FLEXOGHAPHY PRINTING PROCESS BY RAJDEEP
Flexography
PRINTING
PROCESS
Rajdeep
| Printing | 2023
PREFACE
This
book could not have been written without the input and expertise of many
terrific people who provided personal input and company resources as background
for this project. Many also reviewed and commented on the chapters to give them
the up-to-date, real- world flavor that is so important to newcomers to
the process today.
In the platemaking, my thanks go
to K Shivaraman and Dhirender Kaushik, who is a professor of platemaking at
Pusa Institute of Technology.
In flexo process and anilox
roller, my thanks go to Vinod Ganga. He explained and showed me about its
process.
Some other books which have
helped me in writing this book are Designer’s Printing Companion by Heidi
Tolliver-Nigro, Flexography: principles & Practices and Flexography Primer.
HISTORY
Aniline printing, as flexography
was known until 1952, evolved out of rotary letterpress. Its name was taken
from the aniline dyes in the inks that were used at the time.
Early forms of the aniline press
were in use in Europe as far back as 1860, and historians trace the first
modern style of aniline press to 1890, when Bibby Baron and Sons of Liverpool,
England, built what resembled a central-impression cylinder press, with
printing units around the drum.
The first patented aniline press
was produced by C.A. Holweg of Alsace-Lorraine, who was granted British patent
on November 7, 1908.
In the late 1800s, Francis X.
Hooper designed and built a press for stamping ink identification marks onto
the wooden planks of shipping crates, using metal type known as “printing
dies.”
By 1900, combined corrugated
board was being considered as a shipping box material.
When aniline was first developed, its
inks made use of aniline – a poisonous, colorless liquid used in dyes, resins,
and explosives. In the 1950s, aniline inks were replaced with inks based on
non-toxic polyamide resins. In march 1951, a campaign to change the name
“aniline printing.” On October 21, 1952, at the 14th Packaging
Institute Forum, the announcement was made that the “flexographic process”
INTRODUCTION
Letterpress and flexography are
both relief printing, which means that the image areas for printing are raised;
pressure is used to imprint the image or text onto a substrate.
Relief printing works the same
way, using printing plates that have raised and non-raised areas. A printing
plate is the surface, whether metal or a type of polymer, that is etched or
chemically engraved to produce the image to be reproduced by a printing
process. The raised parts of the printing plates both receive ink and come into
contact with the surface being printed. The images are also reversed on the
printing plate, or wrong reading.
Letterpress was the first form
of printing, even predating Gutenberg’s moveable type. Printing was invented
centuries earlier in the orient. The Chinese and Koreans carved pages of text
out of wood and printed on rice paper. Ink was applied to the surface with
rollers and a piece of paper was pressed down onto it. Because of the pressure
involved in the process, you can actually feel a slight indent on words printed
using letterpress printing.
Flexography is a modern version
of letterpress printing. It is a direct rotary printing, similar to
letterpress, that uses resilient relief image plates of rubber or photopolymer material.
The process was developed primarily for printing on packaging substrates –
board, paper, foil and film.
Today, flexography, a very fast-growing
printing process, taking market share from letterpress, gravure, and even
offset, especially in the narrow-web label and folding carton markets. Although
flexography is used primarily in the packaging marketplace because of its
ability to handle non-paper substrates.
The flexographic printing
process uses quick-drying, semiliquid inks. In the age of digital printing,
flexography holds its own in the areas of large orders and long print runs,
particular of packaging products and labeling.
CHARACTERISTICS
AND ADVANTAGES
·
Printers on a wide variety
of absorbent and nonabsorbent substrates.
·
Printers on the reverse side
of stretchable, transparent films.
·
Printers using resilient
rubber or photopolymer image carriers – millions of impressions can be printed.
·
Allows printing of 10 or
more colors because of multiple print stations.
·
Allows continuous pattern
printing (giftwrap, wallpaper, floor coverings) because of its near-total
variable- repeat-length system.
·
Can achieve press speeds of
2,000 feet per minute or more (certain segments of the industry).
·
Printers process color jobs
175-lpi and higher (smooth-coated substrates).
·
Uses fast-drying solvent,
water-based or UV curable inks.
·
Eliminates back-trap contamination,
setoff and tapping problem by allowing wet ink to print over dry ink.
·
Can deliver a predetermined
amount of ink with minimum on-press adjustments with its inking system.
·
Can print using fluorescent
and metallic inks.
·
Allows printing-plate
cylinder to be taken out of press to enable printing to be mounted and proofing
as a prepress operation.
