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

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42 FLEXOGRAPHY: PRINCIPLES & PRACTICES

course, that all handling guidelines are fol-

lowed.

• Resin (Oligomer). The resin in radiation-cur-

able inks is actually called an “oligomer.” As

in conventional inks, the resin is the chemi-

cal backbone of the ink. Among others, it

provides the body, wetting ability, binding

and functional properties of the ink.

• Photoinitiators. In UV inks, the photoinitia-

tor is the chemical which becomes “excited”

and starts the curing reaction when exposed

to ultraviolet light. The excited photoinitiator

passes that energy to the other components.

At that point, any component which be-

comes excited has the ability to attract other

components to itself and transfer energy to

the newly attracted component. Photoiniti-

ators are not required in EB inks. The highly

charged energy of the electron beam is suffi-

cient to activate polymerization.

• Additives. These materials include waxes,

wetting agents and rheology modifiers. They

provide the added customizing touches to

the ink.

• Pigments. Pigment particle size and con-

centration can affect the curing rate of a UV

ink. Pigments are selected for color and

wetability, or oil-absorption ratio; and for

their receptivity to UV light. Among process

colors, yellow and magenta are the easiest to

cure, followed by cyan and black. Because

black tends simply to absorb the wave-

lengths of UV light, more energy is required

for a satisfactory cure.

Polymerization. In conventional UV or EB

chemistry, any component that has reacted

is called a “free radical.” It is the free radicals

that have the energy to keep the curing or

“polymerization” chain reaction going. Each

chemical chain continues growing until one

of two things happen: The excited chains

use up all of the available components or the

UV/EB source is removed and a foreign sub-

stance, such as oxygen, quenches or halts

the reaction.

In contrast, irradiation of the photoinitia-

tor used in a cationic UV ink generates a

strong “Bronsted” acid, which reacts with

the other components (aliphatic epoxides

and vinyl ethers). This reaction varies from

free-radical UV chemistry in that removal of

the UV source does not quench the reaction.

The cationic ink or coating continues to cure

for up to 24 hours after UV exposure.

As flexo printing improves in quality and

application, the need for specialized physi-

cal properties also continues to grow.

Increased chemical or product resistance is

the largest attraction to this process. The

additional benefits of low energy cost, mini-

mized downtime and the reduction of VOCs,

will also continue to drive the market.

FLEXOGRAPHIC INK

MANUFACTURING PROCESS

Flexographic inks, whether solvent- or

water-based, are generally manufactured

using the same processes: mixing, dispersing

and filtering.

Many ink companies produce their inks

from scratch using dry pigments for water-

or solvent-based inks, or press-cakes for

water-based inks. Others choose to purchase

predispersed concentrated bases, and let

them down with vehicles of their choice.

These concentrated bases are normally pro-

duced by the same methods as finished inks.

Many ink makers produce their own con-

centrated bases in-house rather than going

outside for them.

The manufacture of a flexographic print-

ing ink (Figure

) typically begins with the

mixing of the raw materials to produce a uni-

form blend. From there, the product contin-

ues into the dispersion stage where the actu-

al work in breaking up the agglomerates is

completed.

There are a large number of flexographic

ink manufacturers in the United States.

Because of the present health/safety and

environmental regulations, a fair number of

these companies are dedicated to water-

based inks, while other companies still man-

ufacture both water- and solvent-based inks.

In general, the same equipment can be used

for the manufacture of either.

Mixing

The dispersion step begins with a pigment

made up of clusters called agglomerates.

Each agglomerate is made up of smaller

crystals that can have either air or water

absorbed on its surface. Mixing, as the first

step in the dispersion process, separates

these agglomerates into a homogenous

blend with the ink vehicle. The more effi-

ciently this is done, the easier and less

power-consuming the actual dispersion step

becomes. Typically two types of mixers are

employed in the manufacture of flexograph-

ic inks, cavitation and rotor stator mixers.

