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CHAPTER 1

Ink

2 FLEXOGRAPHY: PRINCIPLES AND PRACTICES

ACKNOWLEDGEMENTS

Authors/Editors: David Argent, Progressive Ink

Stanley Field and Dr. Chris Patterson, Flint Ink Corp.

Sam Gilbert, Sun Chemical Corp.

George Sickinger, Borden Chemical, Inc.

Contributors: Conrad Carstens, Borden Chemical, Inc.

Rita A. Conrad, Flint Ink Corp.

Richard H. Lunney, Progressive Ink

Matthew McCardell, Automatan Inc.

Rob Sals, Sun Chemical Corp.

Kelly VandenBosch, X-Rite, Inc.

INK 3

nk is critical to the flexographic print-

ing process. It is the media that trans-

fers an image from plate to substrate.

Ink is used to convey a message and

provide a package with decorative

effects. It can be formulated to meet a

specific need dictated by either press config-

uration or printing surface.

This chapter, the first in this two-part vol-

ume joining flexographic inks with sub-

strates—two products that no printer can do

without—is devoted to ink, as well as its

properties and applications. It discusses the

composition of the vast array of printing

inks, as well as the classification of their

components and the physical characteristics

that each brings to the printing process.

Information is presented in four distinct sec-

tions: end-use requirements, formulation,

prepress practices and on-press procedures.

Mixing, blending, dispensing and filtering

are all covered in this primer on effective ink

management. Strength, sharpness, lay and

color are reviewed. Press configurations and

the influence that they have on ink require-

ments are explained. Anilox roll selection

and maintenance guidelines are also offered.

You will learn that colorants, whether pig-

ments or dyes, are the vehicles that give an

ink its color. They can be conventional or

fluorescent (ultraviolet). Resins, on the

other hand, are responsible for a host of fac-

tors, including ink’s printability, rheaology/

viscosity (flow), adhesion and stability.

Solvents are the carrier agents that transport

ink from the fountain to drum to substrate.

Additives mate the ink with the printing sur-

face. They can enhance gloss level and,

increase opacity. At the same time, additives

can improve heat-, moisture-, fade- and rub-

resistance. Directions for performing press-

side tests to measure each of these qualities

of ink are offered in the respective section.

No discussion of printing inks could be

complete without marrying the different

classifications of the media to the substrate

most applicable for its use. Chapter Two of

this volume is dedicated to that cause. It sup-

plements information presented in Chapter

One, which offers the basic, most-necessary

details on both compatibility and conflict.

Substrates, as you may know, and will see,

are grouped into five categories: paper and

paperboard, corrugated board, laminates,

foils and films. In this instructional volume,

the properties of each are presented side-by-

side with a detailed discussion of the appli-

cations that each is best suited forÑright

down to the ink.

Introduction

ince its beginnings in the 1920s

and 1930s, the growth and

acceptance of the flexographic

process have been closely tied

to advances in flexographic

printing ink. At that time, the

process was called aniline printing because

its inks used aniline oil (a coal-tar derivative)

as the coloring ingredient. It was essentially

a rubber-stamp printing process employing a

smooth ink roll and two rubber doctor rolls

to develop the ink film.

The evolution of flexo inks, from dyes dis-

solved in alcohol and laked with tannic acid,

to solvent-based, and more recently water-

borne and ultraviolet-cured systems, has

been a major factor in the greatly expanded

use of flexography in segments of the paper,

plastics and packaging industries (Figure

)

The earlier inks left much to be desired in

terms of print quality, light fastness, adhe-

sion and scuff resistance, which made them

suited primarily for printing bags and

envelopes. The development of high-viscosi-

ty, solvent-based, pigmented inks prior to

World War II, not only produced flexo work

of greater durability and better print quality,

but also led to the design of the anilox roll,

whose small cells gave better control and

uniformity to the ink (Figure

Flexography’s suitability for the stream of

new packaging substrates that reached the

marketplace, beginning with the introduction

of cellophane in the 1930s, was another key

factor in its increasing recognition and, the

number of applications for the process.