·
Can perform coating and
in-line operations such as laminating and die cutting as a continuous
operation.
·
Can produce the complete
package, such as folding cartons, displays, multiwall bags, labels, in-line.
·
Is cost effective for many
applications.
·
Offers high investment
return on equipment.
·
Enables fast turnaround time
between jobs.
·
Can make short-run work more
profitable.
TPYES
OF FLEXOGRAPHIC PRESSES
There
are four types of flexographic presses are:
·
Central impression
·
Stack
·
In-line
·
Sheetfed
Central-Impression
Presses
Central-impression
presses, sometime called drum or common impression presses, fundamentally
differ from inline press in that they support all of their color station around
a single steel impression cylinder. The web is secured at all times against the
drum, acting as the tension control device on the web. This helps to maintain
extremely tight color registration. CI presses typically have two to eight printing
stations.
The use of the
central-impression-cylinder design allows CI presses to print a wide range of
material then inline presses. For example, CI presses can print polyethylene –
forty percent of the flexible packaging market and other very stretchy films.
Inline presses can print materials that are thick and stable, but very thin
materials like polyethylene must be run through CI presses. On the downside, CI
presses cannot print on both sides of the web in one pass.
Stack
Presses
The
term stack refers to the location of the printing units on the press. In a
stack configuration, the printing units are placed vertically, one on top of
the other, rather than horizontally. Six is the most common stack
configuration, but some flexo presses have as many as eight stacks. There are
four advantages to stack presses:
·
The press has a smaller
footprint (amount of space the press takes up on the floor)
·
The web may be reversed to
allow perfecting
·
The stations are very
accessible
·
The press can print a
variety of substrates
Stack
presses do not have the same level of register control as inline presses, so
they are used for lower-quality work.
In-line
Presses
Inline
presses have separate print stations located in individual units driven by a
common line shaft. Each step in the process occurs in line, one after the
other. These presses are common for printing on pressure-sensitive and standard
labels. They can be made to perfect (two-sided printing) by turning the web
over a turn bar. The average flexo press is capable of printing four to six
colors, but flexo presses capable of printing ten or even twelve colors are
increasingly common. One of the greatest advantages of inline presses is their
ability to print on the reverse side of substrates. This is useful in
applications such as folding cartons, where recipes, promotions, or other
information are often printed on the back side of the box. To produce the same result
on a CI press, an additional downstream station would be required. Inline press
does not, however, have the ability to hold registration as tightly as CI
presses on very stretchy films. There is also slightly higher waste because
more material is running through the press.
Sheetfed
Presses
Although
less common, flexographic press do come in inline, sheetfed versions. These
presses are used most commonly in the printing of corrugated containers and
come with cutting, folding, and gluing units at the end of the press. Sheet
size are widely adjustable, typically 18-100 in. or more. Running speeds
generally limited by the cutting, folding, and gluing units at the end of the
press and not the printing speed of the press itself.
PRESS
TECHNOLOGY
The
flexographic printing press consists of four primary elements: the fountain
roll, the ink metering (or anilox) roll, the plate cylinder, and the impression
cylinder. The fountain cylinder rotates in an ink reservoir to pick up ink. The
ink is then transferred to the anilox roll (which might also be called the form
roll, meter roll, knurled roll, engraved roll, ink application roll, and
ink-transfer roll).
The anilox roll accepts the ink
into tiny engraved cells, which can vary from 80 to 1200 cells per linear inch.
These cells can be engraved mechanically or by using lasers, and can be
tri-helical, pyramid-shaped, quadrangular, or hexagonal. The finer the detail
desired, the more cells are engraved. Anilox rolls are often paired with a
doctor blade, which shaves the excess ink from the anilox roll at a revers
angle. This leaves the surface of the anilox roll clean, with the ink contained
mainly within the cells.
The depth of the anilox cells is
inversely proportional to the number of cells on the roll. The coarser the
anilox (the fewer cells per liner inch), the deeper the cells. The finer the
anilox roll (the more cells per liner inch), the shallower the cells and the
more precise the ink placement.
Once the anilox roll has been
filled with ink and the doctor blade has shaved off the excess, the ink is
transferred from the anilox roll to the plate. The plate is held to the plate
cylinder with special double-sided tape called sticky back. The raised surface
of the plate picks up the ink from the anilox roller and transfers it to the
substrate.
