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

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board. This time line is mentioned just to

emphasize how vast the application of flexog-

raphy in the corrugated field is. In the sector

of flexo folder-gluers, it is fair to estimate an

annual worldwide growth of flexo printing

units at between 500 and 600 print stations.

Bottom-printing units are predominant

although top-printers have certain other

advantages and, again, any kind of ink-

metering system is offered. Since this book

deals primarily with flexography and its

principles, the comments on converting

principles other than flexo will be brief.

An Overview

Once the double-face board is constructed

from roll stock on the corrugator, the fol-

lowing events occur in the sheeting section

in preparation for the flexo folding and glu-

ing operations:

• Scores are embedded onto the board to

produce creases.

• Full-width board is slit into narrower

w i d t h s .

• Edges of the board are trimmed.

• Boards are cut to the desired length to

produce a cut-to-size blank.

• Blanks are stacked and stored ready for

p r i n t i n g .

We now have a corrugated board, cut to

size, with creases running down its length

where the flaps of the box are to be folded

(Figure

8 !

). This corrugated board is

referred to as a blank and is ready to be con-

verted into a box at the flexo folder- g l u e r.

The flexo folder-gluer produces complete-

ly finished boxes from sheets directly off the

corrugator (Figure

8 @

). Standard operations

performed by the flexo folder-gluer include:

• f e e d i n g ;

• printing in two colors;

• creasing and slotting;

• die cutting;

• inside or outside lap glueing;

• folding; and

• squaring, stacking and delivery of cor-

rugated boxes in uniform, accurately

counted piles that are now ready for

s h i p p i n g .

The cut-to-size blanks are delivered to the

“work in progress” area in large stacks. They

are stored here until needed by the flexo

f o l d e r- g l u e r. The function of the flexo folder-

gluer is to convert the scored, corrugated

blanks into finished, ready-to-use boxes.

The converting line has two major sec-

tions, the flexo section and the folding sec-

tion. The flexo section of the machine is

where the corrugated blanks are actually

printed. The blanks first have a printed pat-

PRESSES AND PRESS EQUIPMENT 113

Preformed corrugated

blanks are ready to be

converted into a box at

the flexo folder-gluer.

8 !

114 FLEXOGRAPHY: PRINCIPLES & PRACTICES

tern applied to the top sheet. This pattern is

usually the manufacturer’s logo and name,

along with the product description, instruc-

tions and other basic information.

The first printing station prints graphics in

a single color onto the top surface of the

blank. The print station uses a rotary flexo-

graphic printing process to print the blanks.

An anilox roll and wiper roll meter ink that is

transferred to the printing plate. To make the

print impression, the impression cylinder

presses each sheet against an inked printing

plate mounted on a print cylinder. A pair of

pull rolls then advances the sheets on to the

second printing station. The next and subse-

quent printing stations are identical to the

first. When the subsequent color graphics are

placed onto the top surface of the blank, the

pull rolls of the last printing station feed the

printed blank to the slotter-creaser section.

In the creasing and slotting section of the

machine, the printed blanks are creased and

cut to form boxes (Figure

8 #

). Creases are

impressed into the blank where the box is to

be folded, and slots are cut out to define the

box flaps. Glue tabs are also cut out to pro-

vide a way of joining the two ends of the

cutout box pattern.

The creaser/slotter section consists of:

• Creasers (upper and lower shafts);

• slotters (lead and trailing); and

• scrap recovery system.

The purpose of the creaser heads is to

impress a line in the sheet between the trailing

slots and the lead slots. The creaser section

A look at how the flexo

folder-gluer turns

blanks into printed,

finished boxes.

8 @

consists of an upper and lower steel shaft. The

upper shaft has four rubber-covered female

anvil heads mounted onto it. The lower shaft

houses the male profile-creasing heads.

Creasing is done, from the bottom up, on

the inside of the blank. The male profile

head scores a crease on the blank as it pass-

es between the nip of the two shafts. Each

pair of creaser heads is carefully aligned so

that the crease, or fold line, it produces runs

exactly in the middle of the lead and trailing

slots cut in the blank. These creases form

impressions in the sheet to form the end and

side panels when the blank is folded into a

box at the folding section (Figure

8 $

) .

The operator must manually set the gaps

between the upper and lower creaser heads

according to the caliper of the board being

run. If the nip pressure is too great, cracks

will occur along the score. If the shafts are

too far apart, a weak or non-existent crease

will occur. A weak crease results in improp-

er carton folding and possible rolled creases.

