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

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102 FLEXOGRAPHY: PRINCIPLES & PRACTICES

print defects, like dot bridging caused by

dried ink on the printing plate, are all but

eliminated. The inherently higher viscosity

of UV flexo inks aids in reducing dot gain in

process printing and vignettes. Print quality

and consistency is also aided by the lack of

viscosity changes during the press run. UV-

curing equipment is not cheap, but when

examining the comparative cost versus con-

ventional ovens on new presses, the differ-

ences are small.

Two issues seem to concern people the

most about UV inks. First is the question of

safe handling and usage. All inks are chemi-

cals and as such need to be handled with

care. They can’t be eaten or worn, and they

don’t belong in the eyes. Good hygiene is

important when handling any ink. If treated

with the care and healthy respect that chem-

icals deserve, then inks will be able to do the

job they were intended for. The other con-

cern people have expressed or want assur-

ances on is on how to confirm that the UV

ink is cured. There are several cure tests that

can be performed press-side, as well as more

detailed, time consuming tests such as ana-

lytical measurements, which are correlated

to the more simple press-side tests.

ENERGY-CURED PRODUCTS

The following program is recommended

for the safe use of energy-cured formula-

tions and should be applied in the areas of

ink or coating handling, mixing, and cleaning

of equipment. The necessary items should

be readily available to the working area.

Precautions for handling energy-cured inks

and coatings are as follows:

1.Minimize exposure to UV/EB materials.

2. The use of barrier creams for the hands

as a preventive measure is recommend-

ed for those workers who may handle

the products in common, short-term

exposure situations.

3.Glove protection is recommended for

personnel where continuous or long-

term exposure is expected and can not

be avoided. This is particularly true dur-

ing wash-up procedures where solvents

may be in use.

4.It is recommended that some form of

eye protection be available to press-

room personnel and be used around the

pressroom during the working day.

Glasses act as a first-line defense

against accidentally introducing ink,

coating, or wash solvents directly into

the eye. They also reduce the chances

of rubbing the eyes with contaminated

hands. Should ink or coating be acci-

dentally introduced into the eye, flush

with water for at least five minutes and

follow in-plant first aid procedures.

Consult a physician.

5.It is recommended that all personnel

adopt the practice of cleaning ink or

coating off their skin with soap and

water, and not with solvent, which is

the more common practice. Solvent

cleans quicker, but it removes the nat-

ural fats and oils of the skin and may aid

the penetration of the ink and coatings

into the deeper layers of the skin. This

can intensify irritation problems, rather

than helping prevent them.

6.It is recommended that personnel

cleaning large spills of energy-cured

inks or coatings use gloves. In addition,

used wipers from any clean-up should

be placed in a separate container so

that the wiper does not become a

source of additional contamination.

Solvents may be used with care in

cleaning spills on floors and equipment.

7.Keep ink and coating-handling equip-

ment clean and keep used wipers in a

receptacle. This cuts down on inciden-

tal contact to other workers. Keep ink

and coating containers closed.

8.Discourage the practice of eating on the

job or in the work areas and encourage

in every way possible good personal

hygiene.

PROCESS PRINTING

Process color printing is the technique

used to reproduce a subject using yellow,

magenta, and cyan colors, plus black (for

improved contrast). The process is similar

for all printing methods, but varies accord-

ing to ink-metering techniques.

High quality process printing by flexogra-

phy depends on control of the variables in

the process:

Ink Distribution. The evolution in the ink dis-

tribution and metering systems for process

printing has seen a move from the two-roll to

bladed systems, using approximately a 440 to

550 line, 4.0 BCM volume (billion cubic

microns per square inch) chrome anilox, to

ceramic anilox rolls whose line count may be

anywhere from 550 to as high as 1,500 lines

with a cell volume as low as 0.9 BCM. The

improvement in flexo process printing in

recent years can be traced to this evolution in

ink metering and the concurrent change in

flexo plates and inks, as well as printing skill

and techniques.

