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ment, automatic splicing, hydraulics and air-

pressure devices. More controllable drying

systems had to be developed.

The new substrates also demanded even

better ink formulations. Ink manufactures

found alternatives to the aniline dies and co-

solvent inks appeared, using a mixture of

aliphatic hydrocarbons and alcohol as a sol-

vent. Ink chemists then began using other col-

oring agents that everybody considered safe

but even though printers were using these

newer inks, the name aniline printing stuck.

OFF-PRESS MOUNTING

AND PROOFING

In the 1940s, the first plate-mounting and

proofing machine was introduced to mount

plates accurately and completely off-press.

The mounter-proofer boosted production by

minimizing downtime between jobs.

Accurate proofs of each cylinder and of the

complete multicolor job became a reality,

with the prepress proofs showing whether

each job would print in register. The costly

trial and error of correcting misregister

became but a bad memory from the days of

on-press mounting.

ANILINE PROCESS NAME CHANGE

In 1949, the Federal Bureau of Animal

Industries recognized that the dyes and pig-

mentation being used in the new aniline ink

formulations were the same as those in

other printing processes, and removed the

ban for use on food packaging. Aniline print-

ing could not shake the stigma, however,

especially in the minds of customers, and it

was not long before people objected to the

name aniline because of the bad connota-

tions and plain inaccuracy.

In March 1951, a campaign to change the

name “aniline printing” to a more suitable

one was started. The industry’s response was

enormous, with well over 200 submissions. A

subcommittee of the Packaging Institute’s

Printed Packaging Committee was specially

formed to pick the best submission. On

October 21, 1952, at the 14th Packaging

Institute Forum, the announcement was

made that the “flexographic process” had

been the overwhelming choice. The industry

world-wide embrace the new name, and ani-

line’s bad reputation was history.

MOLDED-RUBBER PLATES

The introduction of phenolic-resin mold-

ing boards for rubber-platemaking in the

1950s marked another breakthrough.

Mechanically etched or photo-etched graph-

ics on a metal master plate could be trans-

ferred to a phenolic-resin molding board.

This board was then used to vulcanize rub-

ber copies of the metal master. Using this

technique, finer, more accurate print copy

could be produced.

Charts were developed for figuring the

stretch of rubber plates when curved and

mounted onto round cylinders, and a special

camera was developed for accurate image

distortion of photographic negatives to

allow for image elongation.

PHOTOPOLYMER PLATES

The 1970s saw the introduction of photo-

polymer printing plates. By the mid-1970s,

five companies in the United States began

selling photopolymers for the production of

photopolymer plates.

Photopolymers began to replace the mold-

ed rubber previously used for the manufac-

ture of printing plates. At first, these pho-

topolymer plate materials were not very

chemically stable and often became brittle

from ozone exposure or tacky from ink addi-

tives. Since then, plate manufacturers have

improved platemaking materials and re-

search in the photopolymer plate field is

ongoing.

INTRODUCTION 15

16 FLEXOGRAPHY: PRINCIPLES & PRACTICES

PLATE MOUNTING

More aggressive adhesives were necessary

to keep the polyester plate backings from

pulling free. In 1975, stickyback was devel-

oped to attach photopolymers to plate cylin-

ders. Several companies came out with

cushion-foam stickyback at that time. These

add more cushioning under the plates and

help improve on-press impression.

INK AND DRYING SYSTEM

Before 1940, dryers in general were a

problem, and gas-flame dryers were danger-

ous. By the early 1950s, safer, more adequate

dryers appeared.

One major contribution to productivity

was the introduction of hot-air circulating

systems for presses. While these initial dryer

designs were crude compared to our modern

drying systems, they set the pace for today’s

units and allowed the use of highly pigment-

ed inks that dried at higher press speeds.

Stack presses had greater distances between

stations, allowing space for inter-station dry-

ers. The new dryers allowed press speeds to

be increased substantially.

In the 1950s, the main resin in many inks

was shellac. However, shellac is a natural

resin that can vary in quality and character-

istics, and therefore a substitute had to be

found. Polyamide resins were developed

and inks based on them appeared in 1955.

