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

Подождите немного. Документ загружается.

PRESSES AND PRESS EQUIPMENT 123

Thinner Printing Plates

Since flexo corrugated presses are a deriv-

ative of letterpress presses, printing plates of

0.25" thickness, mounted on a 0.030" backing

for a press undercut of 0.280", are still found

in the majority of corrugated converting

plants today. Such plates may be produced

from natural molded rubber (a dying art),

synthetic molded rubber, liquid or sheet pho-

tocurable polymer, or laser-engraved rubber.

Even on brand-new machines, cylinders are

sized to receive plates as mentioned above

simply because converters have accumulat-

ed hundreds, if not thousands, of printing

plates for ongoing repeat business.

Thinner printing plates provide the advan-

tages of lower print relief and higher print

image definition – necessary if high-resolu-

tion halftone images are to be printed. For-

tunately, an increasing number of printers

have decided to part with the past and work

with an overall plate thickness starting at

about 0.125" (3 mm), up to about 0.1875" (5

mm). Although the thickness of the plate

material itself may measure only 0.067" (1.7

mm), the thinnest is used on corrugated

today, because corrugated is such a variable

substrate, a “cushioning foam” backing mate-

rial is used to bring the plate to the overall

caliper necessary to meet the undercut built

into the press.

Some compromises are always necessary

to conform to the printing diameter of the

press and the cylinder undercut. For exam-

ple, if an overall plate caliper of 0.185" (4.65

mm) is chosen, a 0.125" (3.15 mm) plate must

be under-packed, or permanently mounted,

with 0.060" foam backing material. Any com-

bination of plate thickness plus foam (cush-

ioning material) can generally be used as

long as the overall caliper corresponds to the

printing diameter of a press. A 1:2 ratio of

cushioning material to plate is preferred.

Quick-change Anilox Roll Systems

An anilox roll is capable of giving only one

amount of ink: the content of its engraved,

or laser-burned cells. Any deviation from

this simple law means variation in ink lay-

down or ink density.

Mixing of large solids with screens or fine

type on the same printing unit is not recom-

mended in flexography. In corrugated post-

printing, for years, printers were compelled

to compromise because the anilox roll was,

for all practical purposes, a fixed part of the

printing press. To change a roll or rolls would

take a team of mechanics from several hours

to an entire weekend. Today, most printers

understand that the characteristics of a par-

ticular anilox roll limit its application to a nar-

row band of substrates as well as selected

graphics. Several anilox rolls may be needed

per printing station to enable the printer to

use the right tool for the job at hand.

With the increasing demand for more and

more intricate graphics, such as process

work with plate screens up to 150 lpi, it has

become necessary to build printing presses

with quick-change anilox roll systems. The

tremendously varied substrates utilized in

corrugated demand ink lay downs from as

little as 2 BCM/in

(e.g., with process print

on heavily clay coated paper) to as much as

14 BCM/in

for very absorbent recycled

liner. These very large differences between

rolls explain why press manufacturers for

the corrugated postprinting field are com-

pelled to build machines with “quick-

change” systems for anilox rolls.

Internal Change, Semiautomatic. Today, ma-

chines with 100% automated anilox roll-

changing systems are available. The semi-

automated system will be discussed first.

The idea of accelerating and simplifying

the mounting and dismounting of anilox

rolls is not a new one, simply because of the

excessive time needed to change anilox rolls

even on one or two printing units. Some

manufacturers provide “roll-out ramps” to

facilitate this process. Even with the ramp,

changing a roll can take one to two hours.

124 FLEXOGRAPHY: PRINCIPLES & PRACTICES

On some top printing machines, roll-

change designs involve an overhead hoist

equipped with pillow-block bearing hous-

ings. The housing can be opened up to lift

out an anilox roll and replace it with anoth-

er. The fastest system of this kind can com-

plete a roll change in about 30 to 45 minutes.