A typical process cylinder today is 600 to

700 lines, quite possibly at a cell volume as

low as 1.3 BCM. Work is being done at higher

lineage, but this can be difficult. With higher

line count, the cell and cell volumes become

much smaller and ink makers have a difficult

time achieving color strength and staying

open with the very thin ink films. Lower line

counts and higher volumes can be used, but

print quality will suffer as high dot gain and

bridging occur. Problems of this type can

result from too much ink being sent to the

plate from the anilox.

Printing Inks. The new inking systems for

process printing continue to require a con-

stant, easily controllable viscosity. This gen-

erally means Newtonian flow, but also

means that at application viscosity, when

transferring from anilox to plate, the ink

should be relatively short in flow to prevent

bridging and dirty print. Process printing vis-

cosities with bladed metering are 26–29 sec-

onds/#2 Zahn cup. For a given ink, if at 26

seconds the strength is too high, add exten-

der varnish (and often a pigmented exten-

der) to adjust color. Solvent additions will

make the ink too sloppy and cause dot gain

and dirty print.

Applied ink density is a function of the ink

metering system, the ink and its viscosity,

and the operator working together. Most

printers do set up their own color-density

targets, which depend on their equipment,

color separations, and experience. FIRST

(Flexographic Image Reproduction Specifi-

cations & Tolerances) recommended density

targets are shown in Table 17.

Most presses are set up so that colors trap

from light to dark – yellow to magenta to cyan

to black. The yellow may or may not be trans-

parent. All subsequent colors must be highly

transparent, so that light will pass easily

through all ink layers. This sequence may be

true for surface print. Reverse print, on clear

film, is just the opposite, although the basic

principle is the same. As we go to higher inten-

sity ink for thinner ink films, more trans-

parency is critical to obtaining true color and

high contrast. The sequence of inks may

change with different inks and applications.

The key is to remain consistent for a given

process and not change the sequence from

one job to another.

The highest ink viscosity that can be run to

achieve the desired density without dirty

print is optimum. The viscosities given above

are averages. These are several benefits from

higher viscosity printing inks:

• dot gain is minimized;

• printing is sharper, cleaner;

• bridging is reduced; and

• density and contrast values are

improved.

INK 103

104 FLEXOGRAPHY: PRINCIPLES & PRACTICES

Unfortunately, viscosity cannot be in-

creased infinitely. There is a best range.

When it gets too high, dirty printing and off-

setting will be problems. Also, at high vis-

cosities, ink drying can be more difficult to

control and transfer may lag due to ink dry-

ing in the cells of the anilox.

Ink drying in process application is quite

complex. There are three distinct relation-

ships to consider:

• Ink-film Thickness and Drying

• Ink Transfer and Drying

• Ink Viscosity and Drying

The first impression would seem to be: use

the slowest solvent possible and all problems

will be solved. This is not the case. There are

many different slow solvents, and they do

not all have the same effect on ink-film thick-

ness, ink viscosity, and transfer. Some slow

solvents will cause ink film to swell, and ink

viscosity to increase – both these effects tend

to reduce ink intensity and make the job look

weak. Some solvents reduce ink tack; this

also reduces strength and may cause dirty

print. Slow solvents also can cause odor

problems if used to excess. The solution is to

use small amounts (5%–10%) of very slow sol-

vents rather than large amounts of slower

solvents,. Blanket suggestions cannot be

made. The solvents used should be recom-

mended by the ink supplier.

Press Speed. Press speed and the drying

speed of the inks are directly related. In the

earlier presses, color intensity and press

speed were directly related. As press speed

increased, color strength of the applied ink

also increased. The advent of bladed presses

eliminated this problem and today, ink

strength is generally not dependent on press

speed. In chambered-blade systems, if ink

flow into the chamber does not keep up with

press speed, ink starvation can occur, result-

ing in a weak looking print. In addition to con-

cern with the ink metering, a press set-up at

300 fpm may require resetting of impressions,

when higher running speeds (say 900 fpm)

are used, in order to maintain color densities.

