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

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PRESSES AND PRESS EQUIPMENT 63

It is important to note that the feedback to

the sensor is through the web. Thus, the

behavior of the web, in conjunction with the

other components in the loop, is most impor-

tant to the proper functioning of the control

system. Not only are the characteristics of

these individual components important, but

the relationship of their characteristics must

be carefully established to ensure proper sys-

tem results. The accuracy of this feedback is

particularly true where high press speeds

require fast guiding system response and sta-

bility to achieve highly accurate web position

control and registration.

Various auxiliary control systems are

available for use with the automatic guiding

systems. With intermediate guides, the most

common auxiliary control system is a fea-

ture that allows the operator to disable the

automatic guiding system and to center the

intermediate guiding device in its travel. This

feature is generally most useful in situations

where the press must be re-threaded. With

unwind and rewind guides, the most com-

mon auxiliary control system is a feature

that enables the operator to manually con-

trol the system, as well as to operate in the

automatic guiding mode. With this system,

the operator can manually position the

unwind or rewind stand during press setup

and then switch the system into the normal

automatic mode of operation when printing.

There are three locations on flexographic

presses where guiding is normally applied at

the unwind, prior to printing and at the

rewind. Guiding at the in-feed section with

Plate Thickness

1.00

.250

.187

.125

.100

.067

.155

.045

.030

0.95

0.90

0.85

0.80

0.75

0.70

51015202530354045505560

The four types of

automatic guiding

systems. All of these

systems rely on a

sensor to monitor

web position and

transmit any shifts to

a servo valve (hydraulic

systems) or a DC drive

motor (mechanical

systems).

either unwind guiding or intermediate guid-

ing corrects for misalignment of mill roll

position and such things as telescoped and

poorly wound rolls.

Automatic Web Guiding Systems

There are four basic types of automatic

control systems (Figure

• pneumohydraulic;

• electrohydraulic;

• pneumomechanical; and

• electromechanical.

All of these systems are closed-loop, type 1,

proportional control systems; this description

means the correction output adjustment is

opposite and in proportion to the error

detected.

Hydraulic types. The two hydraulic types of

automatic control systems function in a sim-

ilar manner. A sensor monitors the lateral

position of the web. The sensor signal is

transmitted either directly to the power unit

servo valve (pneumohydraulic systems) or to

a signal processor, which then sends a signal

to the power unit servo valve (electrohy-

draulic systems). Hydraulic output from the

power unit through the servo valve, propor-

tional to the lateral error of the web, posi-

tions the guide structure, which moves the

web to the correct lateral position in the sen-

sor. These systems are attractive for

extremely heavy loads and harsh environ-

ments.

Mechanical types. The two mechanical types

of control systems also function in a similar

The four types of

web-position control

systems. Note the

varying positions of

the sensors.

64 FLEXOGRAPHY: PRINCIPLES & PRACTICES

Web

Edge Guiding

Web

Fixed Sensor Center Guiding

Web

Moving Sensor Center Guiding

Web

Line or Pattern Guiding

= Sensor

PRESSES AND PRESS EQUIPMENT 65

manner. A sensor, either electronic for

electromechanical systems or pneumatic for

pneumomechanical systems, monitors the

lateral position of the web. The sensor signal

is either transmitted directly to the signal

processor (electromechanical system) or is

first converted from an air pressure signal to

an electrical signal with a transducer (pneu-

momechanical system). The processor then

sends a signal, proportional to the amount of

error detected by the sensor, to the DC drive

motor on the electromechanical actuator.

The actuator positions the guide structure,

which moves the web to the correct lateral

position in the sensor.

These systems are especially attractive for

applications demanding a high frequency re-

sponse and where hydraulics are not

desired.

Web Position Control

There are four types of web-position con-

trol systems (Figure

Edge Guiding. The sensor detects the web

edge and the guide system maintains this

edge at the desired lateral position.

Fixed Sensor Center Guiding. Two sensors are

held in a fixed position; they detect both

edges of the web. The guide system main-

tains the centerline in an exact position and

accommodates small web width variations.

Moving-Sensor Center Guiding. In applications

where there are large web width variations

during a production run, the sensors contin-

uously reposition themselves automatically

to detect both web edges and to maintain the

centerline of the web in an exact position.