The impression cylinder holds the substrate at
just the right tension against the plate to from a clean impression. The
fountain roller, the anilox roller, the plate cylinder, and the impression
cylinder must operate at the same speed to ensure the correct image.
Because flexographic presses
print from roll to roll, the front and back of the press are designed with
units called unwinds and rewinds. The main purpose of these units is to
maintain the proper tension and direction for the substrate as it passes
through the press. If the web is too loose, it will have slack and wrinkles. If
the web is too tight, the image will be stretched out of proportion and the web
could break. The outfeed, or rewind, is located at the end of the press. It
runs at a rate slightly faster than the infeed, pulling tension across the web.
This gives the press operator the ability to fine-tune the tension of the
substrate as it is running through the press.
A drying system at the end of
the flexographic print station dries the ink between one color station and the
next. Dryers generally consist of gas-fired burners along with both supply and
exhaust fans. By raising the temperature, this forces the evaporation of
moisture from the inks. The fans improve air flow, aiding in the evaporation
process.
Because dryers are located
between the color station, they are known as color dryers or interstation
dryers. At each stage, the dryers remove enough moisture from the ink so that
the substrate can be run through the next print station without smudging the
previous layer of ink. Once all of the ink layers have been applied, the
substrate passes through one final dryer, called the main tunnel or overhead
dryer.
In some cases, the presses are
configured with cutters and stackers at the end of the press so the press can
deliver sheets instead of rolls. Other presses are configured with full
die-cutting operations so the press can deliver finished folding cartons, rolls
of labels, and similar products. This is why people often talk about
flexography as creating a finished product at the end of the press.
ANILOX
ROLLER
Introduction
of the Anilox Roll
In
1939, a mechanically engraved, chrome plated, ink-metering roll was introduced
in the aniline industry. Similar to rotogravure print cylinders, anilox rolls
were produced by mechanically engraving the surface of copper-coated rolls with
a controlled pattern of ink-carrying cells. Chromium was then electroplated
over the copper layer to prevent corrosion and increase were resistance. The
name anilox roll was derived from the aniline process.
Then, as now, the anilox roll is
the heart of the flexographic printing system. Its introduction was a milestone
in the development of an accurate inking system, and the older
rubber-roll-to-rubber-roll system began to disappear.
Anilox
Roll
Ceramic
plasma-coating, developed for the aerospace industry, has been adapted for use
on anilox rolls, replacing the chromium plating. Fine, ceramic powder heated to
nearly 9,0000F is sprayed onto anilox cells to make them tough and
long-wearing. The use of reverse-angle steel doctor blades, possible because of
the increase durability of ceramic coating, gives a more precise control of ink
metering.
Since in the 1980s, laser have
been used to etch ceramic-coated anilox rolls, and improvements in the
technology continue. Today, precisely engraved ceramic anilox rolls, with up to
1,200 cells per liner inch, are available to the flexographic printer, allowing
flexo to challenge most other forms of printing.
The
primary function of the anilox roll is to meter and control the flow of ink
from the reservoir to the printing plate.
The anilox roll is a cylinder
that has been engraved with a uniform pattern of cells around and across the
entire surface. Cells may be mechanically engraved with an engraved with an
engraving tool, chemically etched or engraving through the use of a laser beam.
When engrave the cylinder, each cell must be uniform and identical in both size
and depth to ensure that a controlled, uniform ink-film thickness is
transferred to the printing plate.
ANILOX
NOMENCLATURE
The
nomenclature of an anilox roll relates to the number of cells in one liner inch
along the engraving angle.
The engraving on an anilox roll
has been given many names, among then are cells, lines or screens. These names
refer to the actual cells that are engraved on the roller. Anilox cells that
are mechanically engraved are set at 450 angles to the roll axis and
anilox cells are counted along that angle. Laser engraved anilox rolls can be
produced at any given angle, but the industry has settled on the 600
engraving angle after detailed testing for ink receptivity and ink transfer
characteristics. The 600 angle worked best for flexographic
printing. Other angle of engraving is used for specific purpose or industries,
such as the 450 angle which has been the standard for flexo printing
of newspapers, or the 300 angle which seems to be widely accepted
for industrial coating applications.
PLATE/IMAGE
CARRIER
Flexographic
plates form an image from a raised image surface. They are made from a flexible
material, most commonly photopolymer, although some are still made from rubber.
Photopolymer plate materials are light-reactive.
Flexo plates are made up of six basic
components:
1.
The image area – the area or
face that comes in contact with the substrate and forms the printed image
2.
Caliper – the thickness of
the plate
3.
