FTA (изд-во). Flexography: Principles And Practices. Vol.1-6

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PRESSES AND PRESS EQUIPMENT 93

Lamination Nip. If the press has two chill rolls,

the first may be made into the laminating nip

by adding a rubber impression roll. If the press

has only one chill roll, another roll is needed

to form the laminating nip. This roll should be

about 14" in diameter and bored for heating. It

should be driven at the same speed as the chill

roll, preferably with a variable speed drive.

Heating of the roll can be by hot oil or hot

water, as most film laminations are performed

at temperatures less than 175° F. Hot water is

desirable in that it is cleaner than oil. Also,

switching from hot water to cold water pro-

vides another chill roll when not laminating.

A rubber-covered nip roll is required to sup-

ply the laminating pressure. This roll should

be a minimum of 6" in diameter and covered

with 80–85 durometer rubber, 1" thick. The

roll should be hydraulically or air operated to

apply variable laminating pressures up to a

maximum of 100 lb/in

, and should be located

so that the web has at least a 90° wrap on the

steel roll after laminating.

Adhesive Application. Immediately following

the adhesive application station, which is

the last press color station, a 2" diameter

chrome plated steel roll is needed. The pur-

pose of this roll is to smooth the adhesive to

improve laminating clarity. The roll should

be adjustable to the web and driven in

reverse rotation to web direction. A small air

motor is adequate for this function.

If the adhesive is to be applied by a flexo

print unit, the rubber transfer roll on the

plate cylinder must be solvent-resistant and

have good release properties.

To apply the adhesive by gravure in the last

color station, a gravure roll, rubber impres-

sion roll (plate-cylinder position), doctor

blade and change gears for the color station

are needed. The web must pass up between

the gravure roll (anilox roll position) and the

impression roll (plate-cylinder position).

Consequently, change gears are required to

change the direction of rotation of these two

rolls. The doctor blade attachment should

have adjustments for angle and pressure of

wiping, and an air or hydraulic motor for

oscillation. The attachment is mounted in the

area normally occupied by the flexo rubber

fountain roll. The ink pan must be modified

in order to supply adhesive to the gravure

roll. To cut down on the expense of rubber

impression rolls, rubber sleeves can be

placed on existing plate cylinders.

Separate Laminator Section

The next generation of in-line laminating

incorporated a separate and self-contained

laminating machine in-line with the flexo

press (Figure

). It is independent of the

press except that it, and the press, use the

Laminator

Lamination

Unwind

Laminated

Rewind

Printed

Unwind

Rewind

Central Impression

Press

Adhesive Applicator

This In-line laminating

setup incorporates a

separate and self-

contained laminating

machine in-line with

the flexo press.

A flexo press with a

separate laminator

allows the converter

to use both machines

in tandem or independ-

ently, thus offering a

wide variety of

applications.

94 FLEXOGRAPHY: PRINCIPLES & PRACTICES

same rewind, and the electric drives of the

two machines are connected and employ

necessary web tension controls. The lamina-

tor consists of a gravure-type adhesive appli-

cation station, with a smoothing roll, drying

tunnel of sufficient length to dry the web at

printing speed, laminating nip, chill roll(s)

and unwind equipment for the primary and

secondary webs. With this type of setup for

in-line laminations, several precautions must

be taken:

• The web travel between the two mach-

ines and to the rewind should be as

short as possible to prevent loss of web

control.

• The electrical and mechanical drive sys-

tems that synchronize the laminator to

the press must be of the highest quality

with adequate web tension controls to

maintain precise control of the web

throughout the operation.

• The adhesive application station must

be tied into the press functions so that

when the press stops, the nip opens and

the applicator roll continues to rotate.

Automatic or manual controls can be

incorporated for engaging the adhesive

application station.

• The above functions must also be incor-

porated into the laminating machine

drying tunnel controls. When the press

stops, the drying temperatures must be

reduced to prevent damage to the web

in the tunnel.

• The laminating station must also auto-

matically open at press stop to prevent

damage to the rubber impression roll,

as well as the web. Independent control

for closing the nip at press start-up is

acceptable.

For the converter who has a large volume of

laminating with printing, the machine arrange-

ment shown in Figure

could be used. It

shows the press and laminator completely

separated except for electrical and mechani-

cal controls, which can be operated individu-

ally or in tandem. This arrangement allows the

converter to laminate in-line with printing,

print without laminating, laminate without

printing, or use both machines at the same

time but independent of each other.

