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Another type of tension trim system is used

on rewinds. The transducer signal is again

used to control torque within a narrow range,

but the operating torque level is determined

by roll diameter as well as speed. The diame-

ter is calculated by a roll follower as

described earlier.

Tension control is of primary importance in

the processing of continuous webs. Better

quality and profits are possible through

improvements in tension control. Manual sys-

tems and roll followers are the cheapest in

terms of purchase cost, but sophisticated con-

trols involving measurement of tension allow

faster operating speeds and consistent results

with reduced waste and less labor cost.

Advantages of Transducers over Dancer-Roll

Systems. The transducer’s ability to measure

actual web tension is the single biggest dif-

ference from all other types of tension con-

trols, and it provides the transducer system

tremendous advantages. The most obvious

of these advantages is the analog meter or

digital display. No other system offers a dis-

play of actual web tension. It eliminates all

the guesswork.

The next advantage is accuracy. The

closed tension loop enables the controller to

automatically and quickly compensate for

factors affecting tension, including changes

of speed, roll diameter, web characteristics

and brake fade. The result is a typical system

accuracy of 2% to 3%. The third advantage is

ease of use. Setting tension is simple; just

watch the meter while turning the tension

set knob and stop when it displays the

desired reading. No further operator

involvement, skill or experience is needed.

Another advantage is consistency. The

high accuracy, automatic compensation and

ease of tension-setting allow the operator to

run the same tension roll after roll and to

easily repeat the correct tension for each job

at any time. An additional advantage is the

ability of transducer systems to operate suc-

cessfully over a wide range of tensions. This

latitude is made possible by the negligible

movement, low inertia, accuracy and sensi-

tivity of the transducers themselves. Ranges

of 25:1 are possible. In fact, it is not unusual

for the range to be limited by the motor,

clutch or brake, rather than by the tension

equipment.

PRESSES AND PRESS EQUIPMENT 43

44 FLEXOGRAPHY: PRINCIPLES & PRACTICES

Unwind Equipment

The single-position

unwind is the most

common type of

unnwind stand.

Single-position unwind

with loading arms

facilitates the loading

of rolls.

he unwind stand, which can be

simple or complex, plays an

important part in the proper

operation of the press. There are

two general groups of unwind

stands: single-position and flying-

splice stands. Most single roll and older drop-

splice unwind stands are non-driven, while

most modern flying-splice unwinds are of the

driven type. A brief discussion later will cover

both center-shaft and surface-drive types.

SINGLE-POSITION UNWIND

The simplest and most common type of

unwind stand is the single-position stand. It

can be manufactured to handle a great vari-

ety of roll widths and diameters (Figure

To make the stand more usable, designers

normally incorporate a side shift adjustment

so the core shaft may be moved by the oper-

ator either way from its center position. This

adjustment allows the operator to relocate

the web without having to shift the roll on the

core shaft. For wide, large-diameter rolls, a

loading apparatus may be used. Hydraulic

arms may be added to the stand to facilitate

the loading of rolls (Figure

). Other

arrangements, such as overhead hoists or

tram rails, have found applications.

The most important function of an unwind

stand is uniform tension control during the

unwinding of the material to the printing

sections. Also, the unwind stand can be

automatically guided to provide lateral web

position control at the press in-feed. The sec-

ond most common single-position unwind

stand is the shaftless type (Figure

). This

stand uses the roll core as a support for the

material. The advantage of this system is

that the operator does not have to shaft and

unshaft rolls of material. Even with pneu-

matic shafts, this chore becomes quite diffi-

cult when handling wide rolls. A further

sophistication of a shaftless stand is the self-

loading type (Figure

In general, single-position roll stands are

used where large diameter mill rolls contain

sufficient footage to keep the press running

for a reasonable time between changes.

Parent Roll

Brake

Parent Roll

Hydraulic

Cylinder

Arms

When changes become frequent, considera-

tion must be given to flying-splice systems,

which, although more expensive, increase

the printing output.