Floor – the area of the
plate that does not come in contact with the substrate and form the
“non-printed” area of the plate
4. Relief – the distance from the floor to the print surface, or to the
face
5.
Shoulder – the visible edge
of the photopolymer plate between the print surface and the floor
6. Plate baking – the dimensionally stable material that adheres to the
back of the flexible plate to provide stability
Negative
for flexo platemaking may be produced digitally or photomechanically, although
most are made digitally. In conventional platemaking, film is run off an
imagesetter, placed on the plate, and exposed to UV light to selectively cure
the resin on the plate. Once the image is polymerized, the plate is rinsed with
a solvent (or more environmentally friendly solution – in some cases, water),
washing away the non-polymerized areas of the plate. This leaves the hardened,
or polymerized portions as the raised image areas.
Within the last few years,
flexographic printers have begun to embrace computer-to-plate processes for
making flexographic plates. The elimination of the film step increases the quality
of flexographic platemaking by removing one more opportunity for variation in
the process.
Flexographic printing plates are
divided into two broad classes: rubber and photopolymer. The photopolymer
plate, in its sheet and liquid forms.
The first type of flex plate develops
and they are still in use for some application.
Rubber
Plate
The actual
process of making rubber plates is not different from the process use to
produce photonegative’s use in letterpress process. Now the sheet of zinc alloy
coated with light sensitive material, it is also an acid resist. The negative
film of the job is place over the coated sheet and light is pass through
negative, when the light strikes the emulsion to acid resist is hardened during
processing. The unhardened resist the non-image area. The metal sheet is then
etched to lower the non-image area, raised the complete engraving is then moved
to a matrix/mold of the engraving is made by pressing the matrix material
against the engraving is made by pressing the matrix links into the metal
engraving to form a mold, either molten rubber may be poured into the mold to
get a flex plate for printing or sheet of rubber press against the mold to form
a rubber flex plate for printing.
Sheet
Polymer
The
plates are cut to size and placed into exposing units having UV light source.
The back side of the plate is completely exposed to UV light to hardened the
base of the plate. Then the plate is turned over and the negative film of the
job is placed over the exposed side and exposed to UV light. The image area is
hardened during the exposed and the non-image area is washed away during
processing, during developed after processing the plate is post exposed.
Liquid
Polymer
These
plates are made in a special UV exposing unit. In this process clear plastic
protective cover film is mounted over a negative film which is placed emulsion
side up on the exposing unit. Now a layer of liquid polymer then deposited by a
motorized carriage over the cover film. The carriage deposited the liquid
polymer evenly over the cover. Film also controls the thickness of the deposits
as liquid polymer is deposited the carriage the liquid emulsion.
The
substrate sheets are specially coated on one side to bound with the liquid
polymer and it also serves as a back/base of the plate. After exposure, now
exposure is made first on the substrate side. This exposure is made to hardened
the base layer then the other side is exposed with a negative which leaves the
image area hardened, the soft non-image area is washed away during processing.
Finally post exposure is given.
Polymer
Coating
There are two fundamental different method for
structure coating polymer are as follows: -
1.
Subtractive structuring: - In
the first step a homogeneous coating with the polymer material is done on the
surface of the substrate. In the second step the areas of the material film
which is not necessary have to be remove so that the aspire structure remains
(Negative working coating)
2.
Additive structuring: - The
aspire structure is done by taking of polymer material directly by the
substrate.
Photopolymer plates
In
photo polymer plate, the polymer emulsion which is light sensitive consists of
molecules called monomers. When they are exposed to UV, light these monomers
chemically link and cross-linked with each other to form a strong polymer.
These polymers are of a complex chain of monomers, which are liked so strongly
that they behave as a single hard and resistive molecules (image area)
Procedure
These
photo polymer plates which are pre-sensitized plates are available in larger
size. These plates are then cut to required size and then the negative to be
exposed is cleaned with carbon tetra chloride (CTC) to make it dust free. Then
emulsion contact is made and expose in a printing down frame with UV light. The
exposure time depends upon the image and tonal gradation. Then the exposed
plate is developed with Industrial sprit or methyl alcohol or denatured sprit
in a brush developing which contain a sense of plastic brushes soaked in
alcohol, and the brush rotate circumferentially on the expose plate and remove
the unexposed polymer. The non-image area leaving the hardened image area in
relief. After developing, the plate is washed through in running water and
drying for sometimes. After drying the plate is post exposed or re-exposed 02
minutes in printing down frame without film/negative.
