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

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One to four individual print stations can be

mounted on both sides of a vertical frame.

The in-line press has its print stations posi-

tioned in tandem (a straight row). Six to nine

colors are possible with this type of press.

Parts of a Web Press

Flexographic web-fed presses generally

consist of four parts:

• unwind and in-feed section;

• printing section;

• drying section; and

• out-feed and rewind section (or subse-

quent in-line operation)

Unwind and In-feed Section. The roll stock to

be printed must be held under control, so the

web can unwind into the press with proper

alignment and sufficient tension to prevent

slack and wrinkles. If web tension is too

great, stretching and breakage could occur.

An effective unwind and infeed system

may include some or all of the following:

• multiple unwind positions;

• rotating turrets to make reloading

easier;

• semi-automatic chucking;

• precision bearings;

• automatic side-guide control;

• automatic tension control with tension-

sensing devices;

• driven in-feed rolls; and

• automatic (flying) roll splicing.

Printing Section. A single-color station con-

sisting of a fountain roll (or wipe roll), anilox

roll, printing plate roll and impression roll

INTRODUCTION 25

A typical sheet-fed

press for corrugated

postprint.

Slotter

Creaser Print Units Sheet Feeder

26 FLEXOGRAPHY: PRINCIPLES & PRACTICES

are sufficient to constitute a flexo printing

unit. But most presses are multicolor, with

two to eight stations in the printing section.

Drying Section. The drying section usually

includes between-color drying capacity to

print color-on-color. An after-dryer is added

to remove any remaining liquid vehicle

before winding the substrate into a roll. The

most common method of drying is by high-

velocity heated air, although other methods,

such as infrared heating, may be used.

Out-feed and Rewind Section. In many ways,

this is identical to the unwind section, but

with one important difference: The unwind

shaft is braked to apply the necessary ten-

sion to the web, while the rewind shaft must

be driven. As always, the web tension must

be controlled and limited to the minimum

amount necessary to keep the substrate

level, unwrinkled and taut – not necessarily

tight – as it winds on the finished roll. A

rewind section may include:

• multiple rewind positions;

• rotating turrets to facilitate unloading;

• semi-automatic chucking;

• anti-friction bearings;

• web-tension sensing devices;

• tension controls (often programmed to

reduce web tension as the roll diameter

increases);

• driven out-feed rolls;

• chill roll(s);

• automatic transfer;

• side guides;

• slitting devices;

• static eliminators; and

• moving web-inspection devices that

“freeze” the image for close examina-

tion.

THE SHEETFED FLEXO PRESS

Combined corrugated board is supplied in

sheet form. It requires a sheetfed press, which

is generally attached to an in-line die cutting

or slotting and gluing converting section.

Combined corrugated sheets are rigid

enough to be pushed into the printing station

and to remain horizontal from in-feed to fin-

ished stacking. The sheets can be fed into a

pair of feed rolls at speeds as high as 400

sheets or “kicks” per minute without dis-

rupting register. The machines are adjust-

able and can run many different sheet sizes.

Each press has a plate cylinder with a set

size and therefore a repeat cycle that cannot

be changed.

The corrugated postprint press is also a

tandem press and generally has its units

close coupled, in-line, on roll-away tracks

for plate mounting and servicing. Some mod-

ern corrugated presses have permanently

spaced units that allow constant access to

the print stations.

Sheetfed presses can be “bottom printers”

(printing is done on the underside of the

sheet) or “top printers” (printing on the top-

side of the sheet). In bottom printing, a nor-

mal ink fountain is used. With top printing,

the ink fountain is actually a puddle of ink

kept between the wipe and anilox rolls by

one or more applicators that supply a con-

stant flow to the nip. The overflow runs off

at the ends of the rolls into a container and

recycles through the system.

THE BASIC FLEXO PRINT UNIT

In its simplest and most common form, the

flexographic printing system consists of four

basic parts:

• fountain roll;

• ink-metering (anilox) roll;

• plate cylinder; and

• impression cylinder.

Fountain Roll

The fountain roll is generally covered with

natural or synthetic rubber. It is positioned

to rotate in a reservoir of flexo ink, and its

purpose is to pick up and deliver a relatively

heavy flow from the reservoir or “fountain”

to the metering (anilox) roll. The ink on the

land areas of the anilox roll must be

removed to ensure that only the cells carry

ink to the plates. The fountain and anilox

rolls are set to rotate against each other in

such a way as to allow excess ink to form a

puddle behind the nip (point of contact)

while only the ink in the engraved cells

transfers to the printing plates.