Flexography was the only economical way

of printing cellophane, unless the production

run was large enough to justify the more

INK 5

End-Use Requirements

A schematic of the

flexography process.

The enlarged view of the

anilox roll shows the

small cells crucial to

give better control and

uniform ink lay.

Gravure cylinders are

utilized for large runs.

Plate

Cylinder

Impression

Cylinder

Ink

Metering

Roll

Fountain Roll

Ink Pan

6 FLEXOGRAPHY: PRINCIPLES & PRACTICES

expensive gravure cylinders (Figure

Solvent-based ink formulations containing

nitrocellulose (and later polyamide resins)

were developed that were good for printing on

the polyethylene films introduced in the

1950s, as well as for the many modern-day

materials and constructions that followed.

Among these substrates were polyethylene

and polyester films, foil, metallized films, ori-

ented and coextruded polypropylenes, coated

films and many different kinds of laminates.

They required inks that could grip their less-

porous surfaces and hold up under what were

then unheard of application conditions

(Figure

Water-based flexo inks were first tested in

the 1930s for paper and paperboard, but did

not realize significant commercial use until

the 1950s. Demand for water-based inks grew

in the 1960s as acrylic polymer technology

developed, enabling a higher gloss and giving

the inks better water and rub resistance.

Since then, water-borne systems have be-

come faster drying and have moved into

high-speed printing, process printing, newer

substrates and lamination applications. Their

workplace-safe and environmentally friendly

features have become especially attractive as

regulatory actions have increased.

The regulatory climate has also been one of

the important inducements for the use of

radiation-cured flexo inks, which first

appeared in the 1970s. Ultraviolet (UV) light

and electronic beam (EB) systems using such

inks, which are stable and compatible with

the environment, are employed to speed

operations and reduce solvent emissions.

APPLICATIONS

Today, flexo inks still find their greatest use,

in packaging applications (Figure

). These

applications range from their initial use for

printing flexible packaging to corrugated

materials, folding cartons, milk and other liq-

uid cartons, food and rigid plastic containers,

multiwall and paper bags, tags and labels, and

gift wraps. The largest single use today is the

printing of corrugated materials. Why?

Quality graphics have become increasingly

important for packaging and point-of-pur-

chase display products. Flexible packaging is

not far behind and continues to grow steadily

as the industry moves from paper to plastic

materials.

Other applications include household

paper products such as towels, tissues and

napkins as well as wall coverings. Flexo inks

are also replacing letterpress and offset types

in some newspaper operations and are also

used in commercial publication and book

printing. The total value of the flexo market,

as reported by the Printing Industries of

America, exceeds $54 billion. A breakdown

of the business segments is shown in Table 1.

END-USE QUESTIONS TO ASK

Packaging applications, in particular, have

created new demands for the properties and

performance of flexo inks as new materials,

printing technology, storage and shipping

requirements, nontraditional uses and gov-

ernment regulations have reached the mar-

ketplace.

It has become critical for the printed pack-

age to print well under different conditions,

A sampling of items

printed on various

substrates, including

polyethylene and

polyester films, foil,

metallized films,

oriented and coextruded

polypropylenes and

coated films.

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but to also withstand adverse storage and

shipping environments, satisfy a host of end-

use requirements, and maintain a fresh

appearance during consumer use.

The following questions apply when deter-

mining the ink and coating end-use require-

ments:

• How will the package be stored? Will it

be in a warehouse or outside?

• Will it be stretch-wrapped? The protec-

tion properties of the ink and/or varnish

must be tailored accordingly.

• Will the package be exposed to sunlight,

either directly or indirectly (such as

through a window)? The fade resis-

tance of the ink must be formulated to

withstand the amount of exposure.

• How will the package be handled

INK 7

Today, flexogrpahy

is widely used for

flexible and corrugated

packaging, folding

cartons, milk and other

liquid cartons, food and

rigid-plastic containers,

multiwall and paper

bags, tags and labels,

and gift wraps.

