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

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12 FLEXOGRAPHY: PRINCIPLES & PRACTICES

Transparent inks are often used for alu-

minum foil printing to enhance the brilliance

of the foil and produce an eye-appealing

effect.

Tags and Labels

Water-borne and UV flexo inks became

established in the tag and label market seg-

ment earlier than in the wide-web segment.

Initial testing was comparatively easy to

accomplish on narrow-web equipment and

the printing surface predominantly used was

friendlier to these inks. The tag and label

market continues to grow because, like plas-

tic packaging, these products are being used

increasingly as a marketing tool and lend

themselves to diversity and segmentation.

Rub resistance is more critical for the inks

used here because there are many more dif-

ferent substrates and conditions experi-

enced by labels than there are to packaging

products. This diversity often makes water

resistance more important because of the

use of labels in a wider range of applications

including shampoo, pharmaceutical and

soap pouches.

Chemical resistance is often necessary.

Pharmaceutical packaging, for example,

often must withstand isopropyl alcohol that

comes from exposure to a sterile environ-

ment (Figure

High gloss is another common require-

ment of printed label products, since they

usually involve surface rather than reverse

printing. This is usually accomplished with

UV coatings. Lightfast requirements are sim-

ilar to those of wide-web applications.

Household and

Office Paper Products

Household paper products, towels, tis-

sues, napkins and the like are almost exclu-

sively printed with aqueous flexo inks. They

are formulated for a wide range of absorbent

stocks. These inks must not bleed or rub off

in the presence of greasy foods and common

household cleansers. Tack, not normally

measured as a flexo ink property, must

remain low throughout the printing process

to prevent paper fiber and ink from plugging

the printing plate.

Other uses include decorative gift wraps,

ream wraps for copier paper, low-cost forms,

letterheads and envelopes. Many of these

applications do not require lightfastness. The

decorative papers use inks that offer bright

colors and special effects such as metallic or

fluorescent prints (Figure

Publication/Commercial Printing

Water-based flexographic inks have

become a viable alternative in the newspa-

per market. The process has been identified

as an alternative to letterpress and offset

The tag and label

market is a growing

segment in flexography.

Like plastic packaging,

these products are

being used increasingly

as a marketing tool.

They lend themselves

to diversity and

segmentation.

Flexography has also

found a niche in house-

hold and office paper

products, such as

printing envelopes

and letterhead.

lithographic installations. Use has been

enhanced by advances in photopolymer

printing-plate technology and the ability to

satisfy the growing four-color needs of the

industry. Other cited advantages are elimina-

tion of VOCs, faster press speeds, reduced

costs and improved quality.

Over the years, the primary requirements

for newspaper inks were low cost and good

mileage because newsprint, a rough surface

that does not lend itself to quality work, is

generally used for printing copy that is read

once and then thrown away. Most newsprint

is printed with offset inks that dry by pene-

tration, which accounts for their low-rub

resistance. Heat-set inks are also available,

and give better printing results because they

are bound to the sheet after drying and do

not further penetrate the paper.

Flexography has also made inroads in

book and other publishing areas, such as

newspaper inserts and low-budget maga-

zines. Book printing is a specialized field

requiring different equipment and skills with

papers ranging from heavyweight coverstock

to very lightweight, almost tissue-thin paper.

As flexography expands in specific market

segments, it is finding applications in the

commercial printing, of anything from sim-

ple invitations to sophisticated brochures.

Print shops that flexography is employed in

range in scope from a simple one-press oper-

ation to a multifaceted business utilizing a

complicated maze of equipment.

TESTING END-USE

INK PROPERTIES

A number of tests have been established

over the years to evaluate the properties of

inks required for an increasingly broad spec-

trum of uses. They are described in detail in

reference material from the National Asso-

ciation of Printing Ink Manufacturers

(NAPIM), ANSI Standard Test Methods

(ASTM), the Flexographic Technical Asso-

ciation (FTA), and Technical Association of

the Pulp and Paper Industry (TAPPI).

Test procedures for some of the more

important properties are discussed here.

They fall generally into two categories: those

that test the durability of the ink film and

those that test its appearance. These tests

are not a complete list, but are offered as

guidelines for communicating ink require-

ments and demonstrating performance. The

most important criterion for any test proce-

dure is that it be performed consistently to

ensure that the manufacture of inks and

their subsequent application on press meet

the desired specifications and tolerances.

