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72 FLEXOGRAPHY: PRINCIPLES & PRACTICES

The pressure setting of the reverse-angle

doctor blade should be maintained at a min-

imum level consistent with the uniform

transfer of a thin ink film. Tests were con-

ducted many years ago by press manufac-

turers, ink makers, anilox roll manufacturers

and others which concluded that the correct

pressure setting of a reverse angle doctor

blade should not exceed quarter-ounce of

pressure per inch. The tests showed that

when the pressure was increased to half

ounce per inch, a noticeable amount of wear

to the anilox cells was detected.

A variety of doctor blade materials have

entered the industry over the years, ranging

from high tensile, tempered blue steel; to

plastic, some of which is reinforced with

synthetic fibers. Many of these changes

came about because of the conversion from

solvent-based to water-soluble ink systems.

Each flexographic printer should determine

the type of doctor blade material which per-

forms best for the particular operation.

Some plants have found that a stainless-steel

doctor blade works best, while other plants

use non-reinforced plastic materials. Re-

gardless of the material used for the doctor

blade itself, the primary concern must be on

the pressure setting. Keep in mind that

excessive pressure can only create friction,

which in turn will create excessive wear.

Chambered Doctor Blade

The chambered doctor-blade system is the

newest system used by the flexographic

printing industry. The chamber blade unit is

comprised of two doctor blades, one of

which is a reverse-angle doctor blade that

does the job of actually doctoring the ink

from the anilox roll, and the other is a con-

tainment blade that holds the ink in the

chamber. The two blades are preset at spe-

cific angles for proper doctoring and are

affixed to a backing plate that forms the

three-sided unit (Figure

). When this unit

is applied to the anilox roll, a chamber is

formed between the anilox roll and the

three-sided fixture. The sides of the anilox

roll are sealed with pads of felt, rubber or

plastic material. These side-sealing mecha-

nisms may be spring-loaded or pressure-

loaded. When ink is pumped into the cham-

ber, it becomes somewhat pressurized.

Chambered doctor-blade systems help

reduced the solvent or water evaporation

from the ink, keeping the ink flow character-

istics under control. A critical element in the

operation of the chambered doctor-blade sys-

tem is that the pressure settings must be

maintained as recommended by the original

equipment manufacturer. Some of these units

have air-loaded pressure controls, while oth-

ers offer manual controls and include a set

screw installed as a protective stop to prevent

A reverse-angle doctor-

blade system allows a

precise ink-film thick-

ness to be transferred

to the printing plate at

various operating

speeds without press-

side adjustments.

Chambered doctor-

blades are the newest

systems in fleoxogra-

phy. Comprised of two

doctor blades, a

reverse-angle blade to

doctor the ink from the

anilox roll; and a con-

tainment blade, to hold

the ink in the chamber.

Anilox

Roll

Doctor Blade

Metering

Roll

Anilox

Roll

Ink Out

Ink In

excessive pressure. Do not attempt to read-

just these stops. Excessive pressure settings

can destroy an anilox roll in a very short peri-

od of time.

Safety Concerns. When handling doctor

blades, either to clean them or install them,

all safety precautions should be taken.

Regardless of the material, doctor blades are

sharp and can cause serious injury if not han-

dled with care. Steel blades are generally in

the 0.004" to 0.009" thickness range and

should be considered to be as sharp as a razor

blade. Handle them with care and caution.

THE ANILOX ROLL

With the improvement of quality standards

for inks, printing plates and presses, flexog-

raphy is now able to perform process print-

ing which was not possible in the past. One

of the many reasons for this improvement

has been the ability to maintain a specific

volume of ink transfer over a long period of

time. The primary function of the anilox roll

is to meter and control the flow of ink from

the reservoir to the printing plate. With the

development of laser engraving and ceramic

coatings for anilox rolls, the longevity prob-

lem has been answered. To obtain maximum

productivity from the anilox roll, it is impor-

tant to understand how they are produced.

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

engraving tool, chemically etched or

engraved through the use of a laser beam.

Regardless of the method of engraving, 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 engraving on an anilox roll has been

given many names, among them 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 a 45° angle 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 set-

tled on the 60° engraving angle after detailed

testing for ink receptivity and ink transfer

characteristics. Testing revealed that the 60°

angle worked best for flexographic printing.

