Satas D., Tracton A.A. (ed.). Coatings Technology Handbook
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192
NEUMANN
paper bags to be produced in a single pass. From the early
1920s
till approximately
1940,
aniline printing machines were slow speed units of simple design and light weight
construction, and, for the most part, intended to operate in line with paper bag machines.
However, the first four-color roll-to-roll presses also were developed and introduced to
the packaging industry at this time; in addition, printing inks were substantially improved,
the first drying arrangements were installed on printing machines, and these were new
packaging materials such
as
cellophane and other nonabsorptive substrates, which had to
be printed by this process.
In
the course of the
1950s,
this special letterpress technique gained in importance for
a variety of packaging applications. The first pigment inks were developed, and machines
providing improved stability, higher precision, and faster operation were designed, built,
and even mechanized to some extent. It was also in the
1950s
when-again in the United
States-the name
of
“flexography” was created. which rather quickly became general
usage in the trade worldwide. Flexography is defined as “a letterpress printing method
utilizing rubber printing plates and liquid, quick-drying inks.”
Over the past
25
years, this printing process has become a sidespread technology;
it has been improved consistently, especially since
1972,
when the photopolymer printing
plate was developed and introduced. Over the past
5-8
years-and this is attributable
first and foremost
to
the relatively low printing plate costs and the good print quality
achieved, as well
as
to the versatile application possibilities-flexo has developed into
an industrial printing process and
is
now a serious competitor
of
the established printing
methods, offset and gravure.
1.2
The
Flexo
Process
Like the letterpress process, flexo utilizes raised printing plates
of
rubber or soft-elastic
polymer, the raised areas of the plates transferring the ink onto the substrate. The low
viscosity, quick-drying ink, which is diluted by means of solvents or water, is conveyed
by a fountain roller or, more recently, by an ink chamber-doctor assembly and
a
transfer
or anilox roller
to
the printing plates or stereos. as they are also called.
The most widespread mode
of
operation today is the roll-to-roll technology and
printing in line with surface finishing machines. Whereas in former times only bags,
sacks, and paper packaging materials were printed flexographically, the
use
of flexo today
includes a great number
of
plastic packages in the food and hygiene sector, applications
in the aluminum industry, and carrier bags, labels. wallpaper, corrugated board and cartons,
newspapers, handkerchiefs, magazines, envelopes, stationery, and other articles requiring
printing.
As
a
result of its high flexibility, flexo has succeeded in continually conquering new
market areas and has penetrated into many
a
domain previously reserved for offset or
gravure.
2.0
FLEX0
PRESS
SYSTEMS
The most usual flexo press layouts, which are known worldwide, can be subdivided into
four main groups:
end printers
stack presses
FLEXOGRAPHY
193
central impression machines
in-line systems
Color decks and other machine components may often be identical for these different
press versions. Nevertheless,
to
get the best possible final product, it is quite important
and decisive
to
select the correct machine for
a
specific application.
The different machine versions are used for both printing and surface finishing of
a
wide range of products including packaging materials of paper, plastic films, and alumi-
num foil (i.e., from thinnest flexible polyethylene
(PE)
films to heavy-duty cartons). Num-
ber of colors. working width, print repeat length, and machine speed may vary quite
substantially and may, accordingly, call for different machine layouts and conceptions.
2.1
End Printers
Today’s flexo end printers have their origin in the aniline printing units developed around
the turn
of
the century. Initially attached to paper bag machines, they served for one-or
two-color “mechanized stamping”
of
paper bags, and later also for the printing
of
sacks
and sheets. In the middle of our century, several thousand machine combinations were
installed throughout the world, consisting
of
flexo end printers and paper converting ma-
chines for the printing and making of paper bags and sacks in
a
single operation. (Fig.
l).
Following the development of polyethylene, this highly efficient printing technique
was extended to include
PE
items
as
well.
These machine combinations dispense with
a
separate operation and reduce roll
handling requirements; in addition, they save personnel, space, and capital expenditures,
in addition to reducing waste. All these factors together generate improved economics.
Today, end printers are available
in
working widths between
25
and
320
cm.
Depending on the capacity of the converting machines, speeds up
to
400
dmin are
achieved. Usually, three-or four-color stack printers with separate impression cylinders
Figure
1
Flcxo
end
printer
with
four
color
decks.
1
94
NEUMANN
are used, but one-and six-color units are in use as well. Such end printers are employed
exclusively for printing line drawings, solids, and type. They do not lend themselves to
actual quality printing, which is reserved to roll-to-roll flexo presses featuring the appropri-
ate optional equipment.