Substrate adhesion is most commonly mea-

sured using the pressure-sensitive tape adhe-

sion test, which compares the ink-to-sub-

strate bond to the bond of the adhesive

between tape and the ink surface.

To conduct the test, an 8" or wider sample

of the printed or coated substrate to be eval-

uated is placed on a flat surface and fastened

either mechanically with clamps or tape.

Sample tension should be sufficient to pre-

vent wrinkles and hold the sample flat

(Figure

A 1" by 6" pressure-sensitive tape (3M 610

brand or equivalent) is applied full length to

the cross-direction width of the sample. Any

air bubbles are removed with one pass only

INK 13

Substrate adhesion

is most commonly

measured using the

pressure-sensitive

tape adhesion test.

It compares the ink-to-

substrate bond to the

bond of the adhesive

between tape and ink

surface.

14 FLEXOGRAPHY: PRINCIPLES & PRACTICES

of a rubber-covered roller, and then the tape

is immediately removed by pulling at an

angle of 180°.

The tape and substrate are examined for

evidence of adhesion failure. Adhesion is

reported as the estimated percentage of lift

on the substrate.

Crinkle adhesion determines the flexibility

and bonding characteristics of the ink when

a flexible substrate is crinkled. A piece of

the printed substrate is grasped between the

thumb and forefinger of each hand with

about 0.05" of substrate between the two

thumbs. The hands are brought almost

together and rotated fairly rapidly about 10

times (Figure

). Avoid generating too

much heat or cutting into the print with the

fingernails. On coated substrates, it is nec-

essary to determine if the failure is ink adhe-

sion or coating separation.

Scratch resistance is determined by placing a

sheet of the printed substrate on a smooth,

resilient surface, a paper pad for example,

and scratching with the back of the index

fingernail. Fast “swipes” are made using

moderate pressure while avoiding cutting

the ink film (Figure

Block resistance is the ability of a printed sur-

face to separate from an adjacent surface

without sticking or disturbing either. There

are a number of block tests. Figure

shows

the “C” clamp press-side test generally used

to provide a quick, running indication of

whether thorough drying and complete sol-

vent removal is happening on the press. A

widely accepted block test is the IC block

test. Test samples are assembled in an IC

block tester and placed in an oven equipped

with automatic humidity and temperature

controls. This permits the testing of a series

of samples in "sandwich" form and provides

uniformly accurate results.

Lamination adhesion is critical for inks used

on a substrate designed to enhance barrier

and other property combinations, especially

those developed for packaging applications.

The inks do not need gloss or surface condi-

tioners, but they must have good adhesion to

the printed substrate and to the adhesive

Crinkle adhesion

determines the

flexibility and bonding

characteristics of the

ink when a flexible

substrate is crinkled.

Scratch resistance is

determined by placing

a sheet of the printed

substrate on a smooth,

resilient surface, a paper

pad for example, and

scratching with the

back of the index

fingernail.

A “C” clamp, press-

side, block-resistance

test provides a quick,

running indication of

whether thorough

drying and complete

solvent removal is

happening on the press.

materials. They must also withstand the tem-

perature of the lamination operation. Ink

tests for laminations are generally the same

as for single substrates. If the ink is on the

surface of the outer substrate, requirements

are the same as for surface-printed, single

substrates. If the ink is printed on one of the

inner substrate surfaces, though reverse-

printing of an outside transparent film, or

surface-printing of an inner substrate, it must

pass the pressure-sensitive tape test, the crin-

kle test for adhesion, and pressure block-

resistance test with the printed surface in

contact with the backside of the substrate.

Rub resistance is required of many printed

inks to enable the package to withstand han-

dling between the press and the point of

sale. Rub resistance may also be described

as resistance to scuff or abrasion. The

Sutherland rub test has been established as

the standard for measuring this property.

