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82 FLEXOGRAPHY: PRINCIPLES & PRACTICES

an important tool for problem solving in the

future. Numerous data – the press settings,

ink conditions, substrate information, and

pressroom conditions – should be placed

into the log. The printing industry contains a

multitude of variables – substrates, types

and ages of presses, drier capability, press

speeds, ink systems, solvent combinations,

time of year and pressroom conditions. It is

impossible to document every single vari-

able for every single job that is run. Even if

possible, going back to this information in

the future would be a problem. There simply

would not be enough time to go through all

these records. The more data accumulated,

the easier it could become to solve a prob-

lem which was not seen on previous runs.

Inks are the one variable that can be easily

controlled as the other variables change. For

example, an ink system might be running

with few to no problems on a daily basis. One

day, the ink doesn’t appear to be drying the

way it should. Is this an ink problem?

Probably not, but the ink is the one variable

in the printing process which can be most

easily altered. After further analyzing the dry-

ing problem, it is apparent that the between-

station dryers are set to lower temperatures

than normal. After turning these dryers up to

their usual temperature, the problem disap-

pears. This is a typical, yet simple, problem

which is seen in the pressroom. If an ade-

quate log book was kept, the problem-solving

time could be kept to a minimum.

The previous example was a simple prob-

lem. Perhaps the between-station dryers

remedy helped, but the problem was not-

completely fixed. After further analyzing, it

is noticed that it is midsummer and the

humidity is at its worst. The alcohol used to

dilute the ink might be hydrophilic, that is, it

likes water, and is sucking moisture right out

of the air. This creates water buildup in the

ink, causing the drying problem. The ink rep-

resentative now provides a new solvent

combination with an alcohol which is

hydrophobic and the problem disappears.

Keeping an accurate log book (See Appen-

dix B), records (See Appendix C,D,E) and

maintaining press settings is a good way to

troubleshoot. The next step is to discuss actu-

al press settings and what they should be.

Dyne Level of Substrates

Affect On Printability. Surface tension is a

condition existing at the free surface of a liq-

uid, resembling the properties of an elastic

skin under tension. Dynes per centimeter is

a measure of surface tension. One dyne is

the force that a milligram exerts under the

influence of gravity. Substrates as well as

inks have a dyne value. A practical example

of what dyne and surface tension is all about

can be seen in the reaction of water on a

waxed surface.

Plain water will bead up on a waxed sur-

face because the surface tension of the

water is greater than that of the wax. If a sur-

factant, such as detergent or alcohol, is

added to the water, it will spread and wet the

wax surface. This is known as wetting out.

In printing, if the ink beads up on the sur-

face of the substrate, there are serious print-

ing problems. The ink must wet out the sub-

strate completely.

The rule of thumb is: in order for the ink to

wet out the substrate, the ink has to have

lower dyne value than the substrate. As a ref-

erence, the dyne value of substrates should

be somewhere between 36 and 42, with 38 to

40 being the norm. Flexo inks can vary but

as long as the dyne value of the ink is less

than 36, the ink will wet out.

Most polymeric-film substrates have dyne

values lower than 36. In this case, the most

widely used method to increase the dyne

level of the substrate is to use an in-line

corona surface treatment. Corona treating

uses electrical charges to oxidize the surface

on the printing side of the stock and raise the

dyne value. The treatment also may burn off

any surface contaminants such as placisti-

cisers that have leached to the surface. The

term burn is used loosely here. The surface

is not really burnt, but pretreated by remov-

ing surface contaminants. The end result is a

high dyne-value substrate. Caution should

be used when corona treating.

When the dyne value of the stock is not

high enough, the most common problem is

pinholing. When the printed material is

examined microscopically, small holes are

visible in the printed area. The worse the

treatment, the worse the pinholing. In severe

cases where the dyne values are well below

that of the ink, the ink may not wet out at all.

Other printability and adhesion problems

can arise when the surface tension of ink to

substrate are not correct.

Dyne Readings And Water-based Inks. Most

solvents have dyne values in the 20s or 30s.