Plates. Good process printing depends on

achieving a light kiss impression between the

anilox and the plate. Too much impression

can show up as weakness, fast drying, and

dirty print. A kiss impression must be main-

tained between plate and substrate as well, or

the dots printed will be misshapen and the job

will look weak and dirty as ink piles up in the

wrong places on the printing plate.

Substrates. Another variable that must be

dealt with in process printing is the substrate.

Due to the transparent nature of process

inks, the finished appearance and colors of a

given job are highly dependent on the color

and smoothness of the substrate. Any color

in the substrate will appear in the printed ink

area. The color must be adjusted by means of

the separations or by using a first-down white

to hide the substrate color. Poor substrate

smoothness, or anything on the substrate

that will negatively affect ink transfer, will

create problems and sometimes these can be

cured using a first-down varnish or white.

Higher ink viscosities can sometimes help,

but often these cause other problems.

Press Characterization

To determine how each of the print vari-

ables come together to create a finished

halftone color process print, a press charac-

terization or “fingerprint test” of the printing

process is carried out by the actual press

Table 17

PAPER FILM

■ CYAN 1.28–1.42 1.18–1.32

■ MAGENTA 1.18–1.32 1.13–1.27

■ YELLOW 0.95–1.05 0.95–1.05

■ BLACK 1.43–1.57 1.33–1.47

* Flexographic Image Reproduction Specifications and

Tolerances

FIRST* SOLID INK DENSITY

crew. The test should reflect actual day-to-

day operating conditions and optimum press

performance. FIRST contains specific guide-

lines as to how the press characterization

process should be conducted, including a

test target (Figure

). During the character-

ization, the printing parameters for the indi-

vidual press conditions will be established.

Controlling the Print. Using a densitometer,

the solid ink densities should be checked.

Color strength can be adjusted by adding

extender (to lower density) or toner (to raise

density) to bring color density to within

specificatons. The printed sheet should be

inspected carefully. Any obvious print prob-

lems (slur, uneven print, over-impression,

moiré, etc.) should be corrected at this

stage. All pertinent data should be recorded,

including:.

• plate type and thickness;

• viscosity of ink;

• density of ink;

• Anilox cell count and volume;

• impression settings;

• blade pressure; and

• press speed.

Interpretation of Print Results. There are sev-

eral ways to interpret the printed result:

• visually;

• microscopically (microscope with

scale);

• densitiometrically (color densitometer);

and

• spectrophotometrically (spectropho-

tometer).

The human eye is a very sensitive color

measurement device. Unfortunately, visual

sensitivity is based on comparisons of color

hue shifts or densities over time, which the

brain has no way of accurately recording.

The modern reflection densitometer is an

invaluable tool for the process printer. It

allows communication in numerical values by

simplifying the process of establishing refer-

ence standards for predictable results.

Making halftone process printing an every-

day reality is greatly aided when the human

element of color evaluation is reduced. As

subsequent density readings are taken, shifts

in hues or densities may be noticed and cor-

rections can be made in the ink or press set-

tings to reestablish the target readings.

Using a spectrophotometer enables pro-

cess printing to be controlled using color

measurements as the human eye perceives

color. More details are covered in the Process

Printing chapter.

The following observations and records

should be taken every time a halftone

process color job is printed.

Printing Quality. First, the print evaluator

should visually examine the print and the

printed target to make sure it is the best

achievable result. The targets should be

sharp, free of unwanted printing between

dots and lines, and in tight register. There

should be no dot deformation such as, slur-

ring, moire, or gear marks. Using a densito-

meter, solid ink densities should be mea-

sured to assure they meet specifications.

Dot Gain and Tonal Range. Selected screen val-

ues for each color are read with a densito-

meter. These readings may be converted to

dot percentages using the Murray-Davis for-

mula built into the microprocessor of the

densitometer. These readings provide impor-

tant information on dot growth and tonal

reproduction.