Table 1

ESTIMATED

MARKET % FLEXO $

$ IN BILLIONS FLEXO IN BILLIONS

Corrugated $20.0 75% $15.0

Folding Carton 8.0 20% 1.6

Flexible Packaging 16.0 75% 12.0

Labels 6.0 30% 1.8

Tag, Ticket, Tape 0.3 50% 0.2

Paper Bags, 3.0 95% 2.9

Multiwall Sacks

Gift Wrap 0.6 45% 0.3

Wallpaper 0.5 30% 0.2

Set-up Paperboard 0.4 20% 0.1

Boxes

TOTAL $54.8 62% $34.1

Source: 1996 Printing Industries of America.

SIZE OF FLEXO MARKET SEGMENTS

8 FLEXOGRAPHY: PRINCIPLES & PRACTICES

through the customer’s packaging and

shipping processes.

• What kind of conveyor system will

process the package and what type of

stacking pattern will be used? The

answers will determine the coefficient

of friction (COF) or slide angle needed

for the bag or box.

• What kind of exposure to the cus-

tomer’s materials will the print undergo,

either through migration or during fill-

ing of the product? The ink or varnish

will need to resist any product contact

that results.

• Will there be any moisture or possible

condensation to the package? If so, the

ink or coating must possess the neces-

sary water or humidity resistance.

• At what temperature will the customer’s

material be packed?

• How long will the package remain at

this temperature? The temperature con-

ditions will dictate the necessary heat-

or freeze-resistance required.

• What will be the shipping conditions

that the package must undergo?

• How will the printed material be

shipped and/or stored? These will deter-

mine the level of necessary rub- and

abrasion-resistance.

RANGE OF IMPORTANT

INK PROPERTIES

Durability and resistance requirements

and the increasing demand for products of

enhanced quality, have placed increasing

importance on the range of properties that

flexographic inks must offer. Among the

most significant ones are adhesion, color

strength, gloss, opacity, rub-resistance,

dimensional stability, mottle-free lay and

block resistance.

Other important properties are surface

tension, coefficient of friction, heat resis-

tance, sealability, solvent resistance, fade

resistance, odor control, and the ability to

satisfy Food and Drug Administration (FDA)

regulations for direct food contact and spe-

cific food additive uses.

MARKET SEGMENTS

The relative significance of the important

ink properties varies depending on the par-

ticular application. The applications can be

grouped by market segments which call for

specific end-use requirements.

Corrugated Materials

An ink-selection checklist for printing on

corrugated should cover such considerations

as stock, trap, anilox specification, press

design, image design, layout, and special

property requirements such as grease and

gloss skid resistance. These considerations

will apply for the most part in all the market

segments.

Corrugated printing generally falls into

one of three categories: brown box, mottled

white and bleached kraft, and high quality

display (Figure

Brown Box. End-use requirements are usually

not critical in printing the traditional corru-

gated natural kraft box. Historically, ink

selection is driven more by price than by

performance. Color exactness and strength

Corrugated printing

is either brown box,

mottled white (bleached

kraft) or high quality

display.

are not as important as economy of opera-

tion. The inks are generally standard GCMI*

colors that are suited to meet a variety of

printing equipment and bulk production

demands.

Mottled White/Bleached. Printing this kind of

corrugated board requires better quality inks

(having harder resins and faster drying sys-

tems) for higher-speed equipment to obtain

improved finish, dry rub resistance, and

other physical characteristics. They should

work well with photopolymer plates.

Printers seeking high quality results on these

stocks or natural kraft, using newer, high

speed equipment, will require inks that also

offer greater color fidelity and premium per-

formance properties, such as scuff resis-

tance. These inks are generally custom for-

mulated to provide specific functional prop-

erties, such as wax-bleed resistance. Bever-

age cartons require inks with a high coeffi-

cient of friction to provide skid-resistant

printed surfaces. They must also give clean,

sharp prints and dry quickly.

High Quality Display. The printed corrugated

box and other constructions are increasingly

being used as silent salespeople and point-of-

purchase (POP) display materials in retail

stores. They have led to a demand for a level

of quality that can compete with preprint

work.

