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62 FLEXOGRAPHY: PRINCIPLES & PRACTICES

by this ink system, and all of the functional

properties of the ink system would be certi-

fied by the ink supplier. If the color is correct

but the ink is too weak, perhaps due to use

of a lower-volume anilox roll, the only way

to correct the situation is by adding concen-

trated base colors. Many converters will

stock some pigmented bases to address this

problem. Adding base to an ink must be

done only with the permission of the ink

company, since the strength problem may be

corrected at the expense of finished proper-

ties such as ink adhesion.

If this same green was needed in a heat-

resistant formulation, it would need to be

blended from single-pigment heat-resistant

finished inks. Ink suppliers work with their

customers to keep inventories down by

developing multi-purpose ink systems. One

ink system with the correct balance of heat

resistance and gloss can, in many cases,

replace the individually formulated heat-

resistant gloss ink systems.

Matched Finished Inks

With this option, the ink is supplied by the

ink company as a pre-matched finished ink

(for example green gloss film), in which case

it would be certified for color, press readi-

ness and end-use properties when received

by the converter. This way of purchasing

inks is normally reserved for large-use items

that repeat frequently. In these cases, the ink

company has manufacturing equipment bet-

ter sized to produce the ink economically.

While adequate quality

and consistency can

be achieved by ink-

blending rooms using

manual methods, best

practices involve

dispensing systems,

with various levels of

computer control and

automation.

INK BLENDING

The ink-blending operation is a key com-

ponent in the smooth running of a convert-

ing operation. The most expensive part of a

converting operation is the pressroom, with

huge capital investment in presses which

must be kept running on schedule. To do

this, the inkroom must deliver the proper

amount of the proper ink to press every

time. While adequate quality and consisten-

cy can be achieved by ink-blending rooms

using manual methods, the best systems

involve dispensing systems, (Figure

)

with various levels of computer control and

automation.

Progressive converters who have installed

automated dispensing systems have mini-

mized ink-related downtime and dramatical-

ly reduced returned ink inventories. With

these systems in place, expensive return ink

is promptly reused by the system. Ideally,

the ink supplier will furnish the formulas

(color matches) and ink, and the automated

ink dispenser will calculate the precise

amount of each component required to

blend the color and then dose each compo-

nent with precise accuracy. With the soft-

ware and know-how in these systems, it is

possible to produce nearly all the colors

from only eight component colors, plus

black, white and extender. The number of

base components necessary will depend on

the product mix and can be determined in

consultation with the ink supplier.

Software Capability

An ink-blending system can contain data

for formulas, supplier information, basic

components, substrates, printing presses,

orders, ink usage and costs. These databases

enable the plant to have a complete book-

keeping system for flexo inks. Other func-

tions available include bar coding and label-

ing.

When evaluating a computerized ink-blend-

ing system, it is important to examine the

associated software. Many systems do not

have their own return-ink software, but

depend on spectrophotometer software for

managing return inks. Color reformulation

with the spectrophotometer can be per-

formed, but it is not the best solution for

everyone. The spectrophotometer is an aid,

but still requires a strong technical knowledge

of color theory and generation of ink system

color databases by the ink department.

On the other hand, ink-blending systems

with integrated press return software are

very easy to operate. If the operator wants to

blend a color, the system will prompt the

operator that there is rework ink in invento-

ry with some of the same components. If

more than one return ink is available, the

computer will display the ink in both

chronological order and cost-of-ink order.

The operator can choose whether or not to

use the rework ink. Most dispensing systems

have software links available for spec-

trophotometers for use in color formulation

and quality control.

Gravimetric vs. Volumetric

The heart of the dispensing system is the

dispensing unit which accurately dispenses

all basic inks. The basic inks often have dif-

ferent viscosity and flow. Historically, gravi-

metric-blending systems have proved to be

more accurate than volumetric-based sys-

tems. Gravimetric systems typically operate

within an accuracy range of one gram.

Volumetric dispensers operate with an accu-

racy range of 2%, which is as accurate as a

gravimetric dispenser for small batches.

However, if 50 gallons (450 lbs.) of ink are

dispensed, the volumetric batch could vary

by as much as 9 lbs. Air entrapment in the

ink will cause a dispensing error in the volu-

metric system, whereas with the gravimetric

system the weight is constantly checked.