Gray Balance. Gray balance targets are three-

color overprints of cyan, magenta and yel-

low, balanced to appear neutral or gray. They

are typically placed next to a patch of black

with the same visual tone value. This pro-

vides for an extremely sensitive visual con-

trol on the printing process since the eye is

very sensitive to slight shifts in color from a

neutral gray. The gray balance targets are

typically composed of shadow, midtones

and quarter-tones. Gray balance targets can

also be measured using either a densitome-

INK 105

106 FLEXOGRAPHY: PRINCIPLES & PRACTICES

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ter or spectrophotometer and the values

compared against the values obtained at the

press characterization.

For high quality process color reproduc-

tion, the information from the press charac-

terization or fingerprint test, should be used

to produce repeatable results. The separa-

tions developed from the test will work best

only for the printing conditions established

during the test. Ink densities, viscosities,

metering, and the other general press condi-

tions must be maintained during production.

PRESS APPROVALS

When a job is printed, there are several

properties that must be checked on a con-

tinuous basis. These include print quality,

ink adhesion, color, strength, print register,

etc. The following sections describe some of

the more common properties that should be

checked press side during a run.

Print Quality

When a job is printed, the overall print

quality must be checked. This would include

sharpness of fine type and reverse-outs, ink

lay, ink gloss–and in process printing, overall

dot structure. Along with print quality, it

should also be noted that all colors are being

printed in the proper trap sequence. This

would also include ensuring that the correct

colors are in each of the printing decks. A

job should also be inspected to ensure that

there are no spelling errors, missed words or

logos, and that all the colors, primers, and/or

overprints are being printed. On more than

one occasion, jobs have been printed where

a color was accidentally left off the job.

Details of the following tests were covered

in other parts of this volume.

Ink Adhesion

Ink adhesion is perhaps the most common

press-side test done. It is also one of the

most powerful diagnostic tools. An ink-adhe-

sion test should be performed at the begin-

ning of each roll of printed material. Poor

tape adhesion can be the result of any of the

following:

• Wrong side of the film was printed.

• The wrong film is in the press.

• The film has insufficient treatment to

allow acceptable ink adhesion.

• Dryer temperatures are insufficient.

• The wrong ink is in the press.

Ink Color

When printed colors, including white are

printed and evaluated, the hue, chroma, and

lightness should be measured using a spec-

trophotometer. As well as a visual assess-

ment, an objective color measurement

should be performed on all colors of a job to

ensure consistency and acceptability to a

standard. Spectrophotometers are used to

measure the amount of light reflection in the

visible color spectrum. These spectral

curves are much like fingerprints of a color.

They provide a numeric representation that

eliminates the subjective factors associated

with visual assessment. It is possible to have

a metameric color match when the two col-

ors being compared use different pigments.

A metameric match occurs when two colors

match under one set of lighting conditions

but not under another. For example, the col-

ors can match when viewed in a light booth

under standard light (D50) but will not

match under incandescent lighting. The

measurement of color by a spectrophotome-

ter allows for identification of metameric

color matches by reading color under vari-

ous lighting conditions.

Ink Strength or Opacity

The transparency or opaqueness of a color

should also be compared to a standard. The

level of transparency becomes important

when printing on nonopaque substrates,

over high gloss or metallic substrates, and in

INK 107

(opposite page)

FTA Press

Characterization Target.

108 FLEXOGRAPHY: PRINCIPLES & PRACTICES

resulting colors from multiple ink traps.

Opacity is commonly assessed visually. The

level of opacity in a white ink can be deter-

mined objectively by the use of an opacime-

ter. The level of strength that an ink print

exhibits can also affect its color. Certain col-

ors will actually appear a different shade as

the color becomes weak or strong vs. a stan-

dard. An example of this would be a Rubine-

based color match (Figure

). As the ink

strength becomes weaker, the color appears

to be more yellow. Color strength is often

assessed visually and subjectively. A densit-

ometer or spectrophotometer can also be

used to monitor the strength of the ink. A

weaker ink will have a lower density and dif-

ferent, lighter color.

Before color adjustments are made press

side, correct strength should be obtained.

This will avoid the possible addition of col-

ors that can make the match difficult to con-

trol and can result in metamerism.

