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PRESSES AND PRESS EQUIPMENT 73

Web Viewers

rom the beginning of flexographic

printing, press manufacturers

have constantly made improve-

ments in their designs and

process techniques to reach high-

er optimum printing speeds for

improved profitability. As web speeds

increased, many process functions became

more critical, requiring greater operator

attention and skill to obtain maximum effi-

ciency, quality assurance and waste control.

As a result, web viewers were developed and

made available to aid the press operator in

fully utilizing the press potential.

As web speeds exceeded the 250 to 300

feet per minute (fpm) range, above which

operators could no longer accurately inspect

the printed web, web scanners became a

necessity for quality assurance. Today, with

the increased demand for more sophisticated

print requirements, the selection of a suitable

web viewer is of prime importance.

The degree of efficiency obtainable is

greatly dependent upon the ability of the web

viewer to provide a stable image of such

quality that precise definition of detail, regis-

tration, etc., is readily visible at all printing

speeds. When this stable image is accom-

plished, the operator can constantly monitor

the print quality and make the necessary

press adjustments, as well as observe the

adjustment results, while the press is running

at optimum web speeds. This, of course, min-

imizes material waste and press downtime.

STROBOSCOPE

The simplest web-viewing device is a strobo-

scope, similar to an automotive timing light.

This device consists of a lamp and voltage

pulse source, providing a brief instant flash

with a frequency synchronous with the print

repeat length. Being relatively simple in con-

cept and structure, it is the least expensive

option but has severe limitations as a suitable

web viewer.

With a stroboscope, the viewing area is lim-

ited to the size of the lamp beam diameter,

which varies with distances, of course. The

distance also controls the degree of illumina-

tion and definition of print detail. Discounting

operator discomfort and fatigue, image defini-

tion is also less than satisfactory. The lamp of

the stroboscope is designed to flash in syn-

chronization with the print repeat length, or

multiples thereof, so that an identical portion

of the web is constantly viewed. Therefore,

frequency is dependent upon print repeat

length versus web speed and will vary as

either or both are changed.

At low speeds, the operator observes a brief

instant flash, lacking the necessary dwell time

to retain the image between flashes. As web

speed increases, a frequency is reached where

persistence of vision bridges the gap between

flashes, providing the stroboscopic effect, or

a stationary image. As the frequency

approaches and exceeds approximately 25

cycles per second, the stationary effect is lost

and the stroboscope begins to provide a con-

stant illumination typical to the common light

bulb. With these factors in mind, it is readily

apparent that a web viewer with the strobo-

scopic concept is limited in performance.

OSCILLATING MIRROR

Another type of web scanner is a device

74 FLEXOGRAPHY: PRINCIPLES & PRACTICES

By “bending” the

printed web over an

idler roller to simulate

an arc throughout the

sweeping angle, the

“bent-web” viewing

feature eliminates

the sweeping angle

distortion during a

straight-web threading

orientation.

consisting of a rectangular mirror oscillating

in synchronization with the print repeat

length. In practice, the web viewer is mount-

ed at a fixed distance from the web plane

and the sweeping angle of the oscillating

mirror is varied to accommodate the various

print repeat lengths, or multiples thereof.

The oscillating mirror is generally sized to

view a web width of 18" to 20" and can be

mounted on side-motion tracks to traverse

web widths beyond its fixed viewing range.

Due to the width and height restrictions of

the oscillating mirror, only a portion of the

print repeat length is visible through the

viewing area at a given time. As a result, the

synchronous drive is equipped with a modu-

lator so that the oscillating frequency can be

regulated to cause the field of vision to drift

through the entire print repeat length. Any

portion of the print repeat length can be con-

stantly viewed by simply reestablishing syn-

chronization.

This type of web viewer has a definite

advantage over the stroboscope concept.

Auxiliary light fixtures constantly illuminate

the web, rather than utilizing the flashing

lamp, which eliminates the flickering effect

and reduces operator fatigue. In addition,

the visual dwell time is increased due to the

angular movement of the oscillating mirror

throughout the oscillating cycle, improving

Rotating Mirror

Drum

Fixed

Viewing

Angle

Visual Dwell Span

Strip

Mirror

Bent Web

True Arc

the operator’s ability to define print detail.