When combined with the resins, pigments

and additives, the finished solvent ink will

usually have a dyne value in the mid-30s.

Therefore the substrate, having a dyne value

in the upper 30s or lower 40s, will properly

wet out. The dyne value of water however is

around 72.8.

The emulsion resins, used to formulate

water-soluble inks, have a low dyne value.

When water is added, the surface tension is

increased. In order to make the water-based

inks have a suitable dyne level, small

amounts of solvent, usually alcohol, or a sur-

factant are added. Surfactant is a general

word for many different chemical additives

on the market. Another word often used in

place of surfactant is wetting agent.

The term “water-based ink,” does not

mean that it is solvent-free. Usually there

will be small amounts of volatiles added to

lower the dyne and help printability. Ink

companies are always searching for “zero

VOC” inks, and zero VOC inks can be made,

but they have poor printability and usually

offer little in resistance properties. The addi-

tion of these chemicals helps the ink to wet

out on the surface of the substrate usually by

lowering the dyne value.

In-line Surface Treating. With the increased

use of water-based inks, it is more common

to see in-line corona discharge surface treat-

ing on flexo presses (Figure

). The print-

ing of various films is greatly improved when

the film is pretreated in-line with printing,

because it improves ink lay and ink adhesion.

Some concerns, however, remain. When

treating film prior to printing, it is possible to

over treat the film. This can cause increased

water sensitivity, as well as an increased ten-

dency to block in the rewind. There is an

adjustment on the corona discharge to set

the amount of treatment and if turned too

high, some substrates could become dam-

aged or distorted. All that is necessary is to

treat the film enough to raise the dyne higher

than the ink so there is good printability. Not

all substrates can be treated. Check with the

substrate supplier before treating.

The gauge of a substrate, or thickness, can

effect the level of corona treatment. If print-

ing with a substrate, for example polyethyl-

ene, and a switch is made to a higher-gauge

polyethylene, the treatment level must be

altered to that of the heavier film to achieve

the same printability. If 38 dynes was accept-

able on the thin-gauge material, it may be

necessary to treat the thicker substrate to a

different level to obtain the same printability.

INK 83

With the increased use

of water-based inks,

it is common to see

in-line corona discharge

surface treating on flexo

presses.

High-Voltage

Source

Treater Electrodes

Insulating Layer

Corona-

Treated

Side

Film

Treater

Roll

84 FLEXOGRAPHY: PRINCIPLES & PRACTICES

Tension Control

A balance is necessary. If the rewind web

tension is too high, the potential exists for

blocking problems. On the other hand, if the

rewind tension is too low when an adhesive

laminated structure is printed, there exists a

potential for tunneling in the print. Other

problems associated with poor tension con-

trol include:

• loss of color to color registration;

• deformation of the web; and

• poor productivity.

Dryers

By design, solvent-based flexo inks dry by

evaporation. Ink is applied in a thin layer on

a substrate and then typically is hit with heat-

ed forced air. Both additional heat and air dry

the ink and dramatically reduce the amount

of solvent that is retained in that layer. As the

industry has matured, the quantity of solvent

retained in the dried ink has become of para-

mount importance to packaging buyers

because it has been linked to objectionable

odors and can degrade the desired function-

al properties. There is a big gap between dry-

ing the ink on a package and a printed pack-

age with low retained solvents. Heated air

without significant velocity will do little to

disrupt this condition and therefore will not

effectively dry the ink. A high-velocity jet of

air, however, has enough energy to force

through the boundary layer and to continue

the drying process effectively.

Dryer manufacturers stress what they call

the three Ts of drying: Time, Temperature,

Turbulence. The effectiveness of an oven can

be measured by the time available for drying.

This of course translates into the oven length

on a web-fed press. The correct use of tem-

perature is essential to accelerate the evapo-

ration of the solvents from the ink. And final-

ly, efficient air turbulence can overcome the

negative influence of the boundary layer.