FLYING-SPLICE UNWIND

There are many types of flying-splice

stands available. They are classified with

respect to their roll diameters. Flying-splice

stands for the packaging industry generally

are capable of handling up to 24" or, in some

cases, 32" diameter rolls. Other stands for

handling heavier laminates and paper are

available with diameters up to 60". It is not

uncommon to see flying-splice stands capa-

ble of handling 72" diameter rolls where fair-

ly heavy board stock is being run.

Figure

illustrates a typical 24" diame-

ter, simple drop-splice unwind stand for

common packaging materials such as poly-

ethylene and other extensible films.

In this sketch, note that this simple unwind

is semi-automatic in operation since it

requires the operator to “drop” the splice into

the splicing rolls. Drop splices are made by

placing the pre-taped, leading edge of the

new roll into the pneumatic or hydraulically-

loaded marriage rolls that press the tape onto

the expiring web. Operator skill is required to

determine the precise moment to make the

splice, in order to keep the tail of the expiring

roll to a minimum. The operator also has to

index the turret into the proper splicing posi-

tion. Most modern stands are made to accept

both “underwound” and “overwound” rolls.

Indexing the turrets can be done with either

electric or hydraulic motors.

Larger roll diameters and the demand for

more uniform color throughout the roll lead

to a need to drive the full roll during splicing.

Rolls weighing 500 lbs. to 800 lbs. are very

difficult to move, making a successful “drop”

splice at running speeds where matching

web sped is required virtually impossible.

An improvement in this type of stand is

to drive the new roll of material so that it

is traveling at the same linear speed as the

expiring roll. The new roll can be driven

PRESSES AND PRESS EQUIPMENT 45

The shaftless unwind

uses the roll core as a

support tor the material

and is the second most

common single-position

unwind because the

operator does not have

to shaft and unshatf

rolls of material.

A more sophisticated

shaftless unwind

self-loads the material.

Flying splice unwind is

a simple unwind, semi-

automatic in its opera-

tion, since it requires

the operator to “drop”

the splice into the splic-

ing rolls.

Assembly A moves in and out on lead screw B

and up and down on lead screw C

Brake

Core chuck

New Mill Roll

Moveable

Cradle

Splicing

Rolls

Expiring

Mill Roll

Tape for

Splicing

Tension Brake

Parent Roll

Brake

Side Adjusting

Screw

46 FLEXOGRAPHY: PRINCIPLES & PRACTICES

with a center-shaft or surface-drive sys-

tem. In the center-shaft system, the new

roll is accelerated to the proper splicing

speed by an adjustable speed drive, which

is usually a direct current (DC) motor or

an alternating current (AC) eddy current

drive. A hand-held or machine-mounted

tachometer controls the drive motor to

properly match the new web speed with

the expiring web speed.

In the surface-drive method, an air cylin-

der pushes a drive belt into engagement with

the new roll surface. The belt is driven by

mechanical means at line speed. In order to

accelerate the roll smoothly, a pneumatic

clutch gradually engages the belt.

The splicing operation in either case is

normally the same. The leading edge of the

new roll is pre-taped, and held in place by

two or three pieces of “breakaway tape” to

keep it from unwinding as the parent roll is

brought up to speed. When the roll reaches

the proper speed, an air-loaded rubber roll

forces the expiring web against the new par-

ent roll. As the pre-taped leading edge of the

new roll comes in contact with the expiring

web, the breakaway tape releases and the

transfer is completed. As with the “drop”

method, the transfer is at the operator’s dis-

cretion and the amount of tail left on the

expiring web depends on the operator’s skill.

Figures

and

describe the automatic

splicing sequence for an “over” splice.

This type of unwind splicing can be auto-

mated by using a knife to cut the expiring

web after the transfer is made. In this case,

the knife may be mounted on the same pivot

arm that holds the rubber bumper roll. There

is usually a delay between the actuation of

the bumper roll and the knife. This delay is

to ensure that contact has been made with

the pre-taped leading edge of the new roll

before cutting.

This automated system eliminates some

of the operator’s responsibility. It will leave

a minimum tail equal to the circumference

of the new parent roll. The amount of tail

can be minimized by adding an electric eye

to the system to sense the position of the

pre-taped leading edge of the parent roll

and, at the proper time, signal the actuation

of the bumper roll and the splicing knife. Of

course, this type of unit is more expensive.