In a typical dry-rub procedure, a 7.5" test

strip (with a solid printed image l" by l.5" cen-

tered on the sample) is clipped to the four-

pound testing block (printed surface away

from the rubber pads). The printed sample is

mounted (print side up) on the rubber pad of

the base plate. The weight is placed over the

sample with one of the 1" by 2" test-block

rubber pads over the ink. Both surfaces

should be free of dirt. The tester should be

set for 100 strokes and, when the rubs have

been completed, both the inked surface and

the plain surface on the test block should be

examined for signs of transfer.

To measure wet-rub resistance, the test

strips are mounted in the same manner as

for dry rubs on the two-pound test block.

Two to six drops of water are placed on the

printed surface so that they will be covered

by the test block. The block is positioned

and press the test started. After one stroke,

both surfaces are examined for color trans-

fer. Individual strokes are repeated until ink

failure is noted. The number of strokes

required to cause failure is recorded.

Heat resistance is a companion requirement

to rub resistance in many printed packaging

applications and is needed to withstand the

heat applied by sealing equipment close to

the package. The heat sealing devices may

be the sliding, heated flat shoe or crimp-type

jaw designs. Sliding iron and crimp-seal tests

have been established to test required heat

resistance.

The sliding iron test (Figure

) requires

that a hand iron is drawn at the desired tem-

perature (read with a surface pyrometer)

across the printed or lacquered surface a pre-

determined number of times, usually three.

After this, any breakdown of the ink or lac-

quer surface is noted, as is any drag on the

iron. Either of these results indicate unsatis-

factory heat resistance.

In the crimp seal test, either a laboratory

model or actual production crimp jaw on the

heat-seal equipment is set for correct tem-

perature, pressure and dwell time to dupli-

cate production requirements. The jaw tem-

perature required on high-speed automatic

packaging machines will be noticeably high-

er than the temperature to seal the heat-seal

substrate. The sample is sandwiched be-

tween aluminum foil or paper and placed

between the jaws of the heat sealer. The

crimp-sealing operation is repeated several

times on different areas of the substrate to

INK 15

To test for heat

resistance on a printed

substrate, the sliding-

iron test requires that a

hand iron, set at a

certain temperature,

be drawn across the

printed surface a

pre-determined number

of times as any break-

down of the ink or

surface is noted.

16 FLEXOGRAPHY: PRINCIPLES & PRACTICES

provide a more critical evaluation of ink

transfer. Results are acceptable, if there is

no cling or transfer of ink to foil. It helps to

operate the crimp jaw against a plain piece

of paper after the test. Any ink transfer or

sticking of the crimp jaws to the printed sub-

strate indicates unsatisfactory heat resis-

tance or ink flexibility.

In some applications, the printed area

must heat seal to itself or to an unprinted

surface. Heat sealing can be tested with

either the sliding iron or the crimp seal jaws.

The heat seal is pulled by hand or on a seal

puller that gives a numerical rating and com-

pares seal strength to that known to be

acceptable.

Ice-water crinkle test checks the flexibility and

integral strength of a print subjected to ice,

refrigerator or freezer conditions. Samples

are immersed in a beaker of water and

cracked ice for 30 minutes and then

removed. While they are still wet, the sam-

ples are grasped firmly between thumb and

forefinger of each hand with about 1" of print

between the two thumbs. With the hands

together, the samples are rubbed 10 times

rapidly in opposite directions. One complete

cycle consists of both a back and forward

motion of the wrists. The percentage of ink

removed is recorded and compared to the

standard (Figure

To test for freeze-thaw resistance, the printed

samples are immersed in a beaker of water

and put in a freezer for 16 hours. After

removal, the beaker is allowed to thaw to

normal room temperature and then the sam-

ples are given the crinkle test.

Moisture bleed or transfer resistance test is

important for packages that come out of the

freezer and sit on the kitchen counter to

thaw. The printed ink should not bleed onto

the counter as moisture condenses on the

cold package. Using the Sutherland rub test,

a strip of blotting paper 5.5" by 2" is mounted

on the test block with the felt or smooth side

out. The blotter is saturated with water. The

wet blotter is placed on the sample to be test-

ed and left in place for four minutes. The

block is removed without rubbing and exam-

ined for ink transfer to the blotter. An alter-

native procedure is to place the print sample

in a beaker of water for 24 hours. The sample

is then removed and wiped with a white tis-

sue. Both the tissue and the water in the

beaker are inspected for any color bleed.