The fountain roll, sometimes referred to as

a wiper roll, is usually driven slower than the

metering anilox roll. This has the effect of

“wiping” the latter, and thus, doctoring the

ink to an even film.

The fountain roll is subjected to fairly

high nip pressures, either from the roll-load-

ing system or from the hydraulic pumping

action caused by the excess ink at the nip

between the fountain roll and the anilox

roll. Because of these pressures, the foun-

tain roll design is critical to the operation of

the two-roll system.

Whatever the press configuration, the

roller grouping of a typical flexo print sta-

tion consists of either:

• Two-roll ink-metering system. This con-

sists of four rolls: ink-fountain roll,

anilox roll, plate cylinder and impres-

sion cylinder (Figure

);

• Two-roll with a doctor-blade ink-meter-

ing system. This consists of four rolls:

ink-fountain roll, anilox roll with doctor

blade, plate cylinder and impression

cylinder (Figure

);

• Chambered doctor-blade ink-metering

system. This consists of three rolls:

anilox roll with enclosed doctor blade

chamber, plate cylinder and impression

cylinder (Figure

To a flexographic press operator, the ink-

metering system is a means of controlling

the amount of ink being presented to the

plates and subsequently to the substrates.

INTRODUCTION 27

The two-roll ink-

metering system shown

consists of, from front

to back, an ink-fountain

roll, anilox roll, plate

cylinder and impression

cylinder.

The two-roll with doctor-

blade ink-metering

system shown consists

of, from front to back,

an ink-fountain roll,

anilox roll with doctor

blade, plate cylinder

and impression cylinder

The chambered doctor-

blade ink-metering

system shown consists

of, from front to back,

an enclosed doctor-

blade chamber, anilox

roll, plate cylinder and

impression cylinder.

28 FLEXOGRAPHY: PRINCIPLES & PRACTICES

Ink Metering and Anilox Rolls

The purpose of the anilox roll is to transfer

a measured amount of ink to the surface of

the printing plate.

The surface of the anilox roll is covered

with tiny engraved cells spaced anywhere

from 80 to 1,200 per linear inch. The amount

of ink delivered to the plates is metered by

the screen size of the cells. The coarser the

cell count, the larger and deeper the cells are

engraved into the roll. Conversely, the high-

er the screen count, the smaller the cells.

The volume of ink contained in the cells is

measured in billion cubic microns (BCM)

per square inch of surface area.

For example; a 200-line screen anilox with

200 cells per linear inch, has 200 x 200, or

40,000 cells per square inch. Similarly, an

anilox with 400 cells per linear inch would

have 400 x 400, or 160,000 cells per square

inch. As cell counts vary, so do the ink vol-

umes delivered and this affects the color

printed.

Special attention must be given to the

selection (screen count and cell volume) and

quality of the anilox rolls. For any given use,

the substrate on which the printing is done,

the type of work (solids, type, halftones,

etc.) and type of ink will be factors in the

selection of the engraved transfer roll.

Choosing the correct anilox roll for a partic-

ular application may be the most difficult

task faced by the flexographic press opera-

tor (Figure

Control over the anilox-to-printing-plate-

transfer is very important. A light contact

An enlarged section of

anilox roll shows the

cells and the cell para-

meters of land area, cell

opening, cell depth, cell

volume.

Cell Opening

The width of the top of the cell, measured

in microns. As the number of cells per

linear inch is increased, this opening

narrows to make room for more cells.

Land Area

The non-ink area between the cells.

This is where a metering blade

would contact the roll if one is used.

Cell Volume

A measurement of how much

ink an anilox cell is capable of

delivering to the surface of the

printing plate.

Cell Depth

How deep the cell is

beneath the surface of the

anilox roll. This depth is

measured in microns.

between the anilox roll and the printing

plate surface prevents over-inking and keeps

ink from being pressed down on the shoul-

ders of the raised image areas of the printing

plate. It is also important that the anilox roll

and plate cylinder are geared to travel at the

same surface speed.

The construction of the anilox roll may be:

• plain, steel-chromed coarse matte fin-

ished;

• chrome-plated, mechanically engraved

cells;

• ceramic-coated, mechanically engraved

cells;

• coarse, random-coated plain ceramic; or

• ceramic-coated, laser engraved cells.

The number of cells per linear inch achiev-

able by mechanical engraving is limited and

either chrome plating or ceramic coating

reduces the cell volume.

Recent developments in laser engraving of

ceramic-coated rolls have been very suc-

cessful. Roll life has been lengthened, cell

volumes are consistent and cell count

increased to as high as 1,200 cells per linear

inch. Experimental rolls have been pro-

duced with cell counts as high as 2, 000 per

linear inch.