Solid Adhesive Laminating

With the advent of 100% solid adhesives for

film laminations, the interest in in-line lami-

nating has greatly increased. Using the 100%

solids (solventless) adhesives eliminates dry-

ing requirements and consequently reduces

the overall length of the machine. Also,

greater printing and laminating speeds can

be obtained since the problem of insufficient

Laminator

Lamination

Unwind

Laminated

Rewind

Printed

Unwind

Rewind

Central Impression

Press

Adhesive Applicator

PRESSES AND PRESS EQUIPMENT 95

drying, or trapped solvents, is eliminated.

Figure

shows the adhesive application

and laminating unit of a solid adhesive lami-

nator, and it is readily seen that this unit

lends itself to in-line operations. However,

turning bars must be installed between the

press and laminator if it is desired to have

the printing sandwiched between the two

webs. This laminator can be arranged as dis-

cussed in Figures

and

. Due to the

nature of solid adhesives, they require spe-

cial application methods that eliminate the

use of the last station on the flexo press.

The same economic forces that are pushing

the expansion of laminating in-line with flexo

printing are also leading to the development of

various in-line coating processes such as:

• overall heat seal coating;

• thermal-stripe heat seal coating; and

• co-adhesive latex, both overall and reg-

istered heat seal coating.

In general, the same production limita-

tions and precautions outlined for laminat-

ing should be followed when considering in-

line coating with a flexo press.

UV/EB VARNISHING

Varnishes that can be cured with ultravio-

let light have gained tremendous popularity.

The flexographic process is well suited for

applying such varnishes over the entire

printed surface, or selectively where needed.

The most important aspect of applying a UV

coating is to ensure that inks laid down pre-

viously (in-line) are completely dry before

the UV varnish is applied over them.

Without going into specific details of their

chemical nature, UV varnishes are com-

posed of 100% solids (no volatile compo-

nents), all of which have to be cured. A typi-

cal composition is illustrated in Figure

• 85% resin (mixture of viscous

and highly viscous resins);

• 4%–6% photoinitiators;

• 5%–7% wax;

• 2%–6% other supplementary

ingredients.

The selection of resins determines the

flexibility of a varnish coating. Some resins

form a flexible surface but remain rather

sticky. Others form a relatively hard, brittle

surface that cracks during scoring or bend-

ing of the substrate. It is therefore important

to perform tests on different substrates with

different coating film thickness before large

production runs are undertaken.

Curing

UV curing units (polymerization units) are

designed to cure printing inks, varnishes,

and plastic coatings. Intensive UV radiation

Printed

Web

To Rewind

Secondary

Web

Laminator

Adhesive

Applicator

Adhesive

Supply

Adhesive

Metering

Photoinitiator 4% to 6%

Wax 5% to 7%

Other supplementary

ingredients 2% to 6%

Resin 85%

An adhesive and lami-

nating unit of a solid

adhesive laminator.

This type of setup can

be easily placed in-line

with a flexo press.

A typical makeup of a

UV varnish.

96 FLEXOGRAPHY: PRINCIPLES & PRACTICES

causes polymerization and curing in a very

short period.

UV/EB curing makes use of ultraviolet

(UV) light or electron beams (EB), respec-

tively, to polymerize a combination of

monomers and oligomers. The UV/EB mate-

rial may be formulated into an ink, coating,

adhesive or other product. The process is

also known as radiation curing because UV

and EB are radiant energy sources. The

energy sources for UV and visible light cur-

ing are typically medium pressure mercury

lamps, pulsed xenon lamps or lasers. The

coatings cured by these light sources are

usually clear or translucent, though thin

opaque coatings are also possible. Electron

beam accelerators are used to generate the

electron stream capable of curing thicker,

pigmented coatings. Unlike photons of light,

which tend to be absorbed mainly at the sur-

face of materials, electrons have the ability

to penetrate through matter.

For UV curing, UV radiation (light) must

be generated. With a mercury lamp, the light

is generated by heating mercury droplets in

a sealed quartz tube to a gaseous ionized

state. When excited to an ionized gas form,

mercury naturally emits radiation in the

ultraviolet frequency. Either an electrical

current or microwave radiation can be used

to vaporize the mercury. Polished reflectors

are used to direct the light at the web. For

better, deeper or faster curing of some col-

ors or of some specialty formulations, other

materials may be added to the mercury in

the lamp to alter the spectral “signature” or

profile of the emitted light.