2.2
Stack
Presses
The first flexo press working from roll to roll was a stack-type press developed on the
basis of an end printer. Stack presses normally consist of four or six color decks with two
or three color decks incorporated on top of each other on either side of the print unit
frame. The configuration
of
such roll-to-roll presses
is
almost identical with all press
manufacturers. Over recent years
a
high level of standardization of press layout has been
reached (Fig.
2).
The web is fed from a simple or fully mechanized unwind unit for automatic reel
splicing via a constant feed-draw assembly to the print unit frame, where it is printed in
several colors, with
a
drier for drying the applied printing ink arranged after each deck.
Leaving the last color deck, the web passes through a drying tunnel or other final drying
arrangements for thorough drying
of
the ink and is then led via one or two chill rolls,
combined with adjustable nip rolls, to the rewind unit or
to
a fully automatic, dc-motor-
driven winding machine featuring roll splicing facility.
Stack presses are used for printing rigid materials. The between-color register accu-
racy in the machine direction is approximately
+_
0.2
mm
T.I.R.
(total indicator runout).
Four color presses are used in the market, but the majority of central impression
machines are six-color presses permitting four-or six-color halftone process printing using
screen counts of the plate of
48,
60,
or even
70
lines to the centimeter. This obviously
calls
for
a perfect printing plate, appropriate printing inks, and
a
high precision machine.
These requirements have been met, and as a result even flexible materials can today be
printed flexographically with
a
quality approaching that achieved by the gravure and offset
processes.
For the reasons outlined, the demand for central impression machines has drastically
increased over recent years, this design being presently also used
to
print rigid materials
Figure
2
Stack-type flexo press
with
six
color decks.
FLEXOGRAPHY
195
such as papers and compounds. Apart from this, the application of these presses has been
extended also to the wallpaper and corrugated board industry. The biggest machines have
a
2-3 m diameter impression cylinder and cover
a
working width of approximately 2.5
m. They achieve printing speeds of approximately 300 m/min, and 400 m/min is not
unusual with standard design presses.
2.3 In-Line Systems
These machines are similar in configuration to gravure presses. For each color a separate
frame is used, with the option
of
having any desired number of printing stations arranged
one after the other, the drive connection between the individual frames being provided
by universal joint shafts or the like. These machines call for
a
large floor space and involve
substantial capital expenditure. This, in turn, means that they can be economically used
only for long production runs at high speeds. Their main field
of
application is
in
the
printing of aluminum foils, papers, and cartons (Fig.
3).
The essential advantage of this machine configuration lies in the long interunit drying
paths, which permit wet-on-wet printing at high speeds. Modern high speed presses of
this type achieve rates of up to 600 m/min (10
ds)
under production conditions. Stack
presses allow the web to be printed with six colors on one side, with five colors on the
face and one on the reverse or in configurations
of
4: 2 or 3
:
3. Stack presses normally
achieve production speeds
of
400
m/min on paper. They are built in working widths
between 25 and 250 cm.
This press design, which was the best-selling machine version about 25 years ago,
has been continuously superseded by central impression machines over the past decade,
and today only 10-15% of all new web-fed presses are built on the stack principle.
2.4
Central Impression Machines
The first central impression flexo press (i.e.,
a
press with the color decks arranged like
satellites around
a
large-diameter common impression cylinder), was developed in
1953- 1954 and introduced to the trade. The suggestion for the design of this press system
came from the plastic film industry, where the need for
a
printing machine giving better
web control had become obvious. And this need has been satisfied indeed: the web comes
from the unwind, passing through
a
draw unit to the first color deck on the central impres-
sion cylinder, where it is pressed onto the cylinder surface and firmly held to it until it leaves
Figure 3
Flexo
in-line machinc with two
printlng
stations.
196
NEUMANN
Figure
4
Central impression flexo
press
with
six
color
decks.
the last printing deck. This working principle guarantees an immovably fixed position of
the web
so
that no register variations whatsoever are liable to occur during printing. With
the use of an appropriate drive system, maximum longitudinal register tolerances keep
within
0.1
mm T.I.R. (Fig.
4).
Hence it follows that this rotary press offers the best register
accuracy without any mechanical or electronic aids and additional equipment.
The key component of the central impression machine is the common impression
cylinder designed for maximum concentricity
(5
If:
T.I.R.)
and perfectly constant tempera-
ture over the full width of the cylinder (+_OS"C) during printing.