Other angles of engraving are used for spe-

cific purposes or industries, such as the 45°

angle which has been the standard for flexo

printing of newspapers, or the 30° angle

which seems to be widely accepted for

industrial coating applications (Figure

The nomenclature of an anilox roll relates

to the number of cells in one linear inch

along the engraving angle. For example a

165 anilox roll would contain 165 cells in a

linear inch. Occasionally, this cell count may

differ slightly and is rounded out to the clos-

est number. A nominal 200-line anilox roll

for example, may actually contain 203 or 204

cells in a linear inch.

Mechanical Engraving

The anilox roll was originally engraved

mechanically by using an engraving tool.

The engraving tool was forced into the sur-

INK 73

The different anilox

engraving angles.

60° angle works best

for flexographic printing.

45° angle is used for

flexo printing of news-

paper. 30° angle is used

for industrial coating

applications.

30°

45°

60°

74 FLEXOGRAPHY: PRINCIPLES & PRACTICES

face of a steel- or copper-plated cylinder

with sufficient pressure to emboss the sur-

face metal of the cylinder into the cavities

formed by the teeth of the engraving tool.

The result is a reverse pattern of the tool on

the surface of the anilox roll (Figure

Each tooth on the tool begins with a single

punch designed to hold a specific volume of

ink. From the single punch, an engraving tool

is made with every tooth measured for accu-

racy. Thus, when the tool is used to make an

anilox roll, each cell formed is identical.

When the cells have been formed in the

base metal of the anilox roll, the next step is

to protect the surface of the cells from cor-

rosion and wear. This is accomplished by

electroplating nickel over the engraving to

provide corrosion resistance and then apply-

ing a layer of double-hard chrome plating to

provide durability. While chrome plating pro-

vides excellent ink receptivity and release, it

cannot withstand the pressures of doctor-

blade metering and wears rapidly. Attempts

have been made and continue to be made, to

extend the life of anilox rolls by electroplat-

ing specialized coatings to provide durability,

but most have been unsuccessful.

Ceramic-coated Anilox Rolls

Because chrome-plated anilox rolls did not

have the durability to withstand the pressure

settings of a doctor-blade system, ceramic

coating was developed. Ceramic coatings are

applied through a plasma-coating system in

which ceramic particles are melted at very

high temperatures and forced onto the sur-

face of a cylinder by using a gas-fired propel-

lant. The ceramic coating provides an

extremely durable and very hard surface

which resists wear and impact that would

normally damage a chrome-plated roller. The

result is the mechanically engraved, ceramic-

coated anilox roll, in which a ceramic coating

is applied to an engraved surface in place of

the chrome plating.

A major problem with ceramic coating

over an engraving is that the coating is limit-

ed to medium-range anilox cell counts of 200

or 220. Attempts to ceramic-coat finer anilox

cell counts found that cell integrity was lost

because the cells would become filled with

ceramic material. In order to preserve cell

definition, most anilox roll suppliers limited

the ceramic coating to a maximum engrav-

ing of 200 or 220. Most manufacturers limit

the amount of ceramic coating over an

engraving to 0.001" for fine screens and

0.003" for coarser engravings.

Laser Engraving

Since ceramic coating of fine line-engraved

anilox rolls was not practical, a new method

of engraving was required. Furthermore,

with the development of the chambered doc-

tor-blade system a more durable surfaced

anilox roll was a necessity. Through much

research and development, the laser-

engraved ceramic anilox roll was invented

and has become an integral part of the flexo-

graphic printing industry. A laser-engraved

roller begins with a cylinder which is plasma

coated with a thick layer of chromium oxide

ceramic material. Chromium oxide material

was selected for the coating because ink

release and ink receptivity characteristics

are comparable to chrome plating. This coat-

ing material has an extremely hard surface

Enlarged detail of a

mechanically-engraved

anilox roll showing the

reverse pattern made

from the engraving tool

used to emboss the

surface of the roll.

which resists impact and abrasion, providing

a very durable roll surface.