The lead slot knives also advance the blanks

PRESSES AND PRESS EQUIPMENT 115

8 #

Creasing and slotting

shfts shape the blanks

prior to folding.

8 $

A cross-section of the

creasing/slotting

process.

8 #

8 $

116 FLEXOGRAPHY: PRINCIPLES & PRACTICES

into the next section of the machine.

The flexo section may also have a creas-

er/die-cutter unit. If required by the cus-

tomer’s specifications, additional cuts or all

of the cuts needed (such as hand holes, vent

holes, extra flaps or other types of holes on

the box) may be made in the creaser/die-cut-

ter section. The creaser is used to place

creases on the blank for folding extra flaps

on specialty boxes and displays.

The creaser/die-cutter section is made up of

the creaser drum and anvil drum, a set of pull

rolls and the die-cut drum and anvil drum

(Figures

and

). The creaser/die cut-

ter makes it possible to score on both sides of

the sheet or to produce offset scores and slit

scores. It also allows the flexo folder-gluer to

run unscored blanks off the corrugator.

An angled scrap conveyor removes the

cutting waste from beneath the station. On

some machines the creaser/die-cutter sta-

tion is equipped with the powered side roll

out so that it can be removed from the flexo

folder-gluer when not in use.

At the folding section (Figure

), adhesive

is applied to the glue tab or to the bottom of

the fourth panel, depending on requirements.

The blanks are then pulled through the folding

section by a set of vacuum belts. Here, they

first encounter tapered folding bars that start

the first 90° fold of the outer blank flaps. The

final folding occurs in the spiral folding belts.

The formed boxes are transferred from the

folding section to the counter/ejector, where

the boxes are counted and placed into bun-

dles (Figure

). As the boxes enter the

counter/ejector from the folding section,

they drop onto an elevator. As the bundle

height increases, the elevator drops to keep

the top of the bundle a fixed distance from

the in-feed conveyor. When the bundle reach-

es a pre-set count, a set of trombone fingers

extend down and out, on top of the last box

in the bundle. To complete the folding of the

box, the belts help in joining the glue tab to

the opposite side of the sheet to form the

actual box. Compression is applied to the

overlap panel of the folded box so that the

glue tab and fourth panel are bonded togeth-

The creaser/die cutter

makes it possible to

score on both sides of

the sheet or to produce

offset scores and slit

scores. It also allows

the flexo folder-gluer to

run unscored blanks off

the corrugator.

A creaser die-cut tool.

A rotary die-cut shell.

Pull

Rolls

Die Cut

Drum

Anvil

Drum

Scrap

Conveyor

Board

Travel

Creaser

Drum

Cutting Rule

Creaser Rule

Ejection Rubber

Hand

Access

Hole

Center

Line

Mark

Plywood

Form

Leading

Edge

Arrow

er to form the knocked down boxes.

The discharge compression-conveyor sec-

tion applies pressure to the top of the bundle

to help the glue applied to the tab form a

good bond. From the compression section,

the bundles are conveyed away from the

flexo folder-gluer.

PLATEN DIE CUTTING

Today, printer-platen die cutters are gener-

ally built on the principle of spaced-apart sta-

tionary printing units (Figure

). After the

printing units are finished, a belt-conveyor or

vacuum-conveyor transports the printed

sheets into a platen-type die cutter.

The number of printing units most com-

monly used today is three or four. However,

it is possible to go up to five or six colors.

The question of how many units are needed

is not a technical one, but rather a viewpoint

of practicality.

A platen-type die-cutting printing operation

is governed in speed by the platen die cutter.

The die-cutting/stripping process may be

intricate and slow, and any jam-ups cause the

entire line to stop, creating downtime,

lengthy washing of printing plates and waste

at each restart. On the other hand, platen die

cutting is the most accurate die-cutting

process and a virtually unlimited number of

die cuts can be placed on a sheet. Also,

smearing or offsetting with a platen die-cut-

ting operation is virtually non-existent, allow-

ing very high quality printed and varnished

sheets can easily be cut, creased, stripped of

waste, and delivered in piles or bundles.

STACKING

Off-line printing presses, rotary die cutters,

platen die cutters and flexo folder-gluers

need a means to form piles of finished prod-

uct or printed sheets. Many press suppliers

use lay-boy stackers, manufactured for many

different applications.