The drying of water-based inks is very

much the same as the drying of solvent-

based inks, only more difficult. There are

two reasons why water is more difficult to

dry. First, water vapor is typically already

part of the atmosphere. If a previous model

is used, but water-based inks substituted, an

equilibrium state is reached very quickly

because of the presence of atmospheric

moisture, making drying difficult. Again, as

with the solvent-based inks, additional heat

and volume only postpone the inevitable.

The second problem with water-based inks

is the amount of energy required to evapo-

rate the water portion. The amount of ener-

gy required to convert any compound from

its liquid state to its gaseous state is called

the latent heat of vaporization. Water

requires several times more energy to

change its state from a liquid to a gas than

typical flexo organic solvents do. In compar-

ison to ethyl alcohol, water requires three

times the energy to vaporize.

This does not mean that a press running

water-based inks will have to run at one

third the speed of its solvent counterpart.

There are several factors that help improve

the drying performance of water-based inks.

Typically, water-based inks have a higher

percentage of solids. Higher solids translate

to less water to be removed. Second, due to

the nature of the type of resins available in

water systems, a thinner layer of ink is often

employed. In addition, when water is com-

bined in an azeotropic mixture with certain

solvents, the evaporation can be accelerat-

ed. These factors can reduce the dryer

demands. All of these discussions relate to

nonporous substrates. On porous materials,

like paper and paperboard, a percentage of

the water can be absorbed by the substrate

itself reducing the dryer demand. At high

speeds, however, the absorption is reduced,

requiring the dryer to be relied on for thor-

ough and complete drying.

Dryer Temperatures Between Station and

Tunnel. Both stack and central-impression

presses have in-between color dryers and a

final overhead tunnel dryer. Together, these

dryers provide for thorough ink drying. The

between-station dryers are positioned, as the

term would imply, between each deck on the

press. The purpose of these dryers is to dry

the ink film, removing volatiles sufficiently

enough for the next color to be printed on top

of the first. This is known as trapping.

Optimum trapping is achieved when the first-

down color is faster drying than the following

colors. This is particularly important in

process printing. It is not the purpose of the

between-station dryers to dry the ink film

completely. The print passes these dryers in a

split second. There simply is not enough time

for the ink film to be dried completely here.

If the between-station dryers are not set

high enough, the first layer of ink will not be

dry enough to accept the second or third ink.

The result will be poor print quality, along

with the possibility of ink contamination.

For example, if the first ink is a white and

the second is a red, when the red plate

comes in contact with the white ink, some

white ink could transfer to the red plate

since the white ink isn’t dried. The red plate

now carries the white ink to the red anilox.

After some time, the white ink makes its way

into the red ink sump, turning it pink.

If the between-station dryers are set too

high in temperature, the ink surface will dry

almost like a skin or a crust with wet ink

under it. This can create printability prob-

lems such as pock marks or “fisheyes.” Also,

the volatiles which lay under the skin will

not be fully removed when it comes out of

the tunnel dryers, causing retained solvents.

The problem with high amounts of retained

solvents for surface printing is the possibili-

ty of blocking in the rewind. With lamina-

tions, high retained solvents get trapped

between the laminant and printed substrate,

causing poor bond strengths.

The tunnel dryer is usually the large, flat

dryer across the top of the press. There is no

between-station dryer after the last deck on

the press. After the final application of ink,

the substrate goes directly to the tunnel

dryer. Here all the ink films are dried to

remove as many volatiles as time permits.

There will still usually be some small amount

of retained solvents. For all the volatiles to be

removed, the press would have to run slow

enough to allow the printed material to

spend sufficient time in the tunnel. This is

just not feasible.; the press speeds would be

too slow for economical operations. Ink dry-

ing depends not only on air temperature, but

also on negative air flow across the web to be

dried. A balance must be achieved between

air temperature and air flow.

Oven Temperature vs. Web Temperature. Care

must be taken when talking about the tem-

perature of ovens or the temperature it takes

to dry or cure an ink. The temperature of an

oven is usually not the temperature of the

web. It would take a longer period of time for

the web to reach the actual oven temperature

than the press speeds allow. In other words,

if the between-station oven temperature is

200° F, the web passing by in a split second

will reach temperatures substantially lower

than 200° F. However, if the tunnel dryer is

set at 200° F, the web spends considerably

more time in the tunnel dryer. Therefore, the

actual web temperature will be considerably

closer to 200° F than initially passing the

between-station dryers.