The ceramic coating is ground and pol-

ished with a diamond wheel to a super-

smooth finish and then a laser beam is used

to burn a cell directly into the ceramic mate-

rial. Laser engraving equipment is computer-

controlled and is capable of engraving very

fine anilox cells. Cell counts of 1,000

upwards to 1,500 or more per linear inch are

now possible with the use of a laser-engrav-

ing machine. Laser engraving offers much

more flexibility and the same cell count can

be made in different cell depths and vol-

umes. The 60° angle of the laser-engraved

cell forms a hexagonal “nested” cell struc-

ture offering better ink release and cell effi-

ciency than the 45° angled cells.

Volumetric Carrying Capacity

Since all of the cells on an anilox roll are

identical in size, the volumetric carrying

capacity can be determined by the actual

measurement of a single cell using a mathe-

matical formula. Volumetric carrying capaci-

ty is the amount of ink contained in the total

number of cells within a square inch.

Because of the development and improve-

ment of anilox rolls over the years, there

have been many changes in the equipment

used to measure and calculate volume. For

chrome-plated mechanical engravings, opti-

cal measuring instruments are generally

used. A microscope is used to measure the

depth of the cell, the cell opening at the top

and the width of the bottom of the cell

(Figure

). These measurements are read in

microns (25,400 microns to an inch). When

the measurements have been made and the

cell count established, the following formula

is used to calculate the specific volume in bil-

lion cubic microns per square inch.

SPECIFIC VOLUME  CV  C2

CV  D/3  O2  B2  O  B

Where:

CV = volume of single cell

C = cell count (165, 200 etc.)

D = depth of cell

= cell top area

= cell bottom area

Knowing the specific volume or volumet-

ric carrying capacity of the anilox roll allows

the flexo printer to determine if the anilox

roll is carrying the precise amount of ink

needed to properly ink the printing plate. A

loss in volume may be attributed to either

wear of the cell or to plugging of the cell

because of improper or infrequent cleaning.

Liquid Volume Calculations. When a micro-

scope is used to measure an anilox cell, the

measurements are determined by the light

that is reflected back into the lens. Since

ceramic material absorbs light, there is little

reflection and an accurate measurement of

the cell is extremely difficult.

A liquid volume method was developed

based upon the principle that a known vol-

ume of fluid will spread only as far as the

surface roughness or cell structure on the

roller allows it to be spread. A known quan-

tity of ink ( usually 25 microliters) is applied

to the surface of the anilox roll using a pre-

cise pipette or syringe. The ink is then

spread around the circumference with a

hand-held doctor blade. A sheet of white

INK 75

Volumetric carrying

capacity is the amount

of ink contained in the

total number of cells

within a square inch.

Measurements of cell

depth, cell opening at

the top and the cell

width at the bottom of

the cell are then

plugged into the

specific volume

formula.

Bottom Area

Cell Top Area

Cell

Depth

Wall

Width

76 FLEXOGRAPHY: PRINCIPLES & PRACTICES

bond paper is placed over the ink smear and

a footprint of the area covered with ink is

made. The area is measured and a calcula-

tion is made to determine the volumetric

carrying capacity.

While this method is very accurate, it is

dependent upon the ability of the individual

to uniformly apply the bead of ink, spread the

ink, take the footprint and finally measure the

outline. There will be differences in the cal-

culated volume among individuals doing the

volume check because of the level of experi-

ence in performing that task. As a result, the

liquid-volume measurement should be taken

from three different areas on the anilox roll

and the volumes averaged. The average

should be used as a starting point to deter-

mine wear and durability of the anilox roll.

An improvement on the method just ex-

plained incorporates the use of a surface-

treated clear-polyester sheet, like those used

in inkjet printers, and a wide hand-held tack

roller, like the ones used to remove dust

from negative films. The ink-jet film is rolled

around the tack roller with the treated sur-

face outside. The covered roller is pressed

against the surface of the anilox and the ink

from the microliter-syringe is introduced

into the nip (Figure

). The ink-jet film is

rolled against the anilox, spreading and

printing the ink volume simultaneously. This

method removes most of the operator error.

Comparative volumes of new and partially

worn or plugged rolls may be checked by

simply weighing the cut silhouette of the ink

patches.

Vertical Scanning Interferometry. A more sci-

entific approach to accurately measure vol-

ume involves using an interferometric scan-

ning device. This device was designed by

incorporating the technology used in manu-

facturing fiber-optic equipment and comput-

er discs. It is also used in several aerospace

programs by the United States government.