The re-exposure or post exposure
is done to harden the image area, finish the non-image area and also to
increase the life of the plate.
There
are two types of Photo polymer plate: -
1.
Rigid polymer plate
2.
Flexible polymer plate
Steps
in making rigid polymer plate
1.
Plate punching.
2.
Measure the folding margin
3.
Before exposing remove the protective
layer.
4. Keep the negative film emulsion to emulsion contact.
5.
Exposing time 2 to 5 min.
using UV light.
6. Developing by water development or spray development.
7.
Wash the plate in running
water.
8. Dry the plate for sometimes.
9. Re-expose or post expose the plate.
10.Plate is kept in grill for baking for 6 to 8 min. at 248 C.
Steps
to making flexible polymer plate
1.
Back exposure: - 1.5 to 2
min.
2.
Main exposure: - 12 to
13min.
3.
Solvent use: - Perchloethylene
butane 25 to 75%
4. Developing time: - 2 to 3 min.
5.
Image depth: - 1.1mm to
1.5mm
6. Standard thickness: - 1.14, 1.70, 1.00, 2.54, 2.72, 2.48mm
7.
Shore hardness: - 540A
to 560A
8. Spectral sensitivity 300 to 450 nm
Defects in Photo polymer plates
1.
Lack of detail in the image
area.
Cause: - Contact between the film and plate is not proper
Remedy: - Give appropriate vacuum for
good between plate and film
2.
Wash out line is too wide
Cause: - Image under developed
Remedy: - Given appropriate developing time
3.
Lines will not wash out deep
enough
Cause: - Image over expose or film is not black
Remedy: - Given less exposure or use fresh film
4. Polymer layer is cracking or lifting off
Cause: - The plate has been exposed too much
Remedy: - Given opportunity baking time
5.
Plates are curling
Cause: - The plate has dried too much
Remedy: - A quick dip in a warm then let post expose for the same length of
time as when we first made the plate.
Water washable plate
These
plates largely reduce or eliminates many of the concerns of solvent washable plates
like lower flash point, hazardous based and irritation in eyes and lungs. Water
washable plates comes in two version sheet and liquid polymer. The sheet water
washable plate is prepared with almost the same process that use for solvent.
The big difference is in the processing equipment water washable plate
processing unit. Usually, a companied with wash water treatment unit.
Solvent washable plate
Perchloethylene
alternative solvent are now being used by many plates’ maker, the drying time
using PAS, it can be used to develop almost any solvent washable photo polymer
plate are volatile organic compound which can be irritating to eye and lungs
therefore adequately vitalization must be provided by developing safety
glasses, goggles, works should be warned when handling these chemicals, it has
flash point of 1500 to 2000 f and are stable at normal
room temperature. Solvent can be recycled but its life time depends upon the
number and the size of the plate if it generally recycled by using vacuum
distillation process.
·
Photopolymer plate
eliminates the various disadvantage of rubber plates. The plates are exposed
from light sensitive polymer, which are hardened by UV light.
The international standard thickness of rigid
photopolymer plates: -
1.
0.31 mm polymer + 0.27 steel
= 0.58
2.
0.46 mm polymer + 0.27 steel
= 0.73
3.
0.67 mm polymer + 0.27 steel
= 0.94
Direct-Imaged plates
Direct-imaged plates refers to plates made
directly from digital data output from a computer and usually, but not always,
involves a leaser to write the image to be printed.
Laser-engraved Plates
laser-engraved rubber
plates are produced by engraving rubber with a laser unit similar to the used
when producing ceramic anilox rolls. The high-energy laser vaporizes (ablates)
the unwanted rubber in the relief area of the plate, leaving the raised image.
Laser-engraved rubber plates combine the excellent printing characteristics of
rubber and direct imaging from the computer-generated artwork, thereby
eliminating the need for negative films. The engraving process is, however,
time consuming, especially in the deep-relief printing plates used for
direct-corrugated post print applications.
SUBSTRATE
Flexographic
presses can print on a wide range of substrate, such as cellophane,
polyethylene, as well as paper, board, and metallized film. The design of
flexographic presses gives them the flexibility to print on just about
anything, and they do.
Substrates may be chosen for
their printing characteristics (bright whites, coated, matte, linen), most
flexo substrate are chosen for their functionality. Substrates are selected for
their ability to provide moisture or odor barriers, strength, flexibility, and
so on. There is a marriage between print performance and structural
functionality that does not exist in most offset applications.