Although it is possible to obtain shaftless

flying-splice unwind stands, these units are

usually too expensive to be considered on

most flexographic press applications. They

are commonly used where a large mill roll of

board stock is to be run. There are also sig-

nificant differences in unwind stands for

rolls up to 72" in diameter when compared

with others described, with respect to the

splicing method.

In the case of board, a double thickness

Splicing operation

where the turret is in

loading position.

Splicing operation

where the turret is in

splicing position.

Contact with the belt

starter brings the new

roll up to speed.

Pinch

Roll

Bumper Roll

Belt

Starter

Pinch

Roll

Bumper

Roll

Out-Running

Core

Belt

Starter

PRESSES AND PRESS EQUIPMENT 47

of an extremely heavy material can ruin the

flexo printing plates. Therefore, a butt

splice must be made instead of the previ-

ously described lap splices. Butt splices

can be made either manually or automati-

cally. For manual splices, an accumulator

or festoon in a two-position unwind is

required. While one roll is in operation, the

second unwind is being loaded with a new

roll. When the expiring roll reaches its end,

the leading edge of the new roll is manual-

ly taped while the press continues to be fed

by a web that has been stored. The amount

of web to be stored in the accumulator or

festoon is determined by the web speed

and the time needed to make the manual

butt splice.

There is equipment available for making

automatic butt splices. This splice is gener-

ally formed in three operations. At the time

of splicing, the new web is fed through a

set of rolls; both webs are cut to give

square edges and are then positioned end

to end. In this position, they travel to the

next set of rolls, where splicing tape is

applied to both sides of the web. From this

point, they travel to the next set of rolls,

which are marriage rolls, to ensure that the

tape applied at the previous operation is

securely bonded. The web then travels its

normal path.

UNWIND TENSION SYSTEMS

In general, there are two basic types of

unwind tension control systems. Each type

can use the same tension-sensing devices.

The most common tension control is the

unwind braking device employing air, elec-

tric or manually adjustable brakes. The

other system uses either air, electric or

hydraulic motors to drive the unwind shaft

to release a predetermined amount of mate-

rial into a tension-sensing device.

Unwind tension control is necessary for

good register. Control is especially neces-

sary on stack and central impression press-

es. Sufficient unwind web-tension must be

applied to maintain an even flow of material

into the printing section. Further, the tension

value must not be so high that it can cause

slippage in the in-feed draw roll section, or

so low that there is not sufficient tension to

properly track the web. Materials that

require the least tension (in pounds per lin-

eal inch) are the most difficult to handle. In

general, when unwinding materials, the ten-

sion values are roughly half the intermediate

and rewind tensions. Table 2 lists some com-

mon converting materials and some typical

unwind tensions for them.

In the unwind braking system, the brak-

ing power is decreased as the material

unwinds. A controlling device such as a

dancer or a load cell may be used to auto-

matically regulate any type of brake.

The braking system has some control

problems when the product of the tension

range and roll diameter buildup exceeds

100-to-1. Problems occur when:

TENSION



ROLL DIAMETER

DIFFERENCE DIFFERENCE

100

Where:

TENSION



MAXIMUM



MINIMUM

DIFFERENCE TENSION TENSION

ROLL DIAMETER



MAXIMUM ROLL



CORE

DIFFERENCE DIAMETER DIAMETER

On narrow webs where low tension val-

ues must be held, overcoming core-shaft

inertia and gearing-friction loads may take

away all of the brake’s sensitivity and hin-

der its proper control of web tension.

Further, if the press speed is high and the

core shaft and brake gearing have a high

inertia value, then as the roll diameter

decreases, the brake may be turned to a

zero setting. Even at a zero setting, the ten-

sion value in the web can still be too high,

thereby stretching the web to overcome the

high inertia value.

The second most common unwind-tension

48 FLEXOGRAPHY: PRINCIPLES & PRACTICES

system is the driven type. A DC motor drives

the parent roll shaft, feeding the material

into a control system. The feedback signals

from a tension device control the motor

speed, thus maintaining preset tensions. The

horsepower required to drive the unwind

roll by the driven method is basically the

same formula as used when calculating the

horsepower for rewind drives, except that

the roll inertia is a factor to be considered.

