Printing Technology
Useful on the topic
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100 years of offset printing
General pressure:
Printing refers to all reproduction processes for duplicating
print templates. Correctly or reversed print templates are used depending
on the printing process. These are first coated with a dye and then
pressed onto a substrate. The dye is transferred from the artwork to the
substrate. As a rule, the print template can be used several times.
There are the following imperative factors that are required for printing:
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Pressure hull (the printing machine)
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Printing ink
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Printing form
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Substrate
Printing principles
A distinction is made between three printing principles:
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Surface against surface
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Cylinder against surface (round against flat)
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Cylinder against cylinder (round against round)
Printing process
According to the relationship
between the printing elements and the printing form, such as in Plano graphic
printing, letterpress and gravure printing, as well as through printing. According
to this feature, the printing processes are also differentiated in DIN 16500
into the main printing processes: letterpress (image areas of the printing form
are higher than non-image areas, e.g. letterpress, flexographic printing), flat
printing (see also lithography (image areas and non-image areas of the printing
form are (approximately) on one level) ( Offset printing)), gravure (image
areas of the printing form are deeper than non-image areas) and through
printing (image areas of the printing form consist of a stencil on a
color-permeable stencil carrier (screen made of plastic or metal threads),
non-image areas are color-impermeable (screen printing)).
Printing process
The four
main printing methods are:
High pressure:
A printing
process that has been widespread since Johannes Gutenberg, known as classic
letterpress, in which the printing elements are raised on the printing form
(printing with movable letters). High-pressure printing plates can be
designed as crucibles (flat printing plates) and cylinders and can print on
both sheets of paper and paper from rolls.
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One
type of letterpress is flexographic printing; A photopolymer plate is used
as the printing form. The main areas of application for flexographic
printing are packaging films. Flexographic printing is in close
competition with copper gravure printing. Gravure printing achieves better
results, but is more expensive
for small and medium-sized print runs due to its high up-front printing costs.
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A
further development to increase the efficiency in flexographic printing
consists in the use of prefabricated continuous printing forms. These are
rubber mixtures specially developed and adapted to the intended use (solvent
inks, water colors or UV inks). After vulcanization on the carrier sleeve,
the print motif is engraved into the surface using a laser.
Gravure printing:
Printing
process in which the printing elements are designed as a recess in the printing
form. These depressions (cells) are filled with ink during the printing
process, which is then transferred back to the printing material. The
cells can be achieved, for example, by etching or engraving. Rotogravure
printing (gravure printing with cylindrically designed printing forms) is now
economical, especially for mass printed matter and magazines in very large
editions, since the high costs of the printing form are offset by low costs in
production printing.
Flat printing:
(Stone
printing and offset printing) - Offset printing is gaining more and more market
share; modern offset machines are fast and today they are superior to
rotogravure printing in terms of quality. In the future, it can be assumed
that the share of digital printing will grow at the expense of offset printing.
Print through
(stencil printing):
(Stencil printing) - A universal print through
process; the
template is usually attached photo-chemically to the fine mesh made of
synthetic fiber or even steel wire. Screen printing can be used in many
ways to print flat objects of all formats.
Printing process:
Pad printing:
Pad printing is a kind of combination of stamp printing and
transfer printing. The template is transferred from one surface to the
other with the help of a tampon (made of silicone rubber) and can be applied to
the recess of an ashtray or cup, for example. So it is not surprising that
pad printing is used particularly in the production of promotional gifts and
packaging, as well as in the fine printing of model railways.
Digital printing
Digital
printing is a printing process in which the print image is transferred directly
from a computer to a printing machine. The printing system is usually an electro
photographic printing system, such as a laser printer, which is designed for
high numbers of copies. Other NIP processes are also used. In
addition, the printing system can have further devices for cutting and
binding. For large-format digital printing, electrographic processes or
inkjet printers are used.
Digital
printing is cheaper than offset printing for smaller print runs. In
contrast to offset printing, for example, no fixed printing template is
required, so that each sheet can be printed
differently. A document consisting of several pages can be printed in the
correct order without collating (sorting). Personalized data, such as
invoices, credit card statements, account statements or advertising
specifically tailored to the recipient, can be printed easily and
inexpensively. Even with large print runs, there is no longer any cost
advantage for offset printing.