The quartz tube and reflector are con-

tained in a chamber called an irradiator. In

most cases, the chamber will also contain

some means to block energy from the web

when it is stationary. Blocking when the web

is stopped is required because of the high

operating temperature of the lamp, and

because a large amount of infrared (IR)

energy is also emitted from the UV lamp and

directed at the web by the reflector. Various

designs are used to block the energy, includ-

ing shutters and rotating reflectors. Some

lamp designs can be instantly turned off and

back on, making shutters unnecessary.

Safety

Ultraviolet radiation burns the skin and

can cause temporary blindness. For this rea-

son, the curing chambers, or irradiators,

must be tightly constructed to avoid light

escaping in any direction other than toward

the web. Additionally, light shields are often

used to prevent the press operator from

looking into the irradiator, and to prevent

exposure to light reflected by the web.

UV Lamp Cooling. The high temperatures in

the irradiator require that some form of heat

management be incorporated into the

design. The most common methods used to

cool the irradiator and its components are to

blow or pull air through the housing. Some

designs use water cooling. Both air and

water cooling are effective means to control

the internal and external temperature of the

irradiator. Neither address the issue of web

temperature, which is another heat manage-

ment issue of importance to the printer/con-

verter. Because of the large amount of

infrared energy associated with UV systems,

web temperatures may be elevated beyond

acceptable levels in some applications.

Elevated temperatures are most common in

film applications, but can also be an issue

with paper products, especially if multiple

UV lamps are used.

There are three possible strategies to

control web temperature. The first is to

remove the heat from the web. The second

is to avoid heating the product. The third is

to do both. Removing heat from the web

usually requires some form of heat sink, or

chill drum. Chill drums transfer heat,

through contact, from the web to a roller or

series of rollers. The rollers are cooled

with air or water to maintain their capacity

PRESSES AND PRESS EQUIPMENT 97

to transfer heat. The chill drum(s) may be

located at the point of exposure to UV and

IR energy, or downstream from that point.

The temperature that the web can tolerate,

and the temperature rise created by the UV

system, determine the location of the chill-

ing mechanism.

There are a number of ways to reduce or

limit the amount of heat put into the web.

Simply running the lamp system at a low

power level will significantly reduce web

temperature. Increasing press speed will

also reduce web temperature since it will

decrease the exposure time of the web to the

IR energy. Beyond these steps, which often

may not be possible in a production environ-

ment, the UV system may be designed to

limit the amount of IR energy reaching the

web. For example, the diameter of the lamp

determines the amount of IR radiation that is

emitted and the shape of the reflector

defines the concentration of the IR radiation

on the web. Each factor therefore, will affect

the degree of temperature rise of the web.

Air Filters. Another method to restrict the

amount of IR energy reaching the web is to

use filters that affect some portions, or

wavelengths, of energy and not others.

These filters are made from dichroic coat-

ings or materials. A dichroic coating on the

reflector either absorbs or transmits IR radi-

ation while reflecting UV radiation toward

the web. Filters placed between the bulb and

the web transmit UV radiation and reflect IR.

Water may be used as the filtering material,

but more typically, a dichroic material is

used. These filters normally reduce the peak

temperature of the web to an acceptable

level. However, the cumulative effect of mul-

tiple lamp exposures may still result in unac-

ceptable temperature increases. Combining

dichroic filters with methods to remove

residual heat from the web is required in

these situations.

98 FLEXOGRAPHY: PRINCIPLES & PRACTICES

orrugated single-faced board or

combined sheets have been

used in the packaging industry

for over a hundred years. In

1871, an American, Albert L.

Jones, received a patent for

improved corrugated packing paper. From

the outset, the material was recognized for

its strength and cushioning characteristics,

but certainly not for its ability to receive a

printed image. None of the printing equip-

ment in existence could efficiently print on

this spongy packaging substrate.

BEGINNINGS

Crude machinery was used to produce

corrugated board and for decades the world

identified brown boxes or shipping contain-

ers as replacements for wooden crates. After

World War II, some machinery manufactur-

ers, as well as some board converters, start-

ed to build printing units to complement in-

line corrugated converting operations.

Letterpress printing came into existence

around the turn of the century and is still

utilized today for certain display work. The

oil-based inks used in letterpress, however,

take many hours to dry and special mea-

sures are needed to prevent smearing, stick-

ing or offsetting. This method of printing

has always been, and still is, a bottleneck in

corrugated converting plants. The corrugat-

ed industry was truly waiting for the arrival

of flexography, a means to mark or rotary-

stamp corrugated boxes.