Scratch Test

A quick test for scratch resistance during

start-up and press run can identify some

potential problems easily and quickly. If an

ink contains wax additives and the container

of ink was not mixed properly, it is possible

the scratch resistance will be poor. The test

is performed by taking the back of the fin-

gernail and dragging it across the surface of

the print. The scratch resistance comparison

vs. standard is necessary, due to the highly

subjective method involved (Figure

If ink is removed vs. standard, the situation

should be investigated.

Print Register

When running more than one color on a

job, each color must be printed in close posi-

tional relationship to the other colors on the

job. This relationship is called register. If the

press causes the substrate to stretch beyond

acceptable limits or oven temperatures are

too high and cause the substrate to shrink,

of these problems are usually correctable by

press adjustments.

A job can also be out of register if the

mounting of the plates on the plate cylinder is

not done properly. If the register at the left

lead edge of the print is in register and the

right trail edge is out of register, it is likely

that one of the plates may be mounted incor-

rectly. When a job is out of register due to

plate-mounting problems, the plates will need

to be remounted to correct the problem.

Ink Gloss

Gloss is a measure of the degree of

reflectance on the surface of the print

(Figure

). The gloss level of a print is most

commonly measured visually versus a stan-

dard. There is a move toward the use of a

glossmeter to eliminate the subjective visual

interpretation and to allow for numeric

determination of gloss. If the gloss level is

unacceptable, it may be an indication that

the ink printed has not been mixed properly.

This can cause an inconsistent level of slip

additives in the ink container. The higher

level of these additives can decrease gloss.

Ink Crinkle

On flexible packaging materials, such as

polyethylene, polypropylene, polyester, etc.,

Strong vs. weak ink test.

Color strength is

often assessed visually

and subjectively. A

densitometer or

spectrophotometer can

also be used to monitor

the strength of the ink.

Strong Weak

inks must also have acceptable flexibility.

Poor flexibility on these substrates can

result in ink flaking off the package as it is

handled. Prints are most commonly checked

for flexibility by putting about an inch of

print material between the thumb and fore-

finger of each hand and quickly rotating the

prints several times. This is a very subjective

test. The number of times the prints are

rotated and the degree of force in that rota-

tion will affect the results. It is therefore ben-

eficial to do this test with a known accept-

able print and compare those results to the

production print (Figure

). There are also

instruments available for determining the

degree of ink flexibility. These instruments

provide a much more consistent method of

testing to allow for greater repeatability.

Lamination Green Bonds

When a job is laminated, it is important

that bond levels be checked off the press to

ensure that the process is in control. When

adhesive lamination is done, bonds typically

improve over time as the adhesive cures.

Bond levels are determined by taking a 1-

inch wide strip of laminated material and

measuring the force needed to split the two

laminated films apart. It is also important to

note where the failure of the bond occurs.

The point of delamination can indicate if the

ink, adhesive, film, or application method is

the cause of low bonds. The ultimate goal is

to obtain stock destruction.

Coefficient of Friction

Certain packages require a specific level of

slip on the outside and often the inside of the

package. The inside slip level is important

for high speed filling operations, where the

product must go into the package. An

inclined plane or an AGR are two common

instruments used to measure the Coefficient

Of Friction of a surface. Tests are usually

done at the start of a production run and

checked at regular intervals during the run

to be sure the process is in control.

Rub

When rub resistance is required, it is com-

mon to test the rub resistance of the print

either against itself or against another surface

the print will be in contact with (Figure

The test is carried out using a Sutherland rub

tester as described in Chapter 1. This test is

performed with a certain amount of weight

applied to the print for a set number of

strokes/rubs in contact with either the print

or the expected contact surface. In addition

to the Sutherland rub tester, there are other

instruments that can be used to test rub resis-

tance, depending on the shipping and storage

conditions a package will be exposed to. It is

not unusual for optimum rub resistance prop-

erties to develop over several hours.

INK 109

Good scratch vs. bad

scratch test. If an ink

contains wax additives

and the container of ink

was not mixed properly,

it is possible the scratch

resistance will be poor.