On certain bottom-printing machines, a

permanent anilox roll elevator can take an

anilox roll down from its working position

and bring another one up by manually open-

ing clamshell-type bearing housings. Five to

seven minutes are needed with such an

approach. Chambered doctor-blade systems

are a prerequisite with this design in order

not to have to remove rubber rolls or ink

fountains, which would obstruct the lifting

or lowering path of the anilox roll.

Internal Change, Automatic. A number of

press manufacturers have designed, or have

announced plans to design, systems that

would change anilox rolls fully automatical-

ly. One manufacturer is already providing a

top-printing press with a fully automatic sys-

tem. This particular machine is a servo-dri-

ven machine where the anilox roll is driven

directly with a separate servo motor. The

spare anilox roll is stored above the inking

system with its own drive motor already

attached. A rack-and-pinion arrangement

brings the roll to its working position in less

than a minute. The entire exchange is exe-

cuted by the machine operator using push

buttons. This design provides quick ex-

change between two rolls in any printing

unit. It is a much larger job to bring any of

the rolls outside the machine for mainte-

nance and repair.

Another design on a bottom-printer allows

push-button exchange of two rolls in about

two minutes. On this machine, the rolls are

exchanged with a vertical elevator. The

anilox rolls have no journals, but have a

female cone arrangement on each end. The

drive journals are part of the machine struc-

ture and can be moved laterally to clamp the

roll in a horizontal position. Again, changing

of anilox rolls using pushbuttons is a

machine operator’s task, without involving

any plant mechanics or engineers. With this

design, up to three rolls can be exchanged.

One anilox roll is in printing position and

two are in a storage cradle ready to be

exchanged with the roll in printing position.

A third approach is to have several anilox

rolls on a horizontal turret system. This

design is, as of this writing, still on the draw-

ing board. The design resembles a triplex

slitter/scoring system on older corrugators.

Changing Anilox Rolls Between Printing Units.

In order to provide a printing press that is

even more flexible regarding ink lay-down,

one manufacturer has gone a step further.

Not only can anilox rolls be exchanged with-

in one printing unit, but also from one unit to

another. This change is achieved with a roll

storage cradle below each inking unit. The

cradle can be transported by a cart or trolley

to move the anilox rolls from one printing

station to another if and when needed. This

approach makes economic sense, as possibly

fewer anilox rolls may need to be purchased

for the daily mix of work done by a printer.

DRYERS

The popularity of drying by air impinge-

ment is mostly due to safety concerns, as this

approach is free of fire hazards. It is proba-

bly the oldest method used to dry ink and the

technology is well known in many other seg-

ments of flexography, as well as rotogravure.

In sheet-fed flexo on corrugated, especially

on machines with spaced-apart printing

units, warm air dryers are today capable of

fully drying large surface printed or var-

nished sheets at machine speeds exceeding

10,000 sheets per hour.

Infrared Dryers. Within the invisible light

spectrum, infrared radiation produces heat

that, depending on wavelength, penetrates

the substrate to varying degrees. Short-wave

infrared, together with an air blanket, has

proven to be an effective drying source for

inks and varnishes. This technique appears

to be well utilized, especially on closed

linked machines where little room exists to

install other means of drying.

Infrared is a very effective heat source, but

with long enough radiation exposure to the

printed board, could cause a fire hazard.

Therefore, when infrared is used as the heat

source for drying, it is generally arranged to

preheat moving air, which is then extracted

after it has saturated itself with moisture.

Sheet Cleaners

Compared to other printing industries, cor-

rugated plants have a multitude of problems

with dust, beginning at the corrugator. Dust

and slivers are in the stacks of corrugated

sheets and are the No. 1 cause of downtime

and quality problems in postprinting or cor-

rugated.

With printers producing finer and finer

graphics with thinner and thinner ink films,

dust-generated hickeys are a nightmare for

press operators. In the past the problem was

also present, since print quality standards

were low, a heavy ink film was used to cover

up the dust. Dust is by far the biggest exter-

nal obstacle to quality printing, especially in

a corrugated operation, and press builders

and converters alike must address it aggres-

sively. Today, quality printers are more and

more conscious of dust. Increasingly sophis-

ticated sheet-cleaning devices are being

installed on printing presses and have

become more common on corrugators.