Keep in mind some important points. If the

ink representative says that the ink running

requires a 150° F pin-on temperature, he/she

means that the web must reach 150° F. The

ovens will need to be set at considerably

higher temperatures, depending upon other

variables such as press speed. Web tempera-

tures can be best determined by using an

infrared pyrometer. The other point to keep

In mind is how much heat can the substrate

tolerate without being damaged or distorted.

Attempting to run a catalytic coating which

requires 240° F web temperatures to cure,

with a polyethylene’s substrate which can-

INK 85

86 FLEXOGRAPHY: PRINCIPLES & PRACTICES

not tolerate high heat, problems will occur.

The temperature required by the coating

would destroy the film.

Using the same substrate with the dryers

set at 180° F to get good adhesion and low

retained solvent in the ink film, the film may

be able to handle this temperature. Since the

actual web temperature will be significantly

lower, the film can still distort. If the film

begins to distort, the print may appear to be

out of register. The press operator will

respond by trying to get the register in line

mechanically. This problem is film-related

not press-related. It is always important to

be aware of the substrate limitation when

dealing with temperature. Do not always

rely upon the meters on the press, unless

they are checked often for accuracy. If there

is not enough heat on the web, the ink film

will not dry or cure properly. Remember, too

much heat can ruin the substrate.

Also, the dryers should be balanced on a

regular basis. What is done in this process

can depend upon the dryers on the particular

press. When the dryers are properly bal-

anced, the between-station dryers should all

have an equal volume of air blowing through

them. If not, the deck with less air volume

could have trouble drying, while the deck

with more air volume could be running into

skinning problems. Equal amounts of air

should be blowing out along the length of the

dryer – gear side of the press to operator

side. If not, one side of the web may not be

drying efficiently. The air flow also needs to

be directed at the web, not on the plates or

the anilox rolls. If air blows on these rollers,

ink will dry in, causing other problems such

as dirty printing. In a balanced dryer system,

the tunnel dryers need to be accurate in tem-

perature control. The volume and velocity of

air needs to be at its optimum. This is the last

place the ink will have the opportunity to dry

with the aid of heat and forced air. If the dry-

ers are not at maximum efficiency, the end

result could be high retained solvents, block-

ing, or poor bond strengths in the case of

lamination inks.

Conditions of Dryers. Other factors beside

temperature are important to the press,

including the velocity of the air, the volume

of the air and time. The speed at which an ink

dries limits press operating speeds. Ink dries

when the solvents are allowed to evaporate.

Drying can be accelerated by heating the

web, circulating air over the web, or both.

Simply stated, this is what press dryers do.

Drying is also accelerated by using fast-evap-

orating solvents and by lowering ink viscosi-

ty to print a thinner ink film. Porous webs

also speed evaporation because they are

highly absorbent.

To print one color effectively, or many that

don’t touch or overlap one another on the

web, the solvent must be removed and the ink

essentially dry before it reaches the rewind

stand. In designs where inks must overlap or

trap over another (whether for register, large-

area overprinting or halftones) the ink must

dry in sequence as it is printed. Complete dry-

ing is not always necessary between colors,

but sufficient drying must occur to prevent

the subsequent color from rewetting the first

and blending with it or picking it off the web.

Bypassing or skipping vacant decks, when

possible, will give more time for sequential

drying and allow higher operating speeds.

Final drying before rewinding must occur to

prevent roll blocking, ink offsetting on the

back of successive webs or solvent retention

and its residual odors.

Normally, inks are formulated to dry rapid-

ly enough to allow proper sequential drying.

When inks have been reused, circulated

excessively, or adulterated with improper

additives and allowed to get out of chemical

balance, drying problems occur that affect

operating speeds. Depending on the area

size of overprint or trap and the ink film

thickness, many open porous substrates can

be printed at speed without using dryers.