However, the tension values are in the range

of 50% of the rewind values, therefore horse-

power requirements are lower.

One basic controlling system is a dancer

roll, which has a force applied to it by

weights, air cylinders or load cells to estab-

lish a predetermined loading of the web.

While the machine is running, the roll is

expiring and decreasing in diameter. As the

diameter decreases, the movement of the

web arm becomes shorter. This means that

the basic braking force applied initially to

the roll to counteract the dancer system is

no longer in balance. As the roll decreases in

size, the braking force becomes larger in

value and applies more tension to the web.

This unbalanced condition changes the

dancer-roll position and adjusts the electric

brake rheostat, or an air-control valve in the

case of air brakes. The controls decrease the

braking force sufficiently to once again put

the system in balance, maintaining the initial

tension value. With heavy rolls, it may be

necessary to have an auxiliary control cir-

cuit for stopping the mill roll as the press

stops. This control can be called an anti-

flood device.

Figure

shows a typical weight-loader

or pneumatically loaded dancer system.

Another type used is a load-cell controlled

A typical weight-loader

or pneumatically loaded

dancer system.

Table 2

MATERIAL TENSION

(per mil per inch of width)

Acetate 0.25 lb.

Foil (aluminum) 0.25 lb.

Foil (copper) 0.25 lb.

Cellophane 0.375 lb.

Nylon 0.125 lb.

Polyethylene 0.06 lb.

Polyester 0.375 lb.

Polypropylene 0.125 lb.

Polystyrene 0.50 lb.

Saran 0.075 lb.

Vinyl 0.025 lb.

Paper* (per inch of width)

15 lb. 0.20 lb.

20 lb. 0.25 lb.

30 lb. 0.375 lb.

40 lb. 0.625 lb.

60 lb. 1.00 lb.

80 lb. 1.50 lb.

100 lb. 2.00 lb.

*based on 3,000 sq. ft. ream

Paperboard (per inch of width)

8 pt. 1.5 lb.

12 pt. 2.0 lb.

15 pt. 2.25 lb.

20 pt. 2.75 lb.

25 pt. 3.25 lb.

30 pt. 4.0 lb.

For laminated webs, sum the tensions for the

individual webs and add 0.10 lb. per inch of width

TYPICAL UNWIND TENSION

FOR WEB MATERIALS

Web

Fixed Roll

Endless Chain

or Timing Belt

Dancer Roll

Movement

Weight or

Pneumatic

Actuator

Movement

Constant tension is maintained as

the web loop varies in length

Sprocket with

Feedback

Potentiometer

tension system. The load cell or tension

transducer system is widely used for con-

trolling unwind tensions. It has the disad-

vantage of being a short-stroke dancer, so it

does not have the storage capacity to absorb

the shock of splicing. For this reason load

cell systems are normally used with single-

position unwind stands or with automatic

splicing units, which drive the full roll to pre-

vent the shock load that occurs with a man-

ual drop-type splice. The load cell arrange-

ment works on the strain-gauge principle as

previously described (Figure

). Web ten-

sion applies a force to the idler roll which, in

turn, is transmitted to the load cell. The most

minute change in tension causes the strain

gauge to change its electrical signal output.

This signal is amplified and then sent to the

unwind brakes to adjust their braking

power. Similar devices are available that, by

very small movements, create air-pressure

signals that are amplified and transmitted

back to either air or electric brakes.

The basic purpose of the unwind braking

system is to apply just enough holdback

force to the expiring roll to maintain a con-

stant tension into the printing section.

IN-FEED UNIT

The function of the in-feed is to present

the web to the print section at a constant

feed rate and tension. It is normally located

prior to the first print deck on a stack press

or in-line press and is usually a three-roll sys-

tem comprised of two driven steel rollers

and a rubber pressure roller that is air or

hydraulically loaded. The in-feed pulls the

web from the unwind roll and helps estab-

lish the first tension zone in the press.