Stationary Brushes. Stationary brushes, in

combination with a vacuum mouth or fun-

nel, are the simplest method for cleaning

sheets or a web from one or both sides. Such

systems are effective only to clean coarse

matter, though, and do not do a good job

with finer dust. In some cases, the friction of

the bristles on the board causes static elec-

tricity, making the dust adhere even more.

Use of static eliminators does help, but does

not eliminate this problem totally.

Stationary brush-type cleaners are gener-

ally used in confined areas near the feed rolls

and are always installed with a dust-suction

device. Unfortunately, since such equipment

is sometimes installed in very tight spaces,

cleaning the brushes becomes difficult, and

the system becomes less and less effective.

Rotary Brushes. Dust suction systems for

sheet cleaning with rotary brushes is anoth-

er arrangement found on corrugated printing

presses. Very similar to stationary brushes,

the sheets are brushed by the rotary brushes,

which rotate in the opposite direction of the

sheet travel. Again, on certain substrates sta-

tic electricity is created, making the dust

adhere to the surface of the sheets instead of

loosening it for removal by suction. The

brushes also saturate themselves with dust

and if not removed frequently, become inef-

fective. As with all systems involving suc-

tion, it is necessary to evacuate the dust into

filter bags or cyclones. Cyclones, although

cumbersome and large, are the preferred

means. Filter bags, if not cleaned frequently,

reduce the vacuum flow severely.

Static Elimination Cleaners. Corrugated

sheets, besides being dusty, are often

charged with static electricity, which makes

dust removal by vacuum and contacting sta-

tionary or rotary brushes even more difficult.

Static elimination devices are therefore

implemented to counteract the problem.

Stationary-brush sheet cleaners and rotary-

brush sheet cleaners, as described earlier,

are available with a more advanced design

that utilizes static-neutralizing devices. Just

as in every other converting and printing

operation, static electricity is created in the

corrugated field through friction, induction,

rapid changes in temperature, and rupture of

the molecular structure created by slitting or

sheeting. All of these actions create an imbal-

ance of electrons.

The most advanced ionic cleaning systems

PRESSES AND PRESS EQUIPMENT 125

126 FLEXOGRAPHY: PRINCIPLES & PRACTICES

involve a corona field formed between two

electrodes in a quartz enclosure. The sheets

to be cleaned pass through this corona field,

which is generated through high voltage and

high frequency (5,000 to 7,000 cycles/sec and

at 15,000 volts). None of the dust removal

systems are absolutely perfect. However, if

the problem is addressed at its source, i.e., at

the point where dust and electrostatic

charges are produced, it can be controlled.

For more information on ionic substrate

cleaning, see the previous section.

Updating and Upgrading for

Continued Development

As mentioned previously, upgrading exist-

ing printing presses is in many cases as sim-

ple as installing finer anilox rolls and either

reverse-angle doctor blades or a chambered

doctor-blade system. Nevertheless, one has

to be aware that anilox rolls are only a tool

(albeit an expensive tool) and that there is

no universal anilox roll that can successfully

print more than a relatively narrow band of

graphics with a given volumetric capacity.

It is, of course, necessary that the machine

be in good mechanical condition, especially

with regard to register accuracy. Some man-

ufacturers offer vacuum transfers for board

conveyance as retrofits to improve on-press

feeder on an older press can also lead to

improved graphics.

Many on-press inspection devices, from

fiber optics to video cameras, are utilized in

the industry. Viscosity/pH control devices

are also finding more application in the cor-

rugated flexo printing field.

In a few years printing presses will have

“closed loop” controls whereby register cor-

rections, ink density and even color hue,

will be constantly monitored and corrected

without the press operator’s intervention.

JOB PREPARATION

AND PLANNING

With the increasing number of off-line flexo

printing presses and the push to reach higher

and higher quality with very fine screen

process images, it has become increasingly

important for corrugated flexographic print-

ers to better prepare and organize all aspects

of the converting process. For instance, when

four to five colors are printed and UV varnish

has been applied over dark colors, creases

may crack (also called checking) on the fold-

er-gluer, the very last operation performed

before the boxes are shipped. One can easily

imagine the seriousness this problem pre-

sents considering the waste of materials,

man-hours and press time.