Most of the work is done on mottled or

bleached stock and special high-holdout lin-

ers. The graphics are usually intricate and

may include process work, trapping, large

solids and fine type. The flexo ink used here

must usually provide high color intensity and

is often matched to custom spot colors. The

ink makes or breaks the job because it is

clearly the most visible part of the product.

As corrugated board becomes a decora-

tive product unto itself, in the form of boxes,

eye-catching display material, or upgraded

packaging in general; process printing is

being adapted to provide the desired high-

quality four-color results. It offers a full spec-

trum of colors as well as the lifelike repro-

duction of flesh tones.

High-solids flexo process inks have recent-

ly been developed to help deliver high-quali-

ty halftone work, usually on bleached or

clay-coated stock. Unlike standard corrugat-

ed inks, they must be highly transparent and

highly pigmented to afford proper hue and

low grayness, so that they can be applied at

the thin film levels needed for precise meter-

ing. They should possess a compatible sur-

face tension in relation to the anilox roll,

plate and substrate, and be formulated with

process colors (cyan, magenta, yellow, and

black) specifically for the unique printing

conditions that may exist at the time. In

preprint applications, the inks must also

exhibit the heat and rub resistance that will

withstand the heat and pressure of the sub-

sequent corrugating operation. Overprints

are employed to enhance gloss or sheen, as

well as rub resistance. The trend is to use

water-borne types with greater use of ultra-

violet-cured overprints for the higher end of

the market, such as display work.

Flexible Packaging

Printing of flexible packaging falls into a

number of subsections, including laminated

and retort pouches, candy wraps, merchan-

dise bags, potato chip bags, frozen food

bags, cheese wrap and bread bags. Flexo

inks for these applications must generally

exhibit the adhesion qualities and color

strength for corona-treated film and provide

sharp, clean print. For reverse printing, low

levels of paraffin wax and other additives in

the ink formulation are important, since

higher paraffin levels and lower surface

energy (dyne level) subsequently inhibit

adhesion and lamination bond strengths.

The inks should also possess high heat resis-

tance, typically above 3,500° F (1,770° C),

INK 9

* Glass Containers Manufacturers Institute, now known as the Glass Packaging

Institute.

10 FLEXOGRAPHY: PRINCIPLES & PRACTICES

and in some applications withstand boiling

water. Excellent wet-out to films provides

good ink-lay, smoothness, trapping and print

quality.

For surface printing, the inks should exhib-

it good scuff resistance, typically 1,000

Sutherland Rubs with a four-pound weight,

and high gloss. Gloss is especially important

in surface-printed packaging film and is cus-

tomarily accomplished by covering the ink

with a clear overprint. A typical good gloss is

60 to 70 at 600° F. Excellent gloss falls in the

70 and up range. For high-gloss, high-impact

product lines, inks are being overprinted with

UV and cationic lacquers to achieve superior

results. Overprints are used to add gloss to

the printed flexible package, protect the ink

from outside conditions, control the coeffi-

cient of friction so the package moves easily

though the production machinery, and build

barrier protection for the package contents.

The particular combination of properties

will depend upon the specific application.

Lightfastness is required to resist both fluo-

rescent store lighting and outside exposure

to sunlight. Pigment choice is critical in

avoiding degradation from UV wavelengths.

Personal care product film packaging has

its own set of requirements. Printed sham-

poo pouches that are stored in showers and

exposed to indirect and direct contact with

water, for example, obviously need to be

water and moisture resistant. They must

also withstand the chemical properties of

the cleaning product which can be more

deleterious to the printed film than mild

caustic soda.

Inks for food-packaging films must not

only satisfy FDA requirements, but also pos-

sess differing combinations of other proper-

ties depending upon the product and end

use. Among the properties needed are low-

odor levels, freeze-thaw stability, ice-water

crinkle resistance, grease resistance, scratch

resistance, different coefficient of friction

levels and block resistance.