Polyamides give superior gloss and adhere

well to polyethylene. An alcohol-ester sol-

vent added to the resins kept the ink stable

and fast-drying on the press, and made wide-

web speeds up to 750 feet per minute possi-

ble. Polyamides are still called the “all pur-

pose ink” because they print well on most

substrates, absorbent or nonabsorbent.

In the corrugated arena, flexographic

printing with water-reducible inks began in

1957. The first flexo press was shipped to

Columbus, Ohio.

ACCURATE MULTICOLOR

REGISTRATION

Among the first corporations to develop

modern registration systems were the

Harley Company, which came out with a

mechanical mounter and an optical mounter

and proofing machine, and Mosstype, which

introduced an optical mounter-proofer

machine in the 1960s. These optical moun-

ters created a reflected image from a proof-

ing cylinder onto the cylinder. The center-

lines reflected on the cylinder made plate

mounting more accurate. Tighter registra-

tion was made possible and, in turn, better

results were obtained.

The 1960s saw the overall design refine-

ment of aniline press into the flexographic

press seen today. Several European press

manufacturers continued their development

of the central-impression press design,

which evolved in the early 1940s and was

used in the United States and Canada. It was

not until the 1950s, when polyethylene and

polypropylene began to replace cellophane,

that demand really took off. The central-

impression press enabled more control over

stretchable substrates than the stack press

with its unsupported web between print sta-

tions.

Early narrow-web label presses were built

using the three main types of flexo press

designs: stack, central impression and in-line.

Label-press web widths of 4" to 6" dominated

the market for many years. During the 1970s,

though, label printers wanted wider web

widths so they could print larger labels and

more of them across and around cylinders,

and more color stations. Manufacturers

responded to this demand.

Today’s narrow-web, in-line label presses

vary in web width capacity from 4" to 20" and

six- to eight-color stations are very common.

By the 1980s, most label presses were of the

in-line type and currently, they continue to

dominate this market.

RECENT DEVELOPMENTS

The past two decades have seen an explo-

sion of technology in the flexographic print-

ing industry. Without doubt, the biggest

development has been the digital revolution,

which has impacted all aspects of the flexo

process, from the initial design to produc-

tion and printing.

Prepress

Today, nearly all prepress is electronic,

including design generation, image capture

and manipulation, page assembly, and final

output to film or directly to plate. A recent

development has been the lower cost of

measurement devices, particularly in the

measurement of color. This is leading to

entirely new workflows, in which color is

controlled or “managed” from initial cre-

ation to final ink-on-paper.

Presses

Computer control has revolutionized the

operation of the modern press. Digital drives

on the print decks allow for precise, repeat-

able-impression setting. Video web inspec-

tion is common and automatic registration

between color stations is available.

In the 1980s, preprinted linerboard

emerged. Rolls of kraft linerboard with

white-coated surface can now be printed on

advanced stack and CI presses in one to nine

or more colors. Excellent process-color

print quality, with screen sizes of 85- to 150-

line can be printed. The finished rolls are

then combined on a corrugator and finished.

These are high-quality printed boxes – some-

thing that was not possible on traditional

sheetfed combined board.

The introduction of high-tech presses to

corrugated postprint in 1995 has had a

marked effect on the quality of graphics.

Sheetfed presses printing on combined cor-

rugated board can produce multicolored

graphics that rival the quality found on

preprint linerboard.

Anilox

Ceramic plasma-coating, developed for

the aerospace industry, has been adapted for

use on anilox rolls, replacing the chromium

plating. Fine, ceramic powder heated to

nearly 9,000° F is sprayed onto anilox cells

to make them tough and long-wearing. The

use of reverse-angle steel doctor blades, pos-

sible because of the increased durability of

ceramic coating, gives a more precise con-

trol of ink metering.

Since in the 1980s, lasers have been used

to etch ceramic-coated anilox rolls, and

improvements in this technology continue.

Today, precisely engraved ceramic anilox

rolls, with up to 1,200 cells per linear inch,

are available to the flexographic printer,

allowing flexo to challenge most other forms

of printing.

Printing Plates

New polymer plates are being developed

for all areas of flexography, including news-

papers. In the past decade, water processing

of photopolymer plates was introduced.