Moisture vapor transmission resistance test

protects packaged products such as cookies

and snacks from the humidity of the sur-

rounding atmosphere. Instruments are avail-

able to measure the moisture vapor trans-

mission rate (MVTR) of printed packaging

materials. Such units automatically record

The ice-water crinkle

test checks the flexibility

and integral strength of

a print subjected to ice,

refrigerator or freezer

conditions.

Acid and alkali

resistance is tested by

applying a drop of the

appropriate reagent to

the sample print. After

a few moments, it is

allowed to run off the

printed area onto the

unprinted section.

The print area is gently

scratched to observe if

the ink vehicle has been

affected.

1^ 1&

the time required for moisture to pass

through the test sample. Results are reported

in grams per 100 square inches per 24 hours.

Acid and alkali resistance is required of print-

ed ink films that come in contact with dairy

products, juices, and other products that

contain acids. To test, a drop or two of the

appropriate reagent is applied to the sample

print (Figure

). After a few moments, it is

allowed to run off the printed area onto the

unprinted section where any dissolved color

can be seen. The print area is gently

scratched to observe if the ink vehicle has

been affected.

Soap and detergent resistance is essential for

the many flexo-printed materials used to

package and label soaps, detergents, flakes,

and granules. One method of testing is to

pour 10 cc of a concentrated solution of

soap or detergent in water on a 4" by 4" fold-

ed, unbleached and unsized muslin pad

(Figure

). A test print of 3" by 3" or larger

size is placed face up on a smooth glass plate,

and the muslin pad is placed on the print.

Over the pad is placed a flat 12-ounce

machined plate, which is left for a period of

time determined by the product and stock

specifications. The sample is then examined

and graded according to the amount of dis-

coloration on the surface of the muslin and

the change in appearance of the test print.

Similar practical tests have been devised to

determine the resistance of ink formulations

to smudging with soap or detergent pastes

and to soaps in bar or cake form.

Oil resistance is necessary for inks that come

in contact either directly or indirectly with

packaged foods containing oil naturally or as

a process additive. To determine resistance

to oils that may penetrate the substrate,

about 1" of the product oil is put into a wide

mouth fruit jar. The printed sample is placed

on top of the jar with the printed surface up

(or down with impervious films) and the jar

is sealed with an open-top ring jar closure.

The sample is inverted on a glass tray and

placed in an oven for 48 hours at 1,200° F

(490° C) and 90% humidity (Figure

). At

the end of the cycle, the printed surface is

checked for adhesion by both the pressure-

sensitive tape and crinkle tests.

Boiling water resistance is needed for printed

convenience food packages in which food is

heated before serving. For a typical boiling

water test, the printed sample is cut into

strips and placed in 200 ml. of boiling water

for 5 minutes. The printed strips and water

are examined for evidence of bleed or dis-

coloration which connotes ink failure. For

steam resistance needed in sterilization and

food processing applications, tests are run in

equipment ranging from household pressure

INK 17

To test for a substrate’s

soap and detergent

resistance, a small

sample of printed sub-

strate is placed on top

of an unbleached and

unsized muslin pad that

has been saturated with

a concentrated soap or

detergent solution.

Pressure is applied to

the pad and print for a

length of time. The sam-

ple is then examined

and graded according

to the amount of discol-

oration on the surface of

the muslin.

To determine a sub-

strate’s oil resistance,

the product oil and the

print sample is put into

a wide-mouth mason

jar. The sample is

inverted on a glass tray

and placed in an oven at

controlled temperature

and humidity for the

specified time.

1* 1(

18 FLEXOGRAPHY: PRINCIPLES & PRACTICES

cookers to autoclaves at temperatures from

2,150° to 4,000° F (1,020° to 2,040° C). Prints

are then inspected for bleed, discoloration

and ink breakdown.

Plasticizer bleed resistance test becomes

important for printing films such as vinyl

that contain plasticizers. Plasticizers can

cause inks to bleed into the film or onto

another material in contact with the ink. A

commonly used test procedure is to saturate

a piece of white-blotter stock with plasticiz-

er and place it on the test print (Figure

A light weight is put on the sandwich, which

is then placed in an oven at controlled tem-

perature and humidity for the specified time.

The blotter is examined for plasticizer stain.