Line or Pattern Guiding. The sensor detects a

printed line, pattern or some distinguishable

feature on the web. The system then main-

tains the printed line, pattern or feature in an

exact lateral position, regardless of the web

edge position.

Sensor Installation

The sensor should be located in the exit

span as close as possible to the exit guide

roller and no farther downstream than one-

half the exit span length. A dead-bar or idle

roller can be used to stabilize the web and

prevent the web from contacting the sensor.

The dead-bar or idle roller should be posi-

tioned immediately downstream of the sen-

sor and have a web wrap no greater than 10°.

Sensor selection is based on material and

system requirements.

UNWIND GUIDING

A typical unwind installation can be seen

in Figure

. In this application, the un-

winding roll of material is shifted laterally

and the sensor is fixed so that the edge of

the material is aligned to the desired posi-

tion relative to the press. An idler roller

should be provided to shift laterally with the

unwind and provide for a constant plane at

the point of sensing.

Since the web plane between the tangent

to the unwinding roll and the first idler in the

press is constantly changing, it is not practi-

cal to sense in this area. The use of the shift-

ing idler establishes a fixed web plane suit-

able for sensing. Also, by shifting the idler

with the unwind stand, the output from the

controller to the cylinder is imparted to the

web immediately in the vicinity of the sensor.

Unwind stand controls

lateral position of web

A typical unwind edge-

guiding system. The

unwinding roll of

material is shifted

laterally and the sensor

is fixed so that the

edge of the material is

aligned to the desired

position relative to the

press.

66 FLEXOGRAPHY: PRINCIPLES & PRACTICES

This setup eliminates undesirable time lags

before the sensor sees the output of the con-

trol cylinder.

Because of the mechanical configuration

of most unwind stands and the masses like-

ly to be involved, mechanical and hydraulic

resonance considerations generally play an

important part in designing a successful

unwind guiding installation. Lateral bending

of the uprights due to their flexibility, or flex-

ing of the stretcher members of the stand

often contribute to a relatively low spring

rate and resultant low mechanical natural

frequency. All shifting stands should be

designed with the natural frequency of the

controller in mind so that they are in the

vicinity of two or three times the natural fre-

quency of the controller.

Unwind guiding aligns the edge to a prede-

termined guide point. Depending on the sen-

sor selected, unwind guiding sometimes

requires a direct mounted or slave idler, or

idler rolls, for proper operation. The idlers

are required when using a sensor with a nar-

row gap or a line guide sensor. The idler

maintains a constant plane for the material

being guided. The unwind stand is generally

positioned so there is less than one web

width to the first fixed idler in the machine.

The operator positions a roll of material

on the unwind stand. The operator then

positions the sensor where he or she wish-

es the edge of the web to be positioned. In

automatic mode, the unwind stand then

shifts laterally, keeping the edge of the

web at the guide point of the sensor.

The ideal location for the sensor is as

close as possible to the shifting unwind

(See Figure

). This positioning will give

the best dynamic performance and accu-

racy. Systems are available that allow the

sensor to be located after the first fixed

idler. These systems will guide the web,

but at a reduced level of performance.

INTERMEDIATE WEB GUIDES

If there are long spans between the unwind

and first print station, it may be desirable to

install an intermediate web guide immediately

ahead of the first print station, either in lieu of

or in addition to the unwind guide (Figure

This intermediate guide can correct for any

web position errors that may occur in spans

between the unwind and the first print station.

Intermediate guiding devices generally fall

into two categories. The first is a steering

guide, which is a roller having accurate

motion (in the plane of web travel) about an

instant center located several web widths

ahead of the guide. The second type of inter-

mediate guide is a displacement guide, or off-

Oven

Unwind

Rewind

Slitting

Sensor

Intermediate

Web Guide

Central

Impression

Cylinder

Chill Roll

Sensor

Offset

Pivot

Guide

Installing an inter-

mediate web guide

immediately ahead of

the first print station,

either in lieu of or in

addition to, the unwind

guide, there is a great

distance between the

unwind and the first

print station.

PRESSES AND PRESS EQUIPMENT 67

set-pivot guide, which is a two-roll assembly,

the rollers of which are pivoted about a line

tangent to the face of the entry roll.

Steering Guides

Steering-type intermediate guides, as seen

in Figure

, have been used with great suc-

cess for many years. A steering guide pro-

vides web position correction by bending

the web through a long entering span. This

type of application, however, requires a long,

free entry span to distribute the stress dis-

tortions caused by the guide motion. The

span requirement is also a function of the

mechanical properties of the web.