The prime disadvantages for the oscillating

mirror concept are its speed limitations and

lack of image stability. Any mechanical

device that utilizes reciprocating motion,

regardless of its design quality, has many

moving parts that are subject to speed limita-

tions and rapid deterioration of precision fit-

ting or surface wear. Since the oscillating fre-

quency, or reciprocating motion, increases

with speed and is further magnified as small-

er repeat lengths are viewed, this concept is

limited to viewing web repeats of moderate

length (6"–12") and slower-moving webs.

For printing applications within the suited

speed range of 100 to 300 fpm, there is anoth-

er disadvantage that induces image instabili-

ty and distortion. This problem is caused by

the constant variation in optical distance

between the flat web and oscillating mirror

surface, as it sweeps through its angular

movement. Obviously, the occurrence rela-

tive to oscillation frequency provides an

additional need for the inspector to continu-

ally refocus on the image each time a print

repeat length is viewed.

ROTATING DRUM MIRRORS

Figure

illustrates a third generation of

web viewing equipment. This method uses a

rotating drum of mirrors to offset the speed

limitations inherent in the previously dis-

cussed web viewers.

The multisided drum of mirrors is rotated

in synchronization with the viewed web seg-

ment of a given print repeat length. Each

time a viewed web segment passes through

the visual dwell span, one mirror of the

rotating drum reflects the image until the

next mirror picks up the identical area of the

succeeding print repeat length. This process

is repeated continuously as other mirrors

rotate into position in synchronization with

the print repeat. The series of images is then

reflected to a separate strip mirror that is

PRESSES AND PRESS EQUIPMENT 75

cam-operated from the drum shaft, provid-

ing tracking through the visual dwell span.

The rotary drum web viewer generally has

a fixed viewing width of 18" to 20". The basic

viewer is normally mounted on an overhead

track assembly with a sideways motion fea-

ture for traversing the total web width as

required by the specific press design. Since

the viewing angle of the rotary drum viewer is

fixed, the track assembly must also have pro-

visions for positioning the web viewer at var-

ious distances from the web. This capability

allows the rotary drum position relative to the

web to vary so that the visual dwell span is

equal to the print repeat length being viewed.

“Bent-Web” Feature

Also shown in Figure

is the “bent-web”

viewing feature. This feature was developed

to eliminate the optical distance change that

occurs as the viewed web segment sweeps

though the visual dwell span when utilizing a

straight-web threading orientation. The

printed web is “bent” over an idler roller to

provide the similarity of an arc throughout

the sweeping angle. Although not a true arc,

optical distance is relatively constant

throughout the visual dwell span. This fea-

ture practically eliminates the sweeping

angle distortion, providing image stability.

Relative to reciprocating motion, the “bent-

web” technique reduces the transition of the

cam profile enough that the strip mirror

oscillation is practically zero. In operation,

this technique, when combined with a rotat-

ing drum mirror concept, can provide suit-

able web scanning at speeds up to 2,000 fpm.

The “bent” idler located in the visual dwell

span optically disappears at speeds over 250

to 300 fpm. The optical disappearance in the

field of view is relative to the idler size.

Therefore, idler diameters should be kept to

3" or less. Concentricity and balance of the

bent idler are of prime importance to pre-

vent web flutter and image instability.

Automatic Synchronization

The synchronous rotation of the mirror

drum is achieved with two self-synchronizing

motors. One self-synchronizing motor on the

drive assembly, acting as a transmitter, is

coupled to the print cylinder shaft or any

other line shaft that has a ratio equivalent to

one revolution of the print cylinder circum-

ference. This self-synchronizing motor trans-

mits voltage to the second self-synchronizing

motor in the basic web-viewing unit driving

the drum of mirrors. Synchronization is now

achieved since one print repeat length equals

one mirror movement of the drum rotation.

By disrupting the synchronization phase

manually with a hand-wheel, or automatical-

ly by a built-in electric drive, the viewed web

segment can be varied throughout the entire

print repeat length. This provides the opera-

tor with the ability to slowly scan any portion

of the print repeat length.