The cavitation mixer uses an impeller to

produce a vortex. The impeller size and

design varies by manufacturer and vessel

size. Shear is at a minimum with this type of

mixer. Rotor stator mixers have an impeller

rotating in a so-called disintegrating head

(stator). This head is fixed, and there is a

very small clearance between the rotor and

stator. This configuration allows for some

shear to be generated. Some inorganic pig-

ments such as titanium dioxide white can

truly be dispersed during the mixing stage,

but, in general, this process is just a precur-

sor to actual dispersion, separating but not

breaking up the agglomerates.

Dispersion

To achieve dispersion (Figure

), pig-

ment agglomerates are broken up as close to

the individual crystal size as possible. The

degree of dispersion is really how close one

can get to this ideal. The dispersion process

must therefore accomplish three things to be

considered successful:

• It must remove the air or water on the

crystal surface and replace it with the

desired vehicle. This is called wetting.

• The particles must be separated and

uniformly distributed throughout the

vehicle.

• The crystal surface must be stabilized

so that re-agglomeration or flocculation

will not occur.

To accomplish this, a combination of

shear and impact is used. By applying these

forces throughout a viscous liquid, the

agglomerates are literally broken down and

then the absorbed air or water is “wiped off.”

The vehicle then replaces the air and/or

water on the surface of the pigment particle.

Stability is normally accomplished in one of

two ways:

INK 43

The manufacture of a

flexographic printing

ink typically begins

with the mixing of

the raw materials to

produce a uniform

blend. From there, the

product continues into

the dispersion stage

where the actual work

in breaking up the

agglomerates is

completed.

To achieve dispersion,

pigment agglomerates

must be broken up as

close to the individual

crystal size as possible.

Solvent

Resin

Mixing

Dispersion

Filtration

Packing

Pigment

Agglomeration

Air and water

on pigment

particle surface

True Dispersion Flocculation

44 FLEXOGRAPHY: PRINCIPLES & PRACTICES

1.By the introduction of ions or molecules

capable of satisfying the surface charges

on the solid pigment particles. These

ions give each particle a similar, uniform

charge resulting in repulsion, or

2.The use of non-ionic materials that

adsorb onto the pigment surface and

produce steric hindrance. This complex

also results in particle repulsion.

Typically, ink resins will accomplish the

required stability. If they fail, surfactants

specific to the pigments are used in the for-

mulations.

The first real piece of dispersion equipment

used in the manufacture of flexographic inks

was the pebble or ball mill (Figure

). The

mill is a horizontal closed-end cylindrical

vessel filled to about half its height with

porcelain or steel balls. This piece of equip-

ment combines mixing with dispersion. It

has a loading hatch on top and an evacuation

valve on the opposite side. The ball mill is

filled with the raw materials, sealed and

rotated on its horizontal axis. The mill media

is carried up to the top of the mill during the

rotation and cascades down. This action

produces the impact and shear to break up

agglomerates and typically 16 hours will give

the desired results.

These mills are manufactured in a wide

assortment of sizes and use a variety of

media.

There are many advantages obtained using

ball mills for dispersion:

• Dispersion is uniform throughout the

batch.

• Grinding time can accurately control

degree of dispersion.

• Production procedures can be stan-

dardized for the mill used.

• No premix is required.

• There is no loss of volatiles. The vessel

is sealed.

• Highly concentrated bases can be pro-

duced for later letdown.

• The process is not very labor intensive.

The mill is opened, loaded, sealed, run

for 16 hours (normally between 4:00

p.m. and 8:00 a.m.), the degree of dis-

persion checked and unloaded.

There are, however, some major disadvan-

tages to this type of equipment:

• Batch size is limited to mill capacity.

• The time factor is the same regardless

of mill size.

• Power costs are high.