When web materials of unusually high

stiffness or very thin and extensible materi-

als at very low tensions are encountered, it

becomes desirable to employ offset pivot-

type guides as opposed to steering guides to

minimize the web stress distribution in limit-

ed-guide entry and exit spans. An offset-

pivot guide with a large dimension between

the two guide rollers requires only short

entry- and exit-span dimensions. Therefore,

it can be installed under the dryer bridge or

below a viewing platform for the web scan-

ner prior to the rewind. The range of materi-

al types to be run on a press and the press

geometry and auxiliary equipment layout,

combined with various guide arrangements

and costs, determine the choice of specific

guide types and locations.

Steering guides are mechanical devices

using one or more rollers rotating about a

remote center of rotation located up-

stream from the device. The center of ro-

tation is also known as the instant center.

The device steers the web in the entering

span of the guide by changing the angle of

the roll relative to the centerline of the

machine. The angular change corrects

steady-state errors, whereas the translation

due to the remote instant center corrects

transient errors.

The steering guide consists of a single or

pair of fixed raceway bases and a moving

structure. The moving structure, sometimes

referred to as the pivot carrier, holds the roll

and rotates about the remote center of rota-

tion. The roller is attached to pivot brackets,

which are attached to the pivot carrier.

The center of rotation is generally located

two-thirds to three-quarters the length of the

entering span ahead of the guide roller. This

location provides satisfactory dynamic char-

acteristics. The angle of rotation of the guide

roller is small, usually less than 3°. For most

steering guides, the location of the center of

rotation can be adjusted by positioning the

base at the correct angle during installation.

The span immediately upstream of the

guide is known as the entry span. The span

upstream of the entry span is known as the

pre-entry span. The relative length of these

two spans is important and will be explained

later. The last fixed roller prior to the guide

roller is referred to as the entry roller.

Steering Guide Operation

The plane of motion of the steering guide

is normally parallel to the plane of the enter-

ing span (Figure

). If the web on the entry

roller is displaced from the desired guide

point, the pivot carrier rotates, causing the

web to be repositioned to the desired guide

point. As the pivot carrier rotates, the web is

bent laterally. The greatest bending occurs

Raceway

Assembly

Sensor

Steering-type

intermediate guides

provide web position

correction by bending

the web through a long

entering span.

A typical installation for a

steering guide operation.

68 FLEXOGRAPHY: PRINCIPLES & PRACTICES

90°

Fixed

Entering

Idler

L (Entering Span)

Pre-Entry

Span

(2/3 to 3/4 L)

Exiting

Span

Guide Roller

Approximately 5° to 25°

Sensor

Deadbar

or Support

Roller

Center of Rotation

of Guide Roll

(Instant Center)

Web Travel

Centerline

of Machine

and Guide

Assembly

90°

as the web exits the entry roller. The exit is

where the highest stress distortion occurs.

At the point of critical rotation, the stress

on one edge will be zero. The stress on the

other edge will be approximately 13 times

the web’s average stress. This lateral bending

of the web, and the associated stress distor-

tion, is the primary reason the steering guide

requires a long entry span. If the rotation of

the steering guide exceeds the critical angle,

the stress on one edge can become negative,

meaning it has a loose edge. The stress on the

other edge increases to compensate for the

loose edge. The net tension in the web will

remain constant, but if less web width is

under tension, then web stress becomes

higher. The loose edge can result in troughs

and/or wrinkles being formed in the web.

The web-entering span ideally should be

parallel to the guide plane of motion. In a

worst case scenario, the web plane of

motion should be no greater than 30° off the

guide plane of motion.

Instant Center Location. The location of the

center of rotation is very important in

obtaining stability in the control system.

The center of rotation is ideally two-thirds

to three-quarters the length of the entry

span upstream of the guide roller. If the cen-

ter of rotation is less than ideal, the guide

becomes over-steered. The magnitude of

over-steering is inversely related to the dis-

tance the center of rotation is located

upstream. As over-steering increases, the

guide control system becomes more and

more unstable until the system oscillates

PRESSES AND PRESS EQUIPMENT 69

constantly, causing the guide to induce

more error than it is correcting.