Central impression machines are built in working widths of
60-
160
cm. Apart from
standard systems, there are also available today press versions utilizing so-called slide-in
printing units or subunits, thereby reducing idle time on job changeover. Moreover, printing
stations are being used which are designed for changing from flexo to gravure with or
without the use of subunits. This concept adds to the application possibilities
of
the press
and ensures optimal utilization.
3.0
THE MOST IMPORTANT FLEX0 DECK SYSTEM
Having explained the four different flexo press versions today available in the marketplace,
we turn to the most important and most interesting color deck system.
As
a matter of fact, the color deck is the most important component of a flexo press.
Today, only two printing deck systems are still
of
general and topic interest-the three
roller system or fountain roller color deck, and the two-roller system or fountainless color
deck. The latter has gained increasing importance over recent years and enjoys steadily
growing popularity because it achieves higher print quality.
3.1 Three-Roller System (Fountain Roller Color Deck)
The three-roller printing deck was developed many decades ago and has been continually
improved, it is today the most commonly used deck version found in flexo presses. It is
relatively simple to operate and to set up, offers great flexibility and. until some years
ago, was the appropriate design to meet all user requirements. The conventional three-
FLEXOGRAPHY
197
roller printing deck is mounted on a pair of frames or consoles and normally consists of
a pair of angular bearing blocks
to
take the plate cylinder and
a
pair of inking roller bearing
blocks to accommodate the inking rollers (i.e., the anilox and the fountain roller). Also,
an arrangement
to
receive the ink fountain would usually be incorporated. For changing
the plate cylinder to print a different repeat length (or, for the purpose of impression
adjustment), the plate cylinder, together with its angular bearing brackets, is horizontally
loaded
to
or unloaded from the impression cylinder. This is done mechanically. hydrauli-
cally, or by motor. The same applies for the fountain and anilox rollers and for the ink
duct (Fig.
5).
This is accomplished through manual or motorized adjustment by threaded
spindles or, more recently, by computerized systems providing different automation levels.
At the moment the machine stops, (i.e., when printing is interrupted), the plate
cylinder must be lifted clear of the impression cylinder and of the web,
to
prevent the
plate and the substrate from sticking together. Thus the plate cylinder is slightly lifted or
retracted in the horizontal and vertical directions. At the same time the plate cylinder must
stop and come off the anilox roller
so
that no more ink is transferred to the plate. To
prevent the ink from drying on the rollers while the press is standing still, the anilox and
fountain rollers must continue rotating, powered by
a
separate motor.
To
prevent grinding
and damage to the printing plates, the circumferential speeds of the anilox roller must
always be equal
to
the surface speeds of the plate on the print cylinder.
With the three-roller printing deck, the amount of ink to be transferred to the substrate
can be varied by altering the gap between fountain and anilox roller. This is an advantage
impression cylinder
I
anilox roller
1
ink fountain
Figure
5
Three-roller
system
of
fountain
roller
color
deck.
198
NEUMANN
with the three-roller system. The inherent disadvantage is. however, that changes in speed
necessarily result in variations
of
the quality
of
ink transferred to the printing plates, which,
in
turn, results in color deviations
in
the printed image.
3.2
Two-Roller or Fountainless Printing
Deck
The latest technology in flexo deck system design is relatively new and a great deal of
engineering work and extensive experiments were necessary
to
accomplish an acceptable
solution.
In the two-roller system, a plate cylinder and an anilox roller with doctor blade
assembly has superseded the previously used fountain roller (Fig.
6).
Almost all doctor
blade units are designed for laterally reciprocating doctor blade motion. Loading the doctor
blade
to
the anilox roller is mostly done pneumatically. with fine adjustment of the contact
pressure.
Initially, the anilox roller was being doctored only conventionally or positively, but
over the past two years, negative or reverse-angle doctoring has been generally accepted
and preferred. Press operators throughout the trade assert that especially when performing
fine process work
on
high-speed wide-web presses, reverse-angle chambered doctor blades
achieve more consistent and better results. This is attributable
to
the capability of such
decks
to
provide even finer, more uniform. and exactly dehed ink application over the
full web width and throughout the machine's speed range. As an added benefit. anilox
roller and doctor blade wear is minimized, provided the doctor blade is carefully set
to
the anilox roller with minimum contact pressure. Also, ink consumption is drastically
reduced, no surplus ink being transferred from the anilox roller surface
to
the printing
plates. This two-roller
or
fountainless printing deck with the reverse-angle chambered
doctor blade system has been increasingly used throughout the industry and can be consid-
ered
to
be the printing deck system of the future. Most of the attachments and options
i"------
plate cylinder
l
anilox roller
/
l
common
impression cylinder
+
,
,,,l
/
!reverse angle
chambered
doctor
blade
system
/
I
Figure
6
Two-roller
or
fountainless printing
dcck.