In the early 1950s, it was realized that if

one could produce more impacts per unit of

time, then dispersion could be accomplished

sooner. This improvement was actually

accomplished by combining a premix with a

very small but dense media – sand – and agi-

tating it with an impeller. The problem, how-

ever, was how to separate the dispersion

from the sand. Continuing along these same

lines, in 1958, DuPont’s S. Hockberg patent-

ed the process of sand milling (Figure

The premix is pumped upward through the

mill, a vertical cylinder containing the

media. A series of plates is mounted on the

agitator shaft, and the top of the mill is

screened to prevent the media from escap-

ing with the dispersed ink. Flow rate

through the mill is controlled by the pumps

The pebble or ball mill-

combines mixing with

dispersion. This action

produces the impact

and shear to break up

agglomerates. The ball

mill is filled with the

raw materials, sealed

and rotated on its hori-

zontal axis until the mill

media is carried up to

the top of the mill and

then cascades down.

and determines dwell time. This in turn con-

trols the degree of dispersion.

This piece of dispersion equipment over-

came the disadvantages of the ball mill. To

minimize the disadvantages, there have been

numerous innovative improvements made to

the basic sand mill. Among these were the

following:

• Sand has generally been replaced by a

more dense, somewhat larger media.

This media maximizes impact forces

and minimizes the possibility of conta-

mination of the ink with sand grit.

• The screen was closed off to the atmos-

phere, and a true mechanical seal was

added, allowing operation under a

slight positive pressure. This change

minimized any loss of volatile ink com-

ponents and allowed heavier viscosities

to be pumped through the mill without

carrying up the media and overflowing

the mill.

• The vertical mill has evolved into the

horizontal mill (Figure

). This is said

to improve performance by creating

better flow and increasing the media

loading capacity from 50% to 80% of the

mill volume.

• Larger diameter shafts are generally

equipped to carry cooling water. Disk

designs have been modified to increase

the number of impacts in a given time.

Ink companies are utilizing technology to

achieve the highest quality ink possible.

Most equipment is available in lab or pilot

size, so production parameters can be opti-

mized in advance and new raw materials can

be tested under actual conditions.

Filtration

The final process prior to drawing the fin-

ished ink into kits, drums or tote tanks is the

filtration step. Before the use of very fine

screens and low cell volumes, ink manufac-

turers regularly used either cheesecloth or a

fine grade of organdy to filter the finished

inks. This process took out any large parti-

cles that could cause problems on the press.

The anilox cells were large enough so that

any undispersed agglomerates would nor-

mally not cause problems.

Today, the advent of fine-screen halftone

and process printing requires the use of very

fine anilox screens and low volume cells.

This change has driven the need for much

better filtration. The undispersed agglomer-

ates can plug these cells and lodge in doctor-

blade nips, causing the loss of quality print-

ing. There are several types of filters in com-

mon use today. Any one of them, used prop-

erly, will remove any large and fine particu-

late matter that might cause problems.

Bag filters are one of the most common

INK 45

Original sandmilling

process required premix

to be pumped upward

through the mill in a

vertical cylinder. The

media passed through

a series of plates on the

agitator shaft. Pumps

controlled flow rate, set

dwell time and con-

trolled dispersion.

The vertical mill evolved

into the horizontal

mill, where performance

is improved by creating

better flow and

increasing the media

loading capacity from

50% to 80% of the mill

volume.

46 FLEXOGRAPHY: PRINCIPLES & PRACTICES

types of filters used. Felt-type bags with sizes

rated from 100 microns to 25 microns are

available and can be used with gravity flow or

commercial pumping systems. There are cau-

tions to be noted.

• With the use of pumping systems, it is

critical that recommended operating

pressures not be exceeded. If they are,

the filter might be bypassed and unfil-

tered ink will contaminate the batch.

• With gravity flow it is common to see

plant workers “beating” the filters with

ink knives to get faster flow. Beating

deforms the bag and can allow larger

particles through.

The cartridge filter is often used to filter

flexographic ink. Natural or synthetic fibers

are wound around a porous core, and the ink

is pumped through the core and fibers. A

wide variety of micron-size cartridges are

available. Here too, if pressures are exceed-

ed, unfiltered ink can bypass the filter and

contaminate the batch.

A new type of filter that is beginning to see

more use in the manufacture of flexographic

inks is the vibrating screen filter. Ink is

pumped onto a rigid vibrating sieve and the

large particles are retained on the screen. A

large number of sieve sizes are available,

some with new innovations like self-clean-

ing filters.