The interferometric measuring method has

been adopted by the leading manufacturers

of anilox rolls as the most accurate measur-

ing device currently developed. The cost of

these more scientific measurement instru-

ments may be too high for the average flexo-

graphic printer to consider. As a result, many

printers correlate the readings by the anilox

roll manufacturer with actual liquid volume

measurement taken by their inspection per-

sonnel. Charts using both volume measuring

systems can be logged by the printer to

determine the durability of anilox rolls, thus

enabling timely roll changes before a critical

print job is to be scheduled.

The interferometric device uses a three-

dimensional scanning head positioned on an

anilox roll while connected to a computer

and a TV monitor. The device scans all of the

cells in the viewing eyepiece at a rate of

some 5,000 readings per second. Light waves

are bounced against each of the interior

anilox cell walls from the top to the bottom

of each cell and the distance is recorded.

This information goes into the computer,

which has been programmed with the prop-

er algorithm. The computer then calculates

the volumes of each of the cells in the view-

ing area and averages them to determine the

volume for that particular anilox roll. Tests

by the United States Bureau of Standards

have found this interferometric method to

be accurate to within 0.03%.

Liquid-volume

measurement requires

the ink-jet film to be

rolled around the tack

roller with the treated

surface outside. The

covered roller is then

pressed against the

surface of the anilox and

ink from the micro-liter

syringe is introduced

into the nip. The ink-jet

film is rolled against the

anilox, spreading and

printing the ink volume

simultaneously.

Hand

Roller

Microliter

Syringe

Inkjet Film

Anilox Roll

Printed Area

Microliters used

Area ( sq.in.)

= Volume (bcm)

Anilox Selection

Selecting the best anilox roll for the type of

flexographic printing produced can be some-

what perplexing, but knowing a few details

can make this decision much easier. First of

all, there are flexo printers in many different

industries and each one prints on a wide

variety of substrates. Wide-web printers print

on film, paper or foil. Narrow-web printers

use absorbent or chrome-coated glossy

stock. Corrugated printers print on a multi-

tude of surfaces from bleached high-holdout

to highly absorbent kraft linerboard. Each of

these substrates has individual characteris-

tics relating to ink absorbency and drying

that the printer should be aware of. The same

anilox roll cell-count and volume cannot be

used to print the same image on each of

these diverse substrates. As a result, this

textbook cannot provide an accurate deter-

mination of which cell count and volume will

work best for each of these industries.

Wide-web film printers may use a 220-line

anilox roll with a volumetric carrying capac-

ity of 7.7BCM/in

to print a large solid, while

a corrugated box printer may use a 180-line

anilox roll with a volume of 9.5 BCM/in

order to print the same image on kraft liner-

board. With these different substrates and

image requirements, the flexo printer should

consult his anilox roll supplier to customize

the anilox roll and volume to best suit each

specific need.

Table 10 can be used as a guideline for the

selection of anilox cell counts and volumes

for coarse screen rulings. Again, each print-

er must assess which anilox roll performs

best for his own need.

Note: Cell counts and volumes in Table 10

are based upon standard print require-

ments and printing plates for halftone

screens with fewer than 85-lines per inch.

Results are dependent upon a number of

variables including the condition and the

type of printing press, the speed of printing,

the skill of press operator and the surface

finish of the substrate. For advanced and

high graphics requirements, both ink- and

anilox-roll suppliers must be consulted and

work together.

The flexo printing ideology is to print with

the thinnest ink film possible, while main-

taining color density, and to run the press as

fast as possible. This is accomplished only

through the use of high-strength inks that

quickly dry and a fine anilox roll with a small

volumetric carrying capacity. With this in

mind, there are several guidelines to consid-

er when purchasing anilox rolls.

First of all, the printer must assess the con-

dition of the printing press and the experi-

ence of the press crew, so that he/she is

assured the desired image requirements can

be achieved. The press crew must be capable

of operating the press to attain that objective.