It is highly recommended that with each

new project, the printer/converter conduct

certain lab tests such as rub tests, creasing

and bending tests, and moisture content mea-

sures of substrates. Collaboration with trade

shops and art designers must be established

by the printer/converter to have a good

exchange of ideas and practices that can be

applied (or not) to a printing/converting job.

Today, many corrugated boxes are used

where folding carton boxes were once used

exclusively. Such cartons are generally

called “folding-carton-style” corrugated

boxes. The placement and disposition of

graphics on these boxes is done with the

same scale of measure as with folding car-

tons. Graphics must appear parallel to creas-

es and, if possible, not run over box scores.

Basically, graphic designs and dispositions

that leave no room for manufacturing toler-

ances during printing, die cutting and fold-

ing/gluing create unnecessary waste.

As a general guideline, a converter must

know the manufacturing tolerances possible

on his printer, die cutter and folder/gluer, and

design the job for his equipment; not the

other way of trying to adapt the converting

machine to a job, which is an expensive and

incorrect process. As machinery becomes

more accurate and allows for finer and finer

tions, such as plate mounting, cutting die tol-

erances (especially on rotary dies) and waste

stripping tools must be manufactured with

tolerances even finer than those allowed on

the end product for which they are built.

Machinery manufacturers offer many

options on equipment to allow for correcting

errors. These are just a few examples:

• printing plate skewing devices correct

inaccurate printing plate mounting or

mounting of the lead-edge hook on a

bias;

• anilox roll adjustments put the roll out

of parallel to adjust for printing plate

caliper variations; and

• feeders with skewable front gates

adjust for printing performed on a bias.

Such features can sometimes help opera-

tors to “save the job.” Unfortunately, if

machine operators neglect to return such

systems to their zero position after a badly

mounted job has been run, a job that is

mounted correctly by a quality supplier will

not perform correctly without sometimes

lengthy adjustments.

Equipment Maintenance

In corrugated converting operations,

whether starting with rolls of linerboard or in

a sheet plant, maintenance involves many

different machines and many different con-

verting operations. Machinery maintenance

is generally handled by a crew of specialized

mechanics and electricians. In some smaller

operations, machine operators and press

crews are deeply involved in maintaining

their respective pieces of equipment. Both

options work quite well when the people

involved are very familiar with the process or

processes performed on a piece of equip-

ment. An increasing number of equipment

suppliers offer maintenance and process-ori-

ented seminars to help converters maintain

and improve their machinery and operations.

Training for

Continued Improvement

Flexo postprinting, the most-common

printing method for corrugated, high-end

graphics or just plain everyday printing, has

been experiencing a period of renewed

excitement. Different universities have start-

ed to show great interest in teaching flexog-

raphy, right down to on-press training.

Continued emphasis on education is crucial

for meeting the increasing demand for high-

quality flexo products.

PRESSES AND PRESS EQUIPMENT 127

128 FLEXOGRAPHY: PRINCIPLES & PRACTICES

Press Mechanics

he increase in printing speeds

and quality standards in the

flexographic printing industry

today makes it necessary to

consider the importance of

accurately balancing the vari-

ous rolls and cylinders used in the press. To

better understand the problems involved

with an unbalanced cylinder, we must con-

sider that balancing is a process whereby the

distribution of mass in a roll is altered to

eliminate vibration at the supporting bear-

ings. A roll can be manufactured to very

close dimensional tolerances and can be

properly designed structurally so that it is a

rigid integral unit in a static state. Some-

times, however, in spite of all the care and

precautions taken, the press in which the roll

functions does not perform satisfactorily due

to excessive vibration. In the case of plate

cylinders, the addition of the plate mass

itself may cause imbalance vibrations. These

vibrations often limit the printing speeds the

press is mechanically capable of producing.