Two-Roll System with Doctor Blade. In order to

eliminate a number of the deficiencies asso-

ciated with a fountain-roll system, press man-

ufacturers have developed a number of doc-

tor-blade devices to assist the operator in

controlling the distribution of ink. The pur-

pose of the doctor blade is to remove excess

ink (or fluid) from the surface of the

engraved anilox or transfer roll, allowing bet-

ter control of ink transfer to the plate cylin-

der. The device is particularly useful when

printing halftone screens and process colors.

Ideally, the doctor blades should make

contact at a 30° angle with the tangent point

of the anilox roll (Figure

). At this angle,

the blade shaves, or doctors, off the excess

ink, leaving the precise amount of ink con-

tained in the engraved cells of the anilox

rolls. Also, the Total Indicated Runout (TIR)

of the anilox roll must not exceed 0.0005" in

order to maintain proper blade pressure and

doctoring of the ink. To obtain good doctor-

ing in any application, a number of require-

ments must be satisfied:

• The anilox roll should be manufactured

for use with a doctor blade.

• The anilox roll must be in reasonably

good condition.

• The doctor blade must be designed and

manufactured for the specific applica-

tion, taking into account machine width

and speed, the function and location of

the roll being doctored, the location of

the doctor blade on the roll and the sur-

face material of the roll (chrome or

ceramic).

• The doctor blade must be accurately

aligned and adjusted to the anilox roll in

the designed location.

• The doctor blade should be set at the

minimum blade pressure to accomplish

its task.

• The doctor blade and anilox roll must

be given sufficient maintenance to pre-

vent deterioration and misalignment.

Chambered Doctor-blade System. Two doctor

blades usually make up this system. One is a

INTRODUCTION 29

The doctor blade

should ideally make

contact with the anilox

roll at a 30° angle.

Anilox Roll

Plate

Cylinder

Rubber

Roll

2°–10°

30 FLEXOGRAPHY: PRINCIPLES & PRACTICES

reverse-angle metering blade and the second

a trailing containment blade (Figure

The reverse-angle metering blade is typically

made of steel and the trailing containment

blade is often plastic. These blades are set

about 2" apart, but this may vary between

manufacturers. The blades are connected in

a box-like enclosure with flexible sealing

material at both ends. This is then fit snugly

against the sides of the anilox rolls. Ink is

usually pumped into the system at the mid-

dle of the ink pan, but can be pumped in sev-

eral locations on wider presses. A pan is gen-

erally placed beneath the anilox roll for

cleanup purposes. The advantage of this

method: The entire inking system from ink

kit to anilox roll is never exposed to the air

and, the volume of ink flowing through the

pumped system is reduced. This makes tight

viscosity control possible. The system is

quite popular on high-speed, wide-web and

corrugated postprint presses.

Continuous Inking. Since most flexographic

inks are fast-drying, with the exception of

UV-curable inks, the anilox roll in the ink dis-

tribution system must continue rotating

when the press is in a non-printing mode. If

not, the inks will dry in the cells, and con-

trolled transfer will no longer be smooth.

Therefore, when the press is idling, if the

fountain roll and anilox roll are to continue

to rotate the anilox roll must be separated

from the plate cylinder. Otherwise, the

anilox roll will wear the plate along the line

of contact with the stationary printing plate.

In addition, it is essential to separate the

plate cylinder from the web when in the stop

mode. If not, the ink from the plate will dry

on the web, and when the press is restarted,

the web will stick to the plate and may break.

If ink has been applied to the plates before

the press is stopped, it may be necessary to

clean the dried ink from the plates before

restarting the press again. After the plates

have been cleaned, the press can be restarted

and the impression-control sequence com-

menced; the anilox roll comes in contact with

the plate cylinder and the plate cylinder in

contact with the web, and printing resumes.

Plate Cylinders and Sleeves

The plate cylinder is usually steel and is

installed between the ink-metering roll and

the impression cylinder. Printing plates are

mounted to the plate cylinder with sticky-

back. The raised impression areas on the

printing plate pick up ink from the ink-meter-

ing roll and transfer it to the substrate. Other

kinds of printing plates include ferrous (con-

taining iron), metal-backed plates mounted

to a magnetic cylinder, and magnetic-backed

plates mounted to a steel cylinder.