Entry Spans. The length of the pre-entry span

should be shorter than the length of the

entry span. If the pre-entry span is equal to

the entry span or longer, the stress distor-

tions in the entry span can transfer across

the entry roller into the pre-entry span.

In the pre-entry span, stress distortions will

cause a movement in the web, inducing error.

The guide will then try to correct the greater

error, causing even more error. The end result

will be the guide bottoms trying to correct the

amplified error. This chain of events is known

as a pre-entry span condition. Keeping the

pre-entry span shorter than the entry span

prevents this problem. The lengths of the typ-

ical entering spans expressed in web widths

for normal corrections are shown in Table 5.

The required correcting capabilities of a

steering guide can be determined by the

equation:

C  9T(L/W)

/tE

Where:

C = guide correction (inches, mm)

E = modulus of elasticity

(psi, Newtons/mm

)

L = length of entry span (inches, mm)

T = tension (pounds, Newtons)

t = web thickness (inches, mm)

W = maximum web width (inches, mm)

Web Plane. The plane of the web in the exit

span of the steering guide ideally should be

perpendicular to the guide plane of motion.

This arrangement will keep the stresses in

the exit span to acceptable levels. In this

case, the web in the exit span is twisted.

In some cases, a steering guide with a

straight through-pass is used. The plane of

the exit span is then parallel to the guide

plane of motion. These installations work,

but with increased stress distortions in the

exit span and loss of guide accuracy due to

the bending of the web in the exit span. This

loss of accuracy is known as residual error

and can be as much as 20% of the incoming

error, depending on the material and the

length of the exit span.

The equation for exit span length limits the

edge stress amplification to three times the

average stress. This limit prevents possible

web damage. As a rule, if material properties

are not known or if the calculated exit span is

less than one-half the web width, the exiting

span should be greater than one-half the web

width in length. This span reduces the possi-

bility of the steering guide inducing wrinkles

into the web.

In the case of stiff webs, the previous equa-

tions should be used to determine the mini-

mum length of the entering and exiting spans.

Steering Guide Selection

To select the proper steering guide

model, or to determine the actual steering

load capacity of the guide, the following

application parameters must be known:

• web wrap style;

• modulus of elasticity of the

web material;

• maximum web width;

• maximum web tension;

• maximum web thickness;

• web error;

• roller diameter;

• roller face length;

• guide entry span; and

• exit span length.

To prevent the steering guide from

inducing wrinkles in the web, it is recom-

Table 5

MATERIAL ENTRY SPAN IN WEB WIDTHS

■ WOVEN TEXTILES 1 to 2

■ PLASTIC FILMS 1.5 to 3

■ PAPER AND PAPERBOARD 2 to 5

■ METALS 10 to 390

ENTRY SPANS

mended that the exit span be greater than

one-half the maximum web width.

Offset Pivot Guides

An offset pivot guide (OPG), shown in

Figure

, is a mechanical device used to

correct the lateral error in a web of material

as it runs through a process. The OPG cor-

rects the error by displacing the web

between the entry span and the exit span of

the guide. It is a displacement-type guide that

provides web position correction with mini-

mum entry and exit span requirements.

Designed with parallel rollers, the guide piv-

ots such that the web is twisted, thus mini-

mizing web stress. This design allows an

OPG to be used in the least amount of space.

The offset pivot guide consists of a fixed

base and a pivoting structure with one or

more rollers. The pivoting structure, referred

to as a pivot carrier, rotates about a fixed,

imaginary point called the pivot point. The

angle of rotation is limited to 7.5° maximum.

The point of rotation is ideally in the plane of

the entry span or within 10% of the guide

span of the entry span plane.

OPG Operation. If the web on the entering

roller is displaced from the desired guide

point, the pivot carrier will rotate, causing the

web to be repositioned to the desired guide

point. As the pivot carrier rotates, the web in

the entering and exiting spans is twisted. This

twisting causes the stress in the web to be

redistributed so the stresses are higher at the

web edges than in the center of the web.

The web should enter the OPG perpendic-

ular to the plane of motion of the OPG. The

plane of motion is the plane that the pivot

carrier rotates in. Webs entering the OPG at

angles other than perpendicular to the plane

of motion cause an increase in stress distor-

tions and detrimental steering effects in the

web. These steering effects can cause insta-

bilities in the guide control loop or lead to

reduced guiding accuracy, as well as stress

distortions that could damage the web.