Other less or nonabsorbent webs require

combinations of air, heat, between-color dry-

ers, overhead tunnel dryers and the addition

of faster-drying solvents.

If between-color dryers are not available

or not functioning properly, sequential dry-

ing can be achieved by adding faster sol-

vents in first-down colors, unchanged inks in

the middle station and slower solvents in the

last-down colors. Dryers can become inef-

fective if partially clogged with ink, dirt or

web fragments. Air-duct dampers which

have been changed from the factory settings

can give too much or too little air. The bal-

ance of in-feed and exhaust air must be cor-

rectly maintained to draw away and exhaust

the solvent-laden air.

If too much air is blowing out of the

between-station dryers, there is a possibility

of air blowing onto the plates or the anilox

rolls. When this happens, the ink will begin

to dry on the plates or in the cells of the

anilox. When the ink dries on the plates,

dirty printing will be the result. When the ink

dries in the cells of the anilox, the cell vol-

ume will begin to diminish, causing less

transfer of ink and a loss of color strength.

“Mottled” printing can result if the ink is dry-

ing in some areas more than others. The air

velocity correlates closely with the air vol-

ume. Usually, the higher the velocity, the

higher the volume. Velocity is particularly

important during the hot and humid summer

months, when many printers have drying

problem. The key here is to have as high a

velocity as possible to blow the humidity

away from the web. If the velocity is low, the

humidity can remain over the web. This

would be like trying to dry the clothes out-

side in August when the humidity is 80%. As

expected, things dry much more quickly

under lower-humidity conditions. Higher

velocity will help to remove the humidity

away from the web making it easier to dry.

Drying On Absorbent Substrates. Water-based

inks for absorbent surfaces depend on sever-

al factors for drying: evaporation, penetra-

tion, precipitation and chemical action.

Evaporative drying occurs through the

action of air and heat. Heated air is passed

over the surface of the print and removes the

volatile components from the ink. Penetra-

tion drying occurs on absorbent substrates.

The ink is drawn into the surface, often by

capillary action, and the print can no longer

be readily smudged or transferred to another

surface. On suitable papers, this drying

method can be quick (0.1 second). During ink

absorption, a fractionation, or layering, effect

occurs where certain parts of the ink are

preferentially absorbed, leading to a precipi-

tation toward the ink surface. The acidity of

some papers acts as a drying agent for the ink

by neutralizing the solubilizing amines.

The term “absorbent” for substrates

includes many varieties of paper and paper-

board stocks. It varies from lightweight tis-

sue to corrugated board to glassine to

papers with coatings based on many natural

and synthetic binders. The more absorbent

the substrate, the less need there is for effi-

cient dryers. Water-based corrugated inks

run at high speeds without any form of dry-

ers. At the other end of the scale, glassine

and some coated papers behave more like

films and do require good drying.

Chemical drying occurs with paper towel

and tissue inks formulated to benefit from cel-

lulose chemistry. Water-based dyes or pig-

ments and resins capable of reacting chemi-

cally with the cellulose fibers are used in these

inks. Once this reaction has taken place, the

printed product can resist many household

chemicals. Other considerations when printing

water-based inks on absorbent papers include:

• the tendency of water inks to curl or

pucker paper;

• the problem of ink buildup on central-

impression (CI) cylinder drums, caused

by excessive ink penetrations through

the substrate;

• catalytic lacquers will not always cure

over water-based inks; and

INK 87

88 FLEXOGRAPHY: PRINCIPLES & PRACTICES

• ink properties depend on stock ab-

sorbency; lower absorbency often lowers

heat resistance, scuff resistance and

product resistance, but increases gloss.

Press Speeds

Another variable which is vital to good

printing is the press speed. Most printers

want to run as fast as possible, which is

understandable. However, the goal is to run

as fast as possible without sacrificing quali-

ty. Many inks do not run the same on differ-

ent presses, and the same ink on the same

press will run differently at different times of

the year. Press settings have to correlate the

capabilities of the press and the effect of the

other variables already mentioned. The rule

of thumb is to run the press as fast as possi-

ble, yet continue to achieve the necessary

drying and printability. Every press will be

different. Sometimes the same ink can be

run in two different presses in the same

pressroom on the same substrate, yet the

speeds will differ. This speed difference

largely has to do with the capabilities of the

dryers. This again is where a log book can

help determine what the press speeds

should be based on past history.