Most flexographic ink manufacturers

today are using 100-micron filters as their

standard size. For special inks, 50-micron or

even 25-micron filters are used. Also,

mechanical systems are equipped with mag-

netic filters to ensure that no contamination

from metal particles may have been intro-

duced during the shot mill stage.

INK 47

Ink Prepress

Ink is a small part of the

total cost of the printed

product.

The purpose of any

ink-blending operation

is to deliver the right

amount of the right ink

to the right press at the

right time.

n the past 10 years, there has been a

growing trend in the prepress area

toward treating ink management as a

key process. Converters have found

that attention to this area pays high div-

idends in in the form of reduced waste

and increased pressroom productivity.

Professional management of the inkroom is

now an essential part of running a competi-

tive converting operation. The days of the

dingy, dirty manual inkroom are rapidly dis-

appearing and being replaced by computer-

managed information systems and automat-

ed product dispensing. What goes on in the

inkroom today?

Most printing operations have one or two

people controlling the department. Besides

providing ink to the presses, the inkroom

usually controls color standards, ink pur-

chases, inventory, waste, waste reports,

usage records and the very important volatile

organic compound (VOC) reports. In addi-

tion, the inkroom is a management resource,

having essential input into purchases of new

equipment and in many process improve-

ments.

Ink is a small part of the total cost of the

printed product. For example, cost figures

for liquid packaging converters show that

ink is only 2% of the total (Figure

). It is

apparent that ink which is defective or mis-

managed can cause considerable waste in

the converting operation. Problems due to

incorrect ink use can reveal themselves

immediately, or in some cases further down

the value-added chain, where the cost of the

problem is multiplied.

The purpose of any ink-blending opera-

tion is to deliver the right amount of the right

ink to the right press at the right time. This is

a simplistic view of a fairly complex process

and easier said than done (Figure

Waste due to bad ink can have many caus-

es. Some typical causes of production waste

due to ink-related issues are;

• off-shade;

• wrong ink delivered to the press;

• too little or too much ink delivered to

the press; or

• the ink is not at press-ready viscosity.

Board: 71%

Other: 15%

Labor:

Ink: 2%Manufacturing Fixed: 3%

Manufacturing costs

82% of total,

Ink 2% of total

Documented Press Returns

Ink Feedback

Process Changes

Inkroom

Press

Dept.

Consistent Ink

Troubleshooting

Color Standards

Ink Stats

48 FLEXOGRAPHY: PRINCIPLES & PRACTICES

Another common cause of press down-

time, due to ink-related waste issues, is that

the press department has not been instruct-

ed on how to use the ink system in question.

For example, the job may meet all appear-

ance standards yet fail an adhesion test

because the press department was not

aware of the web temperature requirements

to pin on the ink. Thus the ink prepress prod-

uct is not just ink, it is also information, ser-

vice and training. The inkroom must regard

the press department as the customer for

product and service. At the same time, the

press department, as a consumer, has

responsibilities to the inkroom. The inkroom

must receive timely process change infor-

mation, correctly marked press returns and

feedback of ink performance on press.

PREPRESS PROCESS

Ink prepress covers the steps and proce-

dures that take place between receiving the

ink from the ink supplier and delivery of ink

to the press (Figure

). These boundaries

are typical, but not absolute, in terms of what

is necessary for a successful operation. For

instance, some ink departments are respon-

sible for managing ink into the press pumps,

while others may actually do some ink for-

mulation and ink assembly normally associ-

ated with the ink supplier. The main objec-

tives are to understand where the boundaries

have been drawn in the converter operation,

manage the input products from the ink sup-

plier(s), and optimize outputs of products

and services to the press department. Once

the boundaries have been drawn, communi-

cated and understood, ink prepress opera-

tions can be designed to meet all needs.