FLEXOGRAPHY
199
found with flexo presses are identical to those used with offset and gravure presses and
do not feature any specific process-related characteristics.
4.0
PRINTING FORMS
OR
PLATES
Apart from the press design itself, printing plates are of paramount importance to quality
flexo. To better understand flexo technology as a whole, some knowledge of printing
plates and how they are made is indispensable.
Flexography owes its name to the exclusive use of flexible. elastic printing plates.
These were made from natural and lubber compounds for decades, until the appearance on
the market
in
1972
of
the first photopolymer plates for flexo, which have since succeeded
in
capturing quite a substantial market share. For many applications however, rubber plates
are, still used today.
4.1
Rubber and Photopolymer Plate Making
As with any other printing process, good artwork is essential to making good rubber plates.
For single-color printing one negative is needed, and for multicolor printing one negative
for each color with continuous tones being separated into dots. From these negative films
an original plate or a metal etching is prepared by photographic transmission using copper,
magnesium, or zinc. On the basis
of
these metal blocks,
so
called matrices are molded
from board soaked in phenolic resin; this is done in a special molding press. After cooling,
the molded matrix is used for making the rubber plate in the same press, using natural
rubber compounds
in
thicknesses from 2 to
10
mm (normally 2.76 mm) and
of
a Shore
hardness, depending on intended application, between
40"
and 70" Shore A. Minor correc-
tions of plate thickness are achieved by grinding the back of the plate. For the printing
of solids. even today, plates are cut from vulcanized rubber by hand.
It takes less time and work
to
make photopolymer plates than rubber plates, since
the former are prepared directly from the negative, dispensing with metal etching and
matrices.
There are two photopolymer systems, solid and liquid, the solid system being clearly
preferred
to
the liquid technology. The solid system calls for a distinction between single-
layer and multilayer plates. The latter utilize a soft base layer as a buffer layer, which
serves to improve pressure compensation. On the back or
in
the middle, a layer of polyester
film is provided to ensure dimensional stability. The light-sensitive polymer or relief layer
is on top, covered with protective film, which is removed prior
to
exposure.
The plate-making process starts with back exposure to UV light for the purpose of
sensitizing and polymerizing the plate. Next. UV main exposure takes place on the emul-
sion layer, through the negative, the latter being firmly held to the plate surface by vacuum.
The decisive factors for proper polymerization of the printing area or image elements
during exposure are the intensity of UV radiation and the time. Subsequent final exposure
ensures perfect polymerization
of
the relief plate.
During the subsequent wash-out process using a special washout solution, the unex-
posed, nonpolymerized photopolymer is dissolved and removed from the plate. The plate
is then dried by
hot
air and posttreated in an acid solution
to
get the final plate surface,
final exposure of the plate increasing its resistance to solvents. Photopolymer printing
plates are used
in
many different thicknesses and with variable relief depths to cover a
wide range of applications. Hardness ranges from
45
to 60" Shore A.
Other plate-making methods have only minor importance and are not discussed here.
200
NEUMANN
4.2
Printing Plate Mounting and Proofing
For production printing on the flexo press, the finished plates are pasted on the plate
cylinder surface. This is mostly done using double-sided adhesive tape and whatever
auxiliary equipment
or
machines will permit register-true plate mounting
on
the cylinders.
The thickness of the adhesive film depends on the plate thickness; with
a
great
number
of flexo presses, the combined thickness is
3.0
mm.
Other mounting methods also are employed, but these are confined to special areas
and therefore do not need to be discussed here.
Mounting the printing plates always takes place off the printing machine. With
a
view to avoiding unnecessary press downtime, the plate cylinders with the plates mounted
on them are proofed before production begins. This is done using different methods and
different apparatuses and machines. The proofing process is
to
demonstrate and ensure
that the printing plates have been correctly and perfectly mounted, in accurate register
and without blisters. Following this, the proofed plate cylinder can be installed in the flexo
press for trouble-free production printing.
It is also possible to test the design
of
a
new printing subject before plates are made.
As
far
as rubber plates are concerned, a print proof can be produced from the metal block
by letterpress printing, whereas with photopolymer plates a color proofing system can be
used to prepare
a
high quality color proof from the negative.
If the printing plates are
to
be reused, they must be cleaned thoroughly after comple-
tion of the production run and carefully stripped off the print cylinder; they should be
stored lying
as
flat as possible and, especially when photopolymer plates are involved,
they must be wrapped in lightproof material. It is,
of
course,
also
possible
to
store the
complete cylinders with the plates mounted.