Tension in the unwind section is established

by braking the unwind roll. The in-feed also

serves to isolate the tension in the print sec-

tion from the unwind tension and becomes

the nip point for a new tension zone through

the print and drying sections.

On central impression presses, the CI drum

becomes the pulling element and the rubber-

covered nip roll, which secures the web to

the drum, the in-feed. Because of the amount

of web wrap (about 85% of the drum’s cir-

cumference), it is only necessary to secure

the web to the drum to have constant feed.

The in-feed plays a vital role in maintain-

ing tension and in controlling register.

Forgetting to activate the nip roller has cre-

ated more than its share of problems. Also to

be considered is the hardness of the rubber

roller and the amount of pressure used. Most

press manufacturers will specify hardness,

but lacking any specifics, one has to rely on

experience or trial and error.

Also associated with the in-feed is a

spreader roll, which functions to present a

smooth, wrinkle free web to the print sec-

tion. There are a variety of opinions as to the

type of spreader to use or whether one is

really necessary. A spreader roll is normally

a factory option.

Many converters believe a bowed roll or

“banana” roll is the wisest choice. The bowed

roll will be driven either by a belt with an

adjustable pulley, or in some cases, will have

the luxury of an independent DC drive. If a

bowed roll is the preferred choice, the con-

sensus of opinion is that a roll should be of

such a design as to enable the operator to

vary the bow. This option allows the operator

optimum control to facilitate eliminating

problems such as gauge bands. Less effective,

but in many cases preferred, is a non-driven

spiral, grooved roll constructed of rubber, alu-

minum or steel that is designed to spread the

web. The action of the spirals, which are cut

from the center to the end of the roll, has the

same effect but not as dramatic as a bowed

roll, and is available at a lower cost.

OUT-FEED UNIT AND/

OR COOLING DRUM

The function of the cooling drum unit par-

PRESSES AND PRESS EQUIPMENT 49

50 FLEXOGRAPHY: PRINCIPLES & PRACTICES

allels the function of the in-feed. It provides

an even, constant pull through the print sec-

tion and also serves to isolate the print sec-

tion tension zone from the rewind tension

zone. It consists of one or more chill rolls

with a rubber-covered nip roll to secure the

web to the chill roll and prevent slippage.

The drive is variable through a variable-

speed unit or, on the latest presses, by

means of an independent DC drive motor.

Earlier presses drove the chill roll

mechanically at a fixed rate of over-speed.

These drives were speed sensitive and could

not handle a variety of substrates. The vari-

able speed mechanical drive was then devel-

oped, which allowed the operator to adjust

the tension to accommodate the various

substrates, or those that had some imperfec-

tions. The most recent development is the

DC drive, which gives automatic control

over a wide range of tensions. Again, experi-

ence becomes vital in setting the tension in

this zone, but once the experience is gained

it can be repeated. It is only necessary to

input the desired tension; the automatic con-

troller then goes to work. The biggest prob-

lem encountered in this area is failure to

lower the nip roll, resulting in misregister

and other negative-tension problems.

PRESSES AND PRESS EQUIPMENT 51

Rewind Equipment

The double-drum

rewind, the most

common type of surface

rewinder, uses two

equal-diameter winding

drums to drive the web.

here are two basic classifications

for rewinding equipment: center

and surface winding. Some sin-

gle-roll rewinds use a combina-

tion of center and surface drives.

Flying-splice rewinders are nor-

mally of a center-wind type. Although there

are automatic flying-splice surface winders,

they are normally used in the paper industry.

SURFACE WINDERS

Surface rewinding units use the surface of

a moving roll to impart rotation, by friction-

al contact, to the roll being wound. There

are two surface rewind designs common to

the industry.

Double Drum

The double-drum rewind is the most com-

mon type of surface rewinder and is de-

scribed in Figure

. It uses two rolls of

equal diameter to drive the web. The rewind

unit is usually driven by a variable-speed

drive to set the basic tension pattern.