Properties that ensure good ink appearance

are equally important as those that con-

tribute to an ink film’s durability. They

include aspects of color (hue match, intensi-

ty and value) density, tone quality, image

detail, opacity, fade resistance and gloss.

Color measurements are still commonly per-

formed with the naked eye, which remains

one of the most sensitive judges of color and

its variations. Visual color judgments should

be made under standard viewing conditions

such as the American National Standards

Institute PH2-30-1989 viewing standard.

Production prints should be compared to the

color standard on the substrate on which the

ink will be printed. Comparisons of wet inks

can be made by draw-downs (Figure

) in

which an ink drop is spread over a substrate

with a rigid blade or block of steel or by

anilox proof in which the rollout is made

with an anilox hand proofer.

Color measurements can also be taken

using colorimeters and spectrophotometers

(Figure

) which offer consistent and mea-

surable results. These instruments allow

operators to monitor production control,

color difference calculations, color specifi-

cation and tolerance tasks. Measurements

should be performed at the instrument’s

largest viewport area possible. Two samples

each from the front, middle and end of the

pressrun, on both sides of the sheet, and for

each color, should be measured. Readings of

solid-ink densities at several areas of speci-

men surface should be taken to obtain an

indication of uniformity and an average ink-

density value.

Print density can be measured with a reflec-

tion densitometer. The instrument must be

calibrated before testing. The LO (white stan-

dard) and HI (black standard) values for

each color are set and then individual color

patches are read as determined by the instru-

ment. Calibration values are verified for each

standard patch and adjustments made as

necessary. Two substrate samples each from

the front, middle, and end of the press run

are taken. Readings at 10 locations on each

Plasticizer bleed-resis-

tance test procedure

requires saturating a

piece of white-blotter

stock with plasticizer

and placing it on the

test print . A light

weight is put on the

sandwich, which is then

placed in an oven at

controlled temperature

and humidity for the

specified time. The

blotter is examined for

plasticizer stain.

Color measurements

can be taken by

comparing wet inks

done with draw-downs,

where an ink drop is

spread over a substrate

with a rigid blade, block

of steel or by anilox

proof and the rollout is

made with an anilox

hand proofer.

sample for each of the colors, in solid ink-

density areas.

Tone quality can be determined by measuring

dot area (percentage of ink coverage on the

substrate) with a reflection densitometer.

Two samples each from the front, middle

and end of the pressrun for each substrate

are tested for all colors for each sample. The

instrument must be calibrated before test-

ing. The dot area function is selected and the

densities of both the substrate and the per-

cent solid are read to obtain the dot area

percentage. Readings are taken at five ran-

dom spots from each sample color and aver-

aged to obtain the dot area percentage for

the sample.

Image detail can be checked using a digital-

frame grabber equipped with high resolution

optics and a personal computer equipped

with image analysis software to quantify the

following dot characteristics: maximum and

minimum dot area, concentricity, maximum

and minimum diameter and density profile.

At least 50 dots are measured per sample

and readings are taken from five random

spots of each sample color. Measurements

can also be made using a densitometer.

Opacity or contrast ratio can be measured using

a spectrophotometer. The ink sample is print-

ed using an appropriate technique, on a black

line sheet. The sample is measured on both

the white and black portions of the substrate.

The system will then calculate the contrast

ratio or approximate opacity of the film.

Fade resistance requirements will depend on

the application. Inks on an outside billboard,

for example, will require much more fade

resistance than those on a grocery product

with a short shelf life. Rough comparisons

are made by covering a portion of each sam-

ple with an opaque material. The samples are

exposed to sunlight. Equipment such as the

Fadeometer or Weatherometer can be used

to conduct accelerated light-fading tests.

Gloss of a finished print can be normally

judged visually, but instrument readings can

be obtained with a gloss meter (60° angle)

and some spectrophotometers to determine

the specular gloss of the material surface.

Tile standards are cleaned and the instru-

ment is calibrated according to instructions.

Measurements are taken of two samples

each from the front, middle and end of the

run. Reading areas should be consistent in

ink coverage or solid ink densities. The read-

ings should be repeated for all colors,

recorded and averaged.