Up-stackers

On low board-line machines, either top or

bottom printers, an up-stacker is usually uti-

lized. Such stackers, of simple or rugged con-

struction, consist of belt conveyors with the

exit end raised progressively to form a pile

from the bottom up to a pre-selected height.

Two-way jogging of the sheets is offered by

most manufacturers and are equipped with a

simple non-stop device. Generally an opera-

tor is needed to control the operation.

Down-stackers

On high board-line machines (80" and

higher), a down-stacker is generally utilized.

PRESSES AND PRESS EQUIPMENT 117

In this folding section,

material moves right to

left, past low friction

rods, which make the

first fold, and spiral

belts, which make the

last fold.

Upper

Vacuum Belts

Vacuum Box

Spiral Belts

Low Friction

Flexible Folding

Rods

118 FLEXOGRAPHY: PRINCIPLES & PRACTICES

ven gears throughout the gear train in the

press (sheet feeder, feed rolls, pull rolls,

counter-impression rolls, print cylinders and

anilox rolls) are in constant mesh position.

Impression adjustments do not alter the gear

positions. Compensation for these adjust-

ments is allowed by using the permanent

mesh drive coupling.

Registration

The print cylinder can be rotated indepen-

dent from the main drive gear, through a

motor-driven planetary gear or other ar-

rangement, to place printed colors into the

desired position on the sheet. This position-

ing is generally called the register adjust-

ment. The quality of a machine is judged on

its ability to hold register.

machines (the flexo folder-gluer) it was a nat-

ural to transmit the drive from element to ele-

ment by a gear train (Figure

). Generally,

the feeder is the motor-driven unit from

which all subsequent elements receive their

drive-through meshing gears. The simplest

design is to have the print cylinder gear

equipped with a drive gear of a pitch diameter

equal to the printing diameter measured over

the printing plates. When printing units are

closely linked, this gear may mesh with the

drive gear of another printing unit directly, or

more often through an intermediate gear. The

pull-roller shafts that transport the sheets

through the machine must then have a gear

ratio that will produce a surface speed equal

to the printing circumference of the printing

cylinder. When the press is closed, all the dri-

At the counter/ejector,

boxes are counted,

bundled and finally,

compressed so that the

glue tab and fourth

panel bond.

Trombone

Fingers

Bundle

Compression

Section

Elevator

The printed sheets are conveyed into down-

stackers either by belt ramps or with a vacu-

um conveyor at the sheet-traveling level. A

suspended pile-carrying platform cycles

downward until a certain pile height is

reached. A so-called “non-stop device,”

either a roller rack or a belt-type interceptor,

is then introduced to catch the continuously

arriving sheets for the next load.

In a typical operation the boxes enter the

counter/ejector section, where they are

counted, stacked and placed into bundles. As

the bundle height increases, the elevator

drops to keep the top of the bundle a fixed

distance from the in-feed conveyor. When the

bundle reaches the pre-set count, a set of

trombone fingers extends down and out, on

top of the last box in the bundle. The trom-

bone fingers catch the first few boxes of the

next bundle while the elevator drops and dis-

charges the finished bundle into the dis-

charge compression conveyor section. Once

the first boxes are stabilized, a set of auxiliary

fingers extends to allow the trombone fingers

to retract and return to their ready position

above the bundle that is accumulating on the

auxiliary fingers. The elevator then rises to

support the accumulating bundle and the aux-

iliary fingers retract, leaving the bundle on the

elevator. To gain pile height, down-stackers

may be installed into a floor pit.

THE GEAR-DRIVEN PRESS

Since flexo in the sheet-fed post-printing

sector evolved first on in-line, close coupled

PRESSES AND PRESS EQUIPMENT 119

After printing, a feed

conveyor transports the

sheets to the platen die

cutter.

A gear-driven press, is

motor, from which all

subsequent units

receive their power.

Feed Conveyor

Platen

Die-Cut

Stationary Print Units Feeder

Die-cut

Fixed

Out Feed

Conveyor

Print #2

Cylinder

Gear

Impression

Roll Gear

Print #1

Feed Roll Gears

Anilox

Roll

Main Drive

Motor

Feed Section

Pull

Roll

Gears

120 FLEXOGRAPHY: PRINCIPLES & PRACTICES

On a typical close-coupled, gear-driven

press, once the printing plate is mounted,

the machine is closed and started up. Print

registration control can be achieved using

the motor-driven running register and the

manual lateral print register wheel on the

operator side of the print station. The motor-

driven planet gear register is adjusted to

change the placement of the printing design

with respect to the leading edge of the car-

ton. The register control switch moves the

cylinder in either the forward or reverse

direction with respect to the main drive gear.