It is absolutely required that the ink-blend-

ing system be closed and continuously cir-

culating to eliminate variability due to set-

INK 63

64 FLEXOGRAPHY: PRINCIPLES & PRACTICES

tling or separation of ink ingredients. It is

also important that the ink circulate

throughout the whole system including the

dispensing head.

Here are some advantages of in-house

automated ink-blending systems:

Just-in-time (JIT) Ink Production. Automated

dispensing allows the printer to blend inks

immediately for a press run and blend only

the ink that is needed.

Exact Quantity of ink Per Order. Historically,

more ink was ordered than was needed to

avoid any chance of running out of ink and

shutting down a press. With automated dis-

pensing more ink can be produced at the

press of a button. This prevents the constant

buildup of excess ink that must be reworked.

Constant Quality. Automated dispensers en-

sure exact reproducibility on repeat orders.

The color produced for a job last week or

last month will be exactly the same color

when it is blended next time. Ink inventory

build-up is again reduced due to the correc-

tion of wrong colors and rejected batches.

HOW TO ADJUST TOLERANCES

The question of achievable, realistic toler-

ances for special-color inks is the subject of

some debate and will be addressed in future

releases of FIRST. Ideally, on the same stock

and press, running the same conditions, a

repeatability of delta E, of 2.0 CMC 2:1 or less

is desired. This is a tight tolerance which may

or may not be achievable for a given process

and color. It may not even be needed for a

given color and, in the final analysis, a visual

assessment should be made to determine the

required delta E for customer acceptance.

Overall color consistency will improve, how-

ever, simply by measuring the process and

giving feedback to the inkroom and press-

room. This is not an easy task since many

work habits have to be changed. The graph

shown in Figure

is from actual experi-

ence and represents a complete revamping

of product quality coming from the inkroom.

In this situation, the pressroom became con-

fident in the color from the inkroom and was

able to focus on press variables to improve

print quality further.

When the on-press color is approved and

the delta E value is within the established

tolerance, replace the signed-proof spectral

values with the CMC L*C*h° values of the

press proof. This is important since this is

the real target to aim for every time the job

is run. When an ink batch is proofed before

the job goes to press, compare the batch to

the color that was achieved on press. If the

delta E of the initial pressrun was approved

with a delta value greater than the estab-

lished tolerance, the press print spectral val-

ues should not be saved. However, attempt

to get closer the next time the same job is

run and then save the new spectral values.

Once an approved standard and CMC

L*C*h° spectral values are established, it is

necessary to determine the numerical toler-

ances that can be established around the

color. To do this, samples of subsequent job

approvals need to be saved. Job approval

samples (the more the better) for the next

several runs of an identical job should be

saved. Under ideal conditions, two or more

people should visually evaluate all of the

prints at one time against the original signed

The graph shown is

from actual experience

and represents a

complete revamping of

product quality coming

from the inkroom.

In this situation, the

pressroom became

confident in the color

from the inkroom and

was able to focus on

press variables to

improve print quality

further.

JJJFMMMAASOND JJJ FJFM MA ASOND

Year 1 Year 2 Year 3

DEcmc

3.0

2.5

2.0

1.5

1.0

0.5

Time

and approved proofs, as well as against the

original standard. Unacceptable prints should

be discarded, and a spectrophotometer used

to read the others. The remaining values

should be averaged via color computer soft-

ware to make a new numerical standard. This

becomes the newly established tolerance for

batch and press approvals and avoids unnec-

essary ink adjustments on press.

It is now time to make an ink batch for the

press. Clean containers and scales to weigh

the formula accurately should be used.

Electronic scales are more expensive to pur-

chase, but they are also more accurate than

most of the less expensive mechanical

scales. An electronic scale can prove to be a

very wise investment in the long run.

The ink batch should be proofed identical-

ly to the customer-approved proof (same

stock, back-up, overprint, etc.). This proof

should be evaluated visually against both the

signed proof and the original color standard.

The batch proof should be compared numer-

ically to the previously entered standard. If

the light-to-dark difference between the col-

ors is greater than 0.5, the batch proof

should be re-read, or another batch proof

(lighter or darker) made and then re-read.