In the early 1990s the use of electronic

prepress in flexo began to grow. During the

early 1990s, most graphics were still being

produced using cut-and-paste art boards and

photographic negatives to produce the flex-

ographic printing plates. By the end of 1997,

all graphics were computer-generated and

laser-engraved directly to the platemaking

negative. This computer and laser technolo-

gy has led to the development of direct imag-

ing using a laser driven from a computer for

both laser-engraved rubber plates and com-

puter-to-plate (CTP) systems for photopoly-

mers. For rubber plates, the laser ablates

away the rubber in the nonimaged areas and

creates the finished rubber plate directly. In

the photopolymer CTP system, a mask is

applied to the uncured photopolymer. This

mask is ablated away in the non-image area

by the laser and the plate is then processed

conventionally.

INTRODUCTION 17

18 FLEXOGRAPHY: PRINCIPLES & PRACTICES

Plate Mounting

In the 1980s, pin-register systems for pho-

topolymer plates came along, and many

firms introduced accurate register systems

for both narrow- and wide-web press cylin-

ders. The following is just a sampling of the

different solutions available.

• pin register with drilled holes in nega-

tives and plates ;

• microscopically controlled, one-piece

plate mounter;

• macro-lens video camera system with

plate-hole puncher.;

• macro-lens video camera system with

micro-dot register.

Today, we see new and improved systems

for mounting individual small plates across

and around cylinders with pin-register speed

and efficiency. Currently, one-piece plate

mounting is only limited by the sizes of pre-

pared photopolymer sheets provided by sup-

pliers. Wide-web presses may require more

than one plate to be mounted accurately and

quickly. Prepress plate-registration systems

have been perfected and introduced.

Mounting plates on sleeves continues to

grow in popularity. Sleeves come in a variety

of materials, such as metal or composites,

and different constructions, such as varying

wall thickness or cushioned sleeves. Some

advantages of using sleeves are:

• quick, on-press plate remounting when

a job is rerun;

• flexibility of different repeat length with

the same gearing; and

• the ability to change to a thinner plate

using the same undercut plate cylinder.

Sleeves are also used in computer-to-

sleeve (CTS) systems. In these systems, the

sleeve is coated with uncured photopolymer.

In one method, the photopolymer is exposed

on the sleeve using a film negative. In a sec-

ond method, the photopolymer is masked

and a laser ablates the mask similar to a

computer-to-plate (CTP) system.

Inks and Dryers

Growing concern about the environment

has focused national attention on the indus-

try’s impact, and flexo printers have had to

keep a close eye on air emissions from

plants. The Clean Air Act of 1980 mandated

a 35% cutback in these emissions and the

current Environmental Protection Agency

controls are even more stringent.

Catalytic incineration has been introduced

to cut down on emissions; a heat exchanger

allows the hot air from the incinerator to

heat incoming air. This double use of the hot

air slashes energy costs. Another way to cut

back on emissions is to use water-soluble

inks or to reduce solvent content of the inks.

Ink chemists have developed a means of pro-

viding water-soluble inks that work well on

nonabsorbent substrates. Scuff resistance

and good adhesion to nonabsorbent sub-

strates using water-based ink can still be a

problem. On-press corona discharge units

are used after the in-feed web guide to treat

the web immediately before printing,

increasing the adhesion of water-soluble

inks. Many converters are reverse-side print-

ing on transparent films, in which in-line

lamination seals the ink between the lamina-

tions where it can not be scratched.

his section gives an overview of

the flexographic process. The

process starts with the design

itself, which must take into

account the particulars of flexo

printing in order to assure a

smooth, trouble-free work flow all the way

to the final conversion of the printed piece.

BASIC ELEMENTS OF

FLEXOGRAPHY

Starting with the design to be reproduced,

each flexographer involved in the process

must understand the techniques of handling

the different elements of flexo printing as

they relate to a commercially acceptable job

(Figure

). The complete printing job starts

with a team of people which includes the

graphic designer, print buyer, structural engi-

neer – in the case of a package design – and

printer. The printer may handle the prepress

function, but many times this is a separate

company or team member. The team selects

the appropriate printing method, whether it

will be a line job only, contain screens or will

be a full process-color job. The printing

method, in turn, will be determined by the

design considerations for the particular job.

These considerations include the product

and product image, use of space and brand

identity, typography and color usage.