The actual registration position relative to

the main gear is displayed on an indication

dial located on the print section.

The print cylinder can be moved laterally,

across machine direction, in relationship to

the sheet during the setup of the machine by

using the hand wheel located on the operat-

ing side of the print station. The hand wheel

moves the cylinder laterally, either left or

right, ±1", without interfering with the print-

ing plates.

As long as the machine is properly timed

and zeroed between each setup, the print

rect, even before the first blank is run

through the machine. Fine-print register con-

trol is achieved by the operator running a test

board through the machine before an order

run is started. The operator runs a sheet

through the machine, examines the results

on the printed box, and then adjusts the reg-

ister until the test box meets customer spec-

ification. On some of the more modern press-

es, the operator can use the print controls on

the print station or on the counter/ejector

station to adjust the print register in forward

and reverse, or from side to side.

Gear-driven machines are, by a large mar-

gin, predominant in the corrugated industry.

It is easy to understand why gear play must

be controlled to the greatest extent to ensure

ensure that a minimum of play exists, while

still allowing effective lubrication of the gear

train and ensuring that gears are not becom-

ing loose on the shafts they are driving.

Nevertheless, machines with 100% gear-dri-

ven elements are subject to increasing regis-

ter variations as the machine ages.

Typically, on a gear-driven flexo folder-

gluer with four-color stations, in-line register

tolerances of ±0.040" (1 mm) from first to

last color are to be expected. Such inaccu-

racies come into play mainly when machine

speeds need to be varied during production

and therefore, the total accumulated play

between gears enters into account. During

acceleration cycles, this play is toward one

side and during slowdown it is toward the

other side. On older machines, the last print-

ing cylinder of the machine may need to be

rotated back and forth up to a quarter of an

inch due to accumulative gear play. Failure

to properly close and lock the individual

machine sections will result in excessive

backlash and poor print register.

LINE SHAFT-DRIVEN PRESS

A line shaft-driven press already has a cer-

tain advantage over straight gear-driven

machines in that gear play from print unit to

print unit is not cumulative. A rigid line shaft

in this case drives each printing cylinder

through right-angle gearboxes in a worm-

gear arrangement (Figure

). If one dis-

counts some slight torsion forces on the line

shaft, it becomes apparent that only one gear

play enters into account per printing unit

rather than a cumulative effect involving sev-

eral in a gear train. This setup means, at least

in theory, that the gear play of only one print-

ing unit determines print register tolerance.

TRENDS IN PRESS DESIGN

As recently as 20 years ago, manufacturers

did not build a sheet-fed flexo press for cor-

rugated as a separate entity. Instead, flexo

printing units were always put in-line with

slotting, folding and glueing operations

(flexo folder-gluer), flexo rotary die cutting

or flexo platen die cutting. The close-cou-

pled printing units of the printer-rotary die

cutter and flexo folder-gluer machines have

so-called roll-away units on tracks to gain

access to the various printing elements for

maintenance or repair. This setup was disad-

vantageous, especially when printing on

coated stock, as there is lack of space avail-

able for installing effective drying equipment

to guarantee dry-trapping of colors.

With the advent of vacuum-transport for

the sheets instead of roller nips, several

press manufacturers have already success-

fully addressed this space problem. In some

cases, special drying elements are placed

between or after printing units, allowing

complete exposure of the sheets to a drying

source or simply to the ambient air.

Servo-drive Presses

The newest approach in drive technology

on printing machines is the so-called servo

drive, whereby each printing unit has a sep-

arate drive motor, or motors. The motors are

controlled electronically to rotate at a syn-

chronized speed with each other. With this

technology, the angular position of the print

cylinders can be controlled with incredible

accuracy. The scale of measure is truly in the

region of one-thousandths of an inch.

Some manufacturers go as far as driving

every shaft on each printing unit with a sepa-

rate motor, while others use one motor per

printing unit. In the latter case, the auxiliary

functions such as driving the anilox or rub-

ber roll are still a chain of gears.