BALANCING FLEXO ROLLS

Consider the following problems that are a

direct result of vibrations produced by unbal-

anced cylinders:

• excessive printing plate wear;

• excessive bearing wear;

• excessive roll wear;

• reduction of the overall mechanical

efficiency of the printing unit, such as

uneven impressions;

• associated resonant vibration of other

parts of the press or its supporting

structure;

• possible failure of structural compo-

nents of the press; and

• excessive noise created as a side

effect, which often reduces efficiency

of the pressmen.

The cause of imbalance in rolls, although

usually obvious, is frequently overlooked.

Imbalance is caused by the lack of homogene-

ity in a material, whether it is cast, rolled,

forged, extruded or otherwise produced. In

the case of tubular or cylindrical products, an

uneven wall section can cause imbalance.

Evidence such as blow holes, slag occlusions

and variations in the crystalline and chemical

structure of a material are indications that the

raw materials used to produce the rolls are

not homogenous. Variation in the distribution

of mass due to manufacturing tolerances,

which must be allowed on all machine sur-

faces, is a major contributing factor.

Any manufacturing tolerances that permit

eccentricity or lack of squareness of machine

surfaces with respect to the rotational axis

are sources of imbalance. Non-symmetrical

distortion of a roll while running at its operat-

ing speed can also produce excessive vibra-

tion. This distortion is generally the result of

poor design, such as too small a diameter in

relation to face length, or of variations in wall

thickness of the material used to manufacture

the roll.

The need for balancing a flexographic

press roll is evident when we consider that

the center of mass of a roll will not neces-

sarily coincide with the rotational axis as

determined by its supporting bearings. A roll

that is not restrained by bearings will natu-

rally rotate about its center of mass.

However, when the same roll is restrained

by bearings, it is forced to rotate about an

axis other than its center of mass due to the

run-out of the bearings. Consequently, cen-

trifugal forces are introduced, which cause

vibrations. To state it very simply, balancing

is merely an operation that eliminates vibra-

tion by redistributing the mass of the cylin-

der so as to cause its center of rotation to lie

on the same axis as that determined by the

supporting bearings.

Although there is varying terminology

used with respect to balancing, we are con-

cerned primarily with two types: static

imbalance and dynamic imbalance.

Static Imbalance

In static imbalance, only gravity or weight

force is involved, and balancing can be

accomplished without rotation. This type of

balancing is generally not satisfactory for

flexo rolls due to the rotational speeds at

which they operate and the ratio between

their diameter and axial length.

Dynamic Imbalance

Rotating parts ordinarily have both static

and dynamic imbalance. This combined stat-

ic and dynamic imbalance is corrected by

weights placed in two different planes per-

pendicular to the axis of rotation (Figure

The weight compensates not only for the sta-

tic imbalance at rest but also for the dynamic

imbalance caused by centrifugal force when

the roll is rotating.

This two-plane balancing, which corrects

both static and dynamic unbalance, is gener-

ally referred to by engineers as dynamic bal-

ancing. Due to the physical and rotational

characteristics of flexo rolls they should in

every case be in dynamic balance when high

press speeds are used.

Forces on Bearings

The centrifugal force exerted on the

restraining bearings of an unbalanced roll is

proportional to both the weight of the roll

and the distance that its mass center is dis-

placed from the rotational axis.

The general formula for centrifugal force is:

FORCE 

 R  W/g

or FORCE 

 DYNAMIC UNBALANCE

Where:

W = roll weight

g = gravitational constant

R = radius of rotation

 = angular speed of rotation

We must therefore measure imbalance as

the product of weight and displacement,

expressed in terms of ounce-inches. An

imbalance of two ounce-inches is produced

PRESSES AND PRESS EQUIPMENT 129

The process of dynamic

balancing: Placing

weights in two different

planes perpendicular

to the axis of rotation

compensates not only

for the static imbalance

at rest but also for the

dynamic imbalance

caused by centrifugal

force when the roll is

rotating.