In addition, the press crew has to have been

thoroughly trained in the mechanics of the

press. The press must have been fingerprint-

ed to determine press characteristics such

as, pressure settings, registration and dot

INK 77

Table 10

SUBSTRATE ABSORBENT (PAPER) NONABSORBENT (FILM)

IMAGE CELL COUNT VOLUME (BCM) CELL COUNT VOLUME (BCM)

Solids 180 9.5 220 7.5

Line 200 8.3 250 7.0

Halftone 250 7.0 360 4.5

ANILOX SELECTION GUIDE

78 FLEXOGRAPHY: PRINCIPLES & PRACTICES

gain. Once all of these conditions have been

met, the anilox selection process can begin.

After the image requirements have been

determined, the printer can begin to develop

specifications for the anilox roll. If the image

is pictorial and process printing is a require-

ment, then several considerations must be

taken into account. One is that the line

screen of the printing plate must be known,

so that the anilox roll cell count is fine

enough to correctly ink each of the halftone

or process screen dots.

While there are several schools of thought

concerning the ratio of anilox cell count to

halftone or process dots, the prevailing one is

to have the anilox roll four times or more than

that of the halftone line screen. For example,

if the printing is a 110-line screen plate, then

the anilox roll must be 440 or finer in order to

ink each of the dots without having a single

dot dunk into an anilox roll cell. Dot dunking

can be a concern, especially where a printing

plate has very fine highlight dots of 1% , 2% or

even 3%. Should the printing plate contain

highlight dots of 3% or less, then the printer

should consider using an anilox roll with a

higher screen count, i.e., five or even six times

the halftone screen count.

Tables 11 and 12 provide dimensional

information on halftone dot and anilox cell

dimensions. The tables are based on a cell

wall thickness of 5 microns. Table 11 shows

the size, in microns, of 1%, 2% and 3% dots at

various screen rulings. Next, the size of the

dot is the cell count, where the opening of

the anilox cell just equals the size of the dot.

For example, at 120 lpi, a 2 % dot has a diam-

eter of 34 microns. This corresponds to an

anilox-cell count of 654. Hence, a 660 anilox

roll would have a cell size smaller than the 2%

dot. Table 11 shows the cell opening for

some common anilox-cell counts. The tables

can be used to help determine which anilox

roll to use.

Optimum Volumes. Volumetric carrying

capacity is an important consideration when

selecting anilox rolls with screen counts of

440 or higher. With all anilox rolls, the rec-

ommended volumetric carrying capacity

should be determined by the depth-to-open-

ing ratio in order to obtain clean ink recep-

tivity and ink release from the anilox cells. It

is suggested that anilox rolls should have a

depth-to-opening ratio between 23% to 33%,

in order to allow the ink to smoothly enter

and release from the anilox cells. Higher

ratios, where the cells are deeper, may have

a tendency to plug with dried ink after a peri-

od of use. The deeper cell does not evacuate

a sufficient amount of ink to allow total res-

olubility of the remaining ink in the cell when

mixed with fresh new ink. Thus, the ink will

Table 11

SCREEN ANILOX CELL ANILOX CELL ANILOX CELL

RULING 1% DOT COUNT 2% DOT COUNT 3% DOT COUNT

(LPI) (MICRONS) (PER IN) (MICRONS) (PER IN) (MICRONS) (PER IN)

85 34 655 48 481 58 400

100 29 754 41 557 50 464

110 26 817 37 606 45 506

120 24 878 34 654 41 547

133 22 956 31 715 37 599

150 19 1052 27 792 33 666

175 16 1187 23 901 28 760

200 14 1313 20 1004 25 851

DOT SIZE EQUAL TO ANILOX CELL OPENING

dry more rapidly and plug the anilox cell.

Shallower cells are very difficult to produce

and duplication of the cell volume may not be

maintained when re-engraving. Shallower

cells also have distorted cell walls separating

each cell and may not allow smooth entrance

and exit of the ink.

Anilox cell counts and volumes for

process printing can range from a low of 220

lines with a volume of 7.7 BCM to a high of

900 lines with a volume of 1.5 BCM. The

finer the cell count, the smaller the cell size

and smaller volume.

Appendix A lists some recommendations

for cell count and volume.