Lighting and Magnification

Suitable front light fixtures are required to

provide constant illumination of the printed

surface of the web. Fixtures should be posi-

tioned and properly baffled to avoid obstruc-

tion or glare throughout the viewing area. For

applications where transparent or translu-

cent materials are frequently printed, it is

advantageous to utilize a rear lighting fixture

for improved web illumination.

Due to the image stability of a rotating

device, scopes are available for print magnifi-

cation. In the range of five to ten power, criti-

cal inspection of minute details of fine type,

texture and other quality details is feasible.

VIDEO SCANNING

In recent years, coinciding with the evolu-

tion to higher speeds, wider web widths,

longer repeats, gravure-like quality, in-

creased, number of colors and more on-

press coating, the need for better visual

inspection has become critical. Video scan-

76 FLEXOGRAPHY: PRINCIPLES & PRACTICES

ning and video web inspection systems have

filled this need, and are now the most popu-

lar form of online print inspection methods.

They are available from basic viewing sys-

tems to automated defect detection systems.

This discussion will focus primarily on basic

viewing systems.

System Configuration

A typical video inspection system is

shown in Figure

and consists of these

four basic components:

• camera assembly

• camera positioning assembly

• CPU and software

• press timing devices

Camera Assembly. Typically, the camera

assembly contains a high resolution RGB

(red, green, blue) color camera. A motorized

zoom lens and diopter, or close-up lens,

together determine the overall magnification

of the system. The zoom lens also controls

the focus and iris. The iris controls the

amount of light reflected into the camera and

hence the brightness of the image. A strobe

lighting system uses a xenon flash lamp that

produces a high-intensity short-duration

flash of light. This illuminates and provides

stop-motion of the area for the camera to

capture the image.

A typical video

inspection system

consists of a camera

assembly, camera

positioning assembly,

CPU and software, and

press timing devices.

Camera-Positioner Assembly. This is typically

available in three different configurations:

manual, motorized and programmable. If a

manual positioner is chosen, the positioner

must be installed on the press so the opera-

tor has physical access to move the camera

across the web (crossweb). Manual position-

ers are primarily used on narrow-web

machines. Motorized positioners provide

basic left-right jog capability. Programmable

positioners offer multiple, user-programma-

ble positions to be stored and then activated

in a continuous loop.

The camera positioner needs to be installed

after the last line operation that requires

inspection. This is typically accomplished

after the dryer, but before the last dancer

assembly prior to rewind. Most inspection

system suppliers do not supply the brackets

that connect the positioner to the press frame

as standard equipment. Many will provide

these at an additional charge. The brackets

and the chosen viewing area are critical to

ensuring satisfactory viewing results.

The chosen viewing area must meet some

minimal requirements. The distance from the

web surface to the camera/lens is defined by

the camera lens focal length. The focal length

determines the height at which the camera

positioner assembly is positioned above the

web viewing area. This dimension should be

MonitorCPU

Video

Camera

Traverse System

Web

PRESSES AND PRESS EQUIPMENT 77

The video system uses a short-duration,

xenon strobe light to illuminate the cam-

era’s viewing area. This short-duration

strobe flash causes the web material to

appear stationary while the camera cap-

tures an image. The signal input device pro-

vides a timing signal so that the operator

can control the point at which the strobe

fires (Figure

). The timing signal is

directly related to the print cylinder cir-

cumference, which, in effect, divides the

print repeat into sections as illustrated in

Figure

. The timing signal provides the

ability to advance or retard the point on the

web, in the machine direction, where the

strobe will flash. When used in conjunction

with the camera’s crossweb-positioning

capability, any area on the printed repeat

held from sideframe to sideframe to ensure

that the viewed image remains in focus from

one web edge to the other.

The web should be supported so that web-

plane change is kept to a minimum. This may

be accomplished by utilizing two idler rolls

that are relatively close together. Another

method would be to install the camera posi-

tioner assembly as close as possible to one

idler roll that has approximately 90° of wrap,

taking care that the camera viewing area

does not extend beyond the curvature of the

idler roll. If the web is allowed to experience

excessive plane change, the viewed image

may not remain in focus. Care should also be

taken to ensure the centerline of the camera

can be positioned over the web edge and that

no interference is caused by the sideframes.