The total plate-cylinder diameter, includ-

ing stickyback and printing plate, has to

equal the pitch diameter of the driving gear

(Figure

). Therefore, for a given print-

repeat length, the bare cylinder diameter of

the plate cylinder must be reduced or

“undercut” to accommodate the combined

thickness of the stickyback and printing

plate. A trend toward thinner printing plates,

designed to reduce distortion and cupping,

requires the correct plate cylinder diameter

to accommodate the change.

Printing plates are mounted on the print-

ing-plate cylinder. There are four types of

plate cylinders: integral, demountable, mag-

This chambered doctor-

blade shows the

reverse-angle metering

blade and the trailing

containment blade.

Metering

Blade

Containment

Blade

netic and sleeved. The following is a brief

description of each.

Integral. The cylinder body or face, end-caps

and shaft are all one unit. Most cylinder bod-

ies are tubular, with end-caps shrunk-fit into

the tube ends. Small cylinders (less than 3”

in diameter) are generally made from one

solid piece of steel (Figure

Demountable. The cylinder face (or core) is

made (without the shaft) to any desired

diameter, but to fit a prescribed shaft or

mandrel. Mounting or demounting of the

cylinder core on shafts can be done in dif-

ferent ways (Figure

Magnetic. This integral cylinder is built to

generate a magnetic field to receive and hold

printing plates made with steel backing. This

eliminates stickyback.

Sleeves. Sleeves slide onto specially bored

cylinders by allowing high-pressure air to

enter through the side and exit through

holes in the face of the cylinder (Figure

The introduction of air slightly expands the

resilient sleeve and permits it to float into

position.

Impression Cylinder

The impression cylinder is smooth, highly

polished and supports the substrate when it

contacts the printing plate. On most stack

and in-line presses the impression cylinder is

a plain steel roller that supports the web or

substrate within each print station. On a cen-

tral impression (CI) press the impression

cylinder is a single large drum with an

arrangement of satellite print stations. In

INTRODUCTION 31

The total plate-cylinder

diameter, including

stickyback and printing

plate, has to equal the

pitch diameter of the

driving gear

An integral plate

cylinder is one piece,

while a demountable

plate cylinder consists

of the cylinder face and

mandrel.

The plate cylinder is

ready to accept the

sleeve-mounted plate.

Printing

Plate

Bare

Cylinder

Diameter

Cylinder

Undercut

Pitch

Diameter

Mounting

Tape

Demountable Cylinder

Integral Cylinder

32 FLEXOGRAPHY: PRINCIPLES & PRACTICES

both cases the total indicated runout (TIR)

of the support surface will effect the print

quality. TIR’s of better than 0.0005" are com-

mon on most high-quality presses.

The surface speed of the substrate on the

impression cylinder must match the surface

speed of the printing plate and the anilox

roll. Otherwise, slurring, halos, smearing and

reduced plate life will result. For high-quali-

ty printing, the accuracy of cylinder diame-

ters, concentricity, gearing and bearing fit

cannot be overstressed.

Repeat Lengths and Gears

In any printing process, it is necessary to

print cleanly at each color station, with each

station registering properly with one anoth-

er. To prevent smearing, the surface speed of

the plate cylinder, anilox roll and impression

cylinder must be identical; therefore, the

three rolls are geared together to create

equal surface speeds. Keep in mind, the fol-

lowing are necessary for good results:

• The pitch diameter of the plate cylinder

gear must be equal to the diameter of

the top of the printing plate mounted to

the plate cylinder (see Figure

)

• The plate cylinder will have a diameter

that is governed by the repeat length of

the image. (Figure

)

• The pitch diameter of the anilox roll

gear must be identical to the outer

diameter of the anilox roll.

• The pitch diameter of the impression-

roll gear must be equal to the impres-

sion roll diameter plus twice the thick-

ness of the substrate to be printed. In

most applications, the substrate thick-

ness may vary and therefore a compro-

mise is made.

Pitch Diameter. The pitch diameter is the

dimension of a circle through the gear teeth,

where the space between the teeth equals

the thickness measured along the arc of the

pitch circle. Gear pitch is the spacing of gear

teeth measured around the pitch circle. In

the United States, flexographic presses use

either a one-quarter inch (0.025") circular

pitch or 10 diametrical pitch gearing.

Here are some useful equations:



3.1416

Diametrical Pitch  Number of Teeth

When using metric gears, the following

holds true:

Module  Metric Pitch Diameter (mm)

Station Control

The setting of the impressions of the

anilox roll to the plate cylinder and plate

contact with the web requires a certain

amount of “feel” from the operator. To help

the operator, most press manufacturers

design the adjustment to work through very

finely threaded screws.