THE INKROOM DESIGN

A typical inkroom occupies about 1,000

square feet. The drawing shows how an

inkroom this size could be organized

(Figure

). There are two areas: an

office/laboratory area and an ink-handling

area. In the laboratory area, color matching,

proofing, quality control, diagnostic testing

and ink management information system

(MIS) functions are performed. This room

would be connected electronically with the

ink supplier and perhaps with the converter

MIS department and shop floor data system.

Major equipment in the ink lab includes

the following:

• proofer;

• weighing scales;

• color measurement device; and

• computer with ink management software

and modem

Major equipment for ink production would

include the following:

• ink-dispensing system;

Ink prepress covers the

steps and procedures

that take place between

receiving the ink from

the ink supplier and

delivery of ink to the

press.

A typical ink room lay-

out occupies approxi-

mately 1,000 square

feet.

Receive

Ink

Estimate

Usage

Generate

Batch

Ticket

Color Standard

and

Standard Formula

Make

Batch

Stage

Ink

Part 1

End Use

Part 2

Ink

Formulation

Part 3

Ink

Prepress

Part 4

Ink on

Press

Press “A”

Press “B”

Press “C”

Returned

Inks

Blended Inks

in Inventory

Inks Staged

for Press

Dispenser

Light

Booth

Ink Lab

Pressroom

Inkroom

• air mixer;

• drum dolly; and

• hand truck.

It should be noted that the ink-dispensing

system can be as simple as drums on racks

with appropriate valves or as complex as a

computer-controlled dispenser.

INKROOM SYSTEMS

Regardless of the print, end-use and the

types of presses being served, the inkroom

must have procedures in place that define

and control key areas.

Safety

This is always the number-one priority in

any manufacturing operation; but it is partic-

ularly important in the inkroom, due to the

nature of the materials being handled. Proper

handling of chemicals is essential. Formal

training programs for the operators are man-

dated by legislation and must be document-

ed. If flammable materials are present, a spe-

cial emphasis should be placed on maintain-

ing explosion-proof systems, grounding of

containers and arranging exhaust in areas

where open containers are present.

The safety of the inkroom is very important

and must be audited on a regular basis. The

result will make for a safer workplace.

Housekeeping, personal safety equipment,

personal hygiene and adherence to safe work

practices are key factors to be reviewed in an

ink operation.

Color Standard

There should be a process in place that

generates, approves and stores color stan-

dards for use in the inkroom, as well as in

the actual printing operation. This can save a

lot of discussion, time and money, as well as

preserve the printer’s reputation with cus-

tomers. A good system should provide a cur-

rent standard, signed and dated. There

should be at least three samples available:

one on file in the inkroom for color match-

ing, one for pressroom use, and one in a

master file that is used only if the other two

are in question. All standards should be up-

dated at least once a year. At the very least,

they should be inspected and, if still accept-

able, a new date applied.

Proofing System

The heart of any ink-blending operation is

a correlated proofing system that can pre-

dict the ink strength and shade that will be

obtained on the press. Only by having a good

proofing system can the inkroom be confi-

dent in its ability to deliver “press ready”

inks to the printing operation. This will save

hours of press downtime and help eliminate

color variation complaints. The method

should be one that can be used by everyone

in the inkroom with consistent results. This

system will also allow for better matching on

press return ink.

Where a printing operation has a mix of

different presses and inking systems, differ-

ent proofing methods may be required to

ensure that there is a correlation between

the proof from the inkroom and the print

from the press.

Inventory Control

Ink should be used on a first-in, first-out

basis, due to the shelf-life limitations of any

chemical mixture. The issue here is simply

using the oldest ink first. The ink supplier

should have dates on all containers to facili-

tate this process. If there is no date of manu-

facture on the container, get one from the sup-

plier. Some suppliers utilize date-coding sys-

tems that may not be immediately apparent.

There should also be a system of control

that always allows ink to be available while

maintaining the lowest possible inventory. A

basic min-max system can work well with a

weekly inventory and facilitate ordering in

time to fill the pipeline. An area that is often

INK 49

50 FLEXOGRAPHY: PRINCIPLES & PRACTICES

overlooked is press returns. Press returns

should be identified and weighed, and the

containers kept sealed to improve the prob-

ability of reuse. Since waste inks become a

regulated hazardous waste, control of press

returns will save the time and cost of dis-

posing of these materials.