When properly installed, the camera-posi-

tioner assembly will provide complete cross-

web viewing capability.

CPU and Software. The CPU (central pro-

cessing unit) contains the main computer

system and its associated power supplies.

Also contained in the CPU is the frame

grabber, which “grabs” the image that is

captured by the camera and converts it to a

form that can be displayed on a high-reso-

lution video monitor for inspection. The

software gives the computer the instruc-

tions to follow. These include the basic

instructions for capturing and displaying

images and any advanced instructions such

as automatic color monitoring, register

monitoring or bar-code verification.

Signal Input Device. The signal input device

is used to provide the inspection system

with speed and timing information. There

are many different types of signal input

devices. The most commonly used sensors

include proximity sensors, optical encoders

and print mark sensors. The one to choose

will depend on the specific application.

Regardless of which one is used, the instal-

lation of this device is crucial for optimum

system performance.

One Print Cylinder Circumference

Crossweb

Machine Direction

The signal input

device provides a

timing signal so that

the operator can control

the point at which the

strobe fires directly

related to the print

cylinder circumference.

Each mark in the timing

signal represents a

standard number of

strobe pulses. Thus, the

opertor porgrams the

strobe to pulse at a rate

that will allow it to view

the appropriate portion

of the web.

length can be viewed.

As an example, in Figure

each mark in

the machine direction represents 10 timing

pulses. In order to view the center area of

the web, the camera-positioner would be

moved crossweb to the center of the web

and the timing pulse would be set to fire the

strobe at pulse 75, since there are a total of

150 pulses per repeat length.

Proximity Sensor. The proximity sensor is an

inductive device that detects the presence or

absence of ferrous metal. It is most common-

ly used in video inspection systems to detect

and count the teeth on a gear that is relative

to the print cylinder. The proximity sensor

would be installed to sense the teeth of the

bull gear (Figure

). It is critical for proper

operation of the video system that the dis-

The proximity sensor

determines the number

of teeth on a bull gear,

and doing so, measures

the rate of web travel.

A proximity sensor will

detect the beginning of

the repeat and then

develop strobe timing

pulses via software.

This type of sensor is

most commonly used in

non-printing applica-

tions such as inspection

rewinders.

78 FLEXOGRAPHY: PRINCIPLES & PRACTICES

tance between the proximity sensor and the

gear teeth being sensed (sensor gap) is adjust-

ed correctly. Incorrect setting will cause the

system to lose synchronization.

Since the pitch of the bull- and print-cylin-

der gears are known, this determines that

every four teeth detected by the proximity

sensor results in one inch of web travel. The

only setup information the video system

requires to synchronize properly is how many

teeth are on the print cylinder gear for the

current job. This value must be changed each

time a different size print cylinder is used.

Optical Encoder. An optical encoder utilizes

an internal transmitted-light sensor and a

transparent disk with non-transparent

marks equally spaced on the disk to gener-

ate a timing signal. This signal is directly

relative to the shaft of the encoder (i.e.,

each complete rotation of the encoder

shaft generates an exact number of timing

pulses).

This arrangement requires that the

encoder be geared to the press so that, for

each complete revolution of the encoder

shaft, the print cylinder also makes exact-

ly one revolution. Because of this exact 1:1

ratio requirement, the encoder has limited

applications. On a variable-repeat press, it

would require the operator to change the

encoder or gear ratio for every job change.

The optical encoder is most commonly

used on fixed repeat presses.

Print Mark Sensor. A print mark sensor can be

used to synchronize the video system by opti-

cally detecting a printed mark on the web sur-

face. The printed mark must be of significant-

ly opposing contrast to the background color.

The printed mark must only occur once per

printed pattern and no other printing can

occur between the marks (Figure

). In this

mode the video system will detect the begin-

ning of the repeat and then develop strobe

timing pulses via software. This type of sen-

sor is most commonly used in non-printing

applications such as inspection rewinders.

