Offset Printing Process
Side view of the offset printing process.
Multiple ink rollers are used to distribute and homogenize the ink
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:
1. The inking and dampening systems deliver ink and water onto the offset plate covering
the plate cylinder.
2. The plate cylinder transfers the ink onto the blanket covering the offset cylinder.
3. 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 by servomotors 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
Most 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
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.
Variations Several 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
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, unlike letterpress
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
Quality control steps 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.
Plates Negative lithographic printing plate
Materials The plates used in offset printing are thin, flexible, and usually larger
than the paper size to be printed. Two main materials are used: metal plates,
usually aluminum, although sometimes they are made of multimetal, paper,
or plastic; polyester plates, these are much cheaper and can be used in
place of aluminum plates for smaller formats or medium quality jobs, as their
dimensional stability is lower. Computer-to-plate Main article:
Computer-to-plate Computer-to-plate (CTP) is a newer technology which
replaced computer-to-film (CTF) technology, and that allows the imaging
of metal or polyester plates without the use of film. By eliminating the stripping,
compositing, and traditional plate making processes, CTP altered the printing
industry, which led to reduced prepress times, lower costs of labor, and improved
print quality. Most CTP systems used thermal CTP or violet technologies.
Both technologies have the same characteristics in term of quality and plate
durability (longer runs). However often the violet CTP systems are cheaper
than thermal ones, and thermal CTP systems do not need to be operated
under yellow light. Thermal CTP involves the use of thermal lasers to expose
and/or remove areas of coating while the plate is being imaged. This depends
on whether the plate is negative, or positive working. These lasers are generally
at a wavelength of 830 nm, but vary in their energy usage depending on
whether they are used to expose or ablate material. Violet CTP lasers have a
much lower wavelength, 405 nm–410 nm. Violet CTP is “based on emulsion
tuned to visible light exposure”. Another process is computer-to-
conventional plate (CTCP) system in which conventional offset plates can be
exposed, making it an economical option.
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
Perfecting press A perfecting press, also known as a duplex press, is one
that can print on both sides of the paper at the same time. Web and sheet-
fed offset presses are similar in that many of them can also print on both sides
of the paper in one pass, making it easier and faster to print duplex. Offset
duplicators Small offset lithographic presses that are used for fast, good
quality reproduction of one-color and two-color copies in sizes up to 12
by 18″. Popular models were made by A. B. Dick Company, Multilith,
and the Chief and Davidson lines made by A.T.F.-Davidson. Offset duplicators
made for fast and quick printing jobs; printing up to 12,000 impressions per hour.
rs, reports, and sales literature.