With a fine-thread adjustment, it’s easier to

set impression for tone work. Also, the com-

bination of fine threads, and, very often, gear

Number of Teeth

Pitch Diameter

(inches)

Circular

Pitch

Number

of Teeth

Pitch Circle

Diameter

The repeat length is

determined by the plate

cylinder diameter;

the smaller the cylinder

diameter, the shorter the

repeat length.

Repeat

Length

Repeat

Length

reduction on the adjusting screw makes it

possible for a press operator to have a read-

out system to visually tell the amount of

squeeze or impression being set on the plate.

That is, it is preferable to have a dial indica-

tor gauge to visually and repeatably set the

impression to a specified value.

When the print cylinder is stationary and

impression is “off”, the fountain and anilox

will separate from the plate cylinder and the

plate cylinder from the web, both by roughly

⁄

". If impression settings were made with

impression “off”, then when impression is

activated and the plate cylinder moves the

⁄

” toward the impression cylinder, the

plate would be damaged. Therefore, it is

mandatory that station setups are done

when the impressions are set to the “on”

position.

VARIATIONS ON THE

FLEXOGRAPHIC PROCESS

There are many variations on the basic

flexo press, each developed for a specific

purpose.

The Impression Bar (Tympan Bar)

The printer, tor example, may face the

problem of printing on very thin or porous

papers. Ink strike-through onto the impres-

sion cylinder, especially on CI presses,

becomes a daunting obstacle. Ink buildup on

the impression cylinder not only affects

print quality, but can also damage printing

plates. Replacing the impression cylinder

with an impression (Tympan) bar is a solu-

tion (Figure

The bar is usually a length of steel drill rod

measuring one-quarter inch (0.25") to one-

half inch (0.5") in diameter (depending on

the press width) mounted in a sturdy, prop-

erly aligned clamp or holding device. In

some cases, the bar is actually hollow and

water-cooled to prevent nonuniform expan-

sion from overheating near the middle of the

web. With this system, ink that penetrates

the substrate can’t accumulate on the bar

because the moving web constantly wipes it

clean.

The Flexographic Press

as a Coating Section

Coating is the process of laying down

overprint varnishes on top of printing or the

application of adhesives to substrate sur-

faces. Properly adapted, the flexographic

printing process is can be used as a coating

station. Figure

illustrates the arrange-

ment for a typical coating application.

INTRODUCTION 33

A variation of flexo

printing uses a thin

impression bar, to print

on very thin or porous

papers.

This flexo print station

is adapted for use as a

coating station.

Impression

Bar

Web

Plate

Cylinder

Anilox

Cylinder

Fountain

Roll

Impression

Cylinder

Web

Blanket Plate

Cylinder

Smoothing Bar

Anilox

Cylinder

Fountain

Roll

aniline, 13-15

anilox roll, 3, 14, 17, 25, 26, 27, 28-29, 30, 32

cell structure, 23

ceramic-coated, 16, 29

selection, 28

Clean Air Act, 16

corrugated container, 13

corrugated postprint, 3, 6, 17, 30

design rolls, 22

doctor blade, 20, 29

dryers, 16, 18, 25

dry offset, see letterset

dyes, 20

flexography

advantages, 4

applications, 4-5

definition, 3

early development, 13-14

variations, 33

flexo offset, 12

fountain roll, 3, 25, 26-27, 30

gear pitch, 32

impression cylinder, 30

inks

solvent-based, 20-21

UV, 21

water-based, 5, 16, 18, 20-21

ink systems

distribution, 30

metering, 3, 14, 26, 28, 30

letterpress, 6-7

letterset, 11

lithography, 7-8

molded-rubber plates, 15, 22

offset gravure, 11

photopolymer plates, 15, 22

pigments, 9, 14, 20

pin register, 15

plate cylinders, 3, 16, 21, 27, 29, 30-31, 32, 33

plates

distortion, 20, 22

molded-rubber, 15, 22

mounting, 18, 22-23

photopolymer, 15, 22

proofing, 15, 16, 22-23

prepress, electronic 17, 20, 22

prepress proof, 15

presses

central-impression, 13, 14, 16, 23

narrow-web, 16, 21

stack, 3, 16, 17, 21, 31

wide-web, 16, 18

proofs

concept, 19

contract, 20

registration, 16

repeat length, 32

rewind equipment, 24

rotogravure, 8-10

screen printing, 10-11

serigraphy, see screen printing

sleeves, 18, 23, 28-29

substrates, 3, 12, 14-16, 18, 21,

corrugated, 6, 26

polyethylene, 16

polypropylene, 16

unwind equipment

in-feed, 25

out-feed, 26

INTRODUCTION 35

Index