List of some of the many substrates that can
be printed flexographically;
·
Paper and paperboard
·
Flexible packaging
·
Corrugated board
·
Polyester film
·
Polyvinyl chloride (vinyl
films)
·
Cellophane
·
Pressure-sensitive papers
·
Metallized films and paper
·
Latex structure papers
·
Paper cups, tubes and milk
cartons
·
Multi-wall bags
·
Polyethylene
·
Polypropylene
·
Pressure-sensitive coated
films
·
Glassing papers
·
Synthetic papers
Corrugated
boxes: - Flexographic printing’s ability to print on a
wide variety of substrates and surface make it an ideal method for printing on
corrugated boxes used in shipping and storage businesses.
Flexible
Packaging: - The ability to print on flexible,
non-rigid materials used for plastic and paper bags. Flexographic printing is
an ideal choice for industrial converters that make printed plastic and paper
bags.
Food
Packaging: - Flexographic printing is quick drying
and able to use non-toxic inks. These factors make flexographic printing
desirable in the printing of food packaging. It can basically be used on
packaging like milk cartons, beverage containers, food containers, as well as
disposable cups and containers.
Medical
Packaging: - Since flexography is food safe with the
right ink, it can also be used to print packaging for medicines and other
medical supplies. Flexographic printing can be used on foil wrappers, hygienic
bags, plastic, and cardboard packaging.
Newspaper
and Other Print Media: - Even with the prevalence of
e-books and the internet, there is still a market for newspapers, magazines,
and books. The production of flyers, posters, and newspaper inserts also
greatly benefit from flexographic printing’s ability to quickly print large
print jobs.
SUBSTRATE
CLEANING
Dust
is by far the biggest enemy of quality printing, especially in polymer film,
paper or corrugated processing operations, and therefore must be addressed by
press builders and converters alike.
The most advanced ionic cleaning
systems involve a corona field formed between two oppositely charged electrodes
in a quartz enclosure. The substrate to be cleaned is passed through this
corona field, which is generated using an alternate high voltage (15,000 volts)
at high frequency (5,000-7,000 cycles/sec).
At peak voltage a corona field
is built up between the two electrodes (+/-) and everything within the corona
field (air, substrate, dust) because conductive. When this happens, electrons begin
to flow between the two electrodes, the surface is passed between the
electrodes, the surface of the substrate is charged positively on the side
facing the negative electrode and is charged negatively on the side facing the
positive electrode. Any dust particles on the surface of the substrate will
also be charged in similar fashion.
As the polarity of the electrode
change, the corona field breaks down. The air between the surface of the
substrate and the adjustment electrode functions as insulation. Powerful
electrostatic fields are formed, which are charged positive above the sheet to
be cleaned and negative on the underside. The dust particles now show an
opposite polar charge, are repelled by the substrate, float into the positive
field and are sucked away by a fan.
After reversing the polarity of
the electrodes, the corona field forms again as soon as maximum current is
reached. With the repeated polarity change and subsequent breakdown of the
corona field, the cleaning process takes place again. Each change of polarity,
which occurs upwards of 10,000 times per second, results in a cleaning action.
Plate Mounting
More
aggressive adhesive was necessary to keep the polyester plate backing from
pulling free. In 1975, sticky back was developed to attach photopolymers to
plate cylinder. Several companies came out with cushion-foam sticky back at
that time. These add more cushioning under the plates and help improve on-press
impression.
INK
Flexographic
inks have undergone vast changes in the last hundred years. Initially,
flexography was called aniline printing because toxic aniline dyes were used to
create the ink. Aniline inks, made from coal tar, were banned by the FDA (Food
& Drug Administration) for use in food packaging.
Flexo inks are in fluid form. In
order for the ink to be transferred from the anilox roll to the substrate, the
ink must move freely in and out of the cells. For this to happen, the viscosity
must be thin enough for this transfer to occur. The viscosity of the ink will vary
by speed of the press. The faster the press, the thinner the ink. Thus, the ink
must be formulated for the anilox roll, the application, the substrate, and the
press speed.
Flexo inks are formulated from
three primary components: pigments or soluble dyes, a vehicle for carrying the
ink to the substrate, and additives that give the inks performance
characteristics like scuff resistance, fade resistance, dry speed, and color
intensity. Flexo inks are stable and fast drying, allowing web speeds to range
from 25 to 1,500 feet per minute. In tissue applications, web speeds can reach
2,000 fpm.