Journal (a)

Axis of Roll

Axis of Rotation

Dynamic unbalance due to

maximum indicated run-out

on Journal (b)

Dynamic unbalance due to

maximum indicated run-out

on Journal (a)

Position balance weight

at minimum indicated

run-out on Journal (a)

Position balance weight

at minimum indicated

run-out on Journal (b)

Journal (b)

Balance Weight = x

Roll Weight

TIR

130 FLEXOGRAPHY: PRINCIPLES & PRACTICES

by a two-ounce weight displaced 1" from the

rotation axis. For example, a roll weighing

125 lbs. (2,000 oz.) with its center of mass

0.001" from its rotational center as deter-

mined by its bearings, is 2 (2,000 x 0.001)

ounce-inches out of balance.

The magnitude of the centrifugal force

produced by a two-ounce unbalanced condi-

tion is a function of the speed of rotation or

in other words, the revolutions per minute of

the roll. This force, which produces vibra-

tions, increases as the square of the rota-

tional speed. For example, consider again

the roll that has an unbalanced condition of

two ounce-inches. When this roll is operated

at 500 rpm, it will produce a centrifugal force

of approximately 0.9 lbs. However, when we

increase the speed to 1000 rpm, we create a

centrifugal force of 3.6 lbs., or 4 times the

force at 500 rpm.

The example illustrates that there is a

much greater need for accuracy of balance

at higher speeds than at lower speeds. The

degree of balance required in a roll can only

be determined after consideration is given

to the operating speed in revolutions per

minute and the conditions under which the

roll will be used. Some of the factors that

must be considered are:

• the structural rigidity of the roll itself;

• the diameter in relation to the distance

between bearings; and

• the overall rigidity of the supporting

structure of the bearings.

Although all factors must be considered,

the following is recommended:

• All rolls, regardless of press speed,

should be in static balance.

• Rolls operated at 300–500 rpm should

be in dynamic balance within 10 ounce-

inches.

Table 7

ROLL DIAMETER (INCHES)

2 4 6 8 10 12 14 16 18

ROLL CIRCUMFERENCE (INCHES)

6.283 12.566 18.850 25.133 31.416 37.699 43.982 50.265 56.549

PRESS SPEED (FT/MIN) ROLL SPEED (Revolutions Per Minute)

100 191 95 64 48 38 32 27 24 21

200 382 191 127 95 76 64 55 48 42

300 573 286 191 143 115 95 82 72 64

400 764 382 255 191 153 127 109 95 85

500 955 477 318 239 191 159 136 119 106

600 1146 573 382 286 229 191 164 143 127

700 1337 668 446 334 267 223 191 167 149

800 NA 764 509 382 306 255 218 191 170

900 NA 859 573 430 344 286 246 215 191

1000 NA 955 637 477 382 318 273 239 212

1100 NA 1050 700 525 420 350 300 263 233

1200 NA 1146 764 573 458 382 327 286 255

1300 NA 1241 828 621 497 414 355 310 276

1400 NA 1337 891 668 535 446 382 334 297

ROLL SPEED VS. PRESS SPEED

• Rolls operated at 500–1,000 rpm should

be in dynamic balance within 5 ounce-

inches.

• Rolls operated at over 1,000 rpm should

be in dynamic balance within 2 ounce-

inches.

Table 7 converts press speed in feet per

minute into roll speed in revolutions per

minute for various roll diameters and/or cir-

cumferences. Figure

indicates centrifu-

gal force as a function of roll speed for vari-

ous units of imbalance in the base roll.

Allowable Total

Indicated Runout (TIR)

Table 8 gives allowable total indicated

runout (TIR), also called eccentricity, of bear-

ing journals for various rotational speeds.

DEFLECTION OF ROLLS

All rolls deflect, even under their own

weight. In addition to the weight of the roll

there are other factors, such as web tension,

impression loading and out-of-balance forces

that cause the roll to deflect. The following

discussion is limited to a uniformly loaded

roll running at low speed, so that conditions

approximate a uniformly loaded beam freely

supported at the ends.

The general formula for the deflection at

the center of a simply supported roll with a

uniformly distributed load is:

Maximum deflection  5  W  L

384  E  I

Where:

W= the total load on the rolls in lbs.