Banded Anilox Rolls. In order to correctly

determine which anilox roll works best, the

flexo printer should purchase a banded

anilox roll from his supplier. This roll should

be engraved with bands of different anilox

cell counts and volumes. These bands can be

as small as a few inches or as large as a foot

or more. Test plates can be made to order for

a specific industry and some plate makers

may have stock test plates available which

will contain useful information. The use of

banded anilox rolls should provide useful

information to the printer so that specifica-

tions can be determined for the anilox roll

cell count and volume that works best on the

press for particular print jobs. Every flexo

printer should know the capability of their

printing press and a test using a banded

anilox roll should enable that printer to

develop a storehouse of data to assist in pro-

viding better quality printing to the clients.

Anilox Maintenance

As printing has become more demanding,

anilox cells have become smaller and shal-

lower, the drying time of inks has decreased.

Add to this the requirements brought about

by new ink-metering systems, and it is clear

that the ink industry has improved ink quali-

ty. Better dispersions are necessary to obtain

desired strength and clean print from the

shallow ds. If the incorrect ink formula is

used, it is possible for large pigment particles

to plug anilox cells.

Regardless of the type of anilox roll used,

whether it is a fine-screen laser engraving or

a very coarse chrome-plated roll, it still must

be cleaned as often as possible. Considering

that water-borne ink systems are used in the

majority of flexo printing operations and the

characteristics of this ink are similar to latex

paint, it is easy to see that cleaning is a criti-

cal part of extending the life of anilox

rollers. The anilox roll should be scrubbed

with the correct brush and an approved

cleaning solution. Brass-bristle brushes

should only be used for chrome-plated

anilox rolls, while fine-bristle stainless-steel

brush should be used for cleaning laser-

engraved ceramic-coated rollers. Caution

must be exercised in the selection of a clean-

ing solution. Flexo printers should consult

with both their ink supplier and their anilox

roll supplier for an approved flexo cleaning

solution. The use of a high pH or caustic

cleaning solution can void any warranty

expressed by the roll supplier.

Anilox rolls should be cleaned as often as

INK 79

Table 12

ANILOX COUNT CELL OPENING

(CELLS PER INCH) (MICRONS)

250 97

300 80

360 66

400 59

440 53

500 46

600 37

660 33

700 31

800 27

1,000 20

1,200 16

ANILOX CELL OPENING

80 FLEXOGRAPHY: PRINCIPLES & PRACTICES

possible, but in every instance no fewer than

once a day, or once every shift. A cleaning

program should be employed to thoroughly

clean and brush anilox rolls whenever the

printing station is to be idled. Should the

anilox roll become clogged with dried ink,

then an aggressive cleaning method must be

employed, such as pressure washing or

ultrasonic cleaning.

Anilox rolls must be cleaned while the ink

is still wet and fluid. This will reduce the

clean-up time and make the job of cleaning

much easier. Dried ink is difficult to remove

from the bottom of the anilox cell and cured

ink may not come off at all. A maintenance

program should be developed and discussed

with the anilox roll supplier in order to pro-

vide maximum productivity and maximum

longevity of the rollers. Many roll suppliers

even sell a recommended cleaning solution.

Should pressure washing of anilox rolls be

an accepted procedure, then it is suggested

that a complete training program be devel-

oped. Some pressure washers have an auto-

matic traversing wand and vacuum system

where the cleaning media, such as bicarbon-

ate of soda, is blasted onto the anilox roll sur-

face and removed instantaneously. These sys-

tems are regulated as to traversing speed and

pressure settings. Other systems have a man-

ual wand unit and the manufacturers of these

manual-controlled units recommend that the

wand be traversed across the roller at a rate

of 12 in/sec. Slower movement may cause

irreversible damage to the anilox roll surface.

Additionally, there are ultrasonic cleaning

units, soak tanks and other cleaning systems

available to the flexo printer. Ultrasonic

cleaning systems are excellent for cleaning

high line anilox rolls, but recently there has

been evidence that they may cause damage

to the cell structure. Therefore, ultrasonic

cleaning should be used with caution.

Since cleaning of the anilox roll is pivotal

to the longevity of the roll, the use of any

cleaning procedure or system should be

thoroughly discussed and approved by your

roll supplier. Regardless of the type of

anilox, ink cannot be allowed to dry in the

cells. Ink flow over rollers must be main-

tained when the press is stopped, or the rolls

must be cleaned immediately. The specific

cleaning material should be that recom-

mended by the ink supplier.