Flexographic inks have a reputation of
begin very bright and vivid. As such, flexo is often chosen to print packaging
that requires large areas of solid color. Flexo’s fluid inks also have a
reputation – if not formulated properly – of drying too quickly, which causes
the plate to “print dirty.”
Flexo ink makers formulate inks
with a wide variety of strengths. The stronger the ink, the more colorant it
has, and the thinner the converter can make the ink film and still retain the
required density. In other words, the stronger the inks, the more mileage they
can get. Furthermore, the stronger the ink, the finer the anilox roll can be to
still achieve the same delivery. The finer the anilox roll, the finer the finer
the graphics can be.
There are downsides to thinner
inks, however. The pigment is the most expensive part of the formula, so there
is a point of diminishing returns. If the density is too high, it begins
affecting the ink’s fluidity. These inks also become prohibitively expensive. A
balance must be achieved between the cost, performance, and requirements of the
individual application. High-end applications tend to use more expensive inks.
Types
of Flexographic Ink
There
are fewer categories of flexographic inks than offset inks. Following are the
main categories designers should know:
Solvent-Based
Inks
Solvent-based
inks used liquid – usually petroleum or other organic material – as the
vehicle. Solvent-based inks offer excellent printability characteristics for
non-paper substrates, but they contain volatile organic compounds (VOCs) that
have undesirable environmental characteristics. Because the flexographic inks
are often required to adhere to non-paper substrates, the majority of
converters printing on film and foil use solvent-based inks, which have far
better adherence properties then water-based inks. Many also feel that the very
specific performance characteristics required of most packaging inks (for
example, requiring excellent printability, scuff-resistance, heat-resistance,
chemical-resistance, and so on) are better suited to the use of solvent.
Water-Based
Inks
Water-based
inks use water as the vehicle. They have few VOCs and therefore exhibit
preferable environmental characteristics over solvent-based inks. Water-based
inks rely heavily on absorption into the substrate to achieve drying. Since
most flexo applications are printed on non-paper substrates, the use of
water-based inks is less common than solvent-based inks. In paper-based
applications such as folding cartons, the use of water-based inks is common.
Water-based flexo inks are hailed for their superior fluorescents and
metallics.
Energy-Cured
Inks
Energy-cured
inks contain chemicals that react to ultraviolet (UV) or electronic-beam (EB)
exposure. These inks are very rigid in structure, giving them excellent
hardness, gloss, and resistance characteristics. Drying under UV or EB dryers
is almost instantaneous, so jobs printed using energy-cured inks and coatings
can be converted right off the end of the press. Because they are cured with a
controlled light source rather than by air, UV inks also provide more control
over drying (reducing or eliminating problems with inks drying on the anilox
rolls and plates) and allow greater ink-film thicknesses.
Although UV and EB inks used
today are much less toxic than in the past, designers and converters should
check with the ink manufacturers before using them for food packaging. Not all
energy-cured inks are approved for food contact.
For flexo, another benefit of UV
is, because of the very fast cure, these inks allow higher-resolution printing
than solvent and water-based inks. When water and solvent-based inks sit on a
dot, they tends to flow outward and to dry on the plate. As they do, the edges
of the dot begin to dry and build up, causing the top of the dot to spread. UV
doesn’t dry or spread on the plate, eliminating the problem of build-up, and
thus printing crisper and cleaner.
Process-Color
Inks
These
inks are the four basic colors – cyan, magenta, yellow, and black – used to
create the entire color gamut of process printing. In theory, the same
combinations of these colors should result in the same end colors on matter
which inks are used. However, the actual color produced is dependent on the
individual pigments used by the ink manufacturer.
This is an important issue for designers and
printers, since certain advertiser exact color matches. Although exact matches
can often be achieved, it adds time to the process. Pure orange, green, and
purples are notoriously difficult – if not impossible, as in the case of
metallics – to match. For this reason, there is a heavy emphasis on spot colors
rather than process colors. Process color is generally used when color realism
is required, such as in food packaging.
Because of the types of images
for which process color is used, printing with process color inks generally
requires higher line screens and finer anilox rolls. Minimum of 100 lpi to 175
lpi is recommended; 133 lpi is the most common. This translates into anilox
rolls with cell counts of 600-1200 lines per inch.
Spot-Color
Inks
Spot
color inks are formulated in the ink room, not from combinations of CMYK on
press. This allows designers to achieve bright, vivid colors and exact color
match more easily. Spot colors required their own print stations, since they
are used to augment rather than replace CMYK. The use of process color is the
exception in packaging. Most packaging jobs use spot colors in all print
stations of the press.