Normally, one roll is also driven slightly

faster than the other to provide a differential

surface speed, creating a hard roll. The

winding drive rolls can also be fluted to

reduce wrinkling and give additional roll

hardness. As shown in the illustration, the

printed roll can be wound with the printed

surface in or out, depending on the method

of threading. The center of the roll, the core

shaft, moves vertically upward as the roll

diameter builds. The increasing weight of

the roll on the winding drums assists in

obtaining a tightly wound roll.

A rider roll can be supplied with a dou-

ble-drum rewind unit. The rider roll rests

on top of the roll being wound, and can be

pneumatically or hydraulically loaded to

provide a greater nip pressure between the

roll and the winding drums. The pressure

of a rider roll should be gradually lessened

during the rewinding cycle to provide more

uniform pressure.

Hydraulic or pneumatic loading of the roll

core shaft is another accepted method of

increasing nip pressure. The hydraulic cylin-

ders act to maintain a uniform or steadily

decreasing pressure relationship between

the roll being wound and the winding drums.

Maintaining uniform pressure is easier to

accomplish on a single-drum winder be-

cause the method of loading doesn’t have to

overcome or compensate for the increasing

roll weight.

Single Drum

The single-drum rewind unit (Figure

)

uses the same surface-winding principles

as the double-drum unit, but has a single

Rider Roll

Mill Roll

Booster

Winding

Drums

Slitter Station

Spreader

Bar

52 FLEXOGRAPHY: PRINCIPLES & PRACTICES

drum to impart rotation. It is also driven

with a variable-speed drive to set the basic

tension pattern.

The roll being wound moves horizontally

rather than vertically during buildup. Hydrau-

lic or pneumatic pressure is applied to main-

tain a regulated roll pressure between the roll

and the winding drum. This pressure can be

varied to provide different roll densities and

hardness. As in the double-drum rewind, the

printing can be wound inside or out.

One of the main advantages of surface-

rewinding equipment is the ability to obtain

very dense and uniformly wound rolls with

most grades of paper. Also, rewinding can be

accomplished with less horsepower than

required with center-winding equipment.

More attention must be paid to minimum

roll widths with a single-drum rewind

because of shaft deflection. The minimum

roll width is usually restricted to 80% of the

maximum roll width.

Because a rotating surface imparts motion

to the sheet through friction contact, this

equipment is generally limited to materials or

products that don’t have slippery surfaces, are

not easily stretched, and can withstand some

rubbing or scuffing action. A waxed sheet sur-

face, for example, might be wound more

effectively on center-winding equipment.

Either score- or shear-slitting units lend

themselves readily to surface-winding meth-

ods. The slitting station is mounted before

the winding drums to reduce the travel

length of slit webs to ensure there will not be

web interleaving.

Roll unloading equipment is available on

both single- and double-drum rewinding

units. Hydraulic or pneumatic means can be

provided to either lower the roll out of the

winding machine or to push the finished roll

onto a loading platform or dolly.

CENTER WINDERS

Center winding gets its rotary movement

through the core shaft. The center-winding

units are available both as core-shaft and as

shaftless types. On flexo presses, the most

common center-shaft winder is the core-

shaft type.

A common center-shaft winder is the sin-

gle-position type described in Figure

This design employs one core shaft mounted

in frames. The shaft is driven by an electric,

mechanical, hydraulic or combination drive

with some means of adjusting the drive

speed to vary the roll hardness. The single-

position type must be stopped for roll

changes. The rider roll is also driven on more

modern designs.

A rider roll is frequently used in both sin-

gle- and flying-splice center rewind units. Its

purpose is to help get a hard and uniform fin-

The single-drum rewind

unit uses the same sur-

face-winding principles

as the double-drum

unit, but has a single

winding drum to impart

rotation.

A single-position center

shaft rewind unit with

web slitter. This com-

mon design employs

one core shaft mounted

in frames. The shaft is

driven by an electric,

mechanical, hydraulic

or combination drive

with some means of

adjusting the drive

speed to vary the roll

hardness.

Bypass-Reverse Wind

Slitter

Station

Spreader

Bar

Roll

Unloading

Arm

Roll

Cylinder

Winding

Drum

Slitter Station

Expander roll

Roll

Unloading Arm

Rider Roll