the distribution rollers from the ink fountain roller and the ink ductor roller. The ink ductor roller is a movable roller that moves back and forth between the ink fountain roller and an ink distribution roller. As the ductor contacts the ink fountain roller, both turn and the ductor is inked. The ductor then swings forward to contact an ink distribution roller and transfers ink to it. There are generally two types of ink distribution rollers: the ink rotating rollers (or ink transfer rollers), which rotate in one direction, and the ink oscillating rollers (or ink vibrating rollers), which rotate and move from side to side. The ink distribution rollers receive ink and work it into a semiliquid state that is uniformly delivered to the ink form rollers. A thin layer of ink is then transferred to the image portions of the lithographic plate by the ink form rollers.
The ink fountain holds a pool of ink and controls the amount of ink that enters the inking system. The most common type of fountain consists of a metal blade that is held in place near the fountain roller. The gap between the blade and the ink fountain roller can be controlled by adjusting screw keys to vary the amount of ink on the fountain roller. The printer adjusts the keys in or out as the ink fountain roller turns to obtain the desired quantity of ink. In simple presses, the printer must turn these screws by hand. In modern presses, the adjusting screws are moved byservomotors which are controlled by the printer at a press console. Thus the printer can make ink adjustments electronically. If the printer needs to increase or decrease ink in an area of the plate (print), he need only adjust the needed keys to allow more or less ink flow through the blade. The ink flow can also be controlled by the rotation velocity of the ink fountain roller.
A simple indication of the quality of a printing press is the number of distribution and form rollers. The greater the number of distribution rollers, the more accurate the control of ink uniformity. It is difficult to ink large solid areas on a plate with only one ink form roller. With three (generally the maximum), it is relatively easy to maintain consistent ink coverage of almost any image area on the plate. Business forms presses, which print very little coverage, usually only have one or two ink form rollers. Because of this, they cannot print large solid or screen images. Smaller, less sophisticated presses also have the same problem, however, many of the newer presses today are being equipped with larger, better inking systems to meet the growing print demands of the consumer.
Dampening systemMost lithographic plates function on the principle of water and ink receptive areas. In order for ink to adhere only to the image areas on the plate, a layer of moisture must be placed over the nonimage areas. The dampening system accomplishes this by moistening the plate consistently throughout the press run.
Dampening systems are made up of several elements:
the water fountain;
the water fountain roller (or water feed roller);
the water ductor roller in intermittent-flow dampening systems and the water slip roller in continuous-flow dampening systems;
the water distribution rollers;
the water form rollers.
Direct dampening systems employ a water fountain roller which picks up the water from the water fountain. The water is then passed to a water distribution roller. From here the water is transferred to the offset plate via one or two water form rollers.
Indirect dampening systems (or integrated dampening systems) feed the water directly into one of the ink form rollers (ink rollers that touch the offset plate) via a water form roller in contact with it. These systems are known as "indirect" since the water travels to the offset plate passing through the inking system and not directly to the offset plate as direct systems do. Some indirect systems will have the ability to feed the water into the inking system as well as to the offset plate. A fine emulsion of ink and water is then developed on the ink form roller. This is one reason printers need to know about "water pickup" or what percentage of water can be taken up by the ink. These systems are also known as "integrated" dampening systems as they are integrated into the inking system. One of the benefits of these systems, is that they do not use covers thus they react more quickly when dampening changes are made. One generally finds this type of dampening systems on newer and faster press equipment today.
Intermittent-flow dampening systems (direct or indirect) use a water ductor roller to pick up the water and transfer it to a water distribution roller. A drawback of these systems is the slow reaction time in making adjustments due to the back and forth action of the ductor.
Continuous-flow dampening systems (direct or indirect), are used by most newer presses today because they do not have the slow reaction time of intermittent-flow dampening systems. They do not employ the water ductor roller but use the water slip roller (a roller in contact with both the water fountain roller and a distribution roller, contrary to the water ductor roller that moves back and forth between the two) for a continuous flow. The speed of the water slip roller controls the supply. The use of alcohol on these type of dampeners was standard for years. Alcohol (isopropyl alcohol) was used as it increased the water viscosity and made it "more wettable" so that transfer was easier from one roller to the other. However, alcohol substitutes such as glycol ethers, butyl cellusolve, etc., are being used today to accomplish the same task because alcohol contains volatile organic compounds. Roller hardness is also being changed to help accomplish the same job—easy transfer of the water.
VariationsSeveral variations of the printing process exist:
blanket-to-blanket, a printing method in which there are two blanket cylinders per colour through which a sheet of paper is passed and printed on both sides. Blanket-to-blanket presses are considered a perfecting press because they print on both sides of the sheet at the same time. Since the blanket-to-blanket press has two blanket cylinders per colour, making it possible to print on both sides of a sheet, there is no impression cylinder. The opposite blanket cylinders act as an impression cylinder to each other when print production occurs. This method is most utilized on offset presses designed for envelope printing. There are also two plate cylinders per colour on the press;
blanket-to-steel, a printing method similar to a sheet offset press; except that the plate and cylinder pressures are quite precise. Actual squeeze between plate and blanket cylinder is optimal at 0.005″; as is the squeeze or pressure between the blanket cylinder and the substrate. Blanket-to-steel presses are considered one-color presses. In order to print the reverse side, the web is turned over between printing units by means of turning bars.The method can be used to print business forms, computer letters and direct mail advertising;
variable-size printing, a printing process that uses removable printing units, inserts, or cassettes for one-sided and blanket-to-blanket two-sided printing;
keyless offset, a printing process that is based on the concept of using fresh ink for each revolution by removing residual inks on the inking drum after each revolution. It is suitable for printing newspapers;
dry offset printing, a printing process which uses a metal backed photopolymer relief plate, similar to a letterpress plate, but, unlikeletterpress printing where the ink is transferred directly from the plate to the substrate, in dry offset printing the ink is transferred to a rubber blanket before being transferred to the substrate. This method is used for printing on injection moulded rigid plastic buckets, tubs, cups and flowerpots.
The paper is visually inspected to make sure there are no rips or damaged pieces of paper.
The chemical plate is checked so that there is a right amount of chemical solution applied to the plate so that when the offset cylinder touches the paper that there aren't any smudges.
During the printing process an operator has to check that there is no smudging between two or more sheets, where the ink is just coming off. If this does happen then quicker drying ink or a higher quality paper is required.
After the printing has taken place there has to be a check for quality on colour, image, shapes and type and other preference.
Quality control of the registration marks ensures that any colours produced beyond the edges of the bar are corrected immediately. A printing colour for each colour means that this could easily go wrong if a plate is not set up to a precision, meaning it will look out of focus and blurry.