Specialty
Inks
Inks
in this category cover every spectrum of specialty application, including:
·
Security inks with special
properties to prevent or discourage counterfeiting
·
Thermochromic inks that
change color when exposed to heat
·
Photochromic inks that
change color when exposed to light
·
Scratch-off inks like those
used in lottery tickets
COATING
Flexographic
printers use a wide variety of coatings, such as aqueous coating, UV coating,
and EB coating to produce the desired characteristics on the printed piece.
This includes stain or gloss finish, scratch-resistance, and quick drying.
Coatings are applied using one of the printing units on the press.
When choosing ink, the converter
must be very careful to match the ink to the substrate. Ink characteristics
such as adhesion, block-resistance, heat-resistance, rub-resistance, and
light-fastness can vary from substrate to substrate, even when using the same
ink. Although the cost of ink may only be less than five percent of the total
cost of the job, its impact on the printability of the job is far greater.
DRYRES
Solvent-based
flexo inks dry by evaporation. Ink is applied in a thin layer on a substrate
and then typically is hit with heated forced air. Both additional heat and air
dry the ink and dramatically reduce the amount of solvent that is retained in
the layer.
The drying of water-based inks
is very much the same as the drying of solvent-based inks, only more difficult.
There are two reasons why is more difficult to dry. First, water vapor is
typically already part of the atmosphere. If a previous model is used, but
water-based inks substituted, an equilibrium state is reached very quickly
because of the presence of atmospheric moisture, making drying difficult.
Again, as with the solvent-based inks, additional heat and volume only postpone
the inevitable. The second problem with water-based inks is the amount of
anergy required to evaporate the water portion. The amount of energy required
to convert any compound from its liquid state to its gaseous state is called
the latent heat vaporization. Water requires several times more energy to
change its state from a liquid to a gas then typical flexo organic solvents do.
In comparison to ethyl alcohol, water requires three time the energy to
vaporize.
FLEXO
OFFSET
In
this process, a flexographic printing plate is used in place of the gravure
cylinder. The flexo plate, with a positive image, prints to the offset blanket,
which reverses it and prints a positive image to the substrate.
Round, plastic containers are
printed this way. Some special presses have three- or four-color stations
around the offset blanket cylinder. All the colors are registered on the
surface of the blanket, which transfers the multicolored image directly to the
rotating container during each revolution.
The containers are held by vacuum
on a printing spindle. After one is printed, the next, on its own spindle,
comes into position and is printed. Aluminum cans with a clear-base coating and
tapered drinking cups can also be printed this way. There are still many
untapped applications for flexo offset.
HYT 4 COLR FLEXO GRAPHIC PRINTING
MACHINE
Printing width : - 600mm – 1600mm
Printing
material : - OPP:20 -70μm, BOPP:18-60μm,
PE:35-100μm, NY:20-50μm,
PET:12-60μm, CPP:20-60μm,
Paper:20- 150g/m2
Printing
color : - at
your own choice +2 special color
Printing length : - 7.5”-40”(190mm~1000mm)/7.5"-
50"(190mm-1270mm)
Printing plate lifting system :-
Pneumatic press control
Mechanical speed :- 100m/min
Printing speed :- 10-90m/min
Register precision :- Longitude: ±0.15mm
Transverse: ±0.15mm
Rewinder/Unwinder Dia. :- Φ1000mm
Material feeding width :- 1560mm
Master motor :- 3.7Kw
Rewinder motor :- 1.5Kw
Electrical heater :- 30Kw
Blower of heater :- 5Kw
Blower of nature wind
:- 1.5Kw
Inking motor :- 200W
Master frequency converter :- 3.7Kw
Powder brakes :- 10Kg
Tension sensor :- 3~50Kg
Master power :- 50Kw
Net Weight :- 10300Kg
Dimension (L×W×H） :- 5.2×2.5×3.85m
ABOUT THE AUTHOR
I
would like to present my book. I am Rajdeep. I am also come from a printing
technology background but only this thing is not enough for me to write this
book, actually my inspiration was Vinod Kumar Ganga, Vivek Chauhan and Dineshwar
Sharma, he has a really inspiring personality.
Also, I decided to write this
book because of my experiences which I got in my college life or we can say
that my student life. College life teachers are so many things that make us a
genuine person.
Actually, authors are talking about their
achievements but right now I don’t have any because this is my first book.
A
book that may help today’s overworked and stressed students to deal with
themselves in their environment of study.
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