EVOLUTION AND GROWTH

In those early days, no one anticipated that

graphics on corrugated board would evolve

much beyond the most basic techniques.

The first flexographic in-line machines, built

in the mid-to-late 1950s, printed one color,

almost without exception black. Compared

to other industries (tag and label, cups, etc.)

corrugated flexo received little recognition

well into the 1970s.

Printing on combined corrugated board

continued to receive little recognition and

seemed to have little potential for producing

high quality, multicolor graphics. When

process printing was mentioned, it produced

blank stares, or a rebuffing laugh, at best.

To d a y, the sheet-fed freestanding flexo press

for corrugated has developed into a high-

tech “marvel,” producing results that rival

lithographic label and preprint quality.

MARKETS FOR FLEXO PRINTING

The predominant market in the corrugated

industry has always been, and remains today,

the so-called “brown box” market. The major-

ity of linerboard produced on paper making

machines worldwide is brown paper, mean-

ing virgin and recycled kraft paper. A figure of

90% to 93% may be used to quantify this seg-

ment of the total linerboard production.

The remaining 7% to 10% is bleached, clay-

coated, often called "mottled white" paper, or

white top sheets. It is this small segment that

is of primary interest to the high-quality flex-

ographic preprinter or postprinter. This is not

C o r r u g a t e d

Postprint Presses

PRESSES AND PRESS EQUIPMENT 99PRESSES AND PRESS EQUIPMENT 99

to say that brown liners are not conducive to

so-called “value-added printing.” In fact, the

contrary is true. Brown boxes almost always

contain some printing, and in fact, even mul-

ticolor halftone screens are found on brown

boxes. In some cases this is achieved by tint-

ing the board with a white coating on the cor-

r u g a t o r, or by printing a white background

with titanium dioxide-based white flexo ink.

It is safe to say that a very large quantity of

brown boxes, or shipping containers, are pro-

duced with value-added graphics and point-

of-purchase messages.

The crossover from flexographic printing

on brown kraft liners to white-top or coated

liners, as well as bleached liners, is by no

means the frontier for high graphics or

value-added printing. It is therefore difficult

to establish statistics that clearly differenti-

ate between a high-graphics operation and a

"brown box" operation. One can also not

classify value-added or plain brown box

printing by the type of printing equipment

used. Value-added graphics are categorized

primarily according to their quality and have

therefore no significant connection as to

what substrate they are printed on or with

which type of machine they are printed.

PREPRINTING VS. POSTPRINTING

The terms preprinting and postprinting are

used to describe when the printing is per-

formed in the corrugating process. A corru-

g a t o r, in its basic version, produces single-

faced board, consisting of the medium (the

corrugated paper) onto which a liner is

glued. This first liner is called the single-face

l i n e r, which is never, or very seldom, printed.

On this same corrugator, another liner is

glued on the opposite side, on the tips of the

fluted or corrugated medium. This liner is

called the double-face liner (Figure

6 %

) and

may be printed on wide-web flexo presses

before it is applied by the corrugator. We call

this process preprinting, i.e., the printing has

taken place before the combining process.

In the preprinting converting process, the

web of double-faced board is cut into sheets

in register with the printing on the double-

face liner and stacks of preprinted sheets are

produced for subsequent converting into

containers, boxes or die cuts. In some cases,

the printing and die-cutting units are incor-

porated into the corrugating line.

It is noteworthy to mention that preprinting

in the corrugated industry has been in exis-

tence for less time than postprinting. Flexo-

graphic preprinting, first introduced in the

United States on a large commercial basis 20

years ago by a company in Wisconsin, has

grown to tremendous output capacity.

Preprint methods other than flexo, such as

offset and rotogravure, are also utilized. The

preprinting onto linerboard process lends

itself to more refined graphics; however, rela-

tively large press runs are needed to make it

cost-effective. The cost of preprinting short

run promotional graphics is prohibitive

To d a y, there are approximately 45 to 50

preprint presses for linerboard operating in

North America.

Postprinting, as the term defines it, is per-

formed on combined double-face corrugated

sheets after the corrugating process. The

postprint process lends itself readily to the

In corrugated pre-

printing on wide-

web flexography,

the double-face liner

may be printed before

it is applied by the

corrugator.

6 %

“just in time” manufacturing methods being

adopted by today’s industries.