Folding Cartons

Inks for printing folding cartons, which is

a growing segment for flexography at the

expense of sheetfed offset and gravure, gen-

erally require high color intensity, fast-drying

properties, good gloss, plasticity and good

adhesion. They must print on clay-coated

paperboard, polyethylene and foil. Many of

their uses require resistance to chemicals,

grease, detergents, alkali, alcohol, heat and

water. Overprint protective coatings are uti-

lized to reinforce barrier properties at the

most vulnerable carton areas and provide

greater rub and scuff resistance.

For fine-line printing, flexo process inks

are formulated to give high density and good

dot structure. They should also exhibit the

excellent color fidelity and color strength

suited to a wide spectrum of process color

jobs. They must also meet requirements for

color trapping (superimposition of color),

color sequence and ink viscosity (Figure

Metallic inks are available to provide bril-

liance, high gloss and clean prints.

Food Containers

Freedom from residual odor is critical for

flexo inks in terms of their ability to with-

stand product contact, satisfy FDA regula-

tions, and meet the many handling and stor-

Folding carton has

become a growing

segment in flexography,

and must meet

resistance to chemicals,

grease, detergents,

alkali, alcohol, heat and

water. The addition of

overprint protective

coatings provide

greater rub- and scuff-

resistance.

age conditions involved in distributing and

using these products.

Inks for printing milk cartons and other liq-

uid packaging, for instance, should not create

noticeable odors, contribute to off-flavors, or

be affected by conditions of high relative

humidity. Although water-borne inks are

inherently hygroscopic, aqueous flexo sys-

tems, that resist water and withstand the

soapy lubricants used on filling machinery,

have been developed. These properties must

not detract from their ability to print cleanly

on carton stock or provide the required wet-

rub resistance. They must also withstand

waxing, polyethylene extrusion and other

coating treatment which the stock undergoes.

Other printed food containers must meet

similar property requirements depending

upon their individual packaging, product

and end-use. Printed paper cups and plates,

and polystyrene foam stock also fall into this

category. Their inks should give sharp, clean

prints, a smooth nonabrasive, rub-resistant

finish, and should resist water and food con-

tact. Printed cups must also withstand the

heat of hot liquids and waxing or polyethyl-

ene extrusion.

Multiwall/Paper/Plastic Bags

Multiwall sacks and paper bags have been

printed by flexography for many years. Like

corrugated materials, there is increasing

demand for better graphics and hence

improved inks and substrates that will give

brighter, smoother finishes. Plastic bags are

becoming increasingly important in this seg-

ment because they are cheaper and easier to

make. End-use requirements determine the

choice of ink for printing these bags for a

gamut of applications that range from lawn

product and pet food bags to grocery and

retail shopping bags (Figure

). For exam-

ple, inks for use on both clear and white

heavy-gauge multiwall polyethylene fertilizer

bags should be suited to both outside and

reverse printing. They should be resistant to

pine bark, compost and cow manure, as well

chemical blends of fertilizer. They should

also be scuff, fade and weather resistant.

For sugar bags, a prime requirement is good

scuff resistance against the abrading influence

of loose sugar crystals trapped between the

bags during shipment. Many sugar bag stocks

are made of strong bleached kraft and require

ink formulations that provide adequate

strength and color brightness for their

designs. A requirement for inks used to print

satchel-bottom sacks is resistance to “crack-

ing” in the reverse folds of the gusset.

Foods such as cookies and cereal products

contain cooking oils which migrate and soften

inks not modified to resist them. Inks must

resist the specific oils and greases in the prod-

uct to be packed. Some bagged products are

packed hot from the ovens. Inks for this kind

of application must have sufficient heat resis-

tance to prevent blocking of the bags during

the cool-down period.

Printing on potato chip, popsicle and ice

cream freezer bags usually require inks with

good light fastness and resistance to bleeding

and blocking in water, greases, waxes, oil and

most fats. On ice cream wraps and other bags

that receive subsequent molten wax or other

hot melt coatings, the inks must not prevent

sealing, and the colors must not bleed in the

molten wax or coating.

INK 11

End-use requirements

determine the choice of

ink for printing on

multiwall sacks or paper

bags. The gamut of

applications ranges

from lawn product and

pet food bags to grocery

and retail shopping

bags.

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