Artwork Design and Prepress

Design and production art (mechanicals

or black-and-white art) for flexographic

printing are prepared largely the same way

as art for other printing processes. But there

are some differences that must be kept in

mind. With the advent of computer graphics,

direct digital-imaging, digital proofs, laser-

imaged films and in some cases digitally

imaged printing plates, the design copy is

often not seen until it is on the actual pack-

age. What is seen on the computer screen or

on the color proof is not necessarily the

same as the finished printed image. The con-

cept proof is used to indicate the graphic

alignment and general layout of the design.

To see a true representation of the final

INTRODUCTION 19

The Flexographic Process

A successful flexo

printed piece results

from a team effort that

works within the para-

meters of design con-

siderations and printing

processes.

The concept proof is

used to indicate align-

ment of graphic ele-

ments in the package

layout, while the con-

tract proof is used to

show accuracy in color.

Team

Graphic Designer

Print Buyer

Structural Engineer

Printer

Design

Considerations

Typography

Color Usage

Negative/Positive Space

Product Image

Brand Identity

Printing Methods

Line

Screen

Process

America’s Choice Butter

America’s

Choice

Butter

America’s

Choice

Butter

America’s Choice Butter

America’s

Choice

Butter

America’s

Choice

Butter

Concept Proof

Indicates layout of

graphic elements.

Not intended for use as a

target for color matching.

Contract Proof

Indicates layout of

graphic elements.

Intended for use as a

target for color matching.

20 FLEXOGRAPHY: PRINCIPLES & PRACTICES

product, a contract proof is generated which

accurately shows the colors in the final

printed piece (Figure

To do a competent job, the designer and

production artist must be thoroughly famil-

iar with the requirements of the flexograph-

ic printing process, especially in the way it

differs from other printing processes. Most

of these differences relate to:

• choice of printing plate (molded or pho-

topolymer, thick or thin, hard or soft,

digital or conventional);

• distortion characteristics of the plate

material (Figure

);

• shrinkage in molded-rubber plates;

• choice of line screens for halftone and

process color (below 65 lpi to 150 lpi

and above);

• print-element growth (dot and bar code

gain, minimum highlight dot, maximum

shadow dot);

• press design (narrow or wide web,

sheet or roll fed, stack, central impres-

sion or in-line);

• two-roll or doctor-blade inking system;

and

• type of substrate (i.e. film, foil, paper,

paperboard, corrugated, newsprint).

Each industry segment (wide web, narrow

web and corrugated postprint) has different

requirements. Properly prepared designs,

appropriate electronic prepress adjust-

ments, image gain allowances, calibrated

and consistent negatives and plates all make

high quality flexo printing possible. Never-

theless, it should always be kept in mind that

the final print result can be no better than

the original copy.

Inks

Flexography uses low-viscosity inks which

dry very quickly between the print stations of

a press. Solvent-based, water-based and

ultraviolet-curable inks are used in flexo for

a wide variety of requirements. The viscosity,

or thickness of the ink is like that of a free-

flowing liquid such as light oil or a light

syrup. Paste inks have been tried in the

anilox system where quick drying was not so

important, but a doctor blade was found to

be a necessity.

Solvent- and water-based printing inks are

composed of a colorant and a liquid vehicle.

The colorant, whether pigment or dye, pro-

vides the visual sensation of color, and

hence appearance, readability and aesthetic

value. A flexographic ink vehicle, consisting

of resin, solvent and additives, does several

jobs: One is to carry color from the ink foun-

tain to the substrate; others include setting

viscosity, drying speed, pigment strength,

tack and surface tension. It also binds the

colorant to the printed surface in a phenom-

enon known as adhesion.

Pigments are small particles that are insol-

uble in the ink vehicle. They are usually more

opaque than dyes, which are soluble.

Pigments also have better lightfastness than

dyes and are more resistance to materials

likely to come in contact with printed matter.

Many different resins are used, either

alone or in combinations, to give adhesion to

different substrates and the ability to with-

stand specific processing and end-use

requirements, such as heat resistance, rub

resistance, etc. Types of inks for different

Flat images tend to

elongate or distort

when printed, caused by

the curvature created by

the flexible plate.

Normal Image

Distorted Image

uses can therefore be classified by the resins

they contain. Examples include polyamides,

nitrocellulose, water-based and acrylics.

Additives provide special effects, such as

slip, low or high coefficient of friction, or rub

resistance.