Servo-drive technology has truly opened

new horizons in the flexibility of sheet-fed

printing presses. One manufacturer’s printing

press has the following drive arrangement:

• one motor to drive the feeder;

• one motor to drive a roller/vacuum

sheet transport system through all

printing units; and

• one motor to drive each printing unit

(up to eight printing units may be

placed in-line).

On servo-drive machines, register adjust-

ment is achieved through angular displace-

ment of the print-cylinder drive motor in ref-

erence to a mechanical fixed point or to the

motor position on another printing unit. Real

time position of each and every motor is fed

into microprocessors through high quality

encoders fixed directly on the motor shafts.

Servo-drive printing press technology is in a

very advanced development and implemen-

tation state, even though many applications

PRESSES AND PRESS EQUIPMENT 121

In a line-shaft driven

press, a rigid line shaft

drives each print cylin-

der, thus eliminating

accumulative gear play.

Die-cut

Fixed

Out Feed

Conveyor

Print #2

Cylinder

Gear

Impression

Roll Gear

Print #1

Feed Roll Gears

Anilox

Roll

Main Drive

Motor

Line

Shaft

Feed Section

Pull

Roll

Gears

122 FLEXOGRAPHY: PRINCIPLES & PRACTICES

still remain to be explored.

Closed-loop register and quality controls

have already been introduced to the indus-

try, including features such as utilization of

variable caliper printing plates, and auto-

matic printing length adjustment, all of

which can be controlled through computers

controlling the servo drives.

The technology of servo drives, responder

drives or electric line shafts, was introduced,

for even very large in-line machines, about

1980. Even some of the early machines

equipped this way had over 50 independent

motors controlled by computer.

Free-standing Off-line Presses

As outlined in the introductory comments

about evolution and growth, flexo printing in

the corrugated industry was first seen as a

necessary “add-on,” if not “evil,” to equip con-

verting processes with graphics capabilities

for tasks such as identification or handling

instructions. This simple start evolved rather

quickly into a demand for more refined mes-

sages on the outside of boxes and a demand

for more colors.

In the late 1970s, the quality of printed

images produced on a sheet-fed flexo corru-

gated press was being pushed to its limits,

always as an in-line operation. The technolo-

gy did not begin to change until flexo post-

print quality was required to compete with

other processes that were perceived as much

higher quality. The time had come for multi-

color free-standing flexo presses where no

mechanical influence, such as folding rods,

die-cutting anvils, under-stackers and other

machine components would negatively influ-

ence printed images. The need for inter-sta-

tion drying and isolation of the printing

process from dust producing converting

processes led to the self-contained free-stand-

ing sheet-fed flexo press.

Today, multi-color, high quality post-printing

on corrugated is becoming a self-contained

manufacturing step, similar to offset printing,

where the printing process is separated away

from other converting operations. This trend

is true to the point where flexo printing oper-

ations are often enclosed in a clean, dust-free

environment to reach the highest quality and

production standards. With this change in

mind, a majority of press manufacturers are

no longer building machines to match other

equipment in terms of size. However, the

aforementioned arguments for conforming to

the corrugated manufacturing process, as a

whole, are still respected. Bottom-printing is

becoming, more and more, a standard in off-

line, sheet-fed postprinting.

Flexibility of Off-line Presses. The increasing

demand for value-added graphics and more

“defect-free’ printing has stimulated press

manufacturers to produce “pure” printing

presses, isolated if possible, from other man-

ufacturing operations. The idea appears to be

the same as in preprinting, where central im-

pression or stack presses are put in an isolat-

ed, protected environment to keep the oper-

ation clean from dust, noise and other haz-

ards. A sheet-fed off-line flexo press installed

in this manner is indeed very flexible.

Printing quality and operational speed no

longer depend on other manufacturing steps.

Practice has shown that a flexo press can

run roughly twice as fast as a platen die cut-

ter. Therefore, the true capacity of the flexo

press can be exploited since it is, when

installed off-line, no longer a slave to a pos-

sibly slower converting operation. Dust from

other operations does not have to be dealt

with. Many of today’s installations benefit

from air conditioning and noise abatement,

making the press operator’s environment a

more friendly one. Off-line flexographic

presses are the most ideal for achieving high

quality on short or long runs.

A disadvantage of the off-line press may be

that the finished printed sheets need to be

transported through additional conveyors or

other means of transportation to the finishing

machines, such as rotary or platen die cutters.