Economics of Postprinting

Postprinting is by far the most common

method for printing on corrugated. A con-

servative estimate is that there are over

10,000 flexographic printing units of one

kind or another in use in integrated and inde-

pendent corrugated converting plants.

The economic advantages of flexography

on corrugated depend largely on what level

of flexographic printing is performed. For

example, profit margins in brown box plants

can only be achieved if the printing process

is performed at the highest possible produc-

tion speeds, e.g., 600–800 linear feet/minute

or 10,000 boxes/hour. Printing, which is per-

formed in-line, should never slow down the

converting process. On the other hand,

when manufacturing high-quality display

work or halftone-process screen work, the

profit margin depends on how much cheap-

er the printing operation can be performed

compared to labeling or laminating, while

maintaining acceptable quality.

Generally speaking, the medium and

linerboard raw materials constitute the

biggest cost share in any corrugated plant.

By the time the flexo printer receives com-

bined sheets on the printing press, the

sheets represent more than 80% of the

value compared to the final selling price.

When clay-coated or bleached liners are

used, the basic material cost (raw materi-

al) is, of course, even higher.

A typical corrugated

postprint press today

can handle printing

boxes from finished

sizes of 7.5" x 13.5" up

to 104" x 210".

100 FLEXOGRAPHY: PRINCIPLES & PRACTICES

6 ^

PRESSES AND PRESS EQUIPMENT 101

Flexographic postprinting, because of its

inherent simplicity and high-speed produc-

tion capabilities, remains the preferred

method for producing graphics of any level

at a reasonable profit.

RANGE OF PRODUCTS

From the beginning, corrugated shipping

containers, which replaced wooden crates,

in sizes from 4" x 4" up to triple-wall contain-

ers (which may be used today to pack the

body of a car) required marking or stamping

for identification purposes. Such markings

included a minimum of information such as:

“Do Not Drop,” “This Side Up,” or an umbrel-

la graphic warning against exposure to the

sun. The printed markings also sometimes

listed the contents in large bold text, and

only rarely were illustrations of the packed

merchandise shown on the outside of the

box or container. To d a y, there are very few

markets, from food production to computers

and appliances, that do not utilize the power

of graphics on corrugated shipping contain-

ers and point-of-purchase displays.

The brown box or container is, by a large

margin, the most common type of corrugat-

ed container. Sheet sizes needed to produce

such boxes and containers may range from

7.5" x 13.5" up to 104" x 210". Flexo printing

equipment for the entire size range exists

today (Figure

6 ^

) .

102 FLEXOGRAPHY: PRINCIPLES & PRACTICES

Repeat orders of medium-to-long runs lend

themselves especially well to such in-line

production. Today’s flexo folder-gluer or

rotary die cutter is generally equipped with

up to four printing units, which can offer

more intricate graphics and also accommo-

date two-color work. In the case of two-color

work, while two units are printing, two oth-

ers can be prepared for very quick job

changeovers, especially on small runs.

The close coupled printing units of the

printer/rotary die cutter and flexo folder-

gluer machines have so-called roll-away units

on tracks (Figure

) to permit access to var-

ious printing elements for setup or wash-up.

The advantages of producing regular slot-

ted cartons on a flexo folder-gluer, at ever-

increasing production speeds, are well

known and appreciated in the corrugated

converting industry. The disadvantages of in-

line flexo printing at most types of convert-

ing operations are: dust, frequent printing

plate washing, limitations in print coverage,

and limited placement of graphics to avoid

Press Construction

n the corrugated industry, many compo-

nents of the flexo printing process

remained crude until parallel industries

started to introduce new plate materials

such as photopolymers, cushioning

materials, modernized inking systems,

and mechanically engraved anilox rolls

(which today are increasingly being replaced

by laser-engraved rolls).

The most appreciated characteristic of

flexo printing on corrugated is its simplicity.

The thick, soft, flexible plates, which adjust

easily to a relatively uneven substrate,

together with the fast-drying inks, allow in-

line printing to be combined with a multitude

of other converting processes in a single

operational step.

Flexographic printing units, whether on

flexo folder-gluer machines, or in-line with

rotary die cutters or with platen-type die cut-

ters, allow converters to produce boxes, con-

tainers, regular slotted cartons, and some-

times multi-out rotary or platen die-cuts, as

an integrated manufacturing step(Figure

Counter Ejector

Folding Section

In-line flexo folder-gluer

components.