A printer has to be very careful in choos-

ing and using ink. Adhesion, block resis-

tance, heat resistance, rub resistance and

lightfastness may be fine on one substrate

but terrible on another. Different ink sys-

tems require different control. For example,

when using solvent-based inks, selecting the

right solvent is essential. Viscosity control is

important for maintaining color intensity

and print quality for both solvent- and water-

based inks. Other ink considerations:

• Water-based inks require good pH con-

trol and balance.

• Metallic and flourescent inks lead to dif-

ferent problems since they are general-

ly weak and don’t dry as well.

• UV inks are more forgiving in terms of

viscosity control. The wise printer

selects ink with the total job in mind.

UV Flexo

Narrow-web presses have incorporated

ultraviolet-curing equipment into their

design since the 1970s. These units were

originally used for the setting of UV-curable

overprint varnishes. At the beginning of the

1990s, pigmented UV-curable flexo inks

became commercially available from a num-

ber of suppliers. Since that time, UV-flexo

printing has grown rapidly among narrow-

web converters.

UV-curable inks are 100% solids in the

sense that there is no solvent to dry or evap-

orate. The entire ink film deposited on the

substrate remains and is cured or hardened

by the UV light. Their fluid character is

obtained by the use of low molecular-weight

oligomers that are diluted with reactive

monomers. Typically, UV-flexo inks have a

viscosity between 500 and 1,700 centipoise.

Some may have an appreciably higher vis-

cosity. Since they do not have any volatile

dilutents, such as alcohol or amines, they are

more stable than other flexo inks. This char-

acteristic gives them greater color consis-

tency and requires less attention from the

press operator while the job is being run.

UV inks are hardened, or set, through a

process of polymerization or curing initiated

by a sufficient quantity of ultraviolet energy.

Liquid inks are converted into solid-colored

polymers or plastics. Since they are are hard-

ened through a process of polymerization,

they do not release volatile organic com-

pounds (VOCs) when they are used. In areas

of strict environmental regulation, this may

be a significant benefit.

Substrates

Flexography is unique because it was

developed primarily for printing packaging

materials. Board, paper, foil and film pack-

aging substrates are commonly supplied in

roll form for feeding into form-and-fill, over-

wrapping, bag-making and other continuous

web-processing machinery. For these appli-

cations, roll-to-roll or roll-to-cut printing is

required.

Because there are so many kinds of paper,

board, plastics, foil and film, the term “sub-

strate” applies to any surface to be printed. If

the material is reasonably smooth and comes

in roll form, chances are it can be printed by

flexography. As a matter of fact, the vast

number of substrates on which flexography

can print is one of its greatest advantages.

Naturally, for high-quality images, the

smoother the substrate the better.

The Printing Plate

As the first chapter pointed out, flexogra-

phy is like letterpress in that both print from

a raised-image surface (see Figure

Flexographic printing plates, whether mold-

ed from natural or synthetic rubber com-

pounds, or imaged using light-reactive

INTRODUCTION 21

22 FLEXOGRAPHY: PRINCIPLES & PRACTICES

photopolymer resins, are generally made

from flexible, elastomeric materials. The ink

is carried by the raised portion of the plate

and transferred to the substrate. The raised

image is obtained by removing and lowering

the nonprinting areas through cutting, mold-

ing, etching, dissolving or laser engraving.

Molded Printing Plates. Using a mold,

uncured plate gum is vulcanized under heat

and pressure. This mold or matrix is made

by vulcanizing an uncured phenolic-coated

board with a magnesium etching or other

suitable original. Numerous duplicate plates

can be made from a cured mold.

The molded printing plate must evolve

through several stages that include camera-

ready art, photographic negative, a master

engraving, mold and finally, printing plate.

The many steps involved in the manufacture

of molded plates may substantially reduce

the image quality. The increased use of com-

puterized electronic prepress and high defin-

ition photopolymer plates has made the

molded-rubber plate almost obsolete.

The Photopolymer Printing Plate. Unlike rub-

ber printing plates, photopolymer plates are

not molded. The light-reactive polymer resin

is exposed to ultraviolet light to selectively

cure the resin to a solid and processed using

either an aqueous or solvent-based solution.