One Print Cylinder Circumference

Crossweb

Machine Direction

Mark

Sensor

Marks

Proximity

Sensor

Sensor Gap

Bull

Gear

PRESSES AND PRESS EQUIPMENT 79

preset tolerances and warning the operator

when the press exceeds these tolerances, i.e.,

registration, color variance, repeat varia-

tions, etc. By changing illumination they also

have the capability of viewing clear varnish-

es and lacquers without using additives.

The biggest drawback to date has been the

cost of such units. They have obviously

appeared on the latest high-tech presses, but

it seems that it will be some time before they

are available as retrofits for older presses.

CONCLUSION

Video inspection systems are capable of

viewing high press speeds without jitter and

distortion. They are also capable of high

magnification and can detect printing

defects normally seen only with the aid of a

magnifying glass on a stationary web. They

are particularly helpful in detecting registra-

tion errors, doctor blade streaks, dot gain

and UPC code verification.

As these systems have evolved, they have

become capable of automatically monitoring

80 FLEXOGRAPHY: PRINCIPLES & PRACTICES

ne of the key functions of the

flexographic printing process

is the proper treatment and

handling of the substrate mate-

rial. One of the primary consid-

erations is, of course, the dry-

ing of the ink on the substrate after the print

stations. Further considerations include

additional treatment in order to receive the

ink, cleaning of the substrate and control of

static electricity carried by the substrate.

Finally, lamination and varnishing will be

briefly discussed.

DRYERS

Some flexographic presses have integrated

drying systems. A typical arrangement for a

central impression (CI) press is shown in

Figure

. There are individual dryers after

each print station except the last one. These

dryers are known as either between-color or

interstation dryers. The dryer after the last

print station is called the main tunnel or over-

head dryer.

Interstation dryers remove a sufficient

amount of the volatile solvents from the ink so

that the next print station may apply another

color without altering the previous one. On a

four-color stack press there are typically three

interstation dryers. A color CI press usually

has five. On either a stack press or CI press,

there is one main tunnel dryer. The latter

removes the volatile solvents from the last

printing station and completes the heat setting

of all the inks applied. The main tunnel dryer

also removes the final traces of volatile sol-

vents from products where retained solvents

could present problems, such as blocking.

The interstation and main tunnel dryers

have various air flow schemes, depending on

the vintage of a press. For example, a press

Substrate Treatment

and Processing

A typical dryer system

on a central impression

(CI) press. There are

individual dryers after

each print station except

the last one.

Dual

Burners

Interconnecting Ductwork

Drying Tunnel

Dual Exhaust

Blowers

Dual Supply

Blowers

Chill Rolls–PIV Controlled

Between Color Dryer

PRESSES AND PRESS EQUIPMENT 81

might have one supply fan, one burner and

one exhaust fan for the main tunnel dryer,

and a parallel system for the interstation dry-

ers, with each one sharing one supply fan and

one exhaust fan (Figure

). This system

gives the press operator the ability to inde-

pendently control air temperatures in either

the interstation dryers or the main tunnel.

Other examples of airflow schemes are:

• one supply fan and one burner ducted

to both the interstation dryers and main

tunnel dryer, then exhausted through a

common exhaust fan;

• independent supply fans and burners,

but exhausted with a common exhaust

fan; and

• independent supply fans with one com-

mon burner with exhaust through an

independent or common exhaust fan.

As with any rule, there may be exceptions.

Some may exist where the interstation dryer

exhaust is also the main tunnel supply or vice

versa, as in a cascading airflow system.

Again, flexographic press dryers use hot

air to remove the volatile portion of the ink.

This air is ducted and directed to the sub-

strate where the substrate, ink solids and ink

solvents are raised to a temperature that

causes volatilization. The important consid-

erations for any dryer are:

Air Temperature. The higher the tempera-

ture, the quicker drying can occur, but

high temperatures can have drawbacks,

such as possible substrate damage. High

temperatures can also dry the top surface

of the ink, forming a crust that must be

broken to vaporize ink trapped under the

surface against the substrate. This defect

can show up as pock marks or “fisheyes.”