The
transition between a high-performance copier and a digital printing system is
fluid. Digital printing systems often still have options for saving the
artwork. They print with higher image quality and are more reliable than
copiers. In digital color printing, the accurate color reproduction of
templates or certain standardized color tones is also supported much more
extensively. In order to achieve a high level of accuracy, there are
usually complex settings and calibrations, so-called color management.
Digital
printing also offers more ways to control how the pages of a document are arranged
on the sheet. Several smaller pages can be arranged on one large
sheet. Folding and binding the prints to the finished product are prepared
in this way. Trimming is made easier by additionally printed cutting marks
and suitable margins.
Offset printing
Offset
printing is a flat printing process that is widely used in commercial printing,
packaging printing and newspaper printing. It emerged from the stone
printing and is based on the different wetting behavior of different substances.
The printing
areas on the printing plate are fat-friendly (lipophilic) and water-repellent
(hydrophobic), so they accept the printing ink. The non-printing areas, on
the other hand, repel the ink (lipophobic), but accept the water
(hydrophilic). The non-printing areas are first wetted by the dampening
system with a thin dampening solution film. Then the printing ink is
applied to the printing areas by the inking rollers of the inking
unit. The print image is first delivered to a rubber cylinder, from which
it is transferred to the printing material. It is therefore also known as
an indirect printing process.
sheet fed
offset and web offset are differentiated according to the format of the paper
fed in. sheet fed offset is suitable for small and medium runs and is
therefore mainly used in commercial and letterpress printing. With web
offset, the paper comes straight from a large roll. It is mainly used for
large and very large print runs, such as newspaper, catalog and telephone book
printing.
In the past,
offset printing plates were only copied from exposed films (as an intermediate
carrier), but direct imaging (direct imaging) of the printing plate or CTP
(computer to plate) has now established itself almost completely. With a
laser (thermal or optical) the printing plate is exposed and developed in tiny
dots (resolution up to 1,000 dots per centimeter) - today it is also
chemical-free.
Screen printing:
The
artisanal-industrial screen printing, the (textile) film printing and the
(artistic) serigraphy belong to the 4th group of printing processes, the
so-called through-printing process. The printing form of the screen
printing consists of a frame and a fabric, which is stretched over the
frame. Using the fabric geometry (fabric thickness) and the theoretical
color transmission volume of the stretched fabric and the coating thickness of
the fabric, a defined layer thickness is achieved and guaranteed with
repeatability. Parts of the fabric are covered (the barrier layer consists
of an exposed, photosensitive layer). The printing parts are open, so they
form the passage. This is then referred to as a stencil, printing screen
or printing form.
The printing
form is fixed in a device above the printing material. The printing medium
is applied to the screen fabric on a non-motif area. During the printing
process, the printing medium (e.g. a color) is coated into the printing form
fabric ("pre-doctoring"). This is done with the flood squeegee.
The transfer
of the printing medium to the substrate to be printed then takes place with the
printing squeegee, which is guided over the screen in a line with a defined
contact pressure. The jump is overcome by pressing downwards (distance
between the substrate to be printed and the screen mesh). The pressure
squeegee generates a shear force under which the pressure medium at the
pressure squeegee edge maintains a lower viscosity. The rheological
behavior of the printing medium - it becomes liquid due to a shear force and
solidifies again when the shear force subsides - plays an important role in screen
printing. If the viscosity builds up again immediately after the shear
force is removed, this is referred to as structural viscosity. If, on the
other hand, the increase in viscosity is delayed (hysteresis), then one speaks
of thixotropic.
The print
medium now flows through the screen mesh and is absorbed by the print
material. Immediately behind the pressure edge of the printing squeegee,
the printing screen is released again from the substrate by the screen jumping
off. The viscosity of the transferred printing medium increases again
after the transfer due to the absence of the shear force. The
higher-viscosity printing medium therefore does not run on the printed
substrate and does not drip from the screen when it is pre-doctored. The
printed image is clean and clear.
CMYK color model
Mostly the
CMYK color model (cyan, magenta, yellow (yellow), key (black = black)) is used,
whereby a printing plate is required for each color. A large part of the
colors of the color space can be printed with these four colors. For
colors that cannot be printed with CMYK colors (gold, silver, reflective
colors) or should not be printed (such as those of a company logo), there are
also various standardized color palettes, such as HKS color fans or Pantone
(solid colors). Different sizes of grid (extremely small dots that are not
individually visible to the eye) result in a huge number of color nuances that
are visible to the eye from four printing inks.
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