Press speeds are often limited by mechan-

ical effects. However, press speeds can also

be altered by the ink-solvent blends and the

reducer blends. The difference between

these two blends is that the ink-solvent

blend consists of the types and amounts of

solvents present in the virgin ink when pur-

chased. The reducer blends are the solvents

used to reduce the virgin inks to press vis-

cosity. These two blends can be quite differ-

ent. The ink-solvent blend is the composi-

tion of solvents in the ink when manufac-

tured. This could be a large variety, maybe as

many as five or six different solvents. The

purposes of all these different solvents is to

control drying speed and to keep the resin in

solution. The printer does not need to know

all the different solvents in a particular ink,

but should know that sometimes a solvent

will evaporate out of the ink. This usually

happens over long runs or if the lids are left

off the sumps. When these solvents are lost

to evaporation over time, the ink may not

behave as it should. Therefore, a make-up

solvent may be required to keep the resin in

solution and keep print quality at its original

high level. These make-up solvents are usu-

ally faster drying solvents as they are the

first to evaporate, such as heptane. The ink

representative should determine if a make-

up solvent is needed and what it should be.

The reducer blend is usually one or two

solvents, sometimes three, that reduce the

virgin inks to press viscosity. This is usually

done in the pressroom. In flexo, the staple

solvent would be some type of alcohol in

larger amounts and usually a small amount

of ester such as normal propyl acetate. This

is for solvent-based inks only and other sol-

vents could be used. If solvent changes are

required for different ink systems or at dif-

ferent times of the year, these alterations

should be noted in a log book, as well as any

press settings that need to be altered, along

with the other changes.

Rewind Tension

The basic requirement of a good rewind-

tension system is to wind rolls with straight

edges and uniform density, while preserving

the accuracy of register and repeat length. It

is not the purpose of this book to discuss the

different types of rewinders or the mechanics

of them. From an ink standpoint, however, fit

is important to remember that the ink is being

sandwiched between the substrate in the

rewind. The more tension used in the rewind,

the higher the possibility of ink blocking to

the backside of the substrate. This is espe-

cially true if the drying capabilities of the

press are not as good as should be.

Another area of concern is rewind tension

when printing lamination inks for future lam-

ination. In this case, it is not in-line laminat-

ing. The concern here is that lamination inks

have very little or zero waxes in them. The

absence of waxes makes an ink more prone

to block; yet, their presence can often hinder

bond strengths of laminations. Therefore,

waxes are used sparingly in lamination inks

so bond strengths are not affected. Lamina-

tion inks are extensively tested for this, but

these conditions exist and excessive rewind

tension could contribute to ink blocking.

Chill Rollers

After the printed material leaves the tunnel

dryer, the substrate is hot. Before it can be

rewound onto a roll, it must be cooled. This

is usually done with the aid of a chill-roller,

which has water or brine being flushed

through the center. The printed substrate

gets cooled as it passes over the roller, so

that it is as close to ambient temperature as

possible when rewound. This is primarily

done so that the substrate does not expand

or contract after being rewound. This could

cause unwanted pressure in the rewind

resulting in blocking or damaged material. If

the temperature difference between the web

and the chill-rollers is too great, condensa-

tion can occur. Water droplets on the printed

material makes the substrate wet while the

ink film is dried but not set in the rewind.

Often an ink will require some time for

complete setting after being printed, some-

times as much as 24 hours. During this set-

ting time, additives in the ink such as waxes

will bloom to the surface. This is why ink

adhesion often gets better after aging. The

water from the chill-roll condensation can

affect the fresh ink film before it has a

chance to set. This could result in blocking,

poor adhesion or damage to the print.

Drying of Catalyzed Inks

Catalyzed inks – also known as two part

systems – have characteristics which cannot

be achieved with conventional inks, such as

high gloss or superior chemical resistance.