Flat vs. gloss ink.

If the gloss level is

unacceptable, it may be

an indication that the

ink printed has not been

mixed properly.

Good Bad

Flat means stale

Gloss means fresh

110 FLEXOGRAPHY: PRINCIPLES & PRACTICES

Water Resistance

If a print is expected to be in contact with

water, the inks must be designed to resist any

bleeding or removal during this exposure.

This is especially important for water inks;

however, solvent inks may be of concern

also. The degree that an ink will bleed in

water is largely a function of the colorant

used. If an incorrect pigment was used to

tone the ink press side, the resulting print

may bleed when exposed to water. This can

happen with both solvent- and water-based

inks. Test methods for water resistance are

specific to the particular application in-

volved. They must be agreed to by all parties

involved in the process.

Water-based inks have their own concerns

when it comes to water resistance. Commer-

cial water-borne inks are not soluble in water,

but rather in amine and water. If a job is print-

ed and the amine is not removed through dry-

ing, or if too much or the wrong amine was

added to an ink, the printed material may res-

olubilize when exposed to water. Generally,

water resistance improves with time when

water-based inks are printed. With the

increased use of water-borne inks, some print-

ers have set up tests off press to check for

water resistance. This is done by wiping the

print a number of times with a wet tissue. The

pressure and amount of water used, as well as

the number of wipes made will all affect the

results. Once these three variables are con-

trolled, this test can be a good indicator that

the inks were applied and dried properly.

Other Conditions

It would be difficult to anticipate all possi-

ble conditions that may require press side

testing. It is perhaps best to realize that any

job should be carefully reviewed to deter-

mine which testing is feasible to be done

press side. This can be determined by using

a team approach to review what conditions

the package will be exposed to and how the

package is designed to perform.

SUBSTRATES

If a printer was asked what one thing he

would like to see from his ink company, he

would likely answer “one ink which works

on all the substrates” (Figure

). There are

many ink systems out there, because there

are many substrates printed flexographically,

which is why the process is so popular. Each

substrate has unique characteristics which

require different formulations of inks in

order to get the end-use properties desired in

the final package. Common substrates that

inks are specifically formulated to print on

are papers and paperboards, polyethylenes,

foils, cellophanes, vinyls, polypropylenes,

PVCs, coextruded films, polyesters, metal-

Good crinkle vs. bad

crinkle. For flexible

packaging materials,

inks must also have

acceptable flexibility.

Poor flexibility on these

substrates can result

in ink flaking off the

package as it is

handled.

It is common to test the

rub resistance of the

print against itself or

against another surface

the print will be in con-

tact with.

Good Crinkle vs Bad Crinkle

Good Bad

lized films and more being introduced to the

process. Each of these substrates has varia-

tions, depending upon manufacturer and

production processes. Each variation could

require a different ink system, or maybe a

slight modification to an existing ink.

It is important to remember that as sub-

strates vary, the inks often have to vary with

them. For example, if a printer is using a

polyester film and switches to another poly-

ester, which is supposed to be the same,

there is a chance that the same ink will not

work equally well on both substrates. The

ink industry saw this problem recently.

Polyester film suddenly became difficult to

obtain in one part of the world. Some print-

ers bought overseas products which were

supposedly equal to what they had previous-

ly purchased. It was realized that this was

not the case, when their standard polyester

ink systems did not work properly on the

imported substrate. In some cases, before

the ink companies could formulate an ink to

work on the imported material, the polyester

shortage was over and printers could again

purchase the material they originally used.

The point to remember is that if someone

asks you what type of ink should be used on

a specific polypropylene film, don’t be too

quick to respond unless you are familiar

with that exact substrate.

Substrate’s Effect on Color

When color matching, it is very important

to match the color of the ink applied to the

substrate on which it will be printed. The

INK 111

Paper, paperboard,

corrugated board,

polyethylene, vinyl,

cellophane, polypropy-

lene, PVC film and foil

represent several of the

different classifications

of substrates that be

utilized in the flexo-

graphic printing

process.

’S