The chroma or hue of the colors should not

be evaluated if the batch proof light-to-dark

difference is greater than 0.5. This will help

ensure the correct color is achieved on

press. If the proof looks acceptable but chro-

ma or hue deltas greater than 1.0 exist, the

color should be adjusted accordingly. Once

the combined deltas of L, C, and h are under

1.0, the color should appear acceptable. The

ink batch is now ready for the press.

Again, note that the values of 0.5 in light-

ness and 1.0 in chroma and hue may not be

achievable for every process. Testing and

visual examination in conjunction with spec-

trophotometric measurements can establish

realistic and achievable values for a particu-

lar process. Once established, they become

the standards for that process.

Once the ink is in a clean press which is

running up to speed, compare and evaluate

the press print to the customer’s approved

proof and the original color standard.

Evaluate the press print to the customer’s

signed proof with the spectrophotometer. If

the delta E of the press proof is greater than

1.0, examine the L differences between the

colors. If the press print is darker than 0.5,

the color needs to be lightened with solvent

or extender. If the press print is lighter than

0.5 and there is extender in the ink formula,

equal percentages of the base colors need to

be added to strengthen the ink. If the color

formula does not contain extender, adjust

the press settings or change the anilox. The

print should be approved with the combined

color (CMC L*C*h°) differences. Do not look

at the color comparisons between the print

generated in the ink room and the press pull

until the light-to-dark difference between

these colors is under 0.5. This will help elim-

inate some of the small variables such as

strike through, paper color shifts, or other

minor process variables that could cause

slight color changes. The inkroom proofing

method must represent what the press will

later produce for the customer.

Once the inkroom proof is made and the

light-to-dark difference meets the standard,

the proof should be viewed under the light

source required by the customer, if possible.

In many cases, the converter is unaware of

where the customer will evaluate the prints

for color approvals. Therefore, the colors

should be evaluated under a “daylight” light

and an incandescent light to eliminate

metameric potential (Figure

). Make sure

that the inkroom proof and color target are

evaluated with a common back-up material

behind the prints. A color booth should be

available for all color work. The color booth

allows the print to be compared under dif-

ferent standard light sources.

The color computer measures reflectance

spectrophotometric curves for printed col-

INK 65

66 FLEXOGRAPHY: PRINCIPLES & PRACTICES

ors. From these curves, the computer calcu-

lates mathematical values that completely

describe the color. It can store these values

and compare them to other prints, and

determine if they are the same or within

commercial tolerance.

The color computer will not replace eyes,

experience or judgment. It is a tool that

always remembers and never gets tired.

Most printers, ink suppliers and many cus-

tomers use color computers to control their

processes and to certify incoming materials.

The colors should be

evaluated under a

“daylight” light and an

incandescent light to

eliminate metameric

potential. The inkroom

proof and color target

should be evaluated

with a common back-up

material behind the

prints.

Daylight

(D50)

Incandescent

Light

Fluorescent

Light

INK 67

Ink on Press

t is critical to understand the dynamic

interaction between the ink and the var-

ious printing components in order to

minimize variations in printed materials

and maximize efficiency. This chapter

will examine the various press configu-

rations and how they influence ink selection

and performance. It will also look at the ink-

metering system, press dryers, printing

plates, and film movement through the press.

Significant changes take place in the

mechanical conditions on press, so it is

important to understand how these changes

relate to, and effect the inks. Proper ink han-

dling on press is critical to trouble-free print-

ing. Color control, viscosity and pH adjust-

ments for water-based inks will be explored.

These areas are becoming more important

due to the increased usage of water-based

inks and high performance solvent inks.

Both the ink and the press are influenced

by the specific environment in which they are

used or located. Humidity and temperature

play important roles in proper drying and res-

olubility of the ink. These factors affect how

well ovens dry inks.

Any printed material must be evaluated

agianst a quality standard. Beside the obvious

color and strength of a print, it is important to

know how to evaluate density for process

printing and to put objective numbers on

evaluating the line print areas. Other areas of

concern are various end-use testing require-

ments. Among the tests to be discussed will

be rub and slip for surface inks and bond

determinations for lamination applications.