The ink is delivered to the printer along with

a catalyst. At press-side, the ink handlers

add the catalyst to the ink in the recom-

mended amounts. Catalyzed inks ususally

require a substantially higher amount of heat

to dry. If insufficient heat is used and the

print is not thoroughly dried, the ink film will

remain tacky and block. Even if blocking

doesn’t occur, the ink may not have all the

final characteristics needed. For this reason,

it is vital when using catalytic inks that the

proper amount of heat be used.

For these same reasons, it is also impor-

tant that the proper amount of catalyst be

used. Catalyzed inks are formulated and

tested using a specific amount of catalyst.

Too much or not enough could result in any

number of problems such as blocking, poor

gloss, or poor chemical resistance due to

improper curing. The reason for the catalyst

to be added at press-side, rather than during

manufacturing, is because the catalyst

reacts with the ink. This reaction is time-lim-

ited. In other words, once catalyzed, the ink

has a limited life and has to be used usually

within 24 hours. For best results, it should

be used immediately. The stability of the ink

after 24 hours is poor, though the time it

takes for this instability to appear will vary

depending upon the system. The inks will

get heavier in viscosity, sometimes almost

turning gelatinous. Some of these cross-

linked systems utilize a catalyst which dissi-

pates over 24 hours. At that time, more of

the catalyst needs to be added. These inks

can usually be cross-linked only twice

before the ink needs to be discarded.

This is one of the main reasons why print-

ers do not like to use catalytic inks – whatev-

er isn’t used in 24 hours has to be discarded.

There is no way to salvage the ink. This can

lead to large ink costs if the amount of ink in

the press is not limited. Also, the cost to dis-

INK 89

Catalyst, cross-linker and curing agents are synonomous terms.

90 FLEXOGRAPHY: PRINCIPLES & PRACTICES

pose of waste ink can be substantial. Another

reason curing inks are not popular is because

of the risk of using improper amounts of cat-

alyst. Press-side testing can be done to check

for the right amount of catalyst; however,

errors can be made. With conventional inks,

there isn’t a question about the proper

amount of catalyst. Finally, some of the cata-

lysts used are hazardous chemicals, and they

must be handled carefully.

INK VISCOSITY

The viscosity of an ink will affect many

aspects of printability including print

strength, print sharpness, ink lay and color.

Viscosity is one of the easiest variables to

change on a press, and it is the variable that

has the most significant effect on the result-

ing print. Ink viscosity should be checked at

least once an hour and more frequent checks

are generally recommended by the ink sup-

pliers. Dot sharpness in process printing, or

clean printing of fine-type edges when line

printing, are both greatly influenced by ink

viscosity. If the viscosity is too low, the ink

will often show dot growth causing the

image to lose its sharpness and print dirty. It

is very easy to reduce or increase the print

strength by slight adjustments in the print

viscosity. Because of this, viscosity is often

the first thing changed when dealing with

print-strength adjustments. If viscosity

adjustments do not meet the requirements

for print strength, anilox changes are usual-

ly the next step. In water-based inks, howev-

er, the opposite should be done. The correct

anilox is critical and is selected first, and

subsequent viscosity changes are small.

The lay of an ink can be affected by vis-

cosity. If an ink viscosity is too low, the ink

may crawl on the substrate before it dries.

Crawling will result in a print of inconsistent

ink thickness and smoothness. Crawling is

more apparent in dark colors than with pas-

tels or lighter shades of pigments. If crawling

or mottle is seen, the ink viscosity should be

increased, or a pigmented extender should

be added. Ink run with viscosity too high can

also show inconsistent lay and dirty printing.

This is typically the result of ink caking on

the plates or ink not transferring properly to

the substrate. In process printing, it is very

important that the proper viscosity be deter-

mined for an ink before any density adjust-

ments are made. Once this viscosity is iden-

tified, a balanced extender should be added

to meet specific density specifications. The

influence of viscosity on color should be

noted. Small viscosity changes can also pro-

duce shade changes in a print. A red color

may become more yellow as it gets higher in

viscosity and bluer as it goes lower.