Air Velocity and Volume. The greater the

velocity and volume of heated air directed

to the substrate, the quicker the volatile

ink components can be vaporized. The ink

must reach the ink-vapor temperature for

volatilization to occur. The moving air

helps carry the volatile components away,

speeding additional evaporation. Velocity

and volume relate directly to fan horse-

power.

Time. Sufficient time with a proper combi-

nation of air velocity, air volume and air

temperature, allows the substrate and ink

to get hot enough to vaporize the volatile

component of the ink. The length of the air

dryer versus the linear speed of the sub-

strate determines the length of time.

How Dryers Work

Normally, the supply fan takes air, from

either the roof or inside the plant, past a gas-

fired burner and directs this air to the sub-

strate (Figure

). The heated air is pushed

onto the substrate through a series of air

Heated

Air

Supply

Air

Exhaust

Boundary

Layer

Air

Nozzles

Burner Burner

Between

Color

Dryers

Main

Tunnel

Dryers

A typical dryer air-flow

scheme.

How a dryer works:

The heated air is pushed

onto the substrate

through a series of air

nozzles, narrow slots

running perpendicular

to the substrate travel,

across the width of the

substrate. After vaporiz-

ing the volatile compo-

nent of the ink, the air is

exhausted.

nozzles, narrow slots running perpendicular

to the substrate travel, across the width of

the substrate. After vaporizing the volatile

component of the ink, the exhaust fan pulls

the air from the dryer and sends it to either

the atmosphere or a pollution-control

device. When the air is exhausted, ambient

air is pulled into the dryer at the web

entrance and exit. In all cases, the exhaust

volume is greater than the supply volume.

Therefore, the dryer is kept under a slight

negative pressure, which prevents volatile

air from escaping into the plant.

NFPA Guidelines

The exhaust airflow volume from the

press dryer is designed according to

National Fire Prevention Association

(NFPA) guidelines which dictate that the

ventilation or exhaust rate must be

designed and maintained to prevent the

vapor concentration in the dryer from

exceeding 25% of the lower explosive

limit. The NFPA says that an estimated

rate of ventilation or exhaust cannot be

less than 10,000 cubic feet (measured at

70°F) per gallon of solvent evaporated in

the dryer. This rate makes for a 25% lower

explosive limit. This limit is based on

ink/solids/solvent concentrations, types of

solvents, press speed, substrate width and

the number of printing units. All these fac-

tors must be considered to determine the

maximum solvent conditions or the most

solvent the dryer will have to evaporate.

Now that pollution-control equipment is

mandatory, flexographic press dryer airflow

schemes have been revised to reduce the

volume of air processed by this equipment

and released to the environment. The EPA’s

capture requirements on total emissions

must be met, limiting the amount of air that

can be treated and exhausted.

Most press manufacturers offer a recircu-

lation airflow scheme that recycles air con-

taining volatilized solvents (Figure

). The

recirculation principle utilizes a supply fan

sized to provide the necessary volume and

pressure of hot air to the dryer and to

exhaust the majority of the air containing the

vaporized solvents. A smaller exhaust fan

removes the filtered air, products of combus-

tion and any leakage into the dryer. In all

cases, this exhaust must meet the guidelines

of NFPA for lower explosive limits. When

dealing with dryer exhaust rates, consult

your press or dryer manufacturer and the

proper authority in pollution control.

COOLING ROLLS

When subjected to heat, polymeric films can

soften, lose strength and stretch. Although

heat is required to complete some of the coat-

ing and printing operations, temperatures

must be carefully controlled, and this usually

necessitates the use of cooling rolls.

Cooling rolls can be devised by using other

rolls as substitutes, such as uncovered foun-

tain rolls, plate cylinders or impression

cylinders, in positions following the dryers,

between the last dryer and rewind, or where

a temperature reduction is necessary.

Heat Transfer

Regardless of the composition of the major-

ity of substrates, the heat transfer through

A recirculation airflow

scheme that recycles air

containing volatilized

solvents.

82 FLEXOGRAPHY: PRINCIPLES & PRACTICES

Burner

Between

Color

Dryers

Main

Tunnel

Dryers