Commonly used substrates, how they com-

pare, and the advantages and disadvantages

of the various substrates will be covered in

this section as well. While new substrates are

being introduced on a regular basis, it is

important to realize that even though two

substrates may be based on the same chem-

istry, the actual print surface may vary and

the printability of the individual substrates

may be different.

The last section will review the ink chem-

istry and press-application conditions of a rel-

atively new and quickly evolving area in flex-

ographic printing – UV inks.

PRESS CONFIGURATION

Press configuration includes the unwind,

printing, drying and rewind sections of the

press. These sections may all vary somewhat

in structure and composition. As require-

ments for improved quality and more com-

plex print designs are introduced, equip-

ment, materials and configurations are all

being optimized to meet current demands.

Printing sections of a central-impression

(CI) press (Figure

), stack press (Figure

) and in-line press (Figures

6% 6^

) are

basically identical. All but the central-

impression configuration have separate

printing units, each with their own individ-

ual impression roller. The central-impres-

sion configuration utilizes separate printing

stations, but the impression cylinder is

common to all the printing decks. The

structure of the printing section is indepen-

dent of the press design. The amount of ink

applied to a substrate and the manner and

fidelity with which it is applied depends on

the ink-metering process. Ink design is gen-

erally not dependent on press configuration

but is determined by what type of metering

68 FLEXOGRAPHY: PRINCIPLES & PRACTICES

system is being used and the position of the

specific color unit in the press.

INK-METERING SYSTEMS

Ink-metering systems are used to control

the amount of ink transferred from ink reser-

voir to printing plate and subsequently to the

substrate. The ink-distribution system on a

flexographic printing press has three com-

ponents; the ink, the anilox roll and the doc-

toring method. Ink is generally pumped from

the reservoir to the ink pan or doctor-blade

chamber on the press and is picked up by

the cells of the anilox roll through physical

transfer or capillary action. The surface of

the anilox roll is then doctored or wiped

clean so that surface ink film is minimal or

totally removed. Doctoring is performed by

a rotating rubber covered roller or by a doc-

tor blade, so that only the ink in the cells

passes beyond the doctoring nip. The ink in

the cells is then transferred to the printing

plate and on to the substrate.

It is important to understand that the pri-

mary and sole function of the anilox roll is to

meter and control the volume of ink trans-

ferred to the printing plate. The amount or

volume of ink is determined by the number

and size of engraved cells on the surface of

the anilox roll and the method of doctoring

or wiping. Currently, there are three doctor-

ing methods in use today.

Fountain-roll Doctoring

The oldest and most common method used

A typical central

impression press

configuration.

A In Feed Guide

B Nip Roll

C Central Impression Cylinder

D Inter Station Dryer

E Hydraulic Vertical Lock

F Hydraulic Horizontal Lock

G Fine Impression Adjustment

H Impression Indicators

I Metering Roll

J Anilox Roll

K Plate Cylinder

in the industry is known as the two-roll sys-

tem (Figure

). This system uses a rubber or

elastomeric covered cylinder known as the

fountain roll. It is driven by a separate drive

system and rotates at a constant speed, gen-

erally much slower than the anilox roll.

Anilox rollers must be driven at the same sur-

face speed as the plate cylinder in order to

achieve a smooth ink transfer to the printing

plate. This is usually accomplished through a

gear-train arrangement where the impression

cylinder drives the plate cylinder, which in

turn drives the anilox roll, thus ensuring they

all rotate at the same surface speed. Thus, the

anilox roll rotates at a different speed than

the fountain roll. The rotational difference

between these two rollers can vary depend-

ing upon the speed at which the press is being

run. This ratio can range from a low of 3 rota-

tions of the anilox roll to each rotation of the

rubber fountain roll to as high as 10 or 12 to 1.

In the fountain-roll system, ink is pumped

into the ink pan so that the rubber fountain

roll is partially immersed in the ink. In order

to obtain maximum durability of the rollers

and to provide lubrication to this nip, it is

recommended that the fountain roll be

immersed to approximately one-third to

one-half of the fountain roll radius. This

depth of immersion will enhance the func-

tion of the fountain roll to pick up and trans-

fer sufficient ink to the anilox roll.

When the fountain roller is positioned in

contact with the anilox roll, ink picked up by

the surface of the fountain roll is transferred

to the cells of the anilox. The slower rotation

INK 69

A typical stack press

layout.