When the pivot carrier is in a centered

position, it does not influence the lateral

position of the web because the rollers on

the pivot carrier are parallel to the other

rollers in the process line.

Different models of OPGs are available for

different web widths, web tensions and roller

diameters. The guide span (the distance from

outside of the entering guide roller face to

the outside of the exiting guide roller face)

and the roller face lengths are a variable fea-

ture for each model. While the guide span

should ideally be equal to the web width, it

should never be less than one-half the web

width.

As a rule, if the material properties are not

known or if the calculated entry/exit span is

less than one-half the web width, the entering

and exiting spans should be made greater

than one-half the web width in length. This

precaution reduces the possibility of the

OPG inducing wrinkles into the web.

OPG Selection. To select the proper OPG

model and determine the guide angle, correct

exit and entry spans, the following applica-

tion parameters must be known:

• guide correction;

• modulus of elasticity;

• length of entry span ;

• maximum tension;

The offset pivot guide

is a mechanical device

used to correct the

lateral error in a web

of material as it runs

through a process.

Economy of space is a

major advamtage of an

OPG.

70 FLEXOGRAPHY: PRINCIPLES & PRACTICES

PRESSES AND PRESS EQUIPMENT 71

• web thickness;

• maximum web width ;

• roller diameter;

• roller face length;

• guide span;

• base mounting dimension; and

• wrap style and base mounting style.

The actual load capacity of an OPG

depends greatly on the guide span, roller face

length and wrap style.

REWIND GUIDING

Rewind guiding (Figure

) is not truly lat-

eral control of a web. Rewind guiding is actu-

ally a chasing control system. At the rewind,

guiding is accomplished by attaching the sen-

sor to the rewind stand and shifting the stand

to chase the web as it comes from the press.

In actuality, this is not web guiding at all but,

rather, merely positioning the rewinding

mandrel in such a way as to follow the nor-

mal wandering of the web so that its edge is

maintained in fixed, lateral relationship to

the rewinding mandrel at all times.

A fixed idler roller between the moving sen-

sor and rewind stand isolates the web from

stand motion and provides for a constant web

plane at the point of sensing. The same con-

siderations regarding mechanical and

hydraulic resonant frequencies apply to

rewind guiding as to unwind guiding. In addi-

tion, however, care must be exercised in

designing the support for the sensor so there

is sufficient rigidity to avoid flexure and

mechanical resonance in this part of the

equipment. Factors to consider in selecting

an unwind/rewind stand are shown in Table 6.

Rewind Operation

The rewinding roll is positioned to align

the edge of the rewind roll to the edge of the

approaching web. The sensor is located just

ahead of the last fixed idler in the machine

and is typically attached to the rewind stand

with a mechanical arm. The operator either

manually or automatically positions the sen-

sor at the edge of the web being chased. The

sensor chases the edge of the web and, as a

result, the rewind control system then caus-

es the rewind stand to shift laterally as the

web tracks laterally, resulting in a roll of

material with a straight edge.

Systems are available that do not require

the sensor to be mounted to an arm attached

to the rewind stand. Instead, the sensor is

In a typical rewind edge

guiding, the rewinding

mandrel is positioned in

such a way as to follow

the normal wandering of

the web, so that its edge

is maintained in a fixed,

lateral relationship to

the rewinding mandrel

at all times.

Table 6

To select the proper stand, the following appli-

cation parameters must be known:

1. Web material.

2. Maximum web width.

3. Maximum tension.

4. Maximum lateral shift required.

5. Maximum line speed.

6. Maximum roll weight.

7. Maximum roll diameter.

8. Maximum weight of the stand.

9. Coefficient of friction for the slider

bearings.

UNWIND/REWIND

STAND SELECTION

72 FLEXOGRAPHY: PRINCIPLES & PRACTICES

electronically connected to the rewind stand.

The rewind stand is generally positioned so

there is approximately one web width from

the last fixed idler.

In rewind guiding, if the sensor is located

downstream of the last fixed idler or the web

does not have sufficient frictional engagement

with the last fixed idler, a roll with a ragged

edge will be wound. Another problem often

seen in rewound rolls is telescoped wraps.

The guiding system is often blamed for this

defect, when in reality, the roll was not wound

with sufficient tension to prevent lateral slip-

page of one or more layers in the wound roll.

This defect often occurs many layers deep in

the wrap being applied, during winding or

even days after the roll is removed.