Usage Records

Accurate records play a central part in the

control of inventories. They will also help to

reduce waste, since most colors are

matched for a specific job and anything not

used is returned to inventory with the hope

that it can be used later. A basic system con-

sists of recording the weight of ink made, the

weight returned, and the run size. This will

provide a record for each job, allowing for a

more accurate estimate of ink requirements

for future runs. More sophisticated comput-

erized systems are available to track this

information. The objective is the same –

have the right amount of ink on press and

only return the minimum amount of ink to

the inkroom. This will reduce cost, waste

and on-hand inventory.

Information Systems

A great deal of information is usually man-

aged by the inkroom. Some examples are:

Ink Systems. Ink formulations must be

matched to specific substrates based upon

their compatibility and end-use properties.

Ink Additives. Anything added to the ink must

be identified and information provided for

their proper use. Misusing additives can

result in downstream problems.

Press Performance. On press, color data and

corrective actions are important to track.

This is valuable feedback that can save the

department from solving the same problems

over and over.

Scheduling. The ink department should have

a good system that links the ink operation

with the press schedule and press setup,

thereby coordinating the timing of ink needs

on the press and quick reaction to changes

in schedule.

Equipment. There is less equipment in an

inkroom than the pressroom, but it is just as

important and must always be working

properly. Scales must be accurate and in

good working order. The mixer must run

smoothly and be sized to handle the batch-

size requirements of the operation. Hoists

and other lifting equipment must have the

rated capacity to handle all containers pre-

sent, and they must move easily without

restriction. Manual material handling equip-

ment should also be inspected on a regular

basis to ensure they are in good working

condition. Color-control equipment (i.e.,

color booths, spectrophotometers, etc.)

must be calibrated and maintained to ensure

optimum performance.

COLOR MANAGEMENT

Color communicates. Color sells. Color is

the sizzle that drives the sale of virtually

every consumer product in the world. It

evokes a wide range of emotions that draw

the buyer to the product. Color management

professionals know that color is a crucial

part of the selling process because it is such

an important part of the “buying decision.” If

color is used effectively in the manufactur-

ing and marketing of an item, potential buy-

ers will perceive added value in that product.

To use color effectively in a flexo package

design requires an understanding of the

entire process, from initial design to final

package (Figure

). Many elements and

professionals are involved, each relying on

interaction with the other. Print buyers and

designers start the process, taking into

account consumer preferences. Colors must

be specified and communicated, from the

visual appeal of the final package to the

capabilities and tolerances of the flexo-

graphic printing process. In a package

design, color and all other elements must be

within the capability of the production

process.

At each step of production, output from

the previous step becomes the input to the

next process. Colors are communicated

among several different individuals who

may render and reproduce the colors on

many different devices. For final production,

a contract proof, viewed under standard

conditions, will show a close approximation

of the final printed product. Once the client

has agreed on the contract proof, the ink

department is responsible for supplying ink

and color standards that allow the agreed

upon color to be obtained; impression after

impression and press run after press run. To

assure consistent reproduction, color mea-

surement and control is essential. This sec-

tion will touch on this subject in three parts:

color theory, color measurement, and color

matching. Color theory and color manage-

ment application are explained more fully in

the Process Color Printing volume.

COLOR THEORY

Color results from the interaction between

light, an object and the viewer (Figure

The human observer, or viewer, sees this

modified light and perceives it as a distinct

color. All three elements, light, object and

viewer, must be present for color as we know

it to exist.

Light Source and Color

If an object is viewed using other than a

white-light source, the perceived color will

take on the hue of the illumination. While

INK 51

Colors must be

communicated and

specified, taking into

account the capabilities

and tolerances of the

flexographic printing

process. In a package

design, color and all

other elements must be

within the capability of

the production process.

BOB’S

Buyer

Graphic

Designer

Structural

Designer

Pre-Press

Suppliers

Printer/

Converter

Consumer