In the clean-up phase, solvent-based inks

are more forgiving than water-based sys-

tems. Solvent-based inks not only can wait

longer before being cleaned, but the inks

also resolubilize in a wider range of solvents

with less elbow grease. Water-based inks

must be cleaned immediately. A combina-

tion such as this one can be used:

10%–20% Mild Alkali (no ammonia)

60%–70% Water

20% Solvent (propyl alcohol or

a propyl ether derivative)

Excessive alkali and/or amines can cause

pitting in chromed cylinders and so should

not be used. Make sure to rinse well after

cleaning, or it can cause problems in the

next inks used with that cylinder. This is true

for water- and solvent-based inks.

Many inks are not compatible with each

other, so when changing from one ink to

another for a different job, be sure to clean

very thoroughly. This practice will avoid

potential for the inks to kick out in the cell

and cause multiple problems.

INK PUMPS

While there are many different types of

pumps available today, those in wide use are

the peristaltic and the centrifugal pumps.

Peristaltic pumps are used primarily on

short runs, as they are easy to clean and

change over. They do require somewhat

more maintenance, however, and inks do

not circulate well with this pump.

The centrifugal pump is the most widely

used today. It works well on long runs and

requires little maintenance. The pump action

keeps the ink circulating. Regardless of the

type of ink pump used, some common guide-

lines should be followed:

• the pump should be designed with

materials that are compatible with ink

components;

• if water-based inks are being used, it is

important that the pumps be corrosion

resistant; and

• if solvent-based inks are used, the

pumps should have components that

are resistant to the various ink solvents.

Ink-return hoses should be submerged into

the ink sump as low as possible, that is,

always below the ink surface. This reduces

the production of foam in the ink pump.

Although critical for water-based inks, it is

also beneficial for solvent-based inks. The

pump must deliver the required flow of ink

over a wide range of viscosities, fountain

heights and coverage rate, and it is recom-

mended that the pump be as universal as pos-

sible. It must be relatively easy to clean the

pump, and there should not be any internal

surfaces that can hold dry ink. The inking sys-

tem should self-drain when the pump is shut

off, otherwise, ink waste increases and the

required cleaning becomes more difficult.

Flexo ink tends to “layer” if allowed to sit in

a container. Also, solvent additives need to be

mixed thoroughly and quickly, before they are

circulated to the print deck. If using piston,

screw, peristaltic or diaphragm pumps, addi-

tional tank agitation, usually a smaller mixer,

is probably needed. The pump intake should

reach almost to the bottom of the ink reser-

voir to pick up heavier materials and facilitate

maximum pump-out at the end of a run.

When using any type of doctor blade, uni-

form and constant pressure must be main-

tained across its full length. Pulsating pumps

(piston, diaphragm) can cause higher pres-

sure against the doctor blade, lifting it off the

cylinder.

Ink Sump Design

A properly designed ink sump should con-

tain a minimum amount of ink. This will

reduce the amount of press-return ink that

would have to be reworked into a new print

job. In addition, by maintaining a minimum

amount of ink in the sump, the solvent bal-

ance of the ink is better controlled.

An ink sump should also have good circu-

lation through the entire print deck, with no

dead spots in the ink pan. This will reduce

any tendency for the ink to settle in the pan.

This is especially important when using

heavy gravity colorants, such as white or

metallic, and also with fluorescent inks that

are not highly dispersed.

The ink sump should be covered to avoid

loss of solvents during the pressrun. This will

not only save money by reducing the amount

of make-up solvent needed, but it will also

maintain a more constant solvent balance in

the ink, resulting in cleaner printing.

Press-Side Ink Filtration

When inks are manufactured, they are fil-

tered while being filled into their shipping

containers. After the inks are put into the

sump, they can become contaminated by a

variety of factors. If printing on paper, there

can be a major contamination with paper

fibers.

Regardless of what substrate is printed,

contamination is always possible. To remove

this contamination, use in-line press filters.

This prevents dirty printing and/or clogged

anilox rollers. Any ink that has been previ-

ously used in a press and is being reused

must be filtered before being returned to the

press. Clean ink is absolutely necessary to

high print quality. With the current bladed

presses, dirt may get trapped under the

blade and cause anilox damage.

PRESS SETTINGS

Keeping a log of all the jobs that are run is

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