5.0
PRINT SUBSTRATES AND PRINTING INKS
Because of the flexibility and versatility the flexo process offers, there is hardly any
material that has not been printed flexographically, regardless
of
material thickness, elastic-
ity
or
rigidity, surface properties, intended application, or many of numerous other mate-
rial-related characteristics. Moreover, flexo permits printing of metal strips only a few
centimeter wide
or
paper
rolls
as well
as
paper and film webs up to
3
m wide.
Also,
newspaper are printed by flexo today at production rates of approximately
700
mhnin.
Obviously, then, there is
a
need for appropriate printing inks, bearing in mind that
extremely varied demands are made on the ink to be used according
to
the specific material
being printed, its proposed application, and the press speed. Also important
in
this respect
are the very stringent statutory regulations that exist
in
the field
of
food packaging.
5.1
Print Substrates
The materials that can be flexo printed include paper, carton, cellophane, plastic films
(both mono- and multilayer films), aluminum foils, and
a
wide range of special materials.
Ranking first in the packaging industry, now
as
before, are wrapping papers, mostly con-
sisting of blends of different pulps.
Sulfate kraft paper is
a
single-sided glazed pulp paper used primarily in the produc-
tion of bags and sacks of
all
kinds. Sulfite and sulfate papers intended for similar applica-
tions are also flexographically printed. A drastic growth rate has recently been experienced
in
the field of kraft
or
test liner preprint for the corrugated board industry. Another area
FLEXOGRAPHY
201
of
flexo application includes tissue papers used as wrapping and toilet papers, hygienic
papers for handkerchiefs and towels. as well as many other specialty papers. Also worth
mentioning in this respect is the direct printing
of
corrugated board for use as outer package
and the printing
of
cartons (e.g., liquid packs).
The first film
to
be printed flexographically, which raised quite substantial problems
for all concerned, was cellulose film. Depending on its finish, cellulose film was mainly
used in the packaging
of
foodstuffs and textiles. Since the arrival
of
plastic films, cello-
phane has lost a great deal of its former importance.
Polyethylene and polypropylene films possess many desirable properties and now
rank first in the field of flexible packages. They are printed and processed
in
many different
thicknesses
(-
10-250
km), as either sheeting or tubing. Their main applications include
carrier bags, food bags of all kinds, hygienic and sanitary articles and many other products,
fertilizer and peat sacks, garbage sacks, cover films, shrink hoods, and the like. This wide
selection of films is completed by polyvinyl chloride and polyvinyl dichloride films, and
other special films used as mono films or in combination with other materials for packaging
or
to
impart to the container specific properties desired for a certain product to be packaged.
This is achieved by laminating different materials, by coating, and by coextrusion.
All these materials and material combinations are printed by the flexo process. This
also applies for aluminum foil. which may be plain or laminated, lacquered or coated in
thicknesses from
7
mm up. It is used primarily in the sweets and candy industry, as lid
foil, and
in
the packaging
of
butter, soups, coffee, and bread, as well as for many other
package applications.
5.2
Flexo
Inks
The substrates named
in
Section
5.1
call for a great variety of flexo inks offering many
different properties. Flexo inks are similar
in
composition to the inks used in gravure
package printing; they always consist of colorants (dyes or pigments), binding agents
(natural resins, artificial resins, or plastics), and
a
solvent or solvent blend. Whereas flexo
inks used to be based on basic (soluble) dyes, pigment inks are used primarily today
because of more exacting demands on the ink’s fastness. To obtain the desired properties
such as brilliance, adhesion. and qualification for laminating, the correct binding agents
and additives must be selected.
Apart from the aforesaid properties, flexo inks are required to generate a quality
end product. Fast and perfect drying of inks on the substrate during printing is another
aspect of paramount importance, and in this respect the solvent of solvent mixture used
is the decisive factor.
The drying system involves evaporation of solvents after the ink has been applied
to the web. This drying process is substantially accelerated within the printing press using
hot air, which is blown onto the web, and appropriate exhaust arrangements.
The most important solvents are hydrocarbons, alcohols, glycols, esters, and ketones.
Recently, water-soluble pigment inks, once used exclusively in the printing of multiwall
paper sacks, gift wrap, corrugated board, and wallpaper, have been playing an increasingly
important role and also have been adopted in the fields of newspapers and plastic films.
The obvious reason for the growing trend to use water-soluble inks in package printing
and for plastic films, is the new set of laws calling for reduction
of
solvent emissions into
the environment.