Coefficient of friction (COF) properties indicate

the ease or resistance of a surface to move or

slide against another surface, and are impor-

tant both to successful production processes,

such as automatic packaging operations, and

to applications such as skid resistance of

stacks of multiwall bags. To determine the

INK 19

Color measurements

can be taken using

colorimeters and

spectrophotometers,

which offer consistent

and measurable results.

To determine the

resistance to sliding

of a printed sample,

the substrate is

attached, face up,

to the end of the inclin-

able arm of a plane.

A second piece is

attached to a weight

positioned at the right

end of the arm. The arm

is inclined slowly and

steadily. At the point

where the weight block

begins to move down

the incline, the angle is

read on the protractor

scale.

20 FLEXOGRAPHY: PRINCIPLES & PRACTICES

resistance to sliding of a printed sample, a

strip of plain or printed substrate is attached,

face up, to the end of the inclinable arm of the

plane. A second piece is attached to the stan-

dard weight which is positioned at the right

end of the arm. The arm is inclined slowly and

steadily. At the point where the weight block

begins to move down the incline, the angle is

read on the protractor scale (Figure

). A

more accurate measurement of coefficient of

friction uses an IBM friction coefficient tester

to provide numerical values for both static

and kinetic COF. The instrument applies force

to test strips (one clamped to the unit, one

free to move) and calculates slip characteris-

tics (Figure

Odor, or absence thereof, is particularly

important in food and personal product pack-

aging applications. To test, the printed speci-

men is placed in a jar, sealed with a cap and

then placed in an oven at 1,000° F (380° C)

for 2 hours. The steps are repeated for

unprinted control substrate. The jar is opened

and a qualitative assessment of the odor is

recorded.

A friction coefficient

tester takes measure-

ments of both static and

kinetic COF. At the same

time, it also calculates

the slip resistance.

INK 21

Ink Formulation

aving identified and specified

the end-use requirements for

the printed package and

understood the converting

conditions, the ink supplier is

in a position to formulate an

ink system to meet the needs of the job.

These end-use requirements, as discussed in

the first chapter, can impose limits on the

materials available for use by the ink formu-

lator. Therefore, it is necessary for the for-

mulator to have a comprehensive under-

standing of the physical properties of the

raw materials used to produce inks, their

interactions and their limitations in produc-

ing a usable ink. The converter and end-user

must possess similar knowledge.

Printing inks are colored media designed

to reproduce an image on a printing surface.

They are primarily used to convey a mes-

sage, provide protection, or give a decora-

tive effect to the material on which they are

applied. Inks are extremely versatile and

have been used on a wide variety of papers,

plastics, metals, glass and textile surfaces.

The majority of printing inks consist of a col-

orant, either an insoluble solid or dye, sus-

pended or dissolved in a liquid vehicle. The

resulting combination forms a colored fluid

capable of distribution and transfer on a

printing press.

In addition to providing the desired visual

characteristics, inks are formulated to meet

the specific needs of the printing process:

they must dry under specified conditions;

adhere to a given material; and have specific

resistance properties, dictated by the inter-

mediate processing and the final end-use of

the finished product.

Inks for flexography have two prominent

characteristics that set them apart from inks

used in most other printing processes: they

are fluid and quick drying.

THE BASICS OF INK TECHNOLOGY

This section will explore these aforemen-

tioned requirements and the impact that the

individual components and other con-

stituents have on flexographic ink manufac-

turing, print properties, performance, and

the environment.

Color

Sir Isaac Newton, using a glass prism,

demonstrated that white light could be split

up into a “rainbow” of hues: red, orange, yel-

low, green, blue and violet, which he called

the visible spectrum. Newton also observed

that the rays themselves are not colored, but,

when they interact with an object, that the

sensation we refer to as color is perceived.

With very few exceptions, objects do not

emit colored light, but only look colored

when under illumination. An object that

appears black under standard illumination

does so because it absorbs all the light

falling onto it. Conversely, an object which

appears white under the same lighting con-

ditions looks as it does because it reflects all

the light incident upon it. If the object

absorbs some portions of this “standard”

spectrum more than others, it will appear

colored. For example, an object that absorbs

only red light will appear cyan.

It is important to note that the colors per-

ceived depend on the illuminating source.

Different sources of light, e.g., incandescent,