Thus, when printing problems occur, a

holistic approach must be used to identify

the proper corrective actions. When a color

is not acceptable, it is imperative to deter-

mine whether toner should be added or a vis-

cosity adjustment made – all aspects of the

printing process must be considered before

making this determination. Although viscosi-

ty changes may be the quickest approach, the

consequences of these changes must be

reviewed.

First, a note concerning water-based inks.

Viscosity can be related to pH in water-borne

inks. It is critical that inks be adjusted for pH

prior to any adjustment for viscosity. If this is

not done, the addition of a water reducer can

cause the inks to become over-reduced.

Excessive viscosity reduction of water inks

can cause many problems including a weak

color, poor lay, poor drying, offsetting and

poor lamination bonds. Rather, small

amounts of amine to adjust pH may result in

better rheology and lower viscosity. These

factors support the importance placed on

automatic viscometers and viscosity instru-

ment calibration.

Both water-based and solvent-based inks

can have a tendency to be thixotropic.

Thixotropy is a tendency of a liquid to show

a large drop in viscosity when agitated.

Therefore, inks should always be well mixed

and pumped in the ink system before viscos-

ity readings are taken. In addition to mixing

an ink, temperature is a concern when

checking viscosity. The viscosity of liquids is

affected by temperature. This is easy to

understand when one considers the com-

mon example of motor oil. When it is cool,

the oil is much more viscous than when it is

warm. An ink behaves in the same way. If the

viscosity is measured when the ink is rela-

tively cool, higher readings are obtained

than when the ink temperature increases on

press with shear and agitation.

Methods of Measurement

Regardless of the methods used for vis-

cosity measurement, it is important that the

ink be agitated prior to checking viscosity.

This agitation reflects how the ink will

respond while running on the press. Many

inks may exhibit some degree of thixotropy.

If inks are adjusted before agitation, they

may be too low in viscosity once put in the

press and agitated. It is also critical that cal-

ibration of viscosity measuring equipment

be done on a regular basis. Since tempera-

ture will have an effect on viscosity, it is

important that measurements be made at

specific ink temperatures.

Zahn Cup. The most common method of

press-side viscosity determination in flexo is

with a Zahn cup (Figure

). A Zahn cup is a

metal cup of predetermined volume with a

specific size hole on the bottom. The ink’s

viscosity is the amount of time it takes for a

full cup to empty. Zahn cups come in various

numbers. Flexo application ranges are typi-

cally from 2 to 5. The higher the number, the

more viscous the material it can handle.

Application on press in flexo is most com-

monly measured with either a #2 or #3 Zahn

cup. Ink viscosity determines which cup to

use. Readings considered accurate are be-

tween 20–40 seconds on any specific cup. If

the reading is higher or lower than the 20

and 40 second range, a higher number or

lower number cup should be used.

Shell Cup. The Shell Cup (Figure

) is

another type of metal cup used to determine

ink viscosity. Unlike the Zahn, the Shell has

a narrow tube on the bottom of the cup for

the ink to flow through. It is more common-

ly used in gravure applications than in flexo.

However, because it is more accurate than a

Zahn, some printers have moved to the Shell

for on-press ink viscosity checks. Because of

the narrow tube on the bottom of the cup,

care must be taken to be sure the tube is

clean. Many individuals use a pipe cleaner

inserted through the tube to be sure all ink is

removed after taking a viscosity reading.

See Appendix C for conversion from Zahn

cup readings to Shell cup readings.

Viscometers. Press units are often equipped

with automatic viscometers to maintain a

specific viscosity while a job is being run.

This avoids constant measurement by an

individual and results in improved consisten-

cy of ink viscosity. The automatic viscometer

is connected to a make-up solvent-blend,

which is added to the ink as needed. In addi-

tion to Zahn and Shell cups, there are several

other types of instruments available to mea-

sure viscosity, but these are generally limited

to lab environments.

INK 91

The most common

method of press-side

viscosity determination

in flexo is with a Zahn

cup.