L = the length of the roll in inches.

I = the moment of inertia of the roll-

body cross-section, a function of

the fourth power of the shaft diameter

) in inches.

E = the modulus of elasticity of the roll

material in pounds per square inch.

Note:

• The amount of deflection varies directly

with the load; i.e., doubling the load dou-

bles the deflection.

• Deflection varies directly with the cube

of the length; i.e., doubling the face

length while maintaining the same total

load increases the deflection eight times.

• Deflection varies inversely with the

moment of inertia of the cross-section.

• The moment of inertia of the cross-sec-

PRESSES AND PRESS EQUIPMENT 131

Centrifugal force as a

function of roll speed

for various units of

imbalance in the base

roll.

Table 8

SHAFT SPEED ALLOWABLE TIR

RPM INCHES MICRONS

100 0.02000 508.0

200 0.00500 127.0

300 0.00222 56.4

400 0.00125 31.8

500 0.00080 20.3

600 0.00056 14.1

700 0.00041 10.4

800 0.00031 7.9

900 0.00025 6.3

1000 0.00020 5.1

1100 0.00017 4.2

1200 0.00014 3.5

1300 0.00012 3.0

1400 0.00010 2.6

ALLOWABLE TIR FOR VARIOUS

SHAFT SPEEDS

1 oz. inch

2 oz. inch

3 oz. inch

4 oz. inch

5 oz. inch

100 1,000 10,000

Roll Speed (revolutions per minute)

Centrifugal Force (lbs.)

100

0.1

0.01

132 FLEXOGRAPHY: PRINCIPLES & PRACTICES

For a solid roll :

I  0.049  Roll Diameter

For a hollow roll:

I  0.049 OD

 ID



Where:

OD = the outside diameter of the roll

ID = the inside diameter of the hollow roll

Table 9 lists the modulus of elasticity for

different roll materials. For example, if a roll

made of steel tube were to be replaced with a

similar one made of cast iron, then the deflec-

tion would be doubled because the modulus

of elasticity of cast iron is only half that of

steel. Similarly, the same roll made of alu-

minum would have three times the deflection.

To design a roll of cast iron having the same

deflection as a hollow steel roll it would be

necessary to double the moment of inertia for

the roll body, since the modulus of elasticity is

only half that of steel. This design could call

for a solid cast iron roll of approximately the

same diameter, weighing more than twice the

hollow steel roll.

The message here is: Changing rolls with-

in a press, or from press to press is not rec-

ommended. Although the rolls may look the

same, they may have very different deflec-

tion characteristics.

GEAR DRIVES

There are many different types of trans-

mission gearing, but the most common found

on flexographic presses are the spur, heli-

cal, bevel and worm gears.

Spur Gears. A spur gear has straight teeth,

which are machined parallel to the rotating

axis of the gear, at right angles to the face of

the gear (Figure

). It is the most common

type of gear because it is the least expensive.

Spur gears are used to transmit power

between parallel shafts. They are generally

used to transmit power from the plate cylin-

der to the anilox roller.

tion varies as a function of the fourth

power of the diameter.

• Deflection varies inversely with the mod-

ulus of elasticity.

Since a roll is composed of shafts, heads

and a body, the cross-section of the roll is

not uniform. For practical purposes, the

cross-section may be considered to be that

of the main roll body, and the face length as

the distance between the support bearings.

In order to calculate the approximate deflec-

tion of a given roll, one must first determine

I, the moment of inertia of the roll-body

cross-section.

Spur gears have

straight teeth, which are

machined parallel to the

rotating axis of the gear,

at right angles to the

face of the gear.

Table 9

MATERIAL MODULUS

MILLION LB/SQ. IN.

Steel 30

Cast Iron 15

Copper 14

Brass 14

18/8 Stainless Steel 14

Aluminum alloys 10.0–10.3

Air-Seasoned Fir 1.5*

Air-Seasoned Poplar 1.5*

*Varies considerably.

MODULUS OF ELASTICITY

OF DIFFERENT ROLL MATERIALS