D E

A Infeed Tension Nip Rolls

B Metering Roll

C Anilox Roll

D Plate Cylinder

E Impression Roll

F Print Station

G Between Station Dryers

To Main Dryer

70 FLEXOGRAPHY: PRINCIPLES & PRACTICES

of the fountain roller provides a wiping

action, thus doctoring an even ink film on

the anilox surface. It is critical that the hard-

ness of the fountain roll covering be com-

patible with the anilox engraving cell count.

If the covering is too soft, when contact is

made, the covering will displace more readi-

ly and too much ink will pass through to

flood the surface of the anilox. When this

happens, press operators must make addi-

tional press-side adjustments at this nip.

Excessive pressure on the anilox roll to

A typical in-line press

layout.

A typical sheet-fed

corrugated press unit.

D D D

A Unwind

B Web Inverter

C Print Units

D Die Cutting

E Waste Removal

F Lamination

G Rewind

H Between-Station Dryers

CCCC

H H H

Slotter

Creaser Print Units Sheet Feeder

fountain roll nip can cause premature wear

to both rollers and in extreme cases the steel

journals may be bent or broken.

Speed-Sensitive Ink Transfer. The two-roll sys-

tem has worked well for many years and has

produced quality graphics, but it has disad-

vantages. One problem is that as print speed

is increased, more ink is passed through the

nip because of the natural hydraulic action

caused by the viscous ink. Excessive ink

transfer to the printing plate requires more

press adjustments to be made. Nip pressure

is controlled at the side frames of the press.

On very wide presses, there could be some

variation of surface ink film thickness at the

center of the anilox roll, when compared to

the surface ink film thickness at the ends of

the anilox. This is due to the hydraulic action

causing a bow, or deflection in the center of

the fountain roll. The use of a hard-rubber

covering can reduce this excess ink transfer

somewhat. Speed sensitivity has been a major

problem with this system and there have been

many creative attempts to solve this.

One way to reduce the deflection factor is

through use of a crown on the surface of the

rubber fountain roll. Crowning requires the

center of the roll to be made larger than the

edges and the roll diameter tapered toward

the ends. Thus, when the press is run at high-

er speeds, there is a more even distribution

of the ink. Another way to solve this problem

is to skew the rubber fountain roll, so that it

contacts the anilox roll at a slightly different

angle to allow a more even ink transfer.

Reverse-angle Doctor Blade

Some flexographic printers added a

reverse-angle doctor blade to their two-roll

system in order to obtain more positive con-

trol over the ink film transfer over a wider

range of operating speeds. When using a

reverse-angle doctor-blade system, where a

rubber fountain roll transfers ink to the

anilox roll, it is important that the fountain

roll is never positioned in contact with the

anilox. A gap of at least 0.002" must be rigid-

ly maintained during the pressrun. This con-

verts the fountain roll to primarily an ink-

feeding roll that floods the surface of the

anilox. The flooded surface allows the doctor

blade to operate more efficiently due to lubri-

cation provided by the excess ink film on the

anilox roll surface. Smaller gaps between

fountain and anilox rolls may not allow a suf-

ficient ink-film thickness to properly lubri-

cate the doctor blade, causing premature

wear to both the blade and anilox roll.

Reverse-angle doctor blades are primarily

used without a rubber-covered fountain roll.

In this system the surface of the anilox roll is

flooded with ink, either by being partially

submerged in the ink fountain (Figure

or through a pumped ink-applicator system.

Many flexo presses have been built with the

single reverse-angle doctor-blade system.

The doctor blade should make contact with

the surface of the anilox roll at a 30° angle to

the tangent point, with a tolerance of ±2°. At

this angle, the doctor blade shears or shaves

the excess ink from the anilox surface, leav-

ing only the ink in the anilox cells for trans-

fer to the printing plate. The reverse-angle

doctor-blade system allows a precise ink-

film thickness to be transferred to the print-

ing plate at a wide range of operating speeds,

without the need for press-side adjustments.

INK 71

In a two-roll metering

system, the anilox roll

rotates at the same

surface speed as the

plate cylinder, while

the fountain roll rotates

at a constant slow

speed. Both rolls,

though, rotate at the

same surface speed.

Anilox

Roll

Metering

Roll