The term photopolymer refers to a range

of polymers that react to ultraviolet light

energy. These come in precast sheets of

varying size and thickness, or in liquid form

for custom sizing and gauging. Photo-

polymer materials are available in varying

levels of durometer. Ordinarily, the printing

plates are backed with a dimensionally sta-

ble polyester support sheet that helps con-

trol plate distortions during processing,

plate-mounting operations and repeated use.

In making the relief-printing plates, film

negatives of the art are positioned in contact

with the plate material. In the case of digital

plates, the image is created by laser ablation

of an opaque mask on the surface of the

photopolymer. The image is transferred to

the plate material by exposing to ultraviolet

radiation. The portions of the raw material

that receive light through the clear areas of

the negatives or mask are rendered “set” or

hardened, or, more properly, polymerized.

The areas protected from the UV light by the

opaque portions of the negative remain

uncured and are brushed or washed away by

either a water- or solvent-based solution,

leaving the hardened, raised printing areas.

The photopolymer plate becomes the final

printing plate, eliminating the need for an

original and mold of any sort. Each pho-

topolymer plate is a faithful copy of the

image on the negative film and is therefore

an original plate, thus reducing any loss of

image fidelity.

Photopolymer resins are made specifically

for use with alcohol, water, oil or glycol inks,

or combinations of these. Because of their

good print performance and ink-transfer

qualities, photopolymer plates are quite pop-

ular for halftone and process color jobs.

Design Rolls

Design rolls are mainly used to produce

continuous-repeat designs. The procedure

involves vulcanizing rubber to a bare cylinder

and grinding the rubber to a desired diameter

for the exact print repeat length needed, then

hand-cutting the face of the rubber to remove

the nonprinting areas. A far more popular

method laser engraves the image into a

ground rubber roll directly from the comput-

er generated art work. Seamless imaged pho-

topolymer rolls are also available.

Mounting and Proofing Devices

Usually, rubber or photopolymer printing

plates are mounted to double-sided sticky-

back that comes in a variety of adhesive

strengths and are up to 18" wide. Some are

suitable for photopolymer plates, while oth-

ers work better with rubber.

Off-press plate mounting and proofing

devices are a basic requirement for good

flexo printing practices. These machines pro-

vide a means for mounting multicolor jobs in

exact register. They are also used to mak-

eready the printing plates to achieve uniform

impression across and around the cylinder

before installing the job in the press.

Furthermore, full-color proofs can be made

that graphically indicate color trapping, print

copy, print positioning and plate height uni-

formity. The proof can also be folded into a

mockup of the finished job to confirm that all

copy is in the right place when the product is

enclosed in three-dimensional form.

Mounting and proofing registration sys-

tems are generally:

• optical, using a split-mirror principle;

• punched hole and pin; or

• video microscope.

Mounting plates on sleeves enables jobs to

be remounted on press quickly and with

excellent, repeatable registration.

PRESSES

The four most common press-frame

designs are:

• central impression or CI (Figure

)

• stack (Figure

)

• in-line (Figure

)

• sheetfed (Figure

The central-impression press has a com-

mon impression cylinder around which two

to eight print stations can be positioned. The

most common CI press in use today is six

colors. The stack press is built with print sta-

tions literally “stacked” one above the other.

INTRODUCTION 23

A typical six-color

central impression

press supports all of

its color print stations

around a single, large

impression cylinder.

A In Feed Guide

B Nip Roll

C Central Impression Cylinder

D Inter Station Dryer

E Hydraulic Vertical Lock

F Hydraulic Horizontal Lock

G Fine Impression Adjustment

H Impression Indicators

I Metering Roll

J Anilox Roll

K Plate Cylinder

24 FLEXOGRAPHY: PRINCIPLES & PRACTICES

A typical six-color wide-

web stack press, where

individual color print

stations are stacked one

over the other on one or

both sides of a main

press frame.

In a typical narrow-web

in-line press, color print

stations are configured

horizonatally, providing

versatility and access-

ability to the printing

stations.

D E

A Infeed Tension Nip Rolls

B Metering Roll

C Anilox Roll

D Plate Cylinder

E Impression Roll

F Print Station

G Between Station Dryers

To Main Dryer

D D D

A Unwind

B Web Inverter

C Print Units

D Die Cutting

E Waste Removal

F Lamination

G Rewind

